Patent Publication Number: US-11655810-B2

Title: Electrically operated displacement pump control system and method

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 17/313,663 filed May 6, 2021 and entitled “ELECTRICALLY OPERATED DISPLACEMENT PUMP CONTROL SYSTEM AND METHOD,” which in turn is a continuation of International PCT Application No. PCT/US2021/025121 filed Mar. 31, 2021 and entitled “ELECTRICALLY OPERATED DISPLACEMENT PUMP ASSEMBLY,” which claims the benefit of U.S. Provisional Application No. 63/002,674 filed Mar. 31, 2020, and entitled “ELECTRICALLY OPERATED DISPLACEMENT PUMP,” the disclosures of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     This disclosure relates to positive displacement pumps and more particularly to a drive system for positive displacement pumps. 
     Positive displacement pumps discharge a process fluid at a selected flow rate. In a typical positive displacement pump, a fluid displacement member, usually a piston or diaphragm, pumps the process fluid. 
     Fluid-operated double displacement pumps typically employ diaphragms as the fluid displacement members and air or hydraulic fluid as a working fluid to drive the fluid displacement members. In an air operated double displacement pump, the two diaphragms are joined by a shaft and compressed air is the working fluid. Compressed air is applied to one of two chambers associated with the respective diaphragms. The first diaphragm is driven through a pumping stroke and pulls the second diaphragm through a suction stroke when compressed air is provided to the first chamber. The diaphragms move through a reverse stroke when compressed air is provided to the second chamber. Delivery of compressed air is controlled by an air valve, and the air valve is usually actuated mechanically by the diaphragms. One diaphragm is pulled until it causes the actuator to toggle the air valve. Toggling the air valve exhausts the compressed air from the first chamber to the atmosphere and introduces fresh compressed air to the second chamber, thereby causing reciprocation of the respective diaphragms. 
     Double displacement pumps can also be mechanically operated such that the pump does not require the use of working fluid. In such a case, a motor is operatively connected to the fluid displacement members to drive reciprocation. A gear train is disposed between the motor and the shaft connecting the fluid displacement members to ensure that the pump can provide sufficient torque during pumping. The motor and gear train are disposed external to the main body of the pump. 
     SUMMARY 
     According to one aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor; a fluid displacement member configured to pump fluid; and a drive mechanism connected to the rotor and the fluid displacement member. The drive mechanism converts a rotational output from the rotor into a linear input to the fluid displacement member. The drive mechanism includes a screw connected to the fluid displacement member and a plurality of rolling elements disposed between the screw and the rotor. The screw is disposed coaxially with the rotor. The plurality of rolling elements support the screw relative the rotor and drive the screw axially. 
     According to another aspect of the disclosure, a method of pumping includes driving rotation of a rotor of an electric motor; linearly displacing a screw shaft in a first axial direction such that the screw shaft drives a first fluid displacement member attached to a first end of the screw shaft through one of a first suction stroke and a first pumping stroke, wherein the screw is coaxial with the rotor and supported by a plurality of rolling elements disposed between the rotor and the screw shaft; and linearly displacing, by the plurality of rolling elements, the screw shaft in a second axial direction opposite the first axial direction. 
     According to yet another aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor disposed in a pump housing; a fluid displacement member configured to pump fluid and interfacing with the pump housing such that the fluid displacement member is prevented from rotating relative to the pump housing; and a drive mechanism connected to a rotor of the electric motor and to the fluid displacement member and configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. The drive mechanism includes a screw connected to the fluid displacement member. The screw provides the linear input to the fluid displacement member. The screw interfaces with the fluid displacement member such that the screw is prevented from rotating relative to the fluid displacement member. 
     According to yet another aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor disposed in a pump housing and including a stator and a rotor rotatable about a pump axis; a fluid displacement member configured to reciprocate on the pump axis to pump fluid; and a drive mechanism connected to the rotor and to the fluid displacement member and configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. The fluid displacement member interfaces with the pump housing at a first interface. The drive mechanism includes a screw connected to the fluid displacement member at a second interface. The first interface and the second interface prevent the screw from rotating about the pump axis and relative to the fluid displacement member and the pump housing. 
     According to yet another aspect of the disclosure, a double diaphragm pump having an electric motor includes a housing; an electric motor comprising a stator and a rotor with the rotor configured to rotate to generate rotational input; a screw that receives the rotational input and converts the rotational input into linear input; a first diaphragm and a second diaphragm. The screw is located between the first and second diaphragms and each of the first and second diaphragms receiving the linear input such that each of the first and second diaphragms reciprocate to pump fluid. Each of the first and second diaphragms are rotationally fixed by the housing. The first and second diaphragms are rotationally fixed with respect to the screw such that the screw is prevented from rotating, despite the rotational input, by the first and second diaphragms rotationally fixing the screw. 
     According to yet another aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor disposed in a pump housing, the electric motor comprising a stator and a rotor with the rotor configured to rotate about a pump axis, a fluid displacement member configured to pump fluid by linear reciprocation of the fluid displacement member, and a drive mechanism connected to the rotor and to the fluid displacement member. The fluid displacement member interfaces with the pump housing such that the fluid displacement member is prevented from rotating relative to the pump housing. The drive mechanism includes a screw connected to the fluid displacement member and is configured to receive rotational output from the rotor and convert the rotational output from the rotor into a linear input to the fluid displacement member to linearly reciprocate the fluid displacement member. The screw is prevented from being rotated by the rotational output by an interface between the screw and the pump housing. 
     According to yet another aspect of the disclosure, a method of pumping fluid by a reciprocating pump includes driving rotation of a rotor of an electric motor by a stator of the electric motor; causing, by rotation of the rotor, a screw shaft disposed coaxially with the rotor to reciprocate along a pump axis, the screw shaft driving a fluid displacement member through a suction stroke and a pumping stroke; preventing rotation of the fluid displacement member relative to a pump housing of the pump by a first interface between the fluid displacement member and the pump housing; and preventing rotation of the screw shaft about the axis by the first interface and a second interface between the screw shaft and the fluid displacement member. 
     According to yet another aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor disposed in a pump housing and including a stator and a rotor; a fluid displacement member configured to pump fluid; and a screw connected to the fluid displacement member. The screw is operably connected to the rotor such that rotation of the rotor drives linear displacement of the screw along a pump axis. The screw includes a shaft body and a lubricant pathway extending through the shaft body and configured to provide lubricant to an interface between the screw and the rotor. 
     According to yet another aspect of the disclosure, a method of lubricating an electric displacement pump includes providing lubricant to an interface between a screw shaft and a rotor of a pump motor of the pump via a lubricant pathway extending through the screw shaft, wherein the screw shaft is disposed coaxially with the rotor. 
     According to yet another aspect of the disclosure, a displacement pump for pumping a fluid includes an electric motor at least partially disposed in a pump housing and including a stator and a rotor and a first fluid displacement member connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first fluid displacement member. The first fluid displacement member fluidly separates a first process fluid chamber disposed on a first side of the first fluid displacement member from a first cooling chamber disposed on a second side of the first fluid displacement member. The first fluid displacement member simultaneously pumps process fluid through the first process fluid chamber and pumps air through the first cooling chamber. 
     According to yet another aspect of the present disclosure, a double diaphragm pump having an electric motor includes a housing; an electric motor comprising a stator and a rotor with the rotor configured to rotate to generate rotational input; a first diaphragm connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first diaphragm; and a second diaphragm connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the second diaphragm. The first diaphragm fluidly separates a first process fluid chamber disposed on a first side of the first diaphragm from a first cooling chamber disposed on a second side of the first diaphragm. The second diaphragm fluidly separates a second process fluid chamber disposed on a first side of the second diaphragm from a second cooling chamber disposed on a second side of the second diaphragm. The first diaphragm and the second diaphragm reciprocate in a first direction and a second direction. The first diaphragm simultaneously performs a pumping stroke of the process fluid and a suction stroke of the air as the first diaphragm moves in the first direction. The second diaphragm simultaneously performs a suction stroke of the process fluid and a pumping stroke of the air as the second diaphragm moves in the first direction. The first diaphragm simultaneously performs a pumping stroke of the air and a suction stroke of the process fluid as the first diaphragm moves in the second direction. The second diaphragm simultaneously performs a pumping stroke of the process fluid and a suction stroke of the air as the second diaphragm moves in the second direction. 
     According to yet another aspect of the disclosure, a method of cooling an electrically operated diaphragm pump includes driving reciprocation of a first fluid displacement member and a second fluid displacement member by an electric motor having a rotor configured to rotate about a pump axis, wherein the first fluid displacement member and the second fluid displacement member are disposed coaxially with the rotor and connected to the rotor via a drive mechanism; drawing air into a first cooling chamber of a cooling circuit of the pump by the first fluid displacement member, the first cooling chamber disposed between the first fluid displacement member and the rotor; pumping the air from first cooling chamber to a second cooling chamber disposed between the second fluid displacement member and the rotor; and driving the air out of the second motor chamber by the second fluid displacement member to exhaust the air from the cooling circuit. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a rotor and a stator extending about the rotor, a fluid displacement member configured to pump fluid and disposed coaxially with the rotor, a drive mechanism connected to the rotor and the fluid displacement member, and a position sensor disposed proximate the rotor, the position sensor configured to sense rotation of the rotor and to provide data to a controller. The drive mechanism is configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the fluid displacement member; and a controller. The controller is configured to regulate current flow to the electric motor such that the rotor applies torque to the drive mechanism with the pump in both a pumping state and a stalled state. In the pumping state, the rotor applies torque to the drive mechanism and rotates about the pump axis causing the fluid displacement member to apply force to a process fluid and displace axially along the pump axis. In the stalled state, the rotor applies torque to the drive mechanism and does not rotate about the pump axis such that the fluid displacement member applies force to the process fluid and does not displace axially. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; and regulating, by a controller, a flow of current to a stator of the electric motor such that rotational force is applied to the rotor during both a pumping state and a stalled state. In the pumping state, the rotor applies torque to the drive mechanism and rotates about the pump axis causing the fluid displacement member to apply force to a process fluid and displace axially along the pump axis. In the stalled state, the rotor applies torque to the drive mechanism and does not rotate about the pump axis such that the fluid displacement member applies force to the process fluid and does not displace axially. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes providing electric current to an electric motor disposed on a pump axis and connected to a fluid displacement member configured to reciprocate along the pump axis; and regulating, by a controller, current flow to the electric motor to control a pressure output by the pump to a target pressure. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member; and a controller. The drive mechanism is configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. The controller is configured to cause current to be provided to the stator to drive rotation of the rotor, thereby driving reciprocation of the fluid displacement member; and regulate the current flow to the electric motor to control a pressure output by the pump to a target pressure. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a fluid displacement member along a pump axis, the fluid displacement member disposed coaxially with a rotor of the electric motor; regulating, by a controller, a rotational speed of the rotor thereby directly controlling an axial speed of the fluid displacement member such that the rotational speed is at or below a maximum speed; regulating, by the controller, current provided to the electric motor such that the current provided is at or below a maximum current. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a fluid displacement member along a pump axis, the fluid displacement member disposed coaxially with a rotor of the electric motor, wherein the fluid displacement member includes a variable working surface area; and varying, by a controller, current provided to the electric motor such that a first current is provided to the electric motor at a beginning of a pumping stroke of the fluid displacement member and a second current is provided to the electric motor at an end of the pumping stroke, the second current less than the first current. 
     According to yet another aspect of the present disclosure, a dual pump for pumping a fluid includes an electric motor comprising a stator and a rotor with the rotor configured to generate rotational input; a controller configured to regulate current flow to the electric motor; a drive mechanism comprising a screw extending within the rotor and configured to receive the rotational input and convert the rotational input into linearly reciprocating motion of the screw, a first fluid displacement member, and a second fluid displacement member. Rotation of the rotor in a first direction drives the screws to linearly move in a first direction along an axis, and rotation of the rotor in a second direction drives the screws to linearly move in a second direction along the axis. The screw is located between the first and the second fluid displacement members. The screw reciprocates the first and the second fluid displacement members in the first direction along the axis when the rotor rotates in the first direction and in the second direction along the axis when the rotor rotates in the second direction. The first fluid displacement performs a pumping stroke of the process fluid and the second fluid displacement performs a suction stroke of the process fluid as the screw moves in the first direction. The first fluid displacement performs a suction stroke of the process fluid and the second fluid displacement performs a pumping stroke of the process fluid as the screw moves in the second direction. The controller regulates output pressure of the process fluid by regulating current flow to the motor such that the rotor rotates to cause the first and the second fluid displacement members to reciprocate to pump the process fluid until pressure of the process fluid stalls the rotor while the first fluid displacement member is in the pump stroke and the second fluid displacement member is in the suction stroke even while current continues to be supplied to the motor by the controller, the first and the second fluid displacement members resuming pumping when the pressure of the process fluid drops enough for the rotor to overcome the stall and resume rotating. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a first fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a second fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the first and second fluid displacement members and including a screw and configured to convert a rotational output from the rotor into a linear input to the first and second fluid displacement members, and a controller configured to operate the pump in a start-up mode and a pumping mode. During the start-up mode the controller is configured to cause the motor to drive the first and second fluid displacement members in a first axial direction; and determine an axial location of at least one of the first and second fluid displacement members based on the controller detecting a first current spike when the at least one of the first and second fluid displacement members encounters a first stop. Moving the first and second fluid displacement members in the first axial direction moves one of the first and second fluid displacement members through a pumping stroke and moves the other of the first and second fluid displacement members through a suction stroke. Moving the first and second fluid displacement members in a second axial direction opposite the first axial direction moves the one of the first and second fluid displacement members through a suction stroke and moves the other of the first and second fluid displacement members through a pumping stroke. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member; and a controller configured to operate the pump in a start-up mode and a pumping mode. The drive mechanism is configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. During the start-up mode, the controller is configured to cause the motor to drive the fluid displacement member in a first axial direction; and determine an axial location of the fluid displacement member based on the controller detecting a first current spike when the fluid displacement member encounters a first stop. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member in a first axial direction on a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and determining, by a controller, an axial location of the first fluid displacement member based on the controller detecting a current spike due to the first fluid displacement member encountering a first stop and the rotor stopping rotation. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; initiating, by a controller, deceleration of the rotor when the first fluid displacement member is at a first deceleration point disposed a first axial distance from a first target point along the pump axis; determining, by the controller, a first adjustment factor based on a first axial distance between a first stopping point and the first target point, wherein the first stopping point is an axial location where the first fluid displacement member stops displacing in the first axial direction; and managing, by the controller, a stroke length based on the first adjustment factor. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor; a fluid displacement member connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first fluid displacement member; and a controller. The controller is configured to regulate current flow to the electric motor based on a current limit to thereby regulate an output pressure of the fluid pumped by the fluid displacement member; regulate a rotational speed of the rotor based on a speed limit to thereby regulate an output flowrate of the fluid pumped by the fluid displacement member; and set a current limit and a speed limit based on a single parameter command received by the controller. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; regulating, by a controller, a flow of current to a stator of the electric motor based on a current limit; regulating, by the controller, a speed of the rotor based on a speed limit; generating the single parameter command based on a single input from a user; and setting, by the controller, both the current limit and the speed limit based on the single parameter command received by the controller. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member operatively connected to the rotor to be reciprocated to pump fluid; and a controller configured to operate the motor in a start-up mode and a pumping mode. During the pumping mode the controller is configured to operate the electric motor based on a target current and a target speed. During the start-up mode the controller is configured to operate the electric motor based on a maximum priming speed that less than the target speed. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; regulating, by a controller, power to the electric motor to control an actual speed of the rotor during a start-up mode such that the actual speed is less than a maximum priming speed; regulating, by a controller, the power to the electric motor to control an actual speed of the rotor during a pumping mode such that the actual speed is less than a target speed. The maximum priming speed is less than the target speed. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member through a pumping stroke in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and managing, by the controller, a stroke length of the first fluid displacement member during a first operating mode and a second operating mode such that the stroke length during the second operating mode is shorter than the stoke length during the first operating mode. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member through a pumping stroke in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and managing, by the controller, a stroke of the first fluid displacement member during a first operating mode such that a pump stroke occurs in a first displacement range along the pump axis; and managing, by the controller, a stroke of the first fluid displacement member during a first operating mode such that the pump stroke occurs in a second displacement range along the pump axis, wherein the second displacement range is a subset of the first displacement range. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member operatively connected to the rotor to be reciprocated along the pump axis to pump fluid; a controller configured to operate the motor in a first operating mode and a second operating mode. During the first operating mode the controller is configured to manage a stroke length of the fluid displacement member such that a pump stroke of the fluid displacement member occurs in a first displacement range along the pump axis. During the second operating mode the controller is configured to manage the stroke length of the fluid displacement member such that the pump stroke of the fluid displacement member occurs in a second displacement range along the pump axis. The second displacement range has a smaller axial extent than the first displacement range. 
     According to yet another aspect of the present disclosure, a method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a first fluid displacement member and a second fluid displacement member to pump fluid; and monitoring, by a controller, an actual operating parameter of the electric motor; and determining, by the controller, that an error has occurred based on the actual operating parameter differing from an expected operating parameter during a particular phase of a pump cycle. 
     According to yet another aspect of the present disclosure, a displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a drive connected to the rotor, the drive configured to convert a rotational output from the rotor into a linear input; a first fluid displacement member connected to the drive to be driven by the linear input; and a controller. The controller is configured to cause current to be provided to the stator to drive rotation of the rotor, thereby driving reciprocation of the fluid displacement member; and monitor an actual operating parameter of the electric motor; and determine that an error has occurred based on the actual operating parameter differing from an expected operating parameter during a particular phase of a pump cycle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a front isometric view of an electrically operated pump. 
         FIG.  1 B  is a rear isometric view of the electrically operated pump. 
         FIG.  1 C  is a block schematic diagram of the electrically operated pump. 
         FIG.  2    is a block schematic diagram illustrating flowpaths of an electrically operated pump. 
         FIG.  3 A  is an exploded rear isometric view of an electrically operated pump. 
         FIG.  3 B  is an exploded front isometric view of a portion of an electrically operated pump. 
         FIG.  4 A  is a cross-sectional view taken along line A-A in  FIG.  1 B . 
         FIG.  4 B  is an enlarged view of detail B in  FIG.  4 A . 
         FIG.  4 C  is a cross-sectional view taken along line C-C in  FIG.  1 A . 
         FIG.  4 D  is a cross-sectional view taken along line D-D in  FIG.  4 B . 
         FIG.  5 A  is an isometric view of an internal check valve and end cap. 
         FIG.  5 B  is an enlarged cross-sectional view of a portion of an electrically operated pump. 
         FIG.  6 A  is an exploded view of an air check assembly. 
         FIG.  6 B  is an isometric view of an inner side of the air check assembly. 
         FIG.  6 C  is an enlarged cross-sectional view of the air check assembly mounted to a pump. 
         FIG.  7    is a cross-sectional exploded view of a fluid displacement member, fluid cover, and portion of a drive mechanism. 
         FIG.  8 A  is an isometric view of an electrically operated pump. 
         FIG.  8 B  is an isometric view of the electrically operated pump shown in  FIG.  8 A  but with a housing cover removed. 
         FIG.  8 C  is an isometric view of a pump body of the electrically operated pump shown in  FIG.  8 A . 
         FIG.  8 D  is a cross-sectional view taken along line D-D in  FIG.  8 A . 
         FIG.  8 E  is a cross-sectional view taken along line E-E in  FIG.  8 A . 
         FIG.  9 A  is a partially exploded isometric view of an electrically operated pump. 
         FIG.  9 B  is an exploded cross-sectional view of an interface between a fluid displacement member and a drive mechanism. 
         FIG.  9 C  is an isometric view of an end of a screw. 
         FIG.  10    is a cross-sectional block diagram showing an anti-rotation interface. 
         FIG.  11    is a block diagram showing an anti-rotation interface. 
         FIG.  12    is an isometric partial cross-sectional view showing a motor and drive mechanism of an electrically operated pump. 
         FIG.  13    is an isometric view of a drive mechanism with a portion of the drive nut removed. 
         FIG.  14    is an isometric view of a drive mechanism with a portion of the drive nut removed. 
         FIG.  15    is an isometric view of the drive mechanism shown in  FIG.  13    with the body of the drive nut removed to show the rolling elements. 
         FIG.  16 A  is a first isometric view of a motor nut. 
         FIG.  16 B  is a second isometric view of the motor nut. 
         FIG.  17 A  is an enlarged cross-sectional view of a portion of an electrically operated pump. 
         FIG.  17 B  is an isometric view of a portion of a rotor. 
         FIG.  18    is an enlarged cross-sectional view of a portion of an electrically operated pump. 
         FIG.  19    is a block diagram of an electrically operated pump. 
         FIG.  20 A  is a block diagram illustrating a first changeover location relative a target point. 
         FIG.  20 B  is a block diagram illustrating a second changeover location relative the target point. 
         FIG.  20 C  is a block diagram illustrating a third changeover location relative the target point. 
         FIG.  21    is a flowchart illustrating a method of operating a reciprocating pump. 
         FIG.  22    is a flowchart illustrating a method of operating a reciprocating pump. 
         FIG.  23    is a flowchart illustrating a method of operating a reciprocating pump. 
         FIG.  24    is a flowchart illustrating a method of operating a reciprocating pump. 
         FIG.  25 A  is an isometric view of a rotor assembly. 
         FIG.  25 B  is an exploded view of the rotor assembly of  FIG.  25 A . 
         FIG.  25 C  is a cross-sectional view of the rotor assembly of  FIG.  25 A . 
         FIG.  26    is a cross-sectional view of a rotor assembly. 
         FIG.  27    is a cross-sectional view of a rotor assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1 A  is a front isometric view of electrically operated pump  10 .  FIG.  1 B  is a rear isometric view of pump  10 .  FIG.  1 C  is a block schematic diagram of pump  10 .  FIGS.  1 A- 1 C  will be discussed together. Pump  10  includes inlet manifold  12 , outlet manifold  14 , pump body  16 , fluid covers  18   a ,  18   b  (collectively herein “fluid cover  18 ” or “fluid covers  18 ”), fluid displacement members  20   a ,  20   b  (collectively herein “fluid displacement member  20 ” or “fluid displacement members  20 ”), motor  22 , drive mechanism  24 , and controller  26 . Motor  22  includes stator  28  and rotor  30 . 
     Pump body  16  is disposed between fluid covers  18   a ,  18   b . Motor  22  is disposed within pump body  16  and is coaxial with fluid displacement members  20 , as discussed in more detail below. Motor  22  is an electric motor having stator  28  and rotor  30 . Stator  28  includes armature windings and rotor  30  includes permanent magnets. Rotor  30  is configured to rotate about pump axis PA-PA in response to current (such as a direct current (DC) signals and/or alternating current (AC) signals) through stator  28 . Motor  22  is a reversible motor in that stator  28  can cause rotor  30  to rotate in either of two rotational directions (e.g., alternating between clockwise and counterclockwise). Rotor  30  is connected to the fluid displacement members  20  via drive mechanism  24 , which receives a rotary output from rotor  30  and provides a linear, reciprocating input to fluid displacement members  20 . Fluid displacement members  20  can be of any type suitable for pumping fluid from inlet manifold  12  to outlet manifold  14 , such as diaphragms or pistons. While pump  10  is shown as including two fluid displacement members  20 , it is understood that some examples of pump  10  include a single fluid displacement member  20 . Further, while the two fluid displacement members  20  are shown herein as diaphragms, they could instead be pistons in various other embodiments, and the teachings provided herein can apply to piston pumps. 
     Controller  26  is operatively connected to motor  22  to control operation of motor  22 . User interface  27  of controller  26  is shown. During operation, current signals are provided to stator  28  to cause stator  28  to drive rotation of rotor  30 . Drive mechanism  24  receives the rotational output from rotor  30  and converts that rotational output into a linear output to drive fluid displacement members  20 . In some examples, rotor  30  rotates in the first rotational direction to drive fluid displacement members  20  in a first axial direction and rotates in the second rotational direction to drive fluid displacement members  20  in a second axial direction. 
     Drive mechanism  24  causes fluid displacement members  20  to reciprocate along pump axis PA-PA through alternating suction and pumping strokes. During the suction stroke, the fluid displacement member  20  draws process fluid from inlet manifold  12  into a process fluid chamber defined, at least in part, by fluid covers  18  and fluid displacement members  20 . During the pumping stroke, the fluid displacement member  20  drives fluid from the process fluid chamber to outlet manifold  14 . Typically, depending on the arrangement of check valves, the two fluid displacement members  20  are operated 180 degrees out of phase, such that a first fluid displacement member  20  is driven through a pumping stroke (e.g., driving process fluid downstream from the pump) while a second fluid displacement member  20  is driven through a suction stroke (e.g., pulling process fluid upstream from the pump). The two fluid displacement members  20  also simultaneous changeover (e.g., transition between the pumping stroke and the suction stroke) but 180 degrees out of phase with respect to each other. 
     Drive mechanism  24  is directly connected to rotor  30  and fluid displacement members  20  are directly driven by drive mechanism  24 . As such, motor  22  directly drives fluid displacement members  20  without the presence of intermediate gearing, such as speed reduction gearing. Power cord  32  extends from pump  10  and is configured to provide electric power to the electronic components of pump  10 . Power cord  32  can connect to a wall socket. 
       FIG.  2    is a block diagram of pump  10  illustrating fluid flowpaths through pump  10 . Process fluid flowpath PF extends from inlet manifold  12  to outlet manifold  14  through process fluid chambers  34   a ,  34   b  (collectively herein “process fluid chamber  34 ” or “process fluid chambers  34 ”). It is understood that process fluid chambers  34  can be connected to a common inlet manifold  12  and outlet manifold  14 . Cooling fluid circuit CF extends through the interior of pump  10  and routes cooling fluid, such as air, through pump  10  to cool components of pump  10 . The main heat sources of pump  10  include controller  26 , stator  28 , and drive mechanism  24 . Cooling fluid circuit CF directs cooling air through passages proximate the heat generating components to affect heat exchange between the cooling air and heat sources and thereby cool pump  10 . Not all embodiments necessarily include a cooling fluid circuit or otherwise pump cooling air. 
     Cooling fluid circuit CF is configured to direct cooling air through pump  10  to cool heat generating components of pump  10 , such as drive mechanism  24 , controller  26 , and stator  28 . Pump  10  pumps cooling air through cooling fluid circuit CF. Fluid displacement members  20   a ,  20   b  are disposed out of phase, such that one fluid displacement member  20  moves through a pumping stroke for the cooling air as the other moves through a suction stroke for the cooling air, and the check valves  48 ,  50 ,  52  are arranged such that the cooling air enters one side of pump  10  and exits the other side of pump  10 . Relatively cooler air enters pump  10  and relatively warmer air exits pump  10 . Fluid displacement members  20  can be utilized for pumping the cooling air as fluid displacement members  20  are not moved by a working fluid (e.g., compressed air) but are instead electromechanically driven by motor  22  and drive mechanism  24 . Fluid displacement members  20  can thus pump both process fluid and cooling air through pump  10 . 
     Cooling fluid circuit CF includes first cooling passage  36 , second cooling passage  38 , third cooling passage  40 , fourth cooling passage  42 , and cooling chambers  44   a ,  44   b  (collectively herein “cooling chamber  44 ” or “cooling chambers  44 ”). Air check  46  is disposed at the inlet/exhaust of cooling fluid circuit CF and controls flow of cooling air for unidirectional flow through flowpath CF. 
     Air check  46  includes inlet valve  48  and outlet valve  50 . Inlet valve  48  is a one-way valve that allows cooling air to enter cooling fluid circuit CF and prevents cooling air from backflowing out of cooling chamber  44   a  through air check  46 . Outlet valve  50  is a one-way valve that allows cooling air to exit cooling fluid circuit CF and prevents atmospheric air from entering cooling fluid circuit CF through outlet valve  50 . Air check  46  can be configured such that one or both of the exhaust and intake flows are directed over cooling fins formed on pump body  16 , providing further cooling to pump  10 . 
     Internal valve  52  is disposed in cooling fluid circuit CF where second cooling passage  38  and third cooling passage  40  provide cooling air to cooling chamber  44   b . Internal valve  52  is a one-way valve that controls flow of cooling air within cooling fluid circuit CF to cause unidirectional flow through cooling fluid circuit CF. Internal valve  52  is a one-way valve that allows cooling air to flow into cooling chamber  44   b  and prevents retrograde flow from cooling chamber  44   b.    
     First cooling passage  36  extends from an air inlet at inlet valve  48  to cooling chamber  44   a . Cooling chamber  44   a  is disposed between fluid displacement member  20   a  and motor  22  (as shown in  FIGS.  4 A,  4 B, and  4 D ). Second cooling passage  38  and third cooling passage  40  extend from cooling chamber  44   a  to cooling chamber  44   b . Each of second cooling passage  38  and third cooling passage  40  can include one or more individual passages. In some examples, second cooling passage  38  includes a plurality of individual passages. In some examples, second cooling passage  38  includes different numbers of inlet/outlet apertures  38   i / 38   o  and pathways  38   p  extending between the inlet aperture(s)  38   i  and outlet aperture(s)  380 . In one example, second cooling passage  38  includes a single inlet aperture  38   i  in direct fluid communication with cooling chamber  44   a , a plurality of pathways  38   p , and a single outlet aperture  38   o  in direct fluid communication with cooling chamber  44   b . In some examples, third cooling passage  40  includes a plurality of individual passages. In some examples, third cooling passage  40  includes variable numbers of individual passages at different axial locations through third cooling passage  40 . For example, third cooling passage  40  can include a first number of inlet apertures  40   i , a second number of pathways  40   p , and a third number of outlet apertures  40   o . The first number, second number, and third number can each be identical, can all be different, or two can be the same with the third different. 
     In some examples, second cooling passage  38  includes stator passages that remain stationary relative to pump axis PA-PA during operation and third cooling passage  40  includes rotor passages that extends through rotor  30  (best seen in  FIGS.  4 A- 4 D and  12   ) and rotate about pump axis PA-PA during operation. For example, second cooling passage  38  can be formed by portions of pump body  16  and can be disposed at least partially between controller  26  ( FIGS.  1 C and  16   ) and stator  28  (best seen in  FIGS.  4 A- 4 D and  12   ). Third cooling passage  40  can be formed through a body of rotor  30  and can be disposed between stator  28  and drive mechanism  24 . It is understood, however, that second cooling passage  38  and third cooling passage  40  can be of any desired configuration suitable for passing cooling air between cooling chamber  44   a  and cooling chamber  44   b.    
     Internal valve  52  is disposed between second cooling passage  38  and cooling chamber  44   b  and between third cooling passage  40  and cooling chamber  44   b . Internal valve  52  is disposed at the outlet  38   o  of second cooling passage  38  and the outlet  40   o  of third cooling passage  40 . Cooling chamber  44   b  is disposed between fluid displacement member  20   b  and motor  22 . Internal valve  52  allows cooling air to flow into cooling chamber  44   b  while preventing retrograde flow through second cooling passage  38  and third cooling passage  40 . In some examples, internal valve  52  includes a single valve member associated with each of second cooling passage  38  and third cooling passage  40 . For example, a flapper valve member can extend over multiple outlets. In some examples, internal valve  52  includes multiple valve members associated with one or more outlets of second cooling passage  38  and third cooling passage  40 . In some examples, internal valve  52  includes the same number of valve members as there are outlets, such that each outlet has a dedicated valve member. For example, ball valves can be disposed in each outlet, among other options. Fourth cooling passage  42  extends from cooing chamber  44   b  to an exhaust outlet at outlet valve  50 . The cooling air exits flowpath CF through outlet valve  50 . 
     Fluid displacement member  20   a  is disposed between and fluidly isolates process fluid chamber  34   a  and cooling chamber  44   a . Fluid displacement member  20   a  can at least partially define each of process fluid chamber  34   a  and cooling chamber  44 . Fluid displacement member  20   a  shifts in a first axial direction AD 1  to decrease the volume of process fluid chamber  34   a , driving process fluid out of process fluid chamber  34   a , and increase the volume of cooling chamber  44   a , drawing cooling air into cooling chamber  44   a . Fluid displacement member  20   a  shifts in a second axial direction AD 2  opposite the first axial direction AD 1  to increase the volume of process fluid chamber  34   a , drawing process fluid from inlet manifold  12  into process fluid chamber  34   a , and decrease the volume of cooling chamber  44   a , driving cooling air out of cooling chamber  44   a . As such, fluid displacement member  20   a  proceeds through a pumping stroke for the process fluid while simultaneously proceeding through a suction stroke for the cooling air and proceeds through a suction stroke for the process fluid while simultaneously proceeding through a pumping stroke for the cooling air. Fluid displacement member  20   a  simultaneously pumps process fluid and cooling air. 
     Fluid displacement member  20   b  is substantially similarly to fluid displacement member  20   a . Fluid displacement member  20   b  pumps process fluid through process fluid chamber  34   b  and cooling air through cooling chamber  44   b . Fluid displacement member  20   b  is connected to fluid displacement member  20   a  such that pump strokes are reversed. As such, fluid displacement member  20   b  proceeds through a pumping stroke of process fluid chamber  34   b  and a suction stoke of cooling chamber  44   b  when driven in the second axial direction AD 2  and proceeds through a suction stroke of process fluid chamber  34   b  and a pumping stroke of cooling chamber  44   b  when driven in the first axial direction AD 1 . 
     During operation, fluid displacement members  20  shift axially through first and second strokes. During the first stroke, fluid displacement member  20   a  shifts through a pumping stroke for process fluid chamber  34   a  and a suction stoke for cooling chamber  44   a . Fluid displacement member  20   a  drives process fluid out of process fluid chamber  34   a  to outlet manifold  14 . Simultaneously, fluid displacement member  20   a  causes cooling chamber  44   a  to expand, drawing cooling air into cooling chamber  44   a  through inlet valve  48  and first cooling passage  36 . Fluid displacement member  20   b  shifts through a suction stroke for process fluid chamber  34   b  and a pumping stroke for cooling chamber  44   b . Fluid displacement member  20   b  causes the volume of process fluid chamber  34   b  to increase, drawing process fluid into process fluid chamber  34   b  from inlet manifold  12 . Simultaneously, fluid displacement member  20   b  causes cooling chamber  44   b  to contract, thereby driving cooling air from cooling chamber  44   b  and out of flowpath CF through fourth cooling passage  42  and outlet valve  50 . Each of inlet valve  48  and outlet valve  50  are open during the first stroke. As such, air check  46  is in an open state during the first stroke. Cooling chamber  44   b  contracting and cooling chamber  44   a  expanding causes internal valve  52  to remain in or return to a closed state, preventing the cooling air from flowing upstream from cooling chamber  44   b  through second cooling passage  38  or third cooling passage  40 . 
     Fluid displacement members  20  changeover at the end of the first stroke and are driven in the opposite axial direction during the second stroke. Fluid displacement member  20   a  shifts through a suction stroke for process fluid chamber  34   a  and draws process fluid into process fluid chamber  34   a  from inlet manifold  12 . Simultaneously, fluid displacement member  20   a  shifts through a pumping stroke for cooling chamber  44   a . The pressure rise in cooling chamber  44   a  causes inlet valve  48  to shift to a closed state, preventing retrograde flow out of cooling air out of flowpath CF through inlet valve  48 . Fluid displacement member  20   a  drives the cooling air from cooling chamber  44   a  to cooling chamber  44   b  via second cooling passage  38  and third cooling passage  40 . 
     Fluid displacement member  20   b  shifts simultaneously with fluid displacement member  20   a . Fluid displacement member  20   b  shifts through a pumping stroke for process fluid chamber  34   b  and a suction stroke for cooling chamber  44   b . The suction stroke causes outlet valve  50  to shift to a closed state, preventing atmospheric flow into cooling chamber  44   b  through air check  46 . Fluid displacement member  20   b  draws the cooling air from cooling chamber  44   a  into cooling chamber  44   b  via second cooling passage  38  and third cooling passage  40 . Both inlet valve  48  and outlet valve  50  are closed during the second stroke. As such, air check  46  is in a closed state during the second stroke. 
     The pressure in cooling chamber  44   a  and the suction in cooling chamber  44   b  cause internal valve  52  to shift to an open state, thereby opening flowpaths between cooling chamber  44   a  and cooling chamber  44   b  through second cooling passage  38  and third cooling passage  40 . A first portion of the cooling air in cooling chamber  44   a  is pumped through second cooling passage  38  and a second portion of the cooling air in cooling chamber  44   a  is pumped through third cooling passage  40 . The first and second portions of cooling air are routed past heat generating components of pump  10 . The cooling air is moved from one side of pump  10  to the other. More specifically, the cooling air is forced to flow through motor  22 . The cooling air is forced to flow over drive mechanism  24 . In some examples, cooling air is forced to flow through the drive mechanism  24 , such that the flowing air contacts the screw and/or plurality of rolling elements. The cooling air absorbs heat from those components as it flows through second cooling passage  38  and third cooling passage  40 . The suction stroke in cooling chamber  44   b  and pumping stroke in cooling chamber  44   a  cause internal valve  52  to open, thereby allowing the first and second portions of the cooling air to flow into cooling chamber  44   b.    
     After completing the second stroke, fluid displacement members  20  are driven back through the first stroke and continue to pump both cooling air and process fluid. In some examples, fluid displacement members  20   a ,  20   b  are disposed in parallel for process fluid flowpath PF. Each of fluid displacement members  20   a ,  20   b  is downstream of inlet manifold  12  and upstream of outlet manifold  14 . Neither one of fluid displacement members  20   a ,  20   b  is upstream or downstream of the other one of fluid displacement members  20   a ,  20   b . Neither one of fluid displacement members  20   a ,  20   b  receives process fluid from or provides process fluid to the other one of fluid displacement members  20   a ,  20   b.    
     While fluid displacement members  20   a ,  20   b  are disposed in parallel in process fluid flowpath PF, fluid displacement members  20   a ,  20   b  are disposed in series in cooling fluid circuit CF. Cooling chamber  44   a  is disposed upstream of and provides cooling air to cooling chamber  44   b . Fluid displacement member  20   a  forms a pumping element for cooling chamber  44   a  and fluid displacement member  20   b  forms a pumping element for cooling chamber  44   b . Fluid displacement members  20   a ,  20   b  operate in tandem to drive cooling air from cooling chamber  44   a  to cooling chamber  44   b.    
     Cooling fluid circuit CF provides air cooling for pump  10 . The main heat generating components of pump  10 , which include controller  26 , stator  28 , and drive mechanism  24 , are disposed relative to second cooling passage  38  and third cooling passage  40  to facilitate a heat exchange relationship with the cooling air. The inlet and/or outlet of cooling fluid circuit CF can be oriented to direct airflow over fins formed on pump body  16  to further cool pump  10 . Fluid displacement members  20  driving both the process fluid and cooling air provides efficient cooling without requiring additional components, such as fans. 
       FIG.  3 A  is an exploded front isometric view of pump  10 .  FIG.  3 B  is an exploded rear isometric view showing a subset of the components of pump  10 .  FIGS.  3 A and  3 B  will be discussed together. Pump  10  includes inlet manifold  12 , outlet manifold  14 , pump body  16 , fluid covers  18   a ,  18   b , fluid displacement members  20   a ,  20   b , motor  22 , drive mechanism  24 , air check  46 , internal valve  52 , bearings  54   a ,  54   b  (collectively herein “bearing  54 ” or “bearings  54 ”), motor nut  56 , pump check valves  58 , grease caps  60   a ,  60   b  (collectively herein “grease cap  60 ” or “grease caps  60 ”), position sensor  62 , and housing fasteners  64 . 
     Pump body  16  includes central portion  66  and end caps  68   a ,  68   b  (collectively herein “end cap  68 ” or “end caps  68 ”). Central portion  66  includes motor housing  70 , control housing  72 , heat sinks  74 , and stator passages  76  ( FIG.  3 B ). Fluid displacement members  20   a ,  20   b  respectively include inner plates  78   a ,  78   b  (collectively herein “inner plate  78 ” or “inner plates  78 ”); outer plates  80   a ,  80   b  (collectively herein “outer plate  80 ” or “outer plates  80 ”); membranes  82   a ,  82   b  (collectively herein “membrane  82 ” or “membranes  82 ”), and fasteners  84   a ,  84   b . Motor  22  includes stator  28  and rotor  30 . Rotor  30  includes permanent magnet array  86  and rotor body  88 . Drive nut  90  and screw  92  of drive mechanism  24  are shown. 
     End caps  68   a ,  68   b  are disposed on opposite lateral sides of central portion  66  and are attached to central portion  66  to form pump body  16 . Housing fasteners  64  extend through end caps  68  into pump body  16  to secure end caps  68  to pump body  16 . Heat sinks  74  are formed on central portion  66 . In the example shown, heat sinks  74  are formed by fins, but it is understood that heat sinks can be of any configuration suitable for increasing the surface area of pump body  16  to facilitate heat exchange to cool pump  10 . Stator passages are formed on central portion  66  at an interface between motor housing  70  and control housing  72 . Stator passages  76  define portions of second cooling passage  38  ( FIG.  2   ). Stator passages  76  are formed as projections that includes at least four sides exposed to heat generating elements within pump body  16  and cooled air flowing through stator passages  76 . For example, one side of each stator passage  76  can be disposed adjacent stator  28  while three sides of each stator passage  76  can be exposed to heated air within control housing  72 . In some examples, stator passages  76  are enclosed during operation such that the stator passages  76  are not exposed directly to atmosphere. 
     Fluid covers  18   a ,  18   b  are connected to end caps  68   a ,  68   b , respectively. Housing fasteners  64  secure fluid covers  18  to end caps  68 . Inlet manifold  12  is connected to each fluid cover  18 . Inlet ones of pump checks  58  are disposed between inlet manifold  12  and fluid covers  18   a ,  18   b . The inlet ones of pump checks  58  are one-way valves configured to allow the process fluid to flow into process fluid chambers  34   a ,  34   b  ( FIGS.  2  and  4 A ) and prevent retrograde flow from process fluid chambers  34   a ,  34   b  to inlet manifold  12 . Outlet manifold  14  is connected to each fluid cover  18 . Outlet ones of pump checks  58  are disposed between outlet manifold  14  and fluid covers  18   a ,  18   b . The outlet ones of pump checks  58  are one-way valves configured to allow the process fluid to flow out of process fluid chambers  34   a ,  34   b  to outlet manifold  14  and to prevent retrograde flow from outlet manifold  14  to process fluid chambers  34   a ,  34   b.    
     Motor  22  is disposed within motor housing  70  between end caps  68 . Control housing  72  is connected to and extends from motor housing  70 . Control housing  72  is configured to house control elements of pump  10 , such as controller  26  ( FIGS.  1 C and  19   ). Stator  28  surrounds rotor  30  and drives rotation of rotor  30 . Rotor  30  rotates about pump axis PA-PA and is disposed coaxially with drive mechanism  24  and fluid displacement members  20 . Permanent magnet array  86  is disposed on rotor body  88 . 
     Drive nut  90  is disposed within and connected to rotor body  88 . Drive nut  90  can be attached to rotor body  88  via fasteners (e.g., bolts), adhesive, or press-fit, among other options. Drive nut  90  rotates with rotor body  88 . Drive nut  90  is mounted to bearings  54   a ,  54   b  at opposite axial ends of drive nut  90 . Bearings  54  are configured to support both axial and radial forces. In some examples, bearings  54  comprise tapered roller bearings. Screw  92  extends through drive nut  90  and is connected to each fluid displacement member  20 . Screw  92  reciprocates along pump axis PA-PA to drive fluid displacement members  20  through respective pumping and suction strokes. 
     Motor nut  56  connects to a portion of pump body  16  housing stator  28 . Motor nut  56  can be considered to connect to a stator housing of pump  10 , which stator housing can be formed by the motor housing  70  and end caps  68   a ,  68   b . In the example shown, motor nut  56  connects to end cap  68   a  and secures bearings  54  within pump body  16 . Motor nut  56  preloads bearings  54 . Screw  92  can reciprocate through motor nut  56  during operation. Grease cap  60   a  is supported by motor nut  56  and motor nut  56  aligns grease cap  60   a  relative to bearing  54   a . Grease cap  60   b  is disposed adjacent bearing  54   b . Grease caps  60  prevent contaminants from entering bearings  54  and retain any grease that may liquefy during operation. 
     Internal valve  52  is connected to end cap  68   b . Internal valve  52  is connected to end cap  68   b  by grease cap  60   b . Internal valve  52  is disposed on a side of end cap  68   b  facing fluid displacement member  20   b . In the example shown, internal valve  52  is a flapper valve. 
     Fluid displacement member  20   a  is connected to first end of screw  92 . Membrane  82   a  is captured between inner plate  78   a  and outer plate  80   a . Fastener  84   a  extends through each of inner plate  78   a , outer plate  80   a , and membrane  82  and into screw  92  to connect fluid displacement member  20   a  to drive mechanism  24 . An outer circumferential edge of membrane  82   a  is captured between fluid cover  18   a  and end cap  68   a . Membrane  82   a  is captured to prevent fluid displacement member  20   a  from rotating about pump axis PA-PA. 
     Fluid displacement member  20   b  is connected to an opposite axial end of screw  92  from fluid displacement member  20   a . In the example shown, membrane  82   b  is overmolded onto outer plate  80   b . Fastener  84   b  extends from outer plate  80   b  through the inner plate  78   b  and into screw  92  to connect fluid displacement member  20   b  to drive mechanism  24 . An outer circumferential edge of membrane  82   b  is captured between fluid cover  18   b  and end cap  68   b . Membrane  82   b  is captured to prevent fluid displacement member  20   b  from rotating about pump axis PA-PA. While fluid displacement members  20  are described as having different configurations, it is understood that pump  10  can include fluid displacement members  20  having the same or differing configurations. 
     During operation, current signals are provided to stator  28  to drive rotation of rotor  30 . Position sensor  62  is disposed proximate rotor  30 , as discussed in more detail below, and generates position data regarding the rotational position of rotor  30  relative to stator  28 . For example, position sensor  62  can include an array of Hall-effect sensors responsive to the polarity of the permanent magnets in permanent magnet array  86 . Controller  26  utilizes the position data to commutate motor  22 . 
     Drive mechanism  24  converts rotational motion from rotor  30  into linear motion of fluid displacement members  20 . Rotor body  88  rotates about pump axis PA-PA (best seen in  FIG.  4 A ) and drives rotation of drive nut  90 . Drive nut  90  drives screw  92  axially along pump axis PA-PA by engagement of rolling elements, such as rolling elements  98  (best seen in  FIGS.  12  and  13   ), disposed between drive nut  90  and screw  92  and supporting drive nut  90  relative screw  92 . The rolling elements support drive nut  90  relative screw  92  such that drive nut  90  does not contact screw  92  during operation. The rolling elements translate the rotation of drive nut  90  into linear movement of screw  92 . Screw  92  drives fluid displacement members  20  through respective pumping and suction strokes. Rotor  30  is rotated in a first rotational direction to cause screw  92  to displace in a first axial direction. Rotor  30  is rotated in a second rotational direction opposite the first rotational direction to cause screw  92  to displace in a second axial direction opposite the first axial direction. 
     Motor  22  is axially aligned with fluid displacement members  20  and drives reciprocation of fluid displacement members  20 . Rotor  30  rotates about pump axis PA-PA and fluid displacement members  20  reciprocate on pump axis PA-PA. Pump  10  provides significant advantages. Motor  22  being axially aligned with fluid displacement members  20  facilitates a compact pump arrangement providing a smaller package relative to other mechanically-driven and electrically-driven pumps. In addition, motor  22  does not include gearing, such as reduction gears, between motor  22  and fluid displacement members  20 . Eliminating that gearing provides a more reliable, simpler pump by reducing the count of moving parts Eliminating the gearing also provides a quieter pump operation. 
     Rotor  30  and drive mechanism  24 ,  24 ′,  24 ″ are sized to provide a desired revolution to stoke ratio. In some examples, rotor  30  and drive mechanism  24 ,  24 ′,  24 ″ are sized such that one revolution of rotor  30  results in a full stroke of screw  92  in one of first axial direction AD 1  and second axial direction AD 2 . A full revolution in an opposite rotational direction results in a full stroke of screw  92  in the opposite axial direction. As such, two revolutions in opposite directions can provide a full pump cycle for each fluid displacement member  20 . Pump  10  can thereby provide a 1:1 ratio between revolutions of rotor  30  and pumping strokes. In the example shown, pump  10  can provide a 1:1 ratio between revolutions of rotor  30  and pump cycles, as one fluid displacement member  20  proceeds through a pumping stroke during a single stroke and the other fluid displacement member  20  proceeds through a suction stroke during the single stroke. The revolution to stroke ratio depends on the stroke length and the lead (the axial travel for a single revolution) of screw  92 . In some examples, screw  92  has a lead of about 5-35 millimeters (mm) (about 0.2-1.4 inches (in.)). In some examples, screw  92  has a lead of about 10-25 mm (about 0.4-1.0 in.). In some examples, the stroke length is about 12.7-76.2 mm (about 0.5-3 in.). In some examples, the stroke length is about 19-63.5 mm (about 0.75-2.5 in.). In some examples, the stroke length is about 21.6-58.4 mm (0.85-2.3 in.). It is understood that rotor  30  and drive mechanism  24 ,  24 ′,  24 ″ can be sized to provide any desired revolution to stroke ratio. For example, pump  10  can have a revolution to stroke ratio of about 0.25:1 to about 7:1. In some examples, pump  10  has a revolution to stroke ratio of about 0.5:1 to about 3:1. In a more particular example, pump  10  has a revolution to stroke ratio of about 0.8:1 to about 1.5:1. A relatively larger revolution to stroke ratio facilitates greater pumping pressures. A relatively smaller revolution to stroke ratio facilitates greater flow rates. 
     It is understood, however, that rotor  30  and drive mechanism  24 ,  24 ′,  24 ″ can be sized to provide any desired revolution to stroke ratio. It is further understood that controller  26  can control operation of motor  22  such that the actual stroke length is dynamic and varies can during operation. Controller  26  can cause the stroke length to vary between the downstroke and the upstroke. In some examples, controller  26  is configured to control operation between a maximum revolution to stroke ratio and a minimum revolution to stroke ratio. Pump  10  can be configured to provide any desired revolution to stroke ratio. In some examples, pump  10  provides a revolution to stroke ratio of up to about 4:1. It is understood that other maximum revolution to stroke ratios are possible, such as about 1:1, 2:1, 3:1, or 5:1, among other options. It is understood that any of the ranges discussed can be an inclusive range such that the boundary values are included within the range. It is further understood that each of the ranges discussed can vary from the specified range while still falling within the scope of this disclosure. 
     Motor  22  and drive mechanism  24 ,  24 ′,  24 ″ can be configured to displace fluid displacement member  20  at least about 6.35 mm (about 0.25 in.) per rotor revolution. In some examples, motor  22  and drive mechanism  24 ,  24 ′,  24 ″ are configured to displace fluid displacement member  20  between about 8.9-30.5 mm (about 0.35-1.2 in.) per rotor revolution. In some examples, motor  22  and drive mechanism  24 ,  24 ′,  24 ″ are configured to displace fluid displacement member  20  between about 8.9-11.4 mm (about 0.35-0.45 in.). In some examples, motor  22  and drive mechanism  24 ,  24 ′,  24 ″ are configured to displace fluid displacement member  20  between about 19-21.6 mm (about 0.75-0.85 in.). In some examples, motor  22  and drive mechanism  24 ,  24 ′,  24 ″ are configured to displace fluid displacement member  20  between about 24, 24′,  24 ″0.1-26.7 mm (about 0.95-1.05 in.). The axial displacement per rotor revolution provided by pump  10  facilitates precise control and quick responsiveness during pumping. The axial displacement per rotor revolution facilitates quick changeover and provides more efficient pumping while reducing wear on components of pump  10 . 
     Pump  10  is configured to pump according to a revolution to displacement ratio. More specifically, motor  22  and drive mechanism  24 ,  24 ′,  24 ″ are configured to provide a desired revolution to displacement ratio between revolutions of rotor  30  and the linear displacement of fluid displacement member  20 , as measured in inches, for each revolution of rotor  30 . In some examples, the revolution to displacement ratio (rev/in.) is less than about 4:1. In some examples, the revolution to displacement ratio is between about 0.85:1 and 3.25:1. In some examples, the revolution to displacement ratio is between about 1:1-3:1. In some examples, the revolution to displacement ratio is between about 1:1-2.75:1. In some examples, the revolution to displacement ratio between is about 1:1-2.55:1. In some examples, the revolution to displacement ratio is between about 1:1-1.3:1. In some examples, the revolution to displacement ratio is between about 0.9:1-1.1:1. In some examples, the revolution to displacement ratio is between about 2.4:1-2.6:1. The low revolution to displacement ratio provided by pump  10  relative to other electrically-powered pumps, such as crank-powered pumps that require reduction gearing to generate sufficient pumping torque and typically have revolution to displacement ratios of about 8:1 or higher, facilitates more efficient pumping, generates less wear, and provides quick responsiveness for changing stroke direction. Rotor  30  can be driven at a lower rotational speed to generate the same linear speed, thereby generating less heat during operation. 
       FIG.  4 A  is a cross-sectional view of pump  10  taken along line A-A in  FIG.  1 B .  FIG.  4 B  is an enlarged view of a portion of the cross-section shown in  FIG.  4 A .  FIG.  4 C  is a cross-sectional view of pump  10  taken along line C-C in  FIG.  1 A .  FIG.  4 D  is a cross-sectional view taken along line D-D in  FIG.  4 C .  FIGS.  4 A- 4 D  will be discussed together. Pump body  16 , fluid covers  18   a ,  18   b , fluid displacement members  20   a ,  20   b , motor  22 , drive mechanism  24 , process fluid chambers  34   a ,  34   b , cooling chambers  44   a ,  44   b , air check  46 , bearings  54   a ,  54   b , motor nut  56 , grease caps  60   a ,  60   b , and grease fitting  94  of pump  10  are shown. 
     Pump body  16  includes central portion  66  and end caps  68   a ,  68   b . Central portion  66  includes motor housing  70 , control housing  72 , heat sinks  74 , and stator passages  76 . Fluid displacement members  20   a ,  20   b  respectively include inner plates  78   a ,  78   b , outer plates  80   a ,  80   b , membranes  82   a ,  82   b , and fasteners  84   a ,  84   b.    
     Motor  22  includes stator  28  and rotor  30 . Rotor  30  includes permanent magnet array  86  and rotor body  88 . Rotor body  88  includes rotor bores  96 . 
     Drive mechanism  24  includes drive nut  90 , screw  92 , and rolling elements  98 . Drive nut  90  includes nut notches  100   a ,  100   b  (collectively herein “nut notch  100 ” or “nut notches  100 ”) and nut thread  102 . Screw  92  includes first screw end  104 , second screw end  106 , screw body  108 , screw thread  110 , first bore  112 , second bore  114 , and third bore  116 . Second bore  114  includes first diameter portion  118  and second diameter portion  120 . Bearings  54   a ,  54   b  include inner races  122   a ,  122   b  and outer races  124   a ,  124   b , respectively. Motor nut  56  includes motor nut notch  126 , outer edge  128 , and cooling ports  130 . 
     Components can be considered to axially overlap when the components are disposed at a common position along an axis such that a radial line projecting that axis extends through each of those axially-overlapped components. Similarly, components can be considered to radially overlap when the components are disposed at common radial distances from the axis such that an axial line parallel to the axis extends through each of those radially-overlapped components. 
     End caps  68   a ,  68   b  are disposed on opposite lateral sides of central portion  66  and are attached to central portion  66  to form pump body  16 . Motor  22  is disposed within motor housing  70  between end caps  68 . Control housing  72  is connected to and extends from motor housing  70 . Control housing  72  is configured to house control elements of pump  10 , such as controller  26  ( FIGS.  1 C and  19   ). Stator  28  surrounds rotor  30  and drives rotation of rotor  30 . Rotor  30  rotates about pump axis PA-PA and is disposed coaxially with drive mechanism  24  and fluid displacement members  20 . Permanent magnet array  86  is disposed on rotor body  88 . Fluid covers  18   a ,  18   b  are connected to end caps  68   a ,  68   b , respectively. 
     Drive mechanism  24  receives a rotational output from rotor  30  and converts that rotational output into a linear input to fluid displacement members  20 . Motor  22  directly drives reciprocation of fluid displacement members  20  via drive mechanism  24  without any intermediate gearing. Drive nut  90  is connected to rotor body  88  to rotate with rotor  30 . Screw  92  is elongate along pump axis PA-PA and extends through drive nut  90  coaxially with rotor  30 . 
     Rolling elements  98  are disposed between rotor  30  and screw  92 . More specifically, rolling elements  98  are disposed between drive nut  90  and screw  92 . Rolling elements  98  are disposed in raceways formed by opposing nut thread  102  and screw thread  110 . Rolling elements  98  engage screw thread  110  to drive linear displacement of screw  92  along pump axis PA-PA. Rolling elements  98  can be balls or rollers among other options and as discussed in more detail below. Rolling elements  98  are disposed circumferentially about screw  92  and evenly arrayed around screw  92 . Rolling elements  98  are arrayed around, and are arrayed along, an axis that is coaxial with axis PA-PA. Rolling elements  98  separate drive nut  90  and screw  92  such that drive nut does not directly contact screw  92 . Instead, both drive nut  90  and screw  92  ride on rolling elements  98 . Rolling elements  98  maintain gap  99  ( FIG.  12   ) between drive nut  90  and screw  92  to prevent contact therebetween. 
     First bore  112  extends into screw body  108  from first screw end  104 . First bore  112  is elongate along pump axis PA-PA. First bore  112  is coaxial with pump axis PA-PA. Second bore  114  extends into screw body  108  from second screw end  106 . Second bore  114  is elongate along pump axis PA-PA. First diameter portion  118  of second bore  114  extends into screw body  108  from second screw end  106 . Second diameter portion  120  of second bore  114  extends into screw body  108  from first diameter portion  118 . In the example shown, each of first bore  112  and second bore  114  are closed such that first bore  112  and second bore  114  are fluidly isolated. In the example shown, second bore  114  has a greater length than first bore  112 . In the example shown, second diameter portion  120  has a greater length than first bore  112 . 
     Grease fitting  94  is disposed in screw body  108 . Grease fitting  94  is disposed within second bore  114 . More specifically, grease fitting  94  is disposed at the interface between first diameter portion  118  and second diameter portion  120 . Grease fitting  94  is secured to screw body  108 . Grease fitting  94  can be secured within second diameter portion  120  and a portion of grease fitting  94  can extend into first diameter portion  118 . Grease fitting  94  can be a grease zerk, among other options. Second diameter portion  120  can act as a lubricant reservoir. 
     Third bore  116  extends from second bore  114  to an outer surface of screw body  108 . Third bore  116  extends from second bore  114  to an outlet on the outer surface of screw body  108 . The outlet of third bore  116  can be disposed on a portion of screw body  108  intermediate screw thread  110 . Third bore  116  can provide lubricant at a point of least clearance between drive nut  90  and screw body  108 . Third bore  116  can be elongate along an axis transverse to pump axis PA-PA. In some examples, third bore  116  extends orthogonal to pump axis PA-PA. 
     First diameter portion  118  of second bore  114  is sized to receive an applicator of a grease gun. The applicator connects to grease fitting  94  to supply lubricant to the rolling elements  98  between drive nut  90  and screw  92  via second bore  114  and third bore  116 . Drive mechanism  24  does not require disassembly to access and lubricate rolling elements  98 . In some examples, a lubricant drive mechanism can be disposed in second bore  114 . The lubricant drive mechanism can physically interface with lubricant in second diameter portion  120  to exert pressure on the lubricant and drive the lubricant through third bore  116 . For example, a feed tube can extend from grease fitting  94  and a follower plate can be disposed about the feed tube. A spring can drive the follower plate towards third bore  116 . A stop can be disposed in second diameter portion  120  to prevent the follower plate from passing over third bore  116 . In other examples, third bore  116  can be disposed closer to grease fitting  94  and a plate and spring can be disposed on an opposite side of third bore  116  from grease fitting  94 . 
     Bearings  54   a ,  54   b  are disposed at opposite axial ends of rotor  30 . Bearings  54  are configured to support both axial and radial forces. In some examples, bearings  54  are tapered roller bearings. Bearing  54   a  is disposed at a first end of rotor  30  about drive nut  90 . Inner race  122   a  of bearing  54   a  is disposed on and connected to drive nut  90 . Inner race  122   a  interfaces with drive nut notch  100   a  formed on drive nut  90 . Drive nut notch  100   a  is an annular notch formed on an exterior of drive nut  90  at the first axial end of drive nut  90 . Drive nut notch  100   a  interfaces both axially and radially with inner race  122   a . Outer race  124   a  of bearing  54   a  interfaces with motor nut notch  126  formed in motor nut  56 . Outer race  124   a  interfaces both axially and radially with motor nut notch  126 . An array of rollers  123   a  is disposed between inner race  122   a  and outer race  124   a . Each roller  123   a  can be oriented along an axis of the roller  123   a  such that the axis of the roller  123   a  is neither parallel nor orthogonal to the axis of reciprocation of the screw  92 . In some examples, the rollers  123   a  can be oriented such that the axes of the rollers  123   a  extended through or converge at point aligned on the pump axis PA. At least a portion of bearing  54   a  can be disposed directly radially inside of rotor  30 . In the example shown, bearing  54   a  and permanent magnet array  86  axially overlap. As such, a radial line extending from pump axis PA can pass through both bearing  54   a  and permanent magnet array  86 . In the example shown, at least a portion of each of inner race  122   a , outer race  124   a , and rollers  123   a  axially overlaps with permanent magnet array  86 . 
     Bearing  54   b  is disposed at a second axial end of rotor  30  about drive nut  90 . Inner race  122   b  of bearing  54   b  is disposed on and connected to drive nut  90 . Inner race  122   b  interfaces with drive nut notch  100   b  formed on drive nut  90   b . Drive nut notch  100   b  is an annular notch formed on an exterior of drive nut  90  at the second axial end of drive nut  90 . Drive nut notch  100   b  interfaces both axially and radially with inner race  122   a . Outer race  124   b  of bearing  54   b  interfaces with end cap  68   b  both axially and radially. Outer race  124   b  interfaces both axially and radially with cap notch  134  formed in end cap  68   b . An array of rollers  123   b  is disposed between inner race  122   b  and outer race  124   b . Each roller  123   b  can be oriented along an axis of the roller  123   b  such that the axis of the roller  123   b  is neither parallel nor orthogonal to the axis of reciprocation of the screw  92 . In some examples, the rollers  123   b  can be oriented such that the axes of the rollers  123   b  extended through or converge at point aligned on the pump axis PA. At least a portion of bearing  54   b  can be disposed directly radially inside of rotor  30 . In the example shown, bearing  54   b  and permanent magnet array  86  axially overlap. As such, a radial line extending from pump axis PA can pass through both bearing  54   b  and permanent magnet array  86 . In the example shown, at least a portion of each of inner race  122   b , outer race  124   b , and rollers  123   b  axially overlaps with permanent magnet array  86 . 
     Motor nut  56  is connected to pump body  16 . Motor nut  56  covers at least a portion of an axial end of motor  22 . In the example shown, motor nut  56  is connected to end cap  68   a . In the example shown, outer edge  128  interfaces with end cap  68   a  to secure motor nut  56  to pump body  16 . Motor nut  56  and end cap  68   a  can be connected by interfaced threading, among other options. In the example shown, a diameter D 1  of motor nut  56  at outer edge  128  is larger than a diameter D 2  of rotor  30 . As such, motor nut  56  can fully cover an axial end of rotor  30  and partially cover an axial end of stator  28 . Motor nut  56  fully radially overlaps with rotor  30  and partially radially overlaps with stator  28 . In the example shown, a diameter D 3  of central aperture  144  ( FIGS.  15 A and  15 B ) of motor nut  56  is larger than a diameter D 4  of drive nut  90 . 
     Motor nut  56  preloads bearings  54  and axially aligns rotor  30 . Motor nut  56  threads into end cap  68   a  and interfaces with bearing  54   a . Motor nut  56  clamps bearings  54  and rotor  30  between end cap  68   b  and motor nut  56 . Motor nut  56  removes play in bearings  54 . Motor nut  56  aligns bearings  54  and rotor  30  axially on pump axis PA-PA by threading into end cap  68   a . The threaded interface aligns motor nut  56  on pump axis PA-PA. Motor nut  56  aligns rotor  30  relative to stator  28  to maintain an air gap between rotor  30  and stator  28  and to prevent undesired contact between rotor  30  and stator  28 . 
     Grease cap  60   a  is supported by motor nut  56  and encloses an end of bearing  54   a  facing fluid displacement member  20   a . Grease cap  60   a  being attached to motor nut  56  ensures that grease cap  60   a  is properly positioned relative to and aligned with bearing  54   a . In the example shown, a plate of grease cap  60   a  is disposed between motor nut  56  and bearing  54   a  and a support is disposed on an opposite side of motor nut  56  and has prongs extending to and supporting the plate. In some examples, the prongs can snap lock onto motor nut  56  to connect grease cap  60   a  to motor nut  56 . Grease cap  60   b  is substantially similar to grease cap  60   a . Grease cap  60   b  is connected to pump body  16  and encloses an end of bearing  54   b  facing fluid displacement member  20   b . More specifically, grease cap  60   b  is connected to end cap  68   b . Grease caps  60  prevent contaminants, such as dirt or moisture, from entering bearings  54  and capture grease that may liquefy during operation. 
     Fluid displacement members  20   a ,  20   b  are connected to opposite ends  104 ,  106  of screw  92 . In the example shown, fluid displacement members  20  are flexible and include a variable surface area during pumping. More specifically, fluid displacement members  20  are diaphragms, including diaphragm plates  78 ,  80  and membranes  82 . The membranes  82  can be formed from flexible material, such as rubber or other type of polymer. It is understood, however, that fluid displacement members  20  can be of other configurations, such as pistons. 
     In the example shown, fluid displacement member  20   a  includes inner plate  78   a  and outer plate  80   a  disposed on opposite sides of membrane  82   a . A portion of membrane  82   a  is captured between the opposed diaphragm plates  78   a ,  80   a . Fluid displacement member  20   a  is attached to first screw end  104  of screw  92 . Fastener  84   a  extends from fluid displacement member  20   a  into screw  92  to secure fluid displacement member  20   a  to screw  92 . Fastener  84   a  extends through each outer plate  80   a , membrane  82   a , and inner plate  78   a  and into first bore  112  to connect fluid displacement member  20   a  to drive mechanism  24 . Fastener  84   a  engages within first bore  112  to secure fluid displacement member  20   a  to screw  92 . For example, the fastener  84   a  and first bore  112  can include interfaced threading, among other options. 
     In the example shown, fluid displacement member  20   b  is similar to fluid displacement member  20   a . A portion of membrane  82   b  is captured between the opposed diaphragm plates  78   b ,  80   b . Outer plate  80   b  is overmolded by membrane  82   b  such that that outer plate  80   b  is disposed within membrane  82   b . Fastener  84   b  extends from fluid displacement member  20   b  and into screw  92  to connect fluid displacement member  20   b  to drive mechanism  24 . Fastener  84   b  extends from outer plate  80   b , through inner plate  78   b , and into second bore  114  to connect fluid displacement member  20   b  to drive mechanism  24 . Fastener  84   b  engages within second bore  114  to secure fluid displacement member  20   b  to screw  92 . For example, fastener  84   b  and second bore  114  can include interfaced threading, among other options. In the example shown, fastener  84   b  extends into and engages with first diameter portion  118  of second bore  114 . Fastener  84   b  does not extend into second diameter portion  120  in the example shown. 
     Drive nut  90  and rolling elements  98  exert a rotational force on screw  92  while driving screw  92  axially. As discussed above, bearings  54  are configured to support both axial and radial forces. Screw  92  is connected to fluid displacement members  20  such that fluid displacement members  20  prevent screw  92  from rotating about pump axis PA-PA. Fluid displacement members  20  interface with pump body  16  to prevent rotation of fluid displacement members  20  and screw  92  relative to pump axis PA-PA. 
     First screw end  104  of screw  92  interfaces with fluid displacement member  20   a  to prevent screw  92  from rotating relative to fluid displacement member  20   a . In the example shown, first screw end  104  interfaces with inner plate  78   a  to prevent screw  92  from rotating relative to inner plate  78   a . In some examples, first screw end  104  and inner plate  78   a  include mating faces configured to interface to prevent relative rotation. 
     Outer edge  128   a  of membrane  82   a  is secured between fluid cover  18   a  and pump body  16  to provide a fluid-tight seal between wet and dry sides of fluid displacement member  20   a . Fluid cover  18   a  and fluid displacement member  20   a  at least partially define process fluid chamber  34   a . Fluid displacement member  20   a  and pump body  16  at least partially define cooling chamber  44   a . Outer edge  128   a  is clamped such that fluid displacement member  20   a  does not rotate about pump axis PA-PA. Outer edge  128   a  does not rotate about pump axis PA-PA. In the example shown, outer edge  128   a  does not shift axially relative pump axis PA-PA. Outer edge  128   a  includes bead  136  seated within groove  138  formed by opposing trenches of fluid cover  18   a  and end cap  68   a . Bead  136  has an enlarged cross-sectional area as compared to a portion of membrane  82   a  adjacent bead  136 . 
     The wet side of fluid displacement member  20   a  is oriented towards fluid cover  18   a  and at least partially defines process fluid chamber  34   a . Outer plate  80   a  and a portion of fastener  84   a  are exposed to the process fluid in process fluid chamber  34   a . The dry side of fluid displacement member  20   a  is oriented towards motor  22  and at least partially defines cooling chamber  44   a . Inner diaphragm plate  78   a  is exposed to the cooling air in cooling chamber  44   a . In some examples, thermally conductive components of fluid displacement members  20  are exposed to the process fluid and the cooling air to effectuate heat exchange between the fluids, thereby cooling pump  10  with the process fluid. For example, inner plate  78   a  and at least one of outer plate  80   a  and fastener  84   a  can be formed from a thermally conductive material, such as aluminum. 
     Second screw end  106  of screw  92  interfaces with fluid displacement member  20   b  such that screw  92  is prevented from rotating relative to fluid displacement member  20   b . In the example shown, second screw end  106  interfaces with inner plate  78   b  to prevent screw  92  from rotating relative to inner plate  78   b . In some examples, second screw end  106  and inner plate  78   b  include contoured surfaces configured to interface to prevent relative rotation. 
     Outer edge  128   b  of membrane  82   b  is secured between fluid cover  18   b  and pump body  16  to provide a fluid-tight seal between wet and dry sides of fluid displacement member  20   b . Fluid cover  18   b  and fluid displacement member  20   b  at least partially define process fluid chamber  34   b . Fluid displacement member  20   b  and pump body  16  at least partially define cooling chamber  44   b . Outer edge  128   b  is clamped between end cap  68   b  and fluid cover  18   b  such that outer edge  128   b  remains static and does not rotate about pump axis PA-PA. Outer edge  128   b  includes bead  136  seated within groove  138  formed by opposing trenches formed on fluid cover  18   b  and end cap  68   b . Bead  136  has an enlarged cross-sectional width as compared to a portion of membrane  82   b  adjacent bead  136 . 
     The wet side of fluid displacement member  20   b  is oriented towards end cap  68   b  and at least partially defines process fluid chamber  34   b . The dry side of fluid displacement member  20   b  is oriented towards motor  22  and at least partially defines cooling chamber  44   b . In some examples, portions of outer plate  80   b  extend through membrane  82   b  such that those portions are exposed to the process fluid. Fluid displacement member  20   b  can thereby provide additional cooling by a conduction path between the cooling air and the process fluid through fluid displacement member  20   b.    
     Air check  46  is mounted on pump body  16 . Valve housing  142  is mounted on motor housing  70 . Valve housing  142  supports inlet valve  48  and outlet valve  50 . Inlet valve  48  controls flow of cooling air into the cooling circuit CF (best seen in  FIG.  2   ) and outlet valve  50  controls flow of cooling air out of the cooling circuit CF. Filter  140  is disposed upstream of inlet valve  48  and is configured to remove contaminants, such as dust, from the air entering the cooling circuit CF. Valve housing  142  is contoured and oriented to direct the flow of cooling air over heat sinks  74  of pump body  16 , as shown by arrows E in  FIG.  4 B . In some examples, valve housing  142  is configured such that the intake flow of cooling air flows over heat sinks  74  to enter valve housing  142 . In some examples, valve housing  142  is configured such that the exhaust flow of cooling air flows over heat sinks  74  when exiting valve housing  142 . In some examples, both the intake and exhaust flows are directed over heat sinks  74 . 
     First cooling passage  36  is formed in pump body  16 . In the example shown, first cooling passage  36  extends through motor housing  70  and end cap  68   a . First cooling passage  36  extends between air check  46  and cooling chamber  44   a.    
     Second cooling passage  38  is formed in pump body  16 . In the example shown, second cooling passage  38  extends through end cap  68   a , through central portion  66  and specifically stator passages  76 , and through end cap  68   b . Second cooling passage  38  includes outer portions extending through end caps  68  and inner portions defined by stator passages  76 . Second cooling passage  38  includes different numbers of inner portions and outer portions. For example, each the outer portions of second cooling passage  38  can be formed by single bores through each end cap  68  while the inner portions are formed by multiple stator passages  76 . Each end cap  68  can include recesses providing fluid communication between the inlet/outlet bores through end caps  68  and stator passages  76 . Second cooling passage  38  can have a larger flow area through the inner portions than through the outer portions. The enlarged flow area of the inner portions relative to the outer portions decelerates airflow through stator pathways, enhancing heat exchange. 
     Third cooling passage  40  extends between cooling chamber  44   a  and cooling chamber  44   b . In the example shown, third cooling passage  40  extend through motor nut  56 , rotor  30 , and end cap  68   b . More specifically, third cooling passage  40  is formed by cooling ports  130  in motor nut  56 , rotor bores  96  in rotor  30 , and cap bores  132  in end cap  68   b . A portion of third cooling passage  40  thus extends through a rotating component of pump  10 . Rotor bores  96  form the rotating portion of third cooling passage  40 . A non-rotating portion of third cooling passage  40  can be formed by pump body  16 . Third cooling passage  40  can include more rotating bores than static bores. For example, rotor body  88  can include more rotor bores  96  than motor nut  56  has cooling ports  130 . Third cooling passage  40  can have a greater cross-sectional flow area through the rotating bores than through the static bores disposed at one or both axial ends of third cooling passage  40 . The increased cross-sectional area decelerates the cooling airflow through rotor bores  96 , enhancing heat exchange. 
     During operation, electric current is provided to stator  28  to drive rotation of rotor  30 . Drive nut  90  is connected to rotor body  88  and rotates with rotor  30 . Rolling elements  98  drive screw  92  linearly along pump axis PA-PA. Axial pump reaction forces are generated during pumping and experienced along pump axis PA-PA. The pump reaction forces are initially experienced by fluid displacement members  20  and transferred to screw  92 . The pump reaction forces flow through screw to rolling elements  98  and from rolling elements  98  to drive nut  90 . The axial forces experienced by drive nut  90  are transferred to bearings  54  and from bearings  54  to pump body  16 . In the example shown, the axial forces experienced by drive nut  90  and transferred through bearings  54   a ,  54   b  to end caps  68   a ,  68   b , respectively, and from end caps  68   a ,  68   b  to other components forming pump body  16 . Bearings  54  transfer the axial forces to pump housing  16  to isolate motor  22  from the pump reaction forces. The pump reaction forces experienced by fluid displacement members  20  oppose each other during each stroke as one fluid displacement member  20  is pumping while the other fluid displacement member  20  is in suction. 
     If screw  92  is initially driven in first axial direction AD 1  in  FIG.  4 A , then screw  92  pulls fluid displacement member  20   b  through a suction stroke and pushes fluid displacement member  20   a  through a pumping stroke for the process fluid. After reaching the end of the first stroke, rotor  30  is driven in an opposite rotational direction such that screw  92  is driven in second axial direction AD 2 , in the opposite linear direction from the first stroke. When screw  92  is driven in direction AD 2 , screw  92  pulls fluid displacement member  20   a  through a suction stroke and pushes fluid displacement member  20   b  through a pumping stroke for the process fluid. During a suction stroke, the volume of process fluid chamber  34  increases and process fluid is drawn into process fluid chamber  34  from inlet manifold  12 . During the pumping stroke, the volume of process fluid chamber  34  decreases and fluid displacement member  20  drives the process fluid downstream out of process fluid chamber  34  to outlet manifold  14 . 
     Fluid displacement members  20  pump cooling air through the cooling circuit CF (best seen in  FIG.  2   ) of pump  10  simultaneously with pumping the process fluid. As screw  92  is driven in direction AD 1 , the volume of cooling chamber  44   a  expands and air is drawn into cooling chamber  44   a  through inlet valve  48  and first cooling passage  36 . As such, fluid displacement member  20   a  proceeds through a suction stroke for the cooling air while simultaneously proceeding through a pumping stroke for the process fluid. The volume of cooling chamber  44   b  decreases as fluid displacement member  20   b  is pulled in direction AD 1 . Fluid displacement member  20   b  drives cooling air from cooling chamber  44   b  through fourth cooling passage  42  and out from pump  10  through outlet valve  50 . As such, fluid displacement member  20   b  proceeds through a pumping stroke for the cooling air while simultaneously proceeding through a suction stroke for the process fluid. 
     Valve housing  142  directs the flow of cooling air entering and/or exiting the cooling circuit. Valve housing  142  directs the flow over heat sinks  74  formed on pump body  16 . The cooling air flowing over heat sinks  74  enhances heat transfer from pump body  16 . 
     As screw  92  is driven in the second axial direction AD 2 , the volume of cooling chamber  44   a  decreases and the volume of cooling chamber  44   b  increases. Fluid displacement member  20   a  drives the cooling air from cooling chamber  44   a  to cooling chamber  44   b  through second cooling passage  38  and third cooling passage  40 . Fluid displacement member  20   b  draws the cooling air from cooling chamber  44   a  to cooling chamber  44   b  through second cooling passage  38  and third cooling passage  40 . The flow of cooling air causes each of inlet valve  48  and outlet valve  50  to shift to respective closed positions and internal valve  52  to shift to an open position, directing unidirectional flow of the cooling air through the cooling circuit CF. 
     Fluid displacement members  20  are configured to simultaneously pump cooling air and process fluid with opposite axial sides of each fluid displacement member  20  interfacing with the respective pumped fluids. The dry side interfaces with the cooling air and the wet side interfaces with the process fluid. Fluid displacement members  20  are simultaneously driven through both pumping and suction strokes for the two fluids being pumped by that fluid displacement member  20 . As such, fluid displacement members  20  is driven through a suction stroke for the process fluid while being driven through a pumping stroke for the cooling air, and fluid displacement members  20  is driven through a suction stroke for the cooling air while being driven through a pumping stroke for the process fluid. 
     Pump  10  provides significant advantages. Bearings  54  support both axial and radial loads, facilitating coaxial mounting of motor  22  and fluid displacement member  20 . In addition, drive mechanism  24  experiences both radial loads and axial loads during pumping. As such, bearings  54  further facilitate the use of drive mechanism  24 . Motor nut  56  preloads bearings  54  and aligns rotor  30  relative to stator  28 . Motor nut  56  ensures proper alignment of rotating components, thereby preventing unintended contact and increasing the useful life. Motor nut  56  further supports grease cap  60   a  for bearing  54   a , reducing part count and ensuring proper alignment between grease cap  60   a  and bearing  54   a , which prevents premature failure that can occur due to lubricant leakage. 
     Screw  92  is prevented from rotating about pump axis PA-PA. In the embodiment illustrated, screw  92  is prevented from rotating about pump axis PA-PA by fluid displacement members  20 . Screw  92  interfaces with fluid displacement members  20  such that screw  92  is prevented from rotating relative to fluid displacement members  20 . Fluid displacement members  20  interface with pump body  16  to prevent rotation of fluid displacement members about pump axis PA-PA, thereby preventing rotation of screw  92 . Preventing rotation of screw  92  maintains the connection between screw  92  and fluid displacement members  20  throughout operation, preventing undesired loosening between screw  92  and fluid displacement members  20 . Preventing screw  92  from rotating about pump axis PA-PA causes screw  92  to displace linearly as drive nut  90  rotates, facilitating pumping by pump  10 . 
     Grease fitting  94  is disposed in screw  92 . Grease fitting  94  facilitates quick and simple lubricant application to rolling elements  98 . To provide lubricant, the user can remove fluid cover  18   b  from pump body  16  and disconnect fluid displacement member  20   b  from screw  92 . Detaching fluid displacement member  20   b  provides access to second bore  114 . The user can insert the applicator of a grease gun into second bore  114  and connect the applicator to grease fitting  94  to supply lubricant. The lubricant flows through second diameter portion  120  and third bore  116  to the gap between drive nut  90  and screw  92 . As such, the user is not required to fully disassembly pump  10  to access drive mechanism  24  for lubrication. In addition, the user is not required to disassemble drive mechanism  24  to access rolling elements  98  for lubrication, simplifying the lubrication process and preventing the need to access multiple loose and small components, which can be easily lost. 
     Fluid displacement members  20  pump both cooling air and process fluid. The cooling air circulates through pump  10  along a unidirectional cooling circuit CF. Pumping cooling air with fluid displacement members  20  that also pump the process fluid reduces part count by eliminating additional components with additional moving parts, such as pumps or fans, for driving the cooling air. Fluid displacement members  20  being disposed in series provides efficient flow through cooling flowpath CF. Second cooling passage  38  and third cooling passage  40  are positioned to absorb heat from the main heat generating components of pump  10 , including controller  26 , stator  28 , and drive mechanism  24 . At least a portion of second cooling passage  38  is positioned intermediate stator  28  and controller  26  to absorb heat from both sources, increasing cooling efficiency. In addition, at least one of the exhaust and intake flows can be directed over heat sinks  74  to further cool stator  28 . Air check  46  and internal valve  52  facilitate unidirectional flow to ensure a flow of fresh cooling air through the cooling circuit CF. 
       FIG.  5 A  is an isometric view showing internal valve  52  mounted on end cap  68   b .  FIG.  5 B  is an enlarged cross-sectional view of a portion of pump  10  showing internal valve  52 .  FIGS.  5 A and  5 B  will be discussed together.  FIG.  5 A  shows internal valve  52 , end cap  68   b , cap bores  132 , cap bores  146 , valve member  148 , support  152 , member body  156 , projection  158 , outer portion  162 , tapered edges  164 , and end  166 .  FIG.  5 B  also shows internal valve  52 , end cap  68   b , cap bores  132 , valve member  148 , support  152 , member body  156 , projection  158 , outer portion  162 , tapered edges  164 , and end  166 , and in addition shows motor  22 , drive mechanism  24 , rotor  30 , cooling chamber  44   b , bearing  54   b , grease cap  60   b , end cap  68   b , permanent magnet array  86 , grease fitting  94 , rotor bores  96 , rolling elements  98 , plate  150 , prongs  154 , inner portion  160 , radially inner edge  168 , radially outer edge  170 , and radially outer edge  172 . 
     Cap bores  146  extend through end cap  68   b  and form outlets for second cooling passage  38 . Cap bores  132  extend through end cap  68   b  and are outlets for third cooling passage  40 . Cap bores  132  can all be of the same configuration or can be of varying configurations. 
     Cap bores  132  are disposed radially outside of bearing  54   b . Cap bores  132  are disposed radially outside of rotor bores  96  relative to pump axis PA-PA. For example, a centerline CL 1  of cap bores  132  can be radially outside of a centerline CL 2  of rotor bores  96 , a radially inner edge  168  of cap bores  132  can be radially outside of the centerline CL 2  of rotor bores  96 , a radially outer edge  170  of cap bores  132  can be radially outside of a radially outer edge  172  of rotor bores  96 , the centerline CL 1  of cap bores  132  can be radially outside of the radially outer edge  172  of rotor bores  96 , and/or the radially inner edge  168  of cap bores  132  can be radially outside of a radially outer edge  172  of rotor bores  96 . Cap bores  132  can at least partially overlap radially with permanent magnet array  86 . 
     Internal valve  52  is mounted on end cap  68   b  and controls flow into cooling chamber  44   b  from second cooling passage  38  and third cooling passage  40 . In the example shown, internal valve  52  is a flapper valve having flapper valve member  148 . Valve member  148  is a flexible member configured to flex between an open state, allowing flow into cooling chamber  44   b , and a closed state, preventing retrograde flow to second cooling passage  38  and third cooling passage  40  from cooling chamber  44   b . Valve member  148  seals against end cap  68   b  in the closed state. 
     Grease cap  60   b  is disposed adjacent bearing  54   b . Plate  150  of grease cap  60   b  is adjacent bearing  54   b , protects bearing  54   b  from contamination, and captures any grease that liquefies during operation. Support  152  of grease cap  60   b  is disposed on the opposite side of end cap  68   b  from bearing  54   b . In some examples, fasteners (not shown) extend into end cap  68  and support  152  to secure grease cap  60   b  to end cap  68   b . In some examples, prongs  154  extend from support  152  and interface with plate  150  to hold plate  150  relative bearing  54   b . In some examples, prongs  154  snap lock onto a portion of end cap  68   b . A portion of valve member  148  is disposed between support  152  and end cap  68   b  such that valve member  148  is connected to end cap  68   b  by grease cap  60   b . It is understood, however, that valve member  148  can be secured within pump  10  in any manner suitable for facilitating unidirectional flow of cooling air. 
     Valve member  148  includes member body  156  and projection  158 . Member body  156  and projection  158  function as a single part and can be integrally formed as a single part. Member body  156  is secured to end cap  68  by grease cap  60   b . Member body  156  forms a body of valve member  148 . Member body  156  is an annular ring extending about a central aperture in end cap  68   b . Screw  92  of drive mechanism  24  reciprocates through a central opening of member body  156 . In the example shown, the inner diameter D 5  of member body  156  is larger than diameter D 4  of drive nut  90 . 
     Inner portion  160  of member body  156  interfaces with support  152  of grease cap  60   b . Inner portion  160  is clamped between support  152  and end cap  68   b . Outer portion  162  does not interface with an axial face of support  152 . Outer portion  162  extends radially from inner portion and covers cap bores  132 . Outer portion  162  interfaces with end cap  68   b  to seal cap bores  132 . Member body  156  flexes to open the flowpaths through cap bores  132  in response to cooling air being pumped from cooling chamber  44   a  to cooling chamber  44   b . More specifically, outer portion  162  flexes away from end cap  68   b  to open the flowpaths. 
     Projection  158  extends from member body  156  and covers cap bores  146 . Second portion includes tapered edges  164  reducing a width of projection  158  between member body  156  and end  166  of projection  158 . End  166  extends between and connects tapered edges  164 . End  166  can be of any desired profile between tapered edges, such as flat, curved, pointed, etc. Projection  158  interfaces with end cap  68   b  to seal flowpaths through cap bores  146 . Projection  158  flexes away from end cap  68   b  to open the flowpaths through cap bores  146 . 
     While internal valve  52  is described as having a flapper valve member  148 , it is understood that internal valve  52  can be of any desired configuration for facilitating unidirectional flow. For example, internal valve  52  can include one or more of ball valves, diaphragm valves, swing valves, or any other one-way valve. In some examples, internal valve  52  includes the same number of valve members as there are bores  132 ,  146 . For example, a valve element can be disposed in each one of bores  132 ,  146  to facilitate unidirectional flow of the cooling air. In some examples, internal valve  52  includes fewer valve elements than there are outlet bores  132 ,  146 . 
     During operation, cooling air is pumped through second cooling passage  38  ( FIG.  2   ) and third cooling passage  40  ( FIG.  2   ) to cooling chamber  44   b . Valve member  148  extends over both cap bores  146  and cap bores  132  to control flow through second cooling passage  38  and third cooling passage  40 . Valve member  148  lifts off of end cap  68   b  to shift to an open state and allow cooling air flow into cooling chamber  44 . In some examples, a 360-degree portion of outer portion  162  of valve member  148  lifts off of end cap  68   b  to expose the full circumferential array of cap bores  132 . After pumping the cooling air to cooling chamber  44   b , fluid displacement members  20  reverse stroke direction. The increase in pressure in cooling chamber  44   b  and suction in cooling chamber  44   a  drive valve member  148  back to the closed state. The structural configuration of valve member  148  also biases valve member  148  towards the closed state. As such, internal valve  52  can be a normally closed valve. 
     Internal valve  52  provides significant advantages. Internal valve  52  prevents retrograde flow from cooling chamber  44   b  to cooling chamber  44   a . Internal valve  52  thereby ensures continuous circulation of fresh cooling air, providing more efficient cooling. Internal valve  52  being a single piece valve controlling flow through both second cooling passage  38  and third cooling passage  40  provides for simpler assembly, reduces part count, simplifies operation, and decreases costs. Valve member  148  is secured by grease cap  60   b , further decreasing part by providing a dual function for grease cap  60   b.    
       FIG.  6 A  is an exploded view of air check  46 .  FIG.  6 B  is a rear isometric view of air check  46 .  FIG.  6 C  is an enlarged cross-sectional view showing air check  46  mounted on pump body  16 .  FIGS.  6 A- 6 C  will be discussed together. Air check  46  includes inlet valve  48 , outlet valve  50 , filter  140 , valve housing  142 , and air cap  174 . Valve housing  142  includes outer side  176 , inner side  178 , upper end  180 , lower end  182 , mounting cylinders  184   a ,  184   b  (collectively herein “mounting cylinders  184 ”), and wall  186 . Inlet valve  48  and outlet valve  50  respectively include valve members  188   a ,  188   b  and retaining members  190   a ,  190   b.    
     Air check  46  is mounted to pump body  16  and is configured to control airflow into and out of cooling circuit CF ( FIG.  2   ). In some examples, valve housing  142  is disposed on and connected to motor housing  70 . In some examples, valve housing  142  is disposed axially between end caps  68   a ,  68   b  (best seen in  FIGS.  4 A,  4 B and  4 D ). Valve housing  142  can be connected to motor housing  70  by fasteners extending through valve housing  142  into motor housing  70 . Upper end  180  and lower end  182  of valve housing  142  are contoured to direct a flow of cooling air over heat sinks  74  (best seen in  FIG.  3 A ) formed on pump body  16 . In some examples, upper end  180  and lower end  182  are contoured to direct the cooling air flow generally tangentially to pump body  16 . 
     Filter  140  is disposed on outer side  176  of valve housing  142 . Filter  140  is configured to filter contaminants, such as dirt and dust, from air prior to the air entering cooling circuit CF. Air cap  174  is mounted to valve housing  142  and retains filter  140 . In some examples, air cap  174  provides an adjustable restriction such that air cap  174  can be adjusted to control a volume of air flowing into cooling circuit CF. Post  192  of air cap  174  extends through filter  140  and connects with tab  194 . In some examples, tab  194  extends from mounting cylinder  184   b  to secure air cap  174  to valve housing  142 . 
     Mounting cylinders  184  are formed on inner side  178  of valve housing  142 . Mounting cylinder  184   a  projects into inlet bore  196  formed in pump housing  16 . Inlet bore  196  forms an inlet of cooling circuit CF. Mounting cylinder  184   b  projects into outlet bore  198  formed in pump housing  16 . Outlet bore  198  forms an outlet of cooling circuit CF. 
     Mounting cylinders  184   a ,  184   b  receive retaining members  190   a ,  190   b  to secure inlet valve  48  and outlet valve  50  to valve housing  142 . Retaining members  190  extend into mounting cylinders  184  and are configured to remain stationary relative to mounting cylinders  184  during operation. Wall  186  extends around the mounting cylinder  184  associated with inlet valve  48 . Wall  186  interfaces with pump body  16  to isolate the inlet flow through inlet valve  48  from the outlet flow through outlet valve  50 . 
     Valve member  188   a  is disposed on a shoulder of mounting cylinder  184   a  and is secured by retaining member  190   a . A shaft of retaining member  190   a  is secured in mounting cylinder  184   a , such as by a press-fit connection. A head of retaining member  190   a  extends over a portion of valve member  188   a  to retain valve member  188   a  on mounting cylinder  184   a . In the example shown, valve member  188   a  includes a u-cup ring oriented with an open end facing towards pump housing  16  and away from valve housing  142 . Valve member  188   a  forms a one-way seal between valve housing  142  and inlet bore  196 . Valve member  188   a  is configured to allow unidirectional flow into first cooling passage  36 , as shown by arrow IF in  FIG.  6 C . 
     Valve member  188   b  is disposed on a shoulder of mounting cylinder  184   b  and is secured by retaining member  190   b . A shaft of retaining member  190   b  is secured in mounting cylinder  184   b , such as by a press-fit connection. A head of retaining member  190   b  extends over a portion of valve member  188   b  to retain valve member  188   b  on mounting cylinder  184   b . In the example shown, valve member  188   b  includes a u-cup ring oriented with an open end facing towards valve housing  142  and away from pump body  16 . Valve member  188   b  forms a one-way seal between valve housing  142  and outlet bore  198 . Valve member  188   b  is configured to allow unidirectional flow out of fourth cooling passage  42 , as shown by arrow EF in  FIG.  6 C . The inverse orientations of valve members  188   a ,  188   b  relative each other facilitates unidirectional flow through cooling circuit CF. Valve member  188   a  allows cooling air to enter but not exit cooling circuit CF, while valve member  188   b  allows cooling air to exit but not enter cooling circuit CF. 
     During operation, a first stroke occurs during which a suction stroke occurs in a first cooling chamber associated with inlet valve  48  (e.g., cooling chamber  44   a  ( FIGS.  2  and  4 A )) and a pumping stroke occurs in a second cooling chamber associated with outlet valve  50  (e.g., cooling chamber  44   b  ( FIGS.  2  and  4 A )). The suction causes valve member  188   a  to flex and disengage from pump body  16 , thereby opening a flowpath through inlet bore  196  between mounting cylinder  184   a  and pump body  16 . An intake portion of cooling air is drawn into air check  46  through air cap  174  and filter  140 . The intake portion of cooling air flows past valve member  188   a  through inlet bore  196  and into cooling circuit CF. Simultaneously, the pressure in the second cooling chamber causes valve member  188   b  to flex and disengage from pump body  16 , thereby opening a flowpath through outlet bore  198  between mounting cylinder  184   b  and pump body  16 . An exhaust portion of the cooling air is driven downstream through fourth cooling passage  42  and through outlet bore  198  past valve member  188   b . The exhaust portion exits cooling circuit CF through outlet bore  198 . The exhaust portion exits outlet bore  198  and is disposed between valve housing  142  and pump body  16 . The exhaust portion is driven towards upper end  180  and lower end  182  of valve housing  142 . The contouring of upper end  180  and lower end  182  direct the exhaust flow over heat sinks  74  formed on pump body  16 . Inlet valve  48  and outlet valve  50  are simultaneously in open states. 
     After completing the first stroke, a second stroke occurs during which a pumping stroke occurs in the first cooling chamber and a suction stroke occurs in the second cooling chamber. The pressure in the first cooling chamber causes valve member  188   a  to widen and engage with pump body  16  thereby closing the flowpath through inlet bore  196 . Simultaneously, the suction in the second cooling chamber causes valve member  188   b  to widen and engage with pump body  16  thereby closing the flowpath through outlet bore  198 . As such, each of inlet valve  48  and outlet valve  50  are simultaneously in closed states. 
     While inlet valve  48  and outlet valve  50  are described as respectively including valve members  188   a ,  188   b  and retaining members  190   a ,  190   b , it is understood that inlet valve  48  and outlet valve  50  can be of any desired configuration for facilitating unidirectional flow. For example, one or both of inlet valve  48  and outlet valve  50  can include ball valves, gate valves, disk valves, flapper valves, or be of any other suitable configuration. 
     Air check  46  provides significant advantages. Air check  46  provides unidirectional flow into and out of cooling pathway CF. Valve housing  142  directs cooling airflow over heat sinks  74  formed on pump body  16 , providing additional cooling to pump  10 . Inlet valve  48  and outlet valve  50  are simultaneously in the same state, either open or closed. As such, fresh cooling air is entering the cooling circuit CF as warm air is exhausted. 
       FIG.  7    is a cross-sectional view showing fluid displacement member  20 ′. Fluid displacement member  20 ′ is substantially similar to fluid displacement member  20  (best seen in  FIGS.  3 A and  4 A ). Fluid displacement member  20 ′ includes inner plate  78 ′, outer plate  80 ′, membrane  82 , and fastener  84 . Inner plate  78 ′ and outer plate  80 ′ each include heat sinks  200 . Fluid displacement member  20 ′ facilitates additional cooling of pump  10  during operation. 
     Heat sinks  200  of inner plate  78 ′ are formed on a portion of inner plate  78 ′ contacting the cooling air in a cooling chamber, such as cooling chambers  44   a ,  44   b  ( FIGS.  2  and  4 A ). Heat sinks  200  of outer plate  80 ′ are formed on a portion of outer plate  80 ′ contacting process fluid in a process fluid chamber, such as process fluid chambers  34   a ,  34   b . Fastener  84  extends through and is in contact with each of inner plate  78 ′ and outer plate  80 ′. Each of inner plate  78 ′, outer plate  80 ′, and fastener  84  can be made from thermally conductive material, such as aluminum, among other options. Fluid displacement member  20  acts as a heat exchange element between the relatively cool process fluid and relatively warm cooling air. The process fluid can absorb heat generated during pumping, further cooling pump  10 . Heat sinks  200  increase the surface area of the conductive surfaces exposed to the cooling air and the process fluid, providing better heat transfer efficiency. In some examples, the central aperture of membrane  82 , through which fastener  84  passes, is enlarged such that portions of inner plate  78 ′ and outer plate  80 ′ can be in physical contact through that central aperture, increasing the conductive capacity of fluid displacement member  20 . 
     Heat sinks  200  can be applied to any desired configuration of fluid displacement member to increase heat transfer efficiency. For example, fluid displacement member  20   b  (best seen in  FIGS.  3 A and  4 A ) includes a membrane overmolded on the portion of the outer plate that would contact the process fluid. The membrane is typically formed from a material with low thermal conductivity, such as rubber that inhibits heat transfer. Fluid displacement member  20   b  can be configured such that heat sinks extend from the outer plate and through the overmolding to be exposed to the process fluid. Fluid displacement member  20 ′ provides significant advantages by increasing heat transfer efficiency for pump  10 . In addition, fluid displacement member  20 ′ utilizes the process fluid as a heat transfer fluid, simplifying heat transfer by utilizing a fluid already present in the system. 
       FIG.  8 A  is a rear isometric view of electrically operated pump  10 .  FIG.  8 B  is a rear isometric view of pump  10  with housing cover  67  removed.  FIG.  8 C  is an isometric view of pump body  16  of pump  10 .  FIG.  8 D  is a cross-sectional view taken along line D-D in  FIG.  8 A .  FIG.  8 E  is a cross-sectional view taken along line E-E in  FIG.  8 A .  FIGS.  8 A- 8 E  will be discussed together. Pump  10  includes inlet manifold  12 , outlet manifold  14 , pump body  16 , fluid covers  18   a ,  18   b  (collectively herein “fluid cover  18 ” or “fluid covers  18 ”), fluid displacement members  20   a ,  20   b  (collectively herein “fluid displacement member  20 ” or “fluid displacement members  20 ”), motor  22 , drive mechanism  24 , controller  26 , fan assembly  31 , and housing cover  67 . Motor  22  includes stator  28  and rotor  30 . Fan assembly  31  includes impeller  33  and fan motor  35 . 
     Pump body  16  includes central portion  66  and end caps  68   a ,  68   b  (collectively herein “end cap  68 ” or “end caps  68 ”). Central portion  66  includes motor housing  70 , control housing  72 , and heat sinks  74 . Rotor  30  includes permanent magnet array  86  and rotor body  88 . Drive nut  90  and screw  92  of drive mechanism  24  are shown. 
     End caps  68   a ,  68   b  are disposed on opposite lateral sides of central portion  66  and are attached to central portion  66  to form pump body  16 . Fluid covers  18   a ,  18   b  are connected to end caps  68   a ,  68   b , respectively. Inlet manifold  12  is connected to each fluid cover  18  to provide fluid to process fluid chambers  34   a ,  34   b . Outlet manifold  14  is connected to each fluid cover  18  to receive fluid from process fluid chambers  34   a ,  34   b.    
     Motor  22  and control elements  29  (such as controller  26  ( FIGS.  1 C and  19   ) among other elements) are supported by pump body  16 . More specifically, motor  22  and control elements  29  are supported by central portion  66  of pump body  16 . Motor  22  is disposed within motor housing  70  between end caps  68 . Stator  28  surrounds rotor  30  and drives rotation of rotor  30 , such that motor  22  can be considered to be an inner rotator motor. Rotor  30  rotates about pump axis PA-PA and is disposed coaxially with drive mechanism  24  and fluid displacement members  20 . Permanent magnet array  86  is disposed on rotor body  88 . 
     Control housing  72  is connected to and extends from motor housing  70 . In the example shown, control housing  72  and motor housing  70  can be integrally formed as a single housing (e.g., by casting among other options). Control housing  72  is configured to house control elements  29  of pump  10 , such as controller  26  ( FIGS.  1 C and  19   ). 
     Heat sinks  74  are formed on central portion  66 . In the example shown, heat sinks  74  are formed in multiple configurations and include projections and fins, but it is understood that heat sinks  74  can be of any configuration suitable for increasing the surface area of pump body  16  to facilitate heat exchange to cool pump  10 . In the example shown, some of heat sinks  74  define flow passages forming an outer cooling fluid circuit CF 2  for pump  10 . In the example shown, support ones of heat sinks  74  extends between and connect control housing  72  and motor housing  70 . 
     Housing cover  67  is mounted to pump body  16  and at least partially defines flow passages of the cooling fluid circuit CF 2 . Inlet openings  83  and outlet openings  85  are formed through housing cover  67 . In some examples, housing cover  67  is formed as an upper portion connected to pump body  16  on an upper side of central portion  66  (e.g., between outlet manifold  14  and central portion  66  in the example shown), and as a lower portion connected to pump body  16  on a lower side of central portion  66  (e.g., between inlet manifold  12  and central portion  66  in the example shown). As such, housing cover  67  can be formed from multiple discrete components assembled to pump  10  to at least partially define cooling fluid circuit CF 2 . It is understood, however, that housing cover  67  can be formed by as many or as few components as desired. 
     The main heat sources of pump  10  include controller  26 , stator  28 , and drive mechanism  24 . Cooling fluid circuit CF directs cooling air through passages proximate the heat generating components to effect heat exchange between the cooling air and heat sources and thereby cool pump  10 . Cooling fluid circuit CF 2  is configured to direct cooling air around motor housing  70 . Cooling fluid circuit CF 2  directs cooling air circumferentially around pump axis PA. Cooling fluid circuit CF 2  is configured to direct cooling air to provide cooling to elements in both motor housing  70  and control housing  72 . It is understood that not all embodiments necessarily include a cooling fluid circuit CF 2  or otherwise pump cooling air. 
     In the example shown, cooling fluid circuit CF 2  includes an inlet passage  101 , intermediate passage  103 , and outlet passage  105 . In the example shown, there is no valving in cooling fluid circuit CF 2  to direct flow. Instead, fan  31  is configured to actively drive cooling air through cooling fluid circuit CF 2 . Fan  31  is supported by pump body  16 . More specifically, fan  31  is supported by a wall forming control housing  72 . Impeller  33  is disposed within cooling fluid circuit CF 2 . In the example shown, impeller  33  is disposed at an intersection between inlet passage  101  and outlet passage  105 . Fan  31  is thereby at least partially disposed within the cooling fluid circuit CF 2 . More specifically, impeller  33  is disposed in the flowpath between an inlet of cooling fluid circuit CF 2  and an outlet of cooling fluid circuit CF 2 . In the example shown, impeller  33  is unshrouded, but it is understood that impeller  33  can be shrouded in other examples. Fan motor  35  is disposed in control housing  72 . Fan motor  35 , which can be an electric motor, is isolated from the environment surrounding stator  28  by the wall of control housing  72 , such that the cooling arrangement shown is suitable for use in hazardous locations. 
     Inlet passage  101  is defined between motor housing  70  and housing cover  67 . In the example shown, inlet passage  101  includes multiple individual passages partially defined by heat sinks  74 . The individual passages extend circumferentially around motor housing  70 . An axial side of each flowpath is formed by a heat sink  74 . In the example shown, at least some of heat sinks  74  can extend circumferentially, but not axially, on motor housing  70  and about pump axis PA. At least three sides of each flowpath in inlet passage  101  is defined by thermally conductive material (e.g., the motor housing  70  and heat sinks  74 ). The body of motor housing  70  at least partially defines inlet passage  101 . Motor housing  70  is thereby directly exposed to the cooling flow through cooling fluid circuit CF 2 . Motor housing  70  is disposed directly between stator  28  and inlet passage  101  to provide efficient heat transfer from stator  28  to the cooling flow through cooling fluid circuit CF 2 . 
     Intermediate passage  103  is disposed between control housing  72  and motor housing  70 . A wall of control housing  72  at least partially defines intermediate passage  103 . One or more of the heat generating elements in control housing  72  can be mounted to control housing wall  73 . The heat generating elements are thereby mounted control housing wall  73  that is also directly in contact with the cooling air flowing through cooling fluid circuit CF 2 . Mounting the heat generating elements to control housing wall  73  facilitates efficient heat transfer from those components to the cooling flow through cooling fluid circuit CF 2 . Intermediate passage  103  is at least partially defined by the body of motor housing  70 . Motor housing  70  is thereby directly exposed to the cooling flow through cooling fluid circuit CF 2 . Motor housing  70  is disposed directly between stator  28  and intermediate passage  103  to provide efficient heat transfer from stator  28  to the cooling flow through cooling fluid circuit CF 2 . Heat sinks  74  extend between and connect control housing  72  and motor housing  70 . The heat sinks  74  at least partially defining intermediate passage  103  directly contact both control housing  72  and motor housing  70 . Such heat sinks  74  transfer heat from both control housing  72  and motor housing  70 . 
     Outlet passage  105  is defined between motor housing  70  and housing cover  67 . In the example shown, outlet passage  105  includes multiple individual passages partially defined by heat sinks  74 . The individual passages extend circumferentially around motor housing  70 . An axial side of each flowpath is formed by a heat sink  74 . In the example shown, at least some of heat sinks  74  can extend circumferentially, but not axially, on motor housing  70  and about pump axis PA. At least three sides of each flowpath in outlet passage  105  is defined by thermally conductive material (e.g., the motor housing  70  and heat sinks  74 ). The body of motor housing  70  at least partially defines outlet passage  105 . Motor housing  70  is thereby directly exposed to the cooling flow through cooling fluid circuit CF 2 . Motor housing  70  is disposed directly between stator  28  and outlet passage  105  to provide efficient heat transfer from stator  28  to the cooling flow through cooling fluid circuit CF 2 . 
     During operation, fan motor  35  is powered to drive rotation of impeller  33 . Fan  31  draws air into cooling fluid circuit CF 2  through inlet openings  83 . Inlet openings  83  provide locations for air to enter into cooling fluid circuit CF 2  and are in fluid communication with the surrounding environment. As such, the ambient air in the environment of pump  10  can form the cooling fluid of cooling fluid circuit CF 2 . While multiple inlet openings  83  are shown, it is understood that cooling fluid circuit CF 2  can include any desired number of inlet openings  83 , such as one or more. Inlet openings  83  can also be spaced circumferentially along inlet passage  101 . For example, one or more additional or alternative inlet openings  83  can be formed at circumferential locations along housing cover  67  between the location currently shown and the position of fan  31 . 
     Fan  31  draws intake air (shown by arrow IA) through inlet passage  101  and over motor housing  70  and heat sinks  74 . The flow of cooling air (shown by arrows AF in  FIG.  8 D ) passes over heat sinks  74  and motor housing  70  and cools those elements. Fan  31  blows the air downstream through intermediate passage  103  and outlet passage  105 . The cooling air blown by the fan  31  initially flows through intermediate passage  103 . The air flowing through intermediate passage  103  contacts both control housing  72  and motor housing  70  to transfer heat from both the heat generating components in control housing  72  (e.g., controller  26  among others) and from the heat generating components of in motor housing  70  (e.g., stator  28  and drive mechanism  24 ). At least a portion of the flow through cooling fluid circuit CF 2  flows directly between the motor  22  and an electric component  29  mounted to housing wall  73 . A radial line extending from pump axis PA can extend through drive mechanism  24 , stator  28 , a passage through cooling fluid circuit CF 2  and an electric component  29  mounted to housing wall  73 . 
     At least a portion of cooling fluid circuit CF 2  is radially bracketed by two unique heat sources. Specifically, intermediate passage  103  is exposed to thermally conductive element on both radial sides of intermediate passage  103 . The electric elements within control housing  72  form a first heat source cooled by the flow through cooling fluid circuit CF 2  and the stator  28  and drive mechanism  24  within motor housing  70  form a second heat source cooled by the flow through cooling fluid circuit CF 2 . Intermediate passage  103  is disposed directly downstream from impeller  33 . As such, the air entering and then flowing through intermediate passage  103  has the greatest velocity of the flow through cooling fluid circuit CF 2 . The high velocity facilitates quick air exchange and decreases residence time, providing enhanced cooling efficiency in the portion of cooling fluid circuit CF 2  exposed to two independent heat sources. 
     Fan  31  blows the air downstream through intermediate passage  103 . The air flow exits intermediate passage  103  and flows through outlet passage  105 . The air further cools pump  10  as the air flows through outlet passage  105  to outlet openings  85 . The air is exhausted through outlet openings  85  as exhaust air (shown by arrow EA). In some examples, pump  10  includes deflectors and/or contouring to direct heated exhaust air exiting outlet openings  85  away from inlet openings  83 . In some examples, pump  10  includes deflectors and/or contouring such that an air intake is oriented away from outlet openings  85  to void intake of hot exhaust air. Blocker wall  71  extends radially from motor housing  70 . Blocker wall  71  is disposed circumferentially between inlet passage  101  and outlet passage  105 . Blocker wall  71  prevents cool intake air entering inlet passage  101  from crossing into outlet passage  105  and prevents heated exhaust air form outlet passage  105  from crossing into inlet passage  101 . Blocker wall  71  can further act as a heat sink to conduct heat away from stator  28  and drive mechanism  24 . 
     One or more of heat sinks  74  can be formed as a continuous projection extending through multiple portions of the cooling fluid flowpath CF 2 . For example, a single heat sink  74  can extend from blocker wall  71 , through inlet passage  101 , through intermediate passage  103 , and through outlet passage  105  and back to blocker wall  71 . As such, one or more of heat sinks  74  can extend fully circumferentially about motor  22  between a common connection point (e.g., blocker wall  71  in the example shown). 
     The cooling air flow AF is drawn into cooling fluid circuit CF 2  by fan  31  and blown between two independent heat sources contained in control housing  72  and motor housing  70  and downstream out of cooling fluid circuit CF 2 . The cooling air flow AF is routed circumferentially about motor housing  70  and pump axis PA. The cooling air flow AF thereby flows around both the axis of rotation of rotor  30  and the axis of reciprocation of fluid displacement members  20 . In the example shown, the cooling air flow AF contacts motor housing  70  about a full circumferential length of the cooling fluid circuit CF 2 . The cooling air flow AF contacts control housing  72  for a portion of the length of the cooling fluid circuit CF 2 . 
     Cooling fluid circuit CF 2  provides significant advantages. Cooling fluid circuit CF 2  draws cooling air from the environment surrounding pump  10 , providing an unlimited source of cooling air. Fan  31  actively pulls the cooling fluid into cooling fluid circuit CF 2  and blows the cooling fluid downstream through cooling fluid circuit CF 2  to the outlet. Fan  31  actively blows the air through cooling fluid circuit CF 2 , facilitating greater flow and more efficient cooling. Cooling fluid circuit CF 2  provides cooling to both the heating elements of control housing  72  and the heating elements in motor housing  70 . By cooling multiple distinct heat sources, cooling fluid circuit CF 2  simplifies the arrangement of pump  10  and provides for a more compact, efficient pumping assembly. Cooling fluid circuit CF 2  routes the cooling air circumferentially around motor housing  70 , maximizing the heat transfer area between motor housing  70  and the cooling air flow AF. 
       FIG.  9 A  is a partially exploded view of pump  10 .  FIG.  9 B  is an enlarged cross-sectional view showing an interface between drive mechanism  24  and fluid displacement member  20   a .  FIG.  9 C  is an enlarged isometric view of an end  104 ,  106  of screw  92 .  FIGS.  9 A- 9 C  will be discussed together. Inlet manifold  12 , outlet manifold  14 , pump body  16 , fluid covers  18   a ,  18   b , fluid displacement member  20   a , and screw  92  of drive mechanism  24  are shown. Fluid displacement member  20   a  includes inner plate  78   a , outer plate  80   a , membrane  82 , and fastener  84 . Inner plate  78   a  includes receiving chamber  202 , fastener opening  204 , and set screw opening  206 . Receiving chamber  202  includes chamber wall  208 . First end  104  of screw  92  includes first bore  112 , locating bore  210 , and flats  212 . 
     As discussed above, fluid displacement member  20   a  is mounted within pump  10  such that fluid displacement member  20   a  does not rotate about pump axis PA-PA. In the example shown, an outer circumferential edge of membrane  82  is captured between fluid cover  18   a  and pump body  16  to prevent fluid displacement member  20   a  from rotating about pump axis PA-PA. 
     Screw  92  is connected to fluid displacement member  20   a  such that screw  92  is prevented from rotating relative to fluid displacement member  20   a . Outer plate  80   a  is disposed on a side of membrane  82  facing fluid cover  18   a . Inner plate  78   a  is disposed on a side of membrane  82  facing end cap  68   a . Fastener  84  extends through each of outer plate  80   a , membrane  82   a , and inner plate  78   a  and into screw  92  to connect fluid displacement member  20  to screw  92 . 
     Chamber wall  208  projects from an inner side of inner plate  78   a . Chamber wall  208  at least partially defines receiving chamber  202 . Chamber wall  208  is profiled such that to engage screw  92  and prevent screw  92  from rotating relative to fluid displacement member  20 . Fastener opening  204  and set screw opening  206  extend through inner plate  78  into receiving chamber  202 . While receiving chamber  202  is described as defined by a projection from inner plate  78   a , it is understood that receiving chamber  202  can be formed in any desired manner. For example, receiving chamber  202  can be formed by a recess extending into inner plate  78   a.    
     In the example shown, first screw end  104  extends into receiving chamber  202 . First end  104  is profiled complementary to chamber wall  208  to prevent rotation of screw  92  relative to fluid displacement member  20   a . In the example shown, flats  212  are formed on opposite radial sides of first end  104 . Chamber wall  208  includes corresponding features configured to mate with flats  212 . The interface between screw  92  and inner plate  78   a  prevents screw  92  from rotating relative to inner plate  78   a . While fluid displacement member  20   a  and screw  92  are described as having mating flats to prevent rotation, it is understood that fluid displacement member  20   a  and screw  92  can interface in any desired manner suitable for keying screw  92  to fluid displacement member  20   a  and preventing relative rotation. 
     Set screw  214  extends through set screw opening  206  and into locating bore  210 . Set screw  214  extending into locating bore  210  further locks screw  92  to fluid displacement member  20   a . Locating bores  210  extend into screw  92  from first end  104  and second end  106 . In some examples, locating bores  210  extends parallel to first bore  112  and second bore  114 . Locating bores  210  can include threading configured to mate with threading formed on set screw  214 . 
     Screw  92  is connected to fluid displacement member  20   a  such that screw  92  cannot rotate relative to fluid displacement member  20   a . Screw  92  is connected to fluid displacement member  20   b  in substantially the same manner screw  92  connects to fluid displacement member  20   a . In some examples inner plate  78   a  is identical to inner plate  78   b . Fluid displacement members  20   a ,  20   b  thereby prevent rotation of screw  92  relative pump axis PA-PA. 
     The connection between screw  92  and fluid displacement member  20  also prevents loosening of or disconnecting of fastener  84  during operation. The rotational moment exerted on screw  92  during pumping does not cause unthreading of fastener  84  from first bore  112  because screw  92  is prevented from rotating relative to fluid displacement member  20 . Fluid displacement member  20   a  is secured within pump  10  such that fluid displacement member  20  cannot rotate relative to pump axis PA-PA. Fluid displacement members  20  prevent screw  92  from rotating about pump axis PA-PA further facilitating translation of screw  92  along pump axis PA-PA. 
       FIG.  10    is a schematic block diagram showing an interface between pump body  16 ′ and fluid displacement member  20 ″. In the example shown, fluid displacement member  20 ″ is a piston. Pump body  16 ′ includes piston bore  216 . Pump body  16 ′ can be any housing of pump  10  within which a piston reciprocates during pumping, such as an end cap configured to house a reciprocating piston. Piston bore  216  includes housing contour  218 . Fluid displacement member  20 ″ includes piston contour  220 . Piston contour  220  mates with housing contour  218  such that fluid displacement member  20 ″ can travel axially relative to pump body  16 ′ but is prevented from rotating relative to pump body  16 ′. The interface between fluid displacement member  20 ″ and pump body  16 ′ prevents fluid displacement member  20 ″ from rotating relative to axis PA-PA and relative to pump body  16 ′. Screw  92  (best seen in  FIGS.  4 A and  12   ) can be connected to fluid displacement member  20 ″ to prevent relative rotation, similar to the connection shown in  FIGS.  9 A and  9 B . 
       FIG.  11    is a schematic block diagram showing anti-rotation interface  222 . Second end  106  of screw  92  is shown. Slot  224  is formed in pump body  16 . It is understood that slot  224  can be formed on one of an end  104 ,  106  of screw  92  and in pump housing  16 . Slot  224  can be open at the end of screw  92 . 
     Projection  226  extends from screw  92 . In the example shown, projection  226  is formed as part of collar  225  connected to the end of screw  92 . In examples where slot  224  is formed in screw  92 , projection  226  can extend from a static component of pump  10 , such as pump body  16 . Projection  226  extends into and mates with slot  224 . Projection  226  mating with slot  224  prevents screw  92  from rotating relative to pump axis PA-PA as screw  92  reciprocates. Screw  92  reciprocates relative to projection  226 . Projection  226  is shown as a pin, but it is understood that projection can be of any configuration suitable for extending into slot  224  to prevent rotation of screw  92 . For example, projection  226  can be a fin, a detent, or a bump, among other options. 
       FIG.  12    is an isometric partial cross-sectional view of motor  22  and drive mechanism  24 . Motor  22  includes stator  28  and rotor  30  and is mounted in motor housing  70 . Rotor  30  includes permanent magnet array  86  and rotor body  88 . Rotor body  88  includes rotor bores  96 ; rotor ends  228   a ,  228   b  (collectively herein “rotor ends  228 ”); axial extensions  230   a ,  230   b  (collectively herein “axial extensions  230 ”); and axial recesses  232   a ,  232   b  (collectively herein “axial recesses  232 ”). Drive mechanism  24  includes drive nut  90 , screw  92 , and rolling elements  98 . Gap  99  between drive nut  90  and screw  92  is shown. Drive nut  90  includes nut notches  100   a ,  100   b , nut thread  102 , nut ends  234   a ,  234   b , and nut body  236 . First screw end  104 , second screw end  106 , screw body  108 , screw thread  110 , first bore  112 , locating bore  210 , and flats  212  of screw  92  are shown. 
     Rotor  30  is disposed within stator  28  on pump axis PA-PA. Axial extensions  230   a ,  230   b  are disposed at and extend from rotor ends  228   a ,  228   b , respectively. Axial extensions  230   a ,  230   b  extend beyond axial ends of stator  28 . Permanent magnet array  86  is mounted on rotor  30 . Axial ends of permanent magnet array  86  extend onto axial extensions  230 . Axial extensions  230  extending beyond the axial ends of stator  28  facilitates top and/or end mounting of position sensor  62  (best seen in  FIGS.  17 A and  18   ), as discussed in more detail below. Rotor bores  96  extend through rotor body  88  between rotor end  228   a  and rotor end  228   b . Rotor bores  96  extend axially in the example shown. Rotor bores  96  can be of any configuration suitable for effecting cooling flow through rotor  30  and/or reducing weight of rotor  30 . 
     Drive nut  90  extends through rotor  30  and is disposed coaxially with rotor  30 . Drive nut  90  is connected to rotor body  88  such that drive nut  90  rotates about pump axis PA-PA with rotor  30 . Nut thread  102  are formed on an inner radial surface of drive nut  90 . Nut end  234   a  extends in a first axial direction from nut body  236  and nut end  234   b  extends in a second axial direction from nut body  236 . Nut notch  100   a  is formed at an interface between nut end  234   a  and nut body  236 . Nut notch  100   b  is formed at an interface between nut end  234   b  and nut body  236 . Inner races  122   a ,  122   b  of bearings  54   a ,  54   b  (best seen in  FIGS.  4 A,  4 B, and  4 D ) are respectively disposed at nut notches  100   a ,  100   b  and seated on nut ends  234   a ,  234   b . Axial recesses  232   a ,  232   b  are annular recesses disposed between axial extensions  230   a ,  230   b  and nut ends  234   a ,  234   b . Bearings  54  are at least partially disposed in axial recesses  232 . Axial recesses  232  provide space for position sensor  62  to extend under permanent magnet array  86 . 
     Screw  92  extends axially through drive nut  90  and is disposed coaxially with rotor  30  and drive nut  90 . Screw thread  110  are formed on an exterior of screw body  108 . First screw end  104  extends axially from a first end of screw body  108  and second screw end  106  extends axially from a second end of screw body  108 . Flats  212  are formed on each of first screw end  104  and second screw end  106 . Flats  212  form anti-rotational surfaces configured to interface with features on fluid displacement members  20  to prevent screw  92  from rotating relative fluid displacement members  20 . First bore  112  and locating bore  210  extend axially into first screw end  104 . 
     Rolling elements  98  are disposed in raceways formed by screw thread  110  and nut thread  102 . Rolling elements  98  support screw  92  relative drive nut  90  such that each of drive nut  90  and screw  92  ride on rolling elements  98 . Rolling elements  98  support screw  92  relative drive nut  90  such that drive nut  90  and screw  92  are not in contact during operation. Rolling elements  98  maintain gap  99  between drive nut  90  and screw  92  and prevent contact therebetween. 
     Drive nut  90  rotates relative to screw  92 . Rolling elements  98  exert forces on screw  92  at screw thread  110  to cause axial displacement of screw  92  along pump axis. Rotor  30  can be driven in a first rotational direction to drive screw  92  in a first axial direction. Rotor  30  can be driven in a second rotational direction opposite the first rotational direction to drive screw  92  in a second axial direction opposite the first axial direction. 
       FIG.  13    is a partial cross-sectional view of drive mechanism  24 ′. Drive mechanism  24 ′ includes drive nut  90 ′, screw  92 , rolling elements  98 , and ball return  238 . 
     Drive nut  90 ′ surrounds a portion of screw  92  and rolling elements  98  are disposed between drive nut  90 ′ and screw  92 . In the example shown, rolling elements  98  are balls. As such, drive mechanism  24 ′ can be considered to be a ball screw. Rolling elements  98  support drive nut  90 ′ relative screw  92  such that drive nut  90 ′ does not contact screw  92 . Rolling elements  98  are disposed in raceways formed by screw thread  110  and nut thread  102  (best seen in  FIG.  12   ). Ball return  238  is configured to pick up rolling elements  98  and recirculate the rolling elements  98  within the raceway formed by screw thread  110  and nut thread  102 . Ball return  238  can be of any type suitable for circulating rolling elements  98 . In some examples, ball return  238  is an internal ball return such that rolling elements  98  not within raceway pass through body of drive nut  90 ′. 
     Drive nut  90 ′ rotates relative to screw  92  and causes rolling elements  98  to exert an axial force on screw  92  to drive screw linearly. Drive mechanism  24 ′ can thereby convert a rotational input to a linear output. 
       FIG.  14    is an isometric view of drive mechanism  24 ″ with a portion of drive nut  90 ″ removed.  FIG.  15    is an isometric view of drive mechanism  24 ″ with the body of drive nut  90 ″ removed to show rolling elements  98 ′.  FIGS.  14  and  15    will be discussed together. Drive mechanism  24 ″ includes drive nut  90 ″, screw  92 , and rolling elements  98 ′. Drive nut  90 ″ includes drive rings  240 . Each one of rolling elements  98 ′ includes end rollers  242  and roller shaft  244 . 
     Drive nut  90 ″ surrounds a portion of screw  92  and rolling elements  98 ′ are disposed between drive nut  90 ″ and screw  92 . In the example shown, rolling elements  98 ′ include rollers. As such, drive mechanism  24 ″ can be considered to be a roller screw. Rolling elements  98 ′ support drive nut  90 ″ relative screw  92  such that drive nut  90 ″ does not contact screw  92 . Rolling elements  98 ′ are disposed circumferentially and symmetrically about screw  92 . Roller shafts  244  extend between and connect pairs of end rollers  242 . As such, each rolling element  98 ′ can include an end roller  242  at a first end of the shaft  244  and can further include an end roller  242  at a second end of the roller shaft  244 . In some examples, roller shafts  244  include threading configured to mate with screw thread  110  to exert additional driving force on screw  92 . Each end roller  242  includes teeth. End rollers  242  extend between and engages thread  110  and drive ring  240 . The teeth of end rollers  242  engage the teeth of drive ring  240 . 
     Drive nut  90 ″ includes a first drive ring  240  at a first end of drive nut  90 ″ and a second drive ring  240  at a second end of drive nut  90 ″. For each rolling element  98 ′, a first one of the end rollers  242  engages the teeth of the drive ring  240  at the first end of drive nut  90 ″ and the second one of the end rollers  242  engages the teeth of the drive ring  240  at the second end of drive nut  90 ″. As drive nut  90 ″ rotates, engagement between end rollers  242  and drive rings  240  causes each rolling element  98 ′ to rotate about its own axis and causes the array of rolling elements  98 ′ to rotate about pump axis PA-PA. The threads of roller shafts  244  engage and exert a driving force on screw thread  110  to linearly displace screw  92 . 
     Drive nut  90 ″ rotates relative to screw  92  and causes rolling elements  98 ′ to exert an axial force on screw  92  to drive screw  92  linearly. Drive mechanism  24 ″ thereby converts a rotational input to a linear output. 
       FIG.  16 A  is a first isometric view of motor nut  56 .  FIG.  16 B  is a second isometric view of motor nut  56 .  FIGS.  16 A and  16 B  will be discussed together. Motor nut  56  includes motor nut notch  126 , outer edge  128 , cooling ports  130 , central aperture  144 , first side  246  (seen in  FIG.  16 A ), second side  248  (seen in  FIG.  16 B ), flange  250 , and lip  256 . Motor nut notch  126  includes axial surface  252  and radial surface  254 . 
     Central aperture  144  extends through motor nut  56  between first side  246  and second side  248 . Central aperture  144  provides an opening that screw  92  can reciprocate through during operation. First side  246  of motor nut  56  is oriented towards fluid displacement member  20   a  (best seen in  FIGS.  4 A,  9 A, and  9 B ) and second side  248  of motor nut  56  is oriented towards motor  22  (best seen in  FIGS.  4 A- 4 D and  12   ). Motor nut  56  is configured to mount to a pump housing, such as pump body  16  (best seen in  FIGS.  3 A- 4 C ). Outer edge  128  includes threading configured to connect to threading formed in the pump housing. As such, motor nut  56  can be threadedly connected to pump body  16 . Flange  250  projects axially from second side  248  of motor nut  56 . Flange  250  interfaces with pump housing  16  as motor nut  56  is installed to ensure proper alignment between motor nut  56  and pump body  16 . In the example shown, flange  250  aligns with end cap  68   a , and end cap  68   a  aligns with central portion  66 . In some examples, the threading does not extend onto flange  250 . 
     Motor nut notch  126  is formed within central aperture  144 . Motor nut notch  126  is configured to extend around and receive an outer race of bearing  54 . Outer race  124  interfaces with both axial surface  252  and radial surface  254  of motor nut notch  126 . Motor nut  56  preloads bearings  54  of pump  10  via the interface with bearing  54   a.    
     Lip  256  extends radially from first side  246  into central aperture  144 . Lip  256  extends circumferentially about central aperture  144 . Lip  256  defines a narrowest diameter of central aperture  144 . In some examples, lip  256  forms a mounting feature on which a portion of grease cap  60   a  can mount. For example, a support, such as support  152  ( FIG.  5 A ), of grease cap  60  can mount to lip  256  via a snap lock configuration. Cooling ports  130  extend through motor nut  56  between first side  246  and second side  248 . Cooling ports  130  form the upstream-most portions of third cooling passage  40  (best seen in  FIGS.  2  and  4 A ). Cooling ports  130  provide pathways for a portion of the cooling air to enter third cooling passage  40 . 
       FIG.  17 A  is an enlarged cross-sectional view showing the location of position sensor  62  relative motor  22 .  FIG.  17 B  is an isometric schematic view of a permanent magnet array, specifically of permanent magnet array  86 .  FIG.  18    is an enlarged cross-sectional view showing a location of position sensor  62  relative to motor  22 .  FIGS.  17 A- 18    will be discussed together. Motor  22  includes stator  28  and rotor  30 . Rotor  30  includes rotor body  88  and permanent magnet array  86 . Position sensor  62  includes support body  263  and sensing components  264 . Permanent magnet array  86  includes permanent magnets  258  and back irons  260 . 
     Position sensor  62  is mounted within pump  10  and adjacent to rotor  30 . Position sensor  62  is mounted such that rotor  30  moves relative to position sensor  62 . For example, position sensor  62  can be mounted to pump body  16  or stator  28 , among other options. In the example shown in  FIG.  17 A , position sensor  62  is mounted to end cap  68   b . More specifically, sensor body  263  is fixed to end cap  68   b  to secure position sensor  62  at a fixed position about pump axis PA. In the example shown in  FIG.  18   , sensor body  263  is fixed to stator  28  to secure position sensor  62  at a fixed position about pump axis PA. For example, sensor body  263  can be connected to stator  28  by fasteners extending into stator  28 , such as into a potting compound of stator  28 . Sensor body  263  can support other components of position sensor  62 , such as electronic components thereof, relative to motor  22  and other components of pump  10 . 
     Position sensor  62  is communicatively connected to controller  26  ( FIGS.  1 A and  19   ). As discussed above, screw  92  does not rotate as screw  92  translates during operation. As such, rotation of screw  92  cannot be sensed to generate commutation data. Instead, position sensor  62  is disposed proximate permanent magnet array  86  such that the magnetic fields of permanent magnets  258  are sensed by position sensor  62 . Specially, position sensor  62  includes an array of sensing components  264  spaced circumferentially about pump axis PA. For example, the array of sensing components  264  can be an array of Hall-effect sensors responsive to the magnetic fields generated by permanent magnets  258 . For example, position sensor  62  can utilize an array of three Hall effect sensors as the sensing components  264  of position sensor  62 . The position information generated by position sensor  62  provides commutation data that controller  26  utilizes to commutate motor  22 . 
     As shown in  FIGS.  17 A , permanent magnet array  86  includes outer radial edge  266  and inner radial edge  268 . Outer radial edge  266  is oriented towards stator  28  and spaced from stator  28  by an air gap. Inner radial edge  268  is oriented towards pump axis PA-PA. During operation, back irons  260  concentrate flux and direct the magnetic field from permanent magnets on opposite circumferential sides of back iron  260 . The stray flux through rotor  30  affects operation of position sensor  62  and can prevent sensing components  264  from accurately sensing the polarity of permanent magnets  258 . The stray flux is concentrated in the region radially aligned with permanent magnet array  86  (e.g., between inner radial edge  268  and outer radial edge  266 ) and the region radially outside of permanent magnet array  86  (e.g., radially outside of outer radial edge  266 ). 
     Position sensor  62  is mounted such that sensing components  264  are disposed at a mounting region radially inward of permanent magnet array  86  (e.g. radially between pump axis PA and permanent magnet array  86 ) to isolate sensing components  264  from the stray flux during operation. In  FIG.  17 A , position sensor  62  is mounted to and supported by end cap  68 . In  FIG.  18   , position sensor  62  is mounted to and supported by stator  28 . In both the examples shown in  FIGS.  17 A and  18   , sensing components  264  are disposed radially inward of permanent magnet array  86  such that permanent magnet array  86  is radially between sensing components  264  and stator  28 . While sensing components  264  are disposed radially inward of rotor  30 , it is understood that position sensor  62  can span radially over permanent magnet array  68  such that a portion of position sensor  62  is disposed radially inside of permanent magnet array  68  and a portion of position sensor  62  is disposed radially outside of permanent magnet array  68 . 
     Sensing components  264  of position sensor  62  are disposed radially between inner radial edge  268  and pump axis PA-PA. Permanent magnet array  86  is disposed between sensing components  264  and stator  28 . Sensing components  264  are disposed radially inward of inner radial edge  268  of permanent magnet array  86 . Sensing components  264  are disposed radially between bearing  54   b  and inner radial edge  268 . Sensing components  264  extend below permanent magnet array  86  and between permanent magnet array  86  and pump axis PA-PA. Sensing component  264  extend axially into rotor body  88  such that axial extension  230   b  is disposed between sensing component  264  and permanent magnet array  86 . Sensing components  264  extend into axial recess  232   b . Sensing components  264  can axially overlap with permanent magnet array  86  such that a radial line extending from pump axis PA passes through a portion of each of sensing components  264  and permanent magnet array  86 . When mounted in the mounting region, sensing components  264  do not radially overlap with permanent magnet array  86 , such that an axial line parallel to pump axis PA will not pass through both sensing components  264  and permanent magnet array  86 . Locating sensing components  264  radially inward of permanent magnet array  86  shields sensing components  264  from the stray flux. Position sensor  62  can generate data regarding the permanent magnets  258  and provide commutation information to controller  26  with sensing components  264  mounted in the mounting region. Sensing components  264  can be mounted radially inward of permanent magnet array and can generate commutation data from that position. 
     Mounting the position sensor  62  such that sensing components  264  are radially inside of permanent magnet array  86  reduces the effect of the stator flux on position sensor  62 . Sensing components  264  mounting radially inside of permanent magnet array  86  shields sensing components  264  and facilitates sensing by position sensor  62 . Sensing components  264  axially overlap with rotor  30  and extend into a portion of rotor  30 , facilitating a compact arrangement of pump  10 . 
       FIG.  19    is a block diagram of pump  10 . Fluid displacement members  20 , motor  22 , drive mechanism  24 , controller  26 , and user interface  27  are shown. Motor  22  includes stator  28  and rotor  30 . Controller  26  includes control circuitry  272  and memory  274 . 
     Motor  22  is disposed within a pump body and is coaxial with the fluid displacement members  20  of pump  10  in the example shown. Controller  26  is operably connected to motor  22  to control operation of motor  22 . While motor  22  and fluid displacement members  20  are shown as coaxial, it is understood that, in some examples, rotor  30  can be configured to rotate on a motor axis that is not coaxial with a reciprocation axis of the fluid displacement members  20 . In addition, each fluid displacement member  20  can be configured to reciprocation on its own reciprocation axis that is not coaxial with the reciprocation axis of the other fluid displacement member  20 . It is further understood that, while pump  10  is shown as including two fluid displacement members  20 , some examples of pump  10  can include a single fluid displacement member or more than two fluid displacement members. 
     Motor  22  is an electric motor having stator  28  and rotor  30 . Stator  28  includes armature windings and rotor  30  includes a permanent magnet array, such as permanent magnet array  86  (best seen in  FIG.  17 B ). Rotor  30  is configured to rotate about pump axis PA-PA in response to current through stator  28 , which can be referred to as current, voltage, or power. It is understood that a reference to the term “current” can be replaced with a different measure of power such as voltage or the term “power” itself. 
     Position sensor  62  is disposed proximate rotor  30  and is configured to sense rotation of rotor  30  and to generate data in response to that rotation. In some examples, position sensor  62  includes an array of Hall-effect sensors disposed proximate rotor  30  to sense the polarity of permanent magnets forming the permanent magnet array of rotor  30 . Controller  26  commutates motor  22  based on data generated by position sensor  62 . 
     The position sensor  62  counts the magnetic sections of rotor  30  as the permanent magnets pass by the position sensor  62 , each magnet being detected as the magnetic field measured by the position sensor  62  increases above a threshold and then decreases back below the threshold, the threshold corresponding to the position sensor being proximate a magnet. The controller can be configured to know what number of passing magnetic sections corresponds with what angular displacement of the rotor  30 , a full turn of the rotor  30 , linear displacement of the screw  92  (and fluid displacement member  20 ), and/or portion of a pump cycle, among other options. The position sensor  62  does not provide information regarding which rotational direction the rotor  30  is spinning, but the controller  26  knows in which direction the rotor  30  is being driven. The controller  26  can then calculate the position of the screw  92  and/or fluid displacement members  20  along pump axis PA-PA based on counting the number of magnets passing the position sensor  62 . In some examples, the number of magnet passes is added to a running total when the rotor is driven in a first direction (e.g., one of clockwise and counterclockwise) and subtracted from the running total when the rotor is driven in the opposite direction (e.g., the other of clockwise and counterclockwise). 
     Motor  22  is a reversible motor in that stator  28  can cause rotor  30  to rotate in either of two rotational directions. Rotor  30  is connected to the fluid displacement members  20  via drive mechanism  24 , which receives a rotary output from rotor  30  and provides a linear input to fluid displacement members  20 . Drive mechanism  24  causes reciprocation of fluid displacement members  20  along pump axis PA-PA. Drive mechanism  24  can be of any desired configuration for receiving a rotational output from rotor  30  and providing a linear input to one or both of fluid displacement members  20 . 
     Rotating rotor  30  in the first rotational direction causes drive mechanism  24  to displace fluid displacement members  20  in a first axial direction. Rotating rotor  30  in the second rotational direction causes drive mechanism  24  to displace fluid displacement members  20  in a second axial direction opposite the first axial direction. Drive mechanism  24  is directly connected to rotor  30  and fluid displacement members  20  are directly driven by drive mechanism  24 . As such, motor  22  directly drives fluid displacement members  20  without the presence of intermediate gearing, such as speed reduction gearing. 
     Fluid displacement members  20  can be of any type suitable for pumping fluid from inlet manifold  12  to outlet manifold  14 . For example, fluid displacement members  20  can include pistons, diaphragms, or be of any other type suitable for reciprocatingly pumping fluid. It is understood that while pump  10  is described as including multiple fluid displacement members  20 , some examples of pump  10  include a single fluid displacement member  20 . 
     In some examples, fluid displacement members  20  have a variable working surface area, which is the area of the surface that drives the process fluid. The working surface area can vary throughout the stroke. For example, a flexible member forming at least a portion of fluid displacement member  20 , such as membranes  82  (best seen in  FIGS.  3 A and  3 B ), can flex to cause the variable working surface area. In some examples, the flexible member can contact a housing, such as fluid covers  18  (best seen in  FIGS.  3 A and  4 A- 4 C ), disposed opposite the flexible member, thereby reducing the working surface area as fluid displacement member  20  proceeds through a pumping stroke. The pressure output by pump  10  depends on the working surface area of the fluid displacement member  20 . As the working surface area decrease, less current is required to cause pump  10  to operate at a given speed and pressure. 
     Controller  26  is configured to store software, implement functionality, and/or process instructions. Controller  26  is configured to perform any of the functions discussed herein, including receiving an output from any sensor referenced herein, detecting any condition or event referenced herein, and controlling operation of any components referenced herein. Controller  26  can be of any suitable configuration for controlling operation of motor  22 , gathering data, processing data, etc. Controller  26  can include hardware, firmware, and/or stored software, and controller  26  can be entirely or partially mounted on one or more boards. Controller  26  can be of any type suitable for operating in accordance with the techniques described herein. While controller  26  is illustrated as a single unit, it is understood that controller  26  can be disposed across one or more boards. In some examples, controller  26  can be implemented as a plurality of discrete circuitry subassemblies. 
     Memory  274  configured to store software that, when executed by control circuitry  272 , controls operation of motor  22 . For example, control circuitry  272  can include one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other equivalent discrete or integrated logic circuitry. Memory  274 , in some examples, is described as computer-readable storage media. In some examples, a computer-readable storage medium can include a non-transitory medium. The term “non-transitory” can indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium can store data that can, over time, change (e.g., in RAM or cache). In some examples, memory  274  is a temporary memory, meaning that a primary purpose of memory  274  is not long-term storage. Memory  274 , in some examples, is described as volatile memory, meaning that memory  274  does not maintain stored contents when power to controller  26  is turned off. Examples of volatile memories can include random access memories (RAM), dynamic random access memories (DRAM), static random access memories (SRAM), and other forms of volatile memories. Memory  274 , in one example, is used by software or applications running on control circuitry  272  to temporarily store information during program execution. Memory  274 , in some examples, also includes one or more computer-readable storage media. Memory  274  can further be configured for long-term storage of information. Memory  274  can be configured to store larger amounts of information than volatile memory. In some examples, memory  274  includes non-volatile storage elements. Examples of such non-volatile storage elements can include magnetic hard discs, optical discs, floppy discs, flash memories, or forms of electrically programmable memories (EPROM) or electrically erasable and programmable (EEPROM) memories. 
     User interface  27  can be any graphical and/or mechanical interface that enables user interaction with controller  26 . For example, user interface  27  can implement a graphical user interface displayed at a display device of user interface  27  for presenting information to and/or receiving input from a user. User interface  27  can include graphical navigation and control elements, such as graphical buttons or other graphical control elements presented at the display device. User interface  27 , in some examples, includes physical navigation and control elements, such as physically actuated buttons or other physical navigation and control elements. In general, user interface  27  can include any input and/or output devices and control elements that can enable user interaction with controller  26 . 
     Pump  10  can be controlled based on any desired output parameter. In some examples, pump  10  is configured to provide a process fluid flow based on a desired pressure, flow rate, and/or any other desirable operating parameter. In some examples, pump  10  is configured such that the user can control operation of pump  10  based on an operating capacity of pump  10 . For example, the user can set pump  10  to operate at 50% capacity, during which a target operating parameter, such as speed and/or pressure, is half of a maximum operating parameter. In some examples, pump  10  does not include a fluid sensor, such as a pressure sensor or flow rate sensor. In some examples, the pumping system including pump  10  does not include a fluid sensor disposed downstream of pump  10 . In some examples, the pumping system does not include a fluid sensor disposed upstream of pump  10 . 
     Controller  26  controls operation of pump  10  to drive reciprocation of fluid displacement members  20  at a target speed and to output fluid at a target pressure. Pump  10  can include closed-loop speed control based on data provided by position sensors  62 . Position sensors  62  sense rotation of rotor  30  and a rotational speed of rotor  30  can be determined based on the data from position sensors  62 . The rotational speed can provide the axial displacement speed of fluid displacement members  20 . As such, position sensor  62  can also be considered as a speed sensor. The ratio of rotational speed to axial speed is known based on the configuration of the drive mechanism. When utilizing a drive mechanism having a screw, such as drive mechanism  24  having screw  92  (best seen in  FIGS.  4 A and  12   ), axial speed is a function of rotational speed and the lead of screw  92 . Controller  26  can operate pump  10  such that the actual speed does not exceed the target speed. The speed corresponds to flow rate output by pump  10 . As such, a higher speed provides a higher flow rate while a lower speed provides a lower flow rate. 
     Controller  26  controls the pressure output of pump  10  by controlling the current flow to pump  10 . Motor  22  has a maximum operating current. Controller  26  is configured to control operation of motor  22  such that the maximum current, which can be either the maximum operating current or target operating current, is not exceeded. Controller  26  current-limits pump  10  such that the current applied to motor does not exceed the maximum current. The current provided to motor  22  controls the torque output by motor  22 , thereby controlling the pressure and flow rate output by pump  10 . 
     The target pressure and target speed can be provided to controller  26  by user interface  27 . In some examples, the target pressure and target speed can be set by a single input to controller  26 . For example, user interface  27  can include a parameter input that provides both pressure commands and speed commands to controller  26 . For example, user interface  27  can be or include a knob that the user can adjust to set the operating parameters of pump  10 , the knob forming the parameter input. It is understood, however, that the parameter input can be of any desired configuration, including analog or digital slider, scale, button, knob, dial, etc. Adjusting the parameter input provides both pressure commands and speed commands to controller  26  to set the target pressure and target speed. The pressure and speed can be linked together to change proportionally to each other when the input is set/adjusted. For example, adjusting the parameter input to increase the target pressure will also increase the target speed, while adjusting the parameter input to decrease the target pressure will also decrease the target speed. One input thereby results in a change to both the pressure threshold and the speed threshold. The user can thereby adjust both pressure and speed at a single instance in time by providing the single input to the controller  26  by the parameter input. 
     During operation, controller  26  regulates power to stator  28  to drive rotation of rotor  30  about pump axis PA-PA. Controller  26  provides up to the maximum current and drives rotation of rotor  30  up to the target operating speed. Controller  26  can control voltage to control the speed of rotor  30 . The current through motor  12  determines the torque exerted on rotor  30 , thereby determining the pressure output by pump  10 . If the target operating speed is reached, then controller  26  continues to provide current to motor  22  to operate at the target operating speed. If the maximum current is reached, then motor  22  can continue to operate at that maximum current regardless of the actual speed. Pump  10  is thereby configured to pump process fluid at a set pressure. Pump  10  can operate according to a constant pressure mode. 
     Pump  10  is operable in a pumping state and a stalled state. Pump  10  can maintain constant process fluid pressure throughout operation. In some examples, pump  10  is configured to output process fluid at about 100 pounds per square inch (psi). In the pumping state, controller  26  provides current to rotor  30  and rotor  30  applies torque to drive mechanism  24  and rotates about pump axis PA-PA, causing fluid displacement member  20  to apply force to the process fluid and displace axially along pump axis PA-PA. In the stalled state, rotor  30  applies torque to drive mechanism  24  and does not rotate about pump axis PA-PA, such that fluid displacement member  20  applies force to the process fluid and does not displace axially along pump axis PA-PA. A stall can occur, for example, when pump  10  is deadheaded due to the closure of a downstream valve. Pump  10  continues to apply pressure to the process fluid when pump  10  is stalled. As such, motor  22  is powered with pump  10  in either the pumping state or in the stalled state. 
     Controller  26  supplies current to stator  28  such that rotor  30  applies torque to drive mechanism  24 , causing fluid displacement member  20  to continue to exert force on the process fluid. In the stalled state, controller  26  causes a continuous flow of current to motor  22  causing rotor  30  to apply continuous torque to drive mechanism  24 . Controller  26  can determine if motor  22  is stalled based on data provided by position sensor  62  indicating whether rotor  30  is rotating. Drive mechanism  24  converts the torque to a linear driving force such that drive mechanism  24  applies continuous force to fluid displacement member  20 . Rotor  30  does not rotate during the stall due to the back pressure in the system being greater than the target pressure. Rotor  30  applies torque with zero rotational speed when pump  10  is in the stalled state. Pump  10  is entirely mechanically driven in that rotor  30  mechanically causes fluid displacement members  20  to apply pressure to the process fluid during the stalled state. Pump  10  does not include any internal working fluid for applying force to fluid displacement members  20 . The pressure applied is electromechanically generated, by motor  22  and drive mechanism  24 , not fluidly generated by compressed air or hydraulic fluid. Controller  26  can provide more power to motor  22  with motor  22  rotating than when the motor  22  is stalled. Current can remain constant both in the stall and when rotating, but voltage can change to alter the speed. As such, voltage is at a minimum when at zero speed and with pressure at the desired level, because no additional speed is required to get to pressure. Voltage increases to increase the speed of motor  22 , resulting in additional power during rotation. As the motor  22  is commutated, power is applied according to a sinusoidal waveform. For example, motor  22  can receive AC power. For example, the power can be provided to the windings of the motor  22  according to an electrically offset sinusoidal waveform. For example, a motor with three phases can have each phase receive a power signal 120-degrees electrically offset from each other. With motor  22  stalled, the signals are maintained at the point of stall such that a constant signal is provided with motor  22  in the stalled state. As such, at least one phase of motor  22  can be considered to receive a DC signal with motor  22  in the stalled state. Motor  22  can thereby receive two types of electrical signals during operation, a first during rotation and a second during stall. The first can be sinusoidal and the second can be constant. The first can be AC and the second can be considered to be DC. The first power signal can be greater than the second power signal. 
     The continuous current flow regulated by controller  26  causes pump  10  to apply continuous pressure to the process fluid via fluid displacement members  20 . The pressure setting of the motor can correspond with the amount of current (or other measure of power) supplied to the motor, such that a higher pressure setting corresponds with greater current and a lower pressure setting correspond with lesser current. In some examples, a set current can be provided to motor  22  throughout the stall such that the pump  10  can apply a continuous uniform force on the process fluid. For example, the maximum current can be provided to motor  22  throughout the stall. In some examples, controller  26  can vary the current provided to motor  22  during the stalled state. For example, the current can be pulsed such that current is constantly supplied to stator  28 , but at different levels. As such, pump  10  can apply continuous and variable force to the process fluid. In some examples, the current can be pulsed between the maximum current and one or more currents lesser than the maximum current. For example, controller  26  can maintain the current at a lower level and then pulse the current to the maximum based on a schedule, among other options. Pump  10  returns to the pumping state when the back pressure of the process fluid drops sufficiently such that the current provide to motor  22  can cause rotation of rotor  30 . Pump  10  thereby returns to the pumping state when the force exerted on the process fluid overcomes the back pressure of the process fluid. 
     Controller  26  can be configured to operate motor  12  in both a constant current mode and a pulsed current mode during the stalled state. For example, controller  26  can initially supply a constant, steady current to the motor  12  when in the stalled state. The constant, steady current can be supplied for a first period of the stalled state. The controller  26  can provide pulsed current to the motor  12  during a second period of the stalled state. For example, the first period can be associated with a first amount of time (e.g., 5 seconds, 30 seconds, 1 minute, etc.) during which the constant, steady current is supplied. If the pump  10  remains stalled after the first periods times out, then controller  26  can supply the pulsed current. 
     A stall occurs when the driving force on the rotor equals the reaction force of the downstream fluid from one of the two fluid displacement members and the hydraulic resistance to suction of fluid from the other one of the two fluid displacement members. The pump exits the stall when the downstream pressure decreases, such that the forces are no longer in balance and the rotor overcomes the forces acting on the first and second fluid displacement members. It is understood that the pump may not include a pressure sensor that measures downstream fluid pressure and provides feedback to the controller. Rather, pressure is controlled based on a user setting corresponding to a level of current (or other level of power) supplied to the motor and whether that level is able to overcome the downstream pressure. 
     Stalling pump  10  in response to process fluid back pressure provides significant advantages. The user can deadhead pump  10  without damaging the internal components of pump  10 . Controller  26  regulates to the maximum current, causing pump  10  to output a constant pressure. Pump  10  continuously applies pressure to the process fluid, allowing pump  10  to quickly resume operating and outputting constant pressure when the downstream pressure is relieved. Pulsing the current during a stall reduces heat generated by stator  28  and uses less energy. 
     As discussed above, fluid displacement members  20  can have variable working surface areas. As the working surface area changes, the current required to drive rotor  30  to output the desired pressure changes. The current provided to motor  22  gives the torque applied by rotor  30 , which torque translates to force applied across the working surface area of the fluid displacement member  20 , which provides the pressure output. The current required to maintain a target pressure output thereby decreases as the working surface area decreases. As such, less current is required when the working surface area is smaller, such as at the end of a pumping stroke, than when the working surface area is larger. In some examples, the working surface area of fluid displacement members  20  can change by up to 50%. In some examples, the working surface area of the fluid displacement members  20  can change by up to 30%. In some examples, the working surface area of the fluid displacement members  20  can change by at least 10%. In some examples, the working surface area of the fluid displacement members  20  can change by 20-30%. 
     Controller  26  is configured to vary the current supplied to motor  22  to compensate for a variable working surface area of fluid displacement member  20 . As the working surface area decreases, controller  26  reduces the current supplied to stator  28  to maintain the constant pressure output by pump  10 . Controller  26  provides the most current for a stroke during the portion of the stroke when fluid displacement member  20  has the largest working surface area. In some examples, the working surface area of fluid displacement member  20  is largest when fluid displacement member  20  is beginning a pumping stroke. In some examples, the working surface area of fluid displacement member  20  is largest at the end of a pumping stroke. The working surface area of fluid displacement member  20  changes as fluid displacement member  20  proceeds through the stroke. Controller  26  decreases the current provided to motor  22  as fluid displacement member  20  proceeds through a pumping stroke if the working surface area of fluid displacement member  20  decreases through the pumping stroke. Controller  26  increases the current provided to motor  22  as fluid displacement member  20  proceeds through the pumping stroke if the working surface area of fluid displacement member  20  increases through the pumping stroke. Controller  26  provides the least current for that stroke when the working surface area is smallest. 
     In some examples, the working surface area variation can be stored in memory  274  such that controller  26  varies the current based on data recalled from memory  274 . Controller  26  can be configured to cross-check the position of fluid displacement member  20  with data from a position sensor, such as position sensor  62 , so that the current can be varied based on the phase of the stroke to account for greater/lesser working surface area of the fluid displacement member  20  in that phase of the stroke. In some examples, controller  26  varies the current based on target operating speed of rotor  30 . Controller  26  is compensating for the variation in the working surface area during operation by varying the current supplied to motor  22 . As such, pump  10  is configured to provide a constant downstream pressure regardless of the working surface area of fluid displacement members  20 . 
     During operation, controller  26  axially locates and manages a stroke length of fluid displacement members  20 . As discussed above, the axial displacement rate of fluid displacement members  20  is a function of rotation rate of rotor  30 . In examples including screw  92 , the axial displacement rate is a function of the rotation rate and the lead of screw  92 . In some examples, pump  10  does not include an absolute position sensor for providing the axial location of reciprocating components. As such, controller  26  can axially locate the reciprocating components. 
     On system start up, controller  26  can operate in a start-up mode. In some examples, controller  26  causes pump  10  to operate according to a priming routine on system start up. Pump  10  can initially be dry and requires priming to operate effectively. During the priming routine, controller  26  regulates the speed of pump  10  to facilitate efficient priming. For example, controller  26  can control the speed of pump  10  based on a priming speed. The priming speed can be stored in memory  274  and recalled for the priming routine. The priming speed can be based on the target speed set for pump  10  or can be disconnected from the target speed. Controller  26  causes pump  10  to operate based on the priming speed to prime pump  10 . After the priming routine is complete, controller  26  exits the priming routine and resumes normal control of motor  12 . For example, after exiting the priming routine controller  26  can control the speed based on the target speed rather than the priming speed. Controller  26  can be configured to exit the priming routine based on any desired parameter. For example, controller  26  can be configured to exit the operating routine based on a threshold time, number of revolutions of rotor  30 , number of pump cycles or strokes, the current draw of motor  12 , etc. In some examples, controller  26  can actively determine when to exit the priming routine, such as where controller  26  exits the priming routine based on the current draw to motor  12 . For example, controller  26  can determine that pump  10  has been primed based on increased current draw or a spike in current, which indicates that pump  10  is pumping against pressure. 
     In some examples, controller  26  causes pump  10  to operate according to an initialization routine on start-up, during which controller  26  axially locates fluid displacement members  20  within pump  10 . Controller  26  locates fluid displacement members  20  and controls the stroke of fluid displacement members  20 . Controller  26  axially locates fluid displacement members  20  relative to mechanical stops that define axial limits of a pump stoke. A mechanical stop can be the mechanical engagement of pump parts. For example, the mechanical stops can be points of contact between outer plates  80  (best seen in  FIG.  4 A ) and the inner surfaces of fluid covers  18  (best seen in  FIGS.  3 A and  4 A ), among other options. Controller  26  can determine the axial location of fluid displacement members  20  based at least in part on the current provided to motor  22 . 
     Controller  26  determines when fluid displacement members  20  encounter a mechanical stop based on a current spike occurring. A current spike occurs when the current provided to motor  22  reaches the maximum current. However, current spikes can occur when either a mechanical stop or a fluid stop are encountered. The mechanical stop, which can also be referred to as a hard stop, defines an axial limit of travel. A fluid stop, which can also be referred to as a soft stop, is caused by increased back pressure that occurs due to increased fluid resistance. For example, a fluid stop is not attributable to the mechanical engagement of pump, but increased hydraulic resistance of process fluid downstream of the fluid displacement member. For example, a deadhead condition in which process fluid has no outlet can quickly result in current rise in the motor (beyond the current level the controller is programmed to provide at the current input setting) corresponding to a fluid stop. The mechanical stops provide useful data for determining a target stroke length. Fluid stops can occur at any point along the stroke due to increased back pressure. 
     Controller  26  is configured to positively identify stops as mechanical stops prior to exiting the start-up mode and beginning pumping. In some examples, a stop is classified as a fluid stop until threshold requirements are met for classifying the stop as a mechanical stop. Controller  26  can further determine whether the measured stroke length is a true stroke length that can be utilized during pumping based on the relative locations of stops. 
     A stop occurs when motor  22  applies torque to drive mechanism  24  without causing any rotation due to the stop. If any displacement is occurring, then a stop has not been encountered and motor  22  continues to drive fluid displacement members  20 . 
     Current is provided to motor  22  to cause axial displacement of fluid displacement members  20  in either axial direction. During the initialization routine, less than the maximum current can provided to motor  22  to maintain axial displacement at a start-up speed slower than a maximum speed. The start-up speed can be less than about 50% of the maximum speed, among other options. Fluid displacement member  20  displaces at less than the maximum speed to prevent impact damage when a mechanical stop is encountered. 
     Controller  26  locates a first stop. Fluid displacement members  20  shift axially until a stop is encountered, which is indicated at least in part by a current spike detected by controller  26 . As discussed above, controller  26  current-limits motor  22  such that motor  22  does not receive current above the maximum current. In some examples, controller  26  utilizes the maximum operating current during the initialization routine and the target operating current during pumping. Controller  26  can ramp the current to the maximum current when the stop is encountered to verify that the stop is a true stop, and not due to fluid pressure greater that the target operating pressure. Ramping the current in response to increased resistance maintains the axial displacement speed at or below the start-up speed. Motor  22  continues to drive axial displacement of fluid displacement members  20  until the first stop is encountered. Controller  26  can save the stop location in memory  274 . Controller  26  then determines whether the stop is a mechanical stop. 
     In some examples, controller  26  can base the stop classification at least in part on whether displacement is sensed relative the stop location. In examples where fluid displacement members  20  are flexible, fluid displacement members  20  can displace beyond the stop location by a detectable distance. For example, membranes  80  (best seen in  FIGS.  3 A and  4 A ) allow displacement of fluid displacement members  20  beyond the stop location when force is increased in that axial direction. Fluid displacement members  20  may continue to slightly displace as the current is ramped to the maximum current. In some examples, position sensor  62  facilitates detection of displacement as small as 0.010 centimeters (0.004 inches). Controller  26  can classify the stop as a mechanical stop based on fluid displacement member  20  not displacing beyond the stop location. Controller  26  can determine that the stop is not a mechanical stop based on fluid displacement member  20  displacing beyond the stop location by any distance. 
     In some examples, controller  26  can classify the stop by probing the stop location. For example, controller  26  can reverse the rotational direction of rotor  30  to run in a second rotational direction to cause axial displacement away from the stop. Controller  26  can then cause rotation in the first rotational direction to drive fluid displacement members  20  back towards the first stop to generate an additional current spike. Controller  26  can compare the stop location associated with the second current spike in the first axial direction to the stop location associated with the first current spike in the first axial direction. Controller  26  can determine whether the stop is a mechanical stop based on a comparison of the stop locations. If, based on data from the position sensor  62 , a screw  92  can travel a predetermined distance between two stops, then the two stops can be confirmed as mechanical stops. But if the screw  92  cannot travel that predetermined distance between the two stops, then at least one of the stops must be a fluid stop and controller  26  will cause continued probing to locate the mechanical stops. A suspected stop can then be eliminated by probing the stop location in a subsequent cycle by attempting to move past the stop, and if a current spike is not measured at the stop location on a subsequent stroke, then the suspect stop can be eliminated as a candidate for a mechanical stop due to it being a confirmed as a fluid stop. If the stop locations match, such that the stop locations are identical or differences between the stop locations do not exceed a threshold, then controller  26  can classify the stop as a mechanical stop. In some examples, controller  26  can require a threshold number of matching stop locations prior to classifying the stop as a mechanical stop, such as two, three, four, or more identical stop locations. 
     In some examples, controller  26  can classify the stop based on a profile of the current spike generated at the stop. The current can rise to the maximum current at different rates depending on whether the stop is a mechanical stop or a fluid stop. Mechanical stops generate a profile having a steeper slope in the current rise due to the mechanical stop preventing any axial displacement beyond the mechanical stop. Fluid stops generate a gentler slope in the current rise due to the fluid stop allowing some axial displacement between when the pressure is initially encountered and the end of axial displacement. In some examples, reference profiles can be stored in memory  274 . Controller  26  can classify the stop based at least in part on a comparison of the measured current profile to the reference current profile. 
     Controller  26  can locate a second stop relative the first stop to measure a stroke length for use during pumping. Controller  26  provides current to motor  22  to cause rotation in a second rotational direction, such that fluid displacement members  20  are driven axially away from the first stop. Controller  26  cause axial displacement until a second stop is encountered, as indicated by a current spike. In some examples, controller  26  determines whether the second stop is a mechanical stop, such as by comparing current profiles, probing the stop location, or absence of relative axial displacement, among other options. In some examples, controller  26  locates the second stop after positively identifying the first stop as a mechanical stop. 
     In some examples, controller  26  compares the measured stroke length, which is the measured distance between stops, to a minimum stroke length, which can be recalled from memory  274 . If the measured stroke length exceeds the minimum stroke length, then controller  26  can classify both stops as mechanical stops and exit the initialization routine. If the measured stroke length is less than the minimum stroke length, then one or both of the stops is not a true mechanical stop and controller  26  can continue to operate according to the initialization routine. 
     Controller  26  can be configured to exit the initialization routine based on any one or more of controller  26  locating a single mechanical stop, controller locating multiple mechanical stops, and/or a measured stroke length exceeding a reference stroke length, among other options. Controller  26  exits the start-up mode and enters a pumping mode. During the pumping mode, controller  26  provides up to the maximum current to motor  22  to drive reciprocation of fluid displacement members  20  and cause pumping by pump  10 . During the pumping mode, controller  26  can control the stroke of fluid displacement members  20  based on the measured stroke length. 
     If controller  26  cannot positively locate one or more mechanical stops, then controller  26  can continue to operate according to the initialization routine until a mechanical stop is positively located. In some examples, controller  26  can provide a notification to the user, such as via user interface  27 , based on controller  26  not positively locating a mechanical stop. For example, controller  26  can generate the alert based on a certain time period passing without completing the initialization routine. The alert can indicate that pump  10  is deadheaded and the downstream pressure should be relieved and/or that pump  10  requires servicing. 
     Controller  26  can control the stroke of pump  10  relative a target turnaround point TP during pumping. As best seen in  FIGS.  20 A- 20 C  and with continued reference to  FIG.  19   , controller  26  can control the stroke to align fluid displacement member  20  with target point TP when the stroke changes over.  FIGS.  20 A- 20 C  are schematic diagrams showing the axial location of a fluid displacement member  20  relative target point TP. 
     Target point TP is a target location at which fluid displacement member  20  stops displacing in a first axial direction and begins displacing in a second axial direction. For example, target point TP can be a location where fluid displacement member  20  completes a pumping stroke and begins a suction stroke. The relative axial location of target point TP can be stored in memory  274 . 
     During changeover, controller  26  causes motor  22  to begin reversing as fluid displacement member  20  approaches target point TP. Controller  26  begins decelerating motor  22  to align fluid displacement member  20  with target point TP when fluid displacement member  20  stops displacing in the first axial direction at changeover. As motor  22  decelerates, fluid displacement member  20  continues to displace in the first axial direction. Controller  26  determines the final location of fluid displacement member  20  relative target point TP and utilizes that information to adjust the stroke length, such as by adjusting the point of deceleration relative target point TP. Controller  26  can thereby adjust and optimize the stroke length during pumping. 
     As shown in  FIGS.  20 A- 20 C , fluid displacement member  20  can undershoot ( FIG.  20 A ), align with ( FIG.  20 B ) or overshoot ( FIG.  20 C ) target point TP during changeover. The stopping distance required to decelerate and reverse the direction of axial displacement varies depending on the process fluid load on fluid displacement members  20 . A larger load will speed deceleration of motor  22  as the load provides resistance that assists deceleration. As such, the greatest stopping distance occurs when pump  10  is operating dry, without a process fluid load. 
     As shown in  FIG.  20 A , fluid displacement member  20  can undershoot target point TP during a changeover. As show in  FIG.  20 C , fluid displacement member  20  can overshoot target point TP during a change over. Controller  26  determines the undershoot distance X and/or the overshoot distance Y between target point TP and the actual changeover point CP. Controller  26  adjusts the point of deceleration for a subsequent pump stroke based on the distance X, Y. As such, distances X and Y provide an adjustment factor. 
     Controller  26  can modify the deceleration point where motor  22  begins to decelerate based on the adjustment factor. In examples where fluid displacement member  20  undershoots target point TP, controller  26  can shift the axial position of deceleration in the first axial direction AD 1  and towards target point TP. Controller  26  alters the axial location where deceleration begins such that fluid displacement member  20  begins to decelerate closer to target point TP relative the previous stroke. In the example shown, the axial location can be modified by the undershoot distance X such that fluid displacement member  20  is X distance closer to target point TP when deceleration is initiated relative to the previous stroke. 
     In examples where fluid displacement member  20  overshoots target point TP, controller  26  can shift the axial point of deceleration in the second axial direction AD 2  and towards target point TP. Controller  26  alters the axial location where deceleration initiates such that fluid displacement member  20  begins to decelerate further from target point TP relative the previous stroke. In the example shown, the axial location can be modified by the overshoot distance Y such that fluid displacement member  20  is Y distance closer to target point TP when deceleration is initiated relative to the previous stroke. 
     Controller  26  can independently optimize the stroke length in each of the first axial direction AD 1  and the second axial direction AD 2 . For example, controller  26  can determine a first adjustment factor for travel in the first axial direction and a second adjustment factor for travel in the second axial direction. Controller  26  can adjust the stroke length in the first axial direction AD 1  based on the first adjustment factor and can adjust the stroke length in the second axial direction based on the second adjustment factor. 
     In some examples, controller  26  can optimize stroke length in only one of the axial directions. For example, controller  26  can determine an adjustment factor for travel in the first axial direction AD 1  and drive displacement in the second axial direction based on one of a measured stroke length and a stroke length stored in memory  274 . The adjustment factor can be utilized to adjust the axial location of deceleration on the subsequent stroke in the first axial direction AD 1 . 
     Controller  26  can continuously optimize the stroke length in the first axial direction AD 1  and the second axial direction AD 2 . For example, controller  26  can determine a first adjustment factor at the end of travel in the first axial direction AD 1 . Controller  26  can modify the axial location of deceleration for the subsequent stroke in the second axial direction AD 2  based on the first adjustment factor. Controller  26  can determine a second adjustment factor at the end of travel in the second axial direction AD 2 . Controller  26  can modify the return stroke in the first direction AD 1  based on the second adjustment factor. Controller  26  can continue to generate adjustment factors and modify the stroke length based on the adjustment factors throughout operation. 
     In some examples, controller  26  is configured to operate motor  12  in a short stroke mode and a standard stroke mode. During the standard stroke mode, controller  26  can cause the fluid displacement members  20  to displace a full stroke length, as discussed above. During the short stroke mode, controller  26  causes fluid displacement members  20  to have shorter stroke lengths as compared to the full stroke length. For example, controller  26  can control the stroke length to be half (50%) of the full stroke length, among other options (e.g., 25%, 33%, 75% of the full stroke length). Controller  26  thereby controls the stroke length such that the pump stroke occurs in a first displacement range during the standard stroke mode and a second displacement range during the short stroke mode. The second displacement range is shorter than the first displacement range and can be, in some examples, a subset of the first displacement range. For example, the second displacement range can be fully disposed within the first displacement range along the reciprocation axis. 
     Controller  26  can continue to control operation of motor  12  based on the target operating speed during the short stroke mode, such that fluid displacement members  20  continue to shift axially at the same speed. The shorter stroke length results in a greater number of changeovers (where movement changes from a first one of axial directions AD 1 , AD 2  to the other one of axial directions AD 1 , AD 2 ). In some examples, controller  26  can increase the target operating speed during the short stroke mode to increase the linear displacement speed of fluid displacement members  20  and further increase the changeover rate. The more frequent changeover causes pump  10  to operate according to an increased number of pump cycles per unit time during the short stroke mode as compared to the standard stroke mode. In some examples, controller  26  can increase the displacement rate during the short stroke mode to further increase the changeover rate. 
     Downstream pressure pulses can be generated during changeover. Controller  26  operating motor  12  in the short stroke mode provides smoother downstream flow. The pressure fluctuation is reduced by the reduction in the stroke length and corresponding increase in changeover rate. Increasing the changeover and decreasing stroke length provides more, smaller pressure fluctuations as compared to the full stroke length, which results in fewer, larger fluctuations. The smaller fluctuations during the short stroke mode are also closer together in time, resulting in a smoother output from pump  10 . 
     Controller  26  can be further configured to determine the existence of a pumping error based on operating parameters of motor  12 . A pumping error can be an error associated with the fluid moving/flow regulating components of the pump  10 . For example, a diaphragm can experience a leak, a check valve can be stuck closed/open, a check valve can be leaky, etc. During operation, controller  26  monitors operation of motor  12  and can determine an error in the pump  10  based on the data regarding the operating parameters of motor  12 . Controller  26  can determine that the error exists based on an unexpected operating parameter. For example, controller  26  can determine that an error has occurred based on the actual operating parameter of the motor  12  differing from an expected value of the operating parameter for a particular phase of a pump cycle or stroke. 
     In one example, controller  26  can cause reciprocation of a fluid displacement member  20  by motor  12 . Controller  26  monitors the current, or other operating parameter of motor  12 , such as speed, and determines the status of pump  10  based on the value of that actual parameter. For example, controller  26  may experience an unexpected current draw during a portion of the pump cycle and can determine the existence of an error based on that unexpected current draw for that portion of the pump cycle. At a certain point in the pump cycle, controller  26  can detect an unexpected drop/rise in the current, which can be indicative of an error. At a certain point in the pump cycle, controller  26  can detect an unexpected drop/rise in speed, which can be indicative of an error. Controller  26  can be configured to generate an error code and provide the error information to the user, such as by user interface  27 . 
     In some examples, controller  26  can be configured to determine the existence of a pump error based on the operating parameters experienced during the stroke of a first fluid displacement member compared to the stroke of a second fluid displacement member. The operating parameters for each of the fluid displacement members should be the balanced for the same parts of the monitored strokes. Controller  26  can compare operating parameters during a pumping stroke of the first fluid displacement member relative to operating parameters during a pumping stroke of the second fluid displacement member. Controller  26  can determine the existence of an error based on a variation in the operating parameters experienced during the two strokes. In some examples, controller  26  can compare the variation to a threshold and determine the existence of an error based on a magnitude of the variation reaching or exceeding the threshold. In some examples, controller  26  can determine a difference in load experienced by the fluid displacement members  20 , such as based on the current feedback, and determines the existence of an error based on those differences. The controller  26  can base the comparison on the operating parameters experienced at the same point in the pump cycle for each fluid displacement member  20 . For example, the controller  26  can compare the operating parameters for a first diaphragm at the beginning of its pumping stroke to the operating parameters for a second diaphragm at the beginning of its pumping stroke. 
     For example, if the second diaphragm has a leak through the diaphragm or a leaky inlet valve, then less current draw will be experienced during the pressure stroke of the second diaphragm due to the leaking fluid. Controller  26  can sense the differences in load between the first and second diaphragms and determine the existence of an error based on that comparison. While controller  26  is described as detecting errors based on current, it is understood that controller  26  can be configured to detect errors based on any desired operating parameter. For example, controller  26  can determine the existence of a pump error based on the actual speed experienced during the two pump strokes. Monitoring motor operating parameters to determine errors facilitates error detection without requiring calibration. The direct comparison can indicate an error based on variations experienced during pumping. 
       FIG.  21    is a flowchart illustrating method  2100 . Method  2100  is a method of operating a reciprocating pump, such as pump  10  (best seen in  FIGS.  3 A- 4 D ). In step  2102  an electric motor, such as electric motor  22  ( FIGS.  4 A- 4 D ), applies torque to a drive mechanism, such as drive mechanism  24  (best seen in  FIG.  12   ), drive mechanism  24 ′ ( FIG.  13   ), or drive mechanism  24 ″ ( FIG.  14   ). 
     In step  2104 , the drive mechanism applies an axial force to a fluid displacement member, such as fluid displacement members  20  (best seen in  FIGS.  3 A and  4 A ), fluid displacement member  20 ′ ( FIG.  7   ), or fluid displacement member  20 ″ ( FIG.  10   ). The fluid displacement member can be disposed coaxially with the rotor such that the rotor rotates about a pump axis that the fluid displacement member reciprocates along. 
     In step  2106 , a controller, such as controller  26  ( FIGS.  1 C and  19   ), regulates current flow to the motor. The current is applied to cause the rotor, such as rotor  30  (best seen in  FIGS.  3 A- 4 C and  12   ), to apply the torque to the drive mechanism, such as drive mechanism  24  (best seen in  FIG.  12   ), drive mechanism  24 ′ ( FIG.  13   ), or drive mechanism  24 ″ ( FIG.  14   ). The controller regulates the current such that current is supplied both when the pump is in a pumping state and when the pump is in a stalled state. In the pumping state, the rotor is rotating and the fluid displacement member is displacing axially. In the stalled state, a back pressure on the fluid displacement member prevents the fluid displacement member from displacing axially and the rotor from rotating. 
     The controller causes current to be continuously provided to motor such that rotor applies torque to the drive mechanism throughout the pumping and stalled states. As such, the fluid displacement member continues to apply force to the pumped fluid. In some examples, the controller can vary the current to the electric motor. For example, the controller can cause the current to be pulsed to the motor during the stalled state. The pulsed current causes the rotor to apply varying amounts of torque, but the rotor continues to apply some torque throughout the stall. 
     Once the back pressure drops below the target pumping pressure, the fluid displacement member can shift axially. The pump is thus in the pumping state. The controller can regulate current to the motor during the pumping state to operate the pump at the target pressure. 
     Method  2100  provides significant advantages. The user can deadhead the pump without damaging the internal components of the pump. The controller regulates to the maximum current, causing the pump to output at a target pressure. The pump continuously applies pressure to the process fluid in both the pumping state and the stalled state, thereby facilitating the pump quickly resuming pumping when the back pressure is relieved. The pump begins operating in the pumping mode when the back pressure drops below the target pressure. Pulsing the current during a stall reduces heat generated during the stall and conserves energy. 
       FIG.  22    is a flowchart illustrating method  2200 . Method  2200  is a method of operating a pump, such as pump  10  (best seen in  FIGS.  3 A- 4 D ). In step  2202  an electric motor, such as electric motor  22  ( FIGS.  4 A- 4 D ), drives a fluid displacement member, such as fluid displacement members  20  (best seen in  FIGS.  3 A and  4 A ), fluid displacement member  20 ′ ( FIG.  7   ), or fluid displacement member  20 ″ ( FIG.  10   ), axially on a pump axis. Method  2200  can be implemented at any point during pumping. In some examples, method  2200  is a start-up routine that occurs when the pump is initially powered and prior to entering a pumping state. 
     In step  2204  a stop is detected by a controller, such as controller  26  ( FIGS.  1 C and  19   ). A stop can be detected based on the controller detecting a current spike and based on the fluid displacement member stopping axial displacement. A current spike occurs when the current supplied to the motor rises to a maximum current. If a current spike is detected but fluid displacement member is still shifting axially, then a stop has not been encountered. 
     In step  2206 , the controller determines whether the stop is a mechanical stop or a fluid stop. A mechanical stop is a stop that physically defines a stroke limit of the fluid displacement member. For example, the mechanical stop can be an axial location where the fluid displacement member contacts an inner surface of a fluid cover, such as fluid covers  18  (best seen in  FIGS.  3 A and  4 A ). A fluid stop is caused by increased back pressure in the system. Fluid stops can occur at any axial location along the stroke. The controller can determine whether the stop is a mechanical stop in any desired manner. For example, the controller can cause displacement in a second axial direction until another stop is encountered. The controller can compare a distance between the first and second stops to determine a measured stroke length and can further compare that measured stroke length to a minimum and/or other reference stroke length. The controller can drive the fluid displacement member in the first axial direction multiple times to generate a plurality of stop locations in that first axial direction. The plurality of stop locations can be compared to determine the stop type. The controller can compare the slope of a current profile of the current spike to a reference profile to determine the stop type. It is understood that the stop type can be identified in any desired manner. 
     If the answer in step  2206  is NO, such that the stop cannot be positively identified as a mechanical stop, then method  2200  proceeds to step  2208 . If the answer in step  2206  is YES, then method  2200  proceeds to step  2210 . 
     In step  2208 , the controller determines if a measured stroke length, between two stops encountered in opposite axial directions, is greater than a minimum stroke length. If the answer in step  2208  is NO, then method proceeds back to step  2202  and the controller continues searching for the locations of mechanical stops. If the answer in step  2208  is YES, then method  2200  proceeds to step  2210 . 
     In step  2210 , the controller manages a stroke length based on the axial location of one or more stops. For example, the controller can control the stroke length to prevent the fluid displacement member from contacting the mechanical stop. In some examples, the controller can base the stroke length on the minimum stroke length and a single stop. In some examples, the controller can locate multiple mechanical stops and manage the stroke length between those two mechanical stops. 
     Method  2200  provides significant advantages. The pump may not include an absolute position sensor such that the axial locations of the fluid displacement members are not known at start up. The controller locates the stops to provide an optimal stroke length and prevent undesired contact between mechanical stops and fluid displacement members. The locations of at least one stop can be positively identified as mechanical stops prior to entering a pumping mode. Positively identifying at least one mechanical stop prevents damage due to false positives, such as fluid stops. 
       FIG.  23    is a flowchart illustrating method  2300 . Method  2300  is a method of operating a pump, such as pump  10  (best seen in  FIGS.  3 A- 4 C ). In step  2302  an electric motor, such as electric motor  22  ( FIGS.  4 A- 4 D  drives a fluid displacement member, such as fluid displacement members  20  (best seen in  FIGS.  3 A and  4 A ), fluid displacement member  20 ′ ( FIG.  7   ), or fluid displacement member  20 ″ ( FIG.  10   ), in a first axial direction on a pump axis. 
     In step  2304 , the controller initiates deceleration of a rotor of the electric motor, such as rotor  30  (best seen in  FIGS.  3 A- 4 D and  12   ). The controller decelerates the rotor as the fluid displacement members approaches the end of a stroke to cause the fluid displacement member to changeover and begin an opposite stroke. The controller initiates deceleration when the fluid displacement member is at an axial location corresponding to a first deceleration point. In step  2306 , the controller determines a stopping point for the fluid displacement member. The stopping point is the point at which the fluid displacement member stops displacing in the first axially direction. 
     The controller controls deceleration and changeover to align the stopping point with a target point. In step  2308 , the controller determines an offset between the stopping point and the target point. The controller determines an adjustment factor based on the axial spacing between the stopping point and the target point. In step  2310 , the controller manages the stroke length based on the adjustment factor. The controller can adjust a deceleration point where deceleration is initiated based on the adjustment factor. For example, the controller can initiate deceleration at a second deceleration point axially closer to the target point relative the first deceleration point when the fluid displacement member undershot the target point. The controller can initiate deceleration at a second deceleration point axially further from the target point relative the first deceleration point when the fluid displacement member overshot the target point. The controller can be configured to continuously manage the stroke length based on the stopping points and the target points throughout operation. The target points can be at any desired axial location. Continuously monitoring and adjusting the stroke length causes the pump to operate at an optimum stroke. In addition, the stroke length adjustment prevents accumulation of drive errors that can affect the stroke length. 
       FIG.  24    is a flowchart illustrating method  2400 . Method  2400  is a method of operating a pump, such as pump  10  (best seen in  FIGS.  3 A- 4 C ). In step  2402  an electric motor, such as electric motor  22  ( FIGS.  4 A- 4 D ) drives a fluid displacement member, such as fluid displacement members  20  (best seen in  FIGS.  3 A and  4 A ), fluid displacement member  20 ′ ( FIG.  7   ), or fluid displacement member  20 ″ ( FIG.  10   ), in a first axial direction on a pump axis. 
     In step  2404 , a controller, such as controller  26  ( FIGS.  1 C and  19   ), monitors a rotational speed of the rotor and a current provided to the electric motor. For example, the controller can determine the rotational speed based on data provided by a position sensor, such as position sensor  62  (best seen in  FIGS.  3 A,  17 A, and  18   ). The axial displacement speed of the fluid displacement member is a function of the rotational speed of the rotor, such that the rotational speed provides the axial speed. The controller regulates both speed and current to cause the pump to output process fluid at a target pumping pressure. 
     In step  2406 , the controller determines if the current provided to the motor is less than a current limit, which can be a maximum operating current or a target operating current. In some examples, the current limit can change throughout the pumping stroke. For example, the fluid displacement member can have a variable working surface area throughout the pumping stroke. The variable working surface area can increase or decrease as the fluid displacement member is driven through the pumping stroke. As such, less current can be required at the end of the pumping stroke, when the working surface area decreases, than at the beginning of the pumping stroke to achieve the target pumping pressure, or more current can be required at the end of the pumping stroke, when the working surface area increases, than at the beginning of the pumping stroke to achieve the target pumping pressure. The controller can control operation based on a variable current limit. If the answer in step  2406  is NO, such that the actual current is at the current limit, then method  2400  proceeds to step  2408 . In step  2408  the controller continues to provide current to the motor at the current limit to operate the pump. If the answer in step  2406  is YES, then method  2400  proceeds to step  2410 . 
     In step  2410 , the controller determines if the actual speed is less than a speed limit. The speed limit can be a maximum operating speed or a target operating speed. If the answer in step  2410  is NO, such that the current operating speed is at the speed limit, then method  2400  proceeds to step  2412  and the controller can cause the motor to continue to operate at the current speed. If the answer in step  2410  is YES, then method proceeds to step  2414 . In step  2414 , the controller increases the power (such as voltage or current) provided to the motor to accelerate the speed of rotor rotation towards the speed limit. 
     Method  2400  provides significant advantages. In some examples, the pump does not include a pressure sensor. The pump can output process fluid at a target pressure based on the speed of rotation, which correlates to a speed of axial displacement, and the current provided to the motor. The controller controls pumping such that the pump can operate in a constant pressure mode where speed and current are controlled to cause the pump to output at the target pressure. Variable working surface areas of the fluid displacement members can cause pressure variations due to the changing surface area throughout the pump stroke. The controller adjusts the current limit throughout the pump stroke to account for the variable working surface area and cause the pump to operate according to the target pressure. 
       FIG.  25 A  is an isometric view of rotor assembly  300 .  FIG.  25 B  is an exploded view of rotor assembly  300 .  FIG.  25 C  is a cross-sectional view of rotor assembly  300 .  FIGS.  25 A- 25 C  will be discussed together. Rotor assembly  300  is substantially similar to rotor  30  and is configured to rotate about axis PA due to power through a stator, such as stator  28 . Rotor assembly  300  includes permanent magnet array  302 , drive component  304 , rotor body  306 , support rings  308 , bearings  310 , and seal  312 . Permanent magnet array  302  includes permanent magnets  314  and back irons  316 . Drive component  304  includes body  318 , which includes interface strip  320 . Rotor body  306  includes body components  322   a ,  322   b  and receiving chamber  324 . Body components  322   a ,  322   b  respectively include axial projections  326   a ,  326   b  and seal grooves  328   a ,  328   b.    
     Rotor assembly  300  is an assembly configured to form the rotating component of an electric motor, such as motor  22 . Rotor body  306  forms a clamshell housing drive component  304 . Permanent magnet array  302  is disposed on the outer surface of rotor body  306 . Support rings  308  are disposed on opposite axial ends of rotor body  306  and hold permanent magnet array  302  on rotor body  306 . Support rings  308  can be secured to rotor body  306  in any desired manner, such as by fasteners, adhesive, or press-fitting, among other options. Permanent magnet array  302  can be fixed to rotor body  306  by adhesive, such as a potting compound. The potting compound can further fix support rings  308  to rotor body  306 . It is understood that some examples of rotor assembly  300  do not include support rings  308 . Bearings  310  are substantially similar to bearings  54   a ,  54   b  and are disposed on axial projections  326   a ,  326   b  body components  322   a ,  322   b . Bearings  310  are configured to support both radial and axial loads. For example, bearings  310  can be tapered roller bearings. 
     Body components  322   a ,  322   b  form the clamshell of rotor body  306  and define receiving chamber  324 . Seal  312  is disposed in seal grooves  328   a ,  328   b  and between body components  322   a ,  322   b . Seal  312  prevents the potting compound from migrating between body components  322   a ,  322   b.    
     Drive component  304  is disposed in receiving chamber  324 . Receiving chamber  324  is defined by body components  322   a ,  322   b . Body components  322   a ,  322   b  are fixed to drive component such that drive component  304  rotates with body components  322   a ,  322   b . Body components  322   a ,  322   b  radially overlap with the axial ends of drive component  304  to axially fix drive component  304  within receiving chamber  324 . Drive component  304  does not rotate relative body components  322   a ,  322   b . For example, body components  322   a ,  322   b  can be press-fit onto body  318  and that interference fit can fix drive component  304  to body components  322   a ,  322   b . In some examples, drive component  304  is fixed to body components  322   a ,  322   b  by adhesive. It is understood that other fixation options are possible. 
     Interface strip  320  is disposed circumferentially around body  318  of drive component  304 . Interface strip  320  further secures body components  322   a ,  322   b  to drive component  304 . For example, interface strip  320  can be knurled, grooved, or of any other configuration suitable for fixing drive component  304  to body components  322   a ,  322   b . In some examples, interface strip  320  is formed across a full length of body  318 . In some examples, drive component  304  does not include interface strip  320 . 
     Drive component  304  can be a drive nut, similar to drive nut  90 , configured to provide the rotating component of a drive mechanism, similar to drive mechanisms  24 ,  24 ′,  24 ″, that converts the rotation of rotor assembly  300  into a linear output. Bore  330  extends axially through rotor assembly  300  and, in the example shown, is defined by drive component  304 . 
     Rotor assembly  300  provides significant advantages. Rotor body  306  being of a clamshell configuration facilitates a larger diameter of drive component  304 , and thus a larger diameter of bore  330  through drive component  304 . The larger diameter of bore  330  facilitates use of more robust driving components, such as balls and rollers, and facilitates the use of a larger diameter linear displacement member, such as screw  92 . A more robust, larger linear displacement member can generate greater pumping pressures and react greater loads. 
       FIG.  26    is a cross-sectional view of rotor assembly  300 ′. Rotor assembly  300 ′ is substantially similar to rotor assembly  300  ( FIGS.  25 A- 25 C ), except rotor assembly  300 ′ is configured to provide a rotary, instead of linear, output from the motor of rotor assembly  300 ′. Drive component  304 ′ includes body  318 ′ and shaft  332 . Shaft  332  projects beyond an axial end of rotor body  306  and forms an output shaft of rotor assembly  300 ′. Shaft  332  provides a rotary output from rotor assembly  300 ′. While drive component  304 ′ is shown as including a single shaft  332 , it is understood that drive component  304 ′ can include a second shaft extending from an opposite axial end of drive component  304 ′ from shaft  332 . 
       FIG.  27    is a cross-sectional view of rotor assembly  300 ″. Rotor assembly  300 ″ is substantially similar to rotor assembly  300 ′ ( FIG.  26   ) and rotor assembly  300  ( FIGS.  25 A- 25 C ). Similar to rotor assembly  300 ′, rotor assembly  300 ″ is configured to provide a rotary output from the motor of rotor assembly  300 ″. Drive component  304 ″ includes body  318 ″. Body  318 ″ defines bore  330 ′. Body  318 ″ is configured to receive a shaft within bore  330 ′. Drive component  304 ″ is configured to transmit rotational forces to drive rotation of the shaft by an interface between the surface of bore  330 ′ and the shaft. For example, the shaft and bore  330 ′ can include a keyed interface or the bore  330 ′ can include a contour configured to interface with a contour of the shaft, among other options. 
     While the pumping assemblies of this disclosure and claims are discussed in the context of a double displacement pump, it is understood that the pumping assemblies and controls can be utilized in a variety of fluid handing contexts and systems and are not limited to those discussed. Any one or more of the pumping assemblies discussed can be utilized alone or in unison with one or more additional pumps to transfer fluid for any desired purpose, such as location transfer, spraying, metering, application, etc. 
     Discussion of Non-Exclusive Examples 
     The following are non-exclusive descriptions of possible embodiments of the present disclosure. 
     A displacement pump for pumping a fluid comprising an electric motor including a stator and a rotor, the rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid by linear reciprocation of the fluid displacement member; and a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the fluid displacement member. The drive mechanism includes a screw connected to the fluid displacement member and disposed coaxially with the rotor; and a plurality of rolling elements disposed between the screw and the rotor, wherein the plurality of rolling elements support the screw relative the rotor and are configured to be driven by rotation of the rotor to drive the screw axially. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The drive mechanism comprises inner threading that rotates with the rotor; and outer threading on the screw; wherein each rolling element of the plurality of rolling elements interfaces with both of the inner threading and the outer threading, and the inner threading does not contact the outer threading. 
     The screw extends within each of the rotor and the stator; the screw, the plurality of rolling elements, and the rotor are coaxially aligned along the pump axis; and the screw, the plurality of rolling elements, and the rotor are arranged directly radially outward from the pump axis in the order: the screw, then the plurality of rolling elements, and then the rotor. 
     A first fluid displacement member configured to pump fluid and a second fluid displacement member; wherein the fluid displacement member is the first fluid displacement member; wherein the screw is fixed to both of the first and the second fluid displacement members; and wherein the first and the second fluid displacement members are respectively located on opposite ends of the screw such that the screw is directly between the first and the second fluid displacement members. 
     The rotor turns in a first rotational direction to drive the screw linearly along the pump axis in a first direction to simultaneously move the first fluid displacement member through a pumping stroke and the second fluid displacement member through a suction stroke, and the rotor turns in a second rotational direction to drive the screw linearly along the pump axis in a second direction to simultaneously move the first fluid displacement member through a suction stroke and the second fluid displacement member through a pumping stroke. 
     The first fluid displacement member is a first diaphragm, the second fluid displacement member is a second diaphragm, and both the rotor and the plurality of rolling elements are located axially between the first diaphragm and the second diaphragm. 
     The plurality of rolling elements includes balls. 
     The plurality of rolling elements includes toothed rollers. 
     The drive mechanism further includes a drive nut connected to the rotor such that rotation of the rotor drives rotation of the drive nut, and wherein the plurality of rolling elements are disposed between the drive nut and the screw. 
     The plurality of rolling elements are arranged in an elongate annular array, the annular array of rolling elements disposed coaxially with the fluid displacement member. 
     The fluid displacement member comprises a diaphragm. 
     The diaphragm includes a diaphragm plate connected to the screw and a flexible membrane extending radially relative to the diaphragm plate. 
     The rotor is supported by a first bearing and a second bearing; the first bearing is capable of supporting both axial and radial forces; and the second bearing is capable of supporting both axial and radial forces. 
     Each bearing includes an array of rollers, each roller orientated along an axis of the roller at an angle such that the axis of the roller is neither parallel nor orthogonal to the axis of the screw. 
     The first bearing is a tapered roller bearing and the second bearing is a tapered roller bearing. 
     The first bearing is disposed at a first axial end of the rotor and the second bearing is disposed at a second axial end of the rotor. 
     A locking nut connected to a stator housing supporting the stator, the locking nut preloading the first and second bearings. 
     The locking nut is disposed adjacent to the first bearing. 
     The locking nut engages an outer race of the first bearing. 
     The locking nut is threadingly connected to the stator housing. 
     The locking nut includes exterior threading. 
     The locking nut supports a grease cap of the first bearing. 
     The first bearing and the second bearing support a drive nut disposed between the plurality of rolling elements and the rotor, wherein the drive nut is connected to the rotor to rotate with the rotor. 
     The drive nut is connected to a first inner race that forms an inner race of the first bearing and to a second inner race that forms an inner race of the second bearing. 
     The fluid displacement member includes a first fluid displacement member connected to a first end of the screw and a second fluid displacement member connected to a second end of the screw. 
     The stator is configured to drive the rotor in both a first rotational direction and a second rotational direction opposite the first rotational direction to drive reciprocation of the screw. 
     A method of pumping includes driving rotation of a rotor of an electric motor; linearly displacing a screw in a first axial direction such that the screw drives a first fluid displacement member attached to a first end of the screw through a first stroke, wherein the screw is coaxial with the rotor and supported by a plurality of rolling elements disposed between the rotor and the screw, and wherein the first stroke is one of a pumping stroke and a suction stroke; and linearly displacing the screw in a second axial direction opposite the first axial direction by the plurality of rolling elements. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Driving rotation of the rotor includes: rotating the rotor in a first rotational direction to drive the screw in the first axial direction; and rotating the rotor in a second rotational direction opposite the first rotational direction to drive the screw in the second axial direction. 
     Linearly displacing the screw in the first axial direction further causes the screw to drive a second fluid displacement member attached to a second end of the screw through a second stroke opposite the first stroke. 
     A displacement pump for pumping a fluid comprising an electric motor disposed in a pump housing, the electric motor comprising a stator and a rotor, the rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid by linear reciprocation of the fluid displacement member, the fluid displacement member interfacing with the pump housing such that the fluid displacement member is prevented from rotating relative to the pump housing; and a drive mechanism connected to the rotor and to the fluid displacement member, the drive mechanism comprising a screw connected to the fluid displacement member, the drive mechanism configured to receive rotational output from the rotor and convert the rotational output from the rotor into a linear input to the fluid displacement member to linearly reciprocate the fluid displacement member; wherein the screw is prevented from being rotated by the rotational output by being rotationally fixed with respect to the fluid displacement member. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A first fluid displacement member configured to pump fluid and a second fluid displacement member; wherein the fluid displacement member is the first fluid displacement member; wherein the screw is rotationally fixed to both of the first and the second fluid displacement members such that the first and the second fluid displacement members prevent rotation of the screw. 
     The first fluid displacement member comprises a first diaphragm and the second fluid displacement member comprises a second diaphragm. 
     The fluid displacement member comprises a diaphragm having a diaphragm plate and a membrane extending between the diaphragm plate and the pump housing; wherein the screw is connected to the diaphragm plate and the membrane interfaces with the pump housing. 
     At least a portion of the membrane is clamped between the pump housing and a fluid cover, and the diaphragm and the fluid cover define a pumping chamber. 
     The portion of the membrane is an outer edge of the membrane. 
     The portion of the membrane includes a circumferential bead. 
     An end of the screw extends into a receiving chamber formed on the diaphragm plate. 
     The end of the screw includes a first contoured surface and the receiving chamber includes a second contoured surface configured to mate with the first contoured surface to prevent the screw from rotating relative to the diaphragm plate. 
     A set screw extends into the diaphragm plate and the screw. 
     The set screw extends axially. 
     A diaphragm screw extends through the diaphragm plate and into the screw to secure the screw to the diaphragm plate. 
     An end of the screw extends into a receiving chamber formed on the diaphragm plate and a diaphragm screw extends through the diaphragm plate and into the screw. 
     The fluid displacement member includes a first fluid displacement member secured to a first end of the screw and a second fluid displacement member secured to a second end of the screw. 
     A displacement pump for pumping a fluid includes an electric motor disposed in a pump housing and including a stator and a rotor rotatable about a pump axis; a fluid displacement member configured to reciprocate on the pump axis to pump fluid, the fluid displacement member interfacing with the pump housing at a first interface; and a drive mechanism connected to the rotor and to the fluid displacement member and configured to convert a rotational output from the rotor into a linear input to the fluid displacement member, wherein the drive mechanism includes a screw connected to the fluid displacement member at a second interface; wherein the first interface and the second interface prevent the screw from rotating about the pump axis and relative to the fluid displacement member and the pump housing. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The fluid displacement member includes one of a diaphragm and a piston. 
     The first interface includes a portion of the fluid displacement member clamped between the pump housing and a fluid cover connected to the pump housing, the fluid cover and the fluid displacement member at least partially defining a process fluid chamber. 
     The second interface includes a first surface contour at an end of the screw contacting a second surface contour formed on the fluid displacement member. 
     A method of pumping fluid by a reciprocating pump includes driving rotation of a rotor of an electric motor by a stator of the electric motor; causing, by rotation of the rotor, a screw disposed coaxially with the rotor to reciprocate along a pump axis, the screw driving a fluid displacement member through a suction stroke and a pumping stroke; preventing rotation of the fluid displacement member relative to a pump housing of the pump by a first interface between the fluid displacement member and the pump housing; and preventing rotation of the screw about the axis by the first interface and a second interface between the screw and the fluid displacement member. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Preventing rotation of the fluid displacement member relative to the pump housing of by the interface between the fluid displacement member and the pump housing includes securing a membrane of the fluid displacement member to a pump housing. 
     Securing the membrane of the fluid displacement member to the pump housing includes clamping a circumferential edge of the membrane between a fluid cover of the pump and the pump housing. 
     Preventing rotation of the fluid displacement member relative to the pump housing of by the interface between the fluid displacement member and the pump housing includes preventing rotation of a piston by an interface between a first surface contour of the piston and a second surface contour defining at least a portion of a piston bore, wherein the piston forms the fluid displacement member and is configured to reciprocate within the piston bore. 
     A double diaphragm pump having an electric motor includes a housing; an electric motor comprising a stator and a rotor, the rotor configured to rotate to generate rotational input; a screw that receives the rotational input and converts the rotational input into linear input; a first diaphragm and a second diaphragm, the screw located between the first and second diaphragms, each of the first and second diaphragms receiving the linear input such that each of the first and second diaphragms reciprocate to pump fluid; wherein each of the first and second diaphragms are rotationally fixed by the housing; and wherein the first and second diaphragms are rotationally fixed with respect to the screw such that the screw is prevented from rotating, despite the rotational input, by the first and second diaphragms rotationally fixing the screw. 
     A displacement pump for pumping a fluid includes an electric motor disposed in a pump housing, the electric motor comprising a stator and a rotor, the rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid by linear reciprocation of the fluid displacement member, the fluid displacement member interfacing with the pump housing such that the fluid displacement member is prevented from rotating relative to the pump housing; and a drive mechanism connected to the rotor and to the fluid displacement member, the drive mechanism comprising a screw connected to the fluid displacement member, the drive mechanism configured to receive rotational output from the rotor and convert the rotational output from the rotor into a linear input to the fluid displacement member to linearly reciprocate the fluid displacement member; wherein the screw is prevented from being rotated by the rotational output by an interface between the screw and the pump housing. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The interface is formed by a projection disposed in a slot, wherein the projection extends from one of the screw and the pump housing, wherein the slot formed in the other one of the screw and the pump housing. 
     A displacement pump for pumping a fluid includes an electric motor disposed in a pump housing and including a stator and a rotor; a fluid displacement member configured to pump fluid; and a screw connected to the fluid displacement member, the screw operably connected to the rotor such that rotation of the rotor drives linear displacement of the screw along a pump axis. The screw includes a screw body; and a lubricant pathway extending through the screw body and configured to provide lubricant to an interface between the screw and the rotor. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A drive nut disposed radially between the rotor and the screw body, the drive nut receiving a rotational output from the rotor and driving the screw linearly. 
     The drive nut includes a plurality of rolling elements disposed between the rotor and the screw, the rolling elements engaging the screw to drive the screw linearly. 
     The plurality of rolling elements includes at least one of balls and toothed rollers. 
     The lubricant pathway includes a first bore extending into the screw body and a second bore extending into the screw body and intersecting with the first bore. 
     The first bore extends into the screw body from a first axial end of the screw body. 
     The second bore extends on a second bore axis, the second bore axis transverse to the pump axis. 
     The second bore axis is orthogonal to the pump axis. 
     The second bore extends between the first bore and an exterior surface of the screw. 
     An outlet of the second bore is disposed at an end of the second bore opposite the first bore and is intermediate threads of the screw. 
     A grease fitting is disposed in the first bore and connected to the screw body. 
     The first bore extends into the screw body from a first axial end of the screw body, and wherein the first bore includes a first diameter portion having a first diameter and extending from the first axial end and a second diameter portion having a second diameter and extending from the first diameter portion, the first diameter being larger than the second diameter. 
     The grease fitting is disposed at an intersection between the first diameter portion and the second diameter portion. 
     The fluid displacement member is connected to the screw by a fastener extending into and connecting with the first diameter portion. 
     The fastener and first diameter portion are connected by interfaced threading. 
     The second bore has a third diameter smaller than the second diameter. 
     The fluid displacement member is a first fluid displacement member connected to a first axial end of the screw body, and wherein a second fluid displacement member connected to a second axial end of the screw body. 
     The screw further comprises a first bore extending into the first axial end of the screw body; and a second bore extending into the second axial end of the screw body; wherein the first bore forms a portion of the lubricant pathway. 
     A grease fitting disposed in the first bore; wherein the first fluid displacement member is connected to the screw by a first fastener extending into the first bore; and wherein the second fluid displacement member is connected to the screw by a second fastener extending into the second bore. 
     The second bore is fluidly isolated from the first bore. 
     The lubricant pathway includes an inlet. 
     The inlet is a grease zerk located within the screw. 
     The inlet is accessible for introducing grease while the screw is located within the rotor. 
     A first fluid displacement member configured to pump fluid and a second fluid displacement member; wherein the fluid displacement member is the first fluid displacement member; wherein each of the first fluid displacement member and the second fluid displacement member are connected to the screw. 
     The first fluid displacement member comprises a first diaphragm and the second fluid displacement member comprises a second diaphragm. 
     A method of lubricating an electric displacement pump includes providing lubricant to an interface between a screw and a rotor of a pump motor of the pump via a lubricant pathway extending through the screw, wherein the screw is disposed coaxially with the rotor. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Disconnecting a fluid displacement member from the screw. 
     Disconnecting the fluid displacement member from the screw includes removing a fastener from a bore extending into the screw. 
     Removing the fastener from the bore extending into the screw includes unthreading the fastener from the bore. 
     The bore forms a portion of the lubricant pathway such that the step of providing lubricant to the interface between the screw and the rotor includes providing lubricant through the bore extending into the screw. 
     Providing lubricant to the interface between the screw and the rotor includes providing lubricant through a bore extending into the screw, the bore configured to receive a fastener to secure a fluid displacement member to the screw. 
     Providing lubricant to the interface between the screw and the rotor includes inserting an applicator of a lubricant gun into the bore and engaging the applicator with a grease fitting disposed within the bore. 
     A displacement pump for pumping a fluid includes an electric motor at least partially disposed in a pump housing and including a stator and a rotor; a first fluid displacement member connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first fluid displacement member; wherein the first fluid displacement member fluidly separates a first process fluid chamber disposed on a first side of the first fluid displacement member from a first cooling chamber disposed on a second side of the first fluid displacement member; wherein the first fluid displacement member simultaneously pumps process fluid through the first process fluid chamber and pumps air through the first cooling chamber. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A second fluid displacement member connected to the rotor to be driven by the rotor, the second fluid displacement member fluidly separating a second process fluid chamber disposed on a first side of the second fluid displacement member from a second cooling chamber disposed on a second side of the second fluid displacement member; wherein the second fluid displacement member is configured to simultaneously pump process fluid through the second process fluid chamber and pump air through the second cooling chamber. 
     A first check valve is disposed upstream of the first cooling chamber to allow flow into the first cooling chamber, at least one passage extends between the first cooling chamber and second cooling chamber, and a second check valve is disposed downstream of the second cooling chamber to allow flow out of the second cooling chamber. 
     The at least one passage includes at least one rotor passage that rotates with the rotor. 
     The at least one passage includes at least one stator passage that remains static relative to the stator. 
     The at least one stator passage is disposed between the stator and a control housing. 
     An internal check valve disposed at an outlet of the at least one passage such that the internal check valve prevents air from backflowing into the at least one passage from the second cooling chamber. 
     The internal check valve is a flapper valve. 
     A flapper of the flapper valve is secured to the pump housing by a grease cap associated with a bearing supporting the rotor. 
     The at least one passage includes a first passage and a second passage, wherein at least a portion of the first passage is formed by at least one rotor passage through the rotor, wherein the second passage includes and at least one stator passage, and wherein the internal check valve controls flow out of both the at least one rotor passage and the at least one stator passage. 
     The first check valve is mounted to a valve plate and the second check valve is mounted to the valve plate. 
     A flow directing member, the flow directing member configured to direct one of an exhaust flow of the air exiting the second check valve and an inlet flow of air flowing to the first check valve such that the one of the exhaust flow and the inlet flow flows over an exterior of the pump housing. 
     The exterior of the pump housing includes at least heat sink increasing a surface area of the exterior of the pump housing to facilitate heat transfer, and wherein the flow directing member directs the one of the exhaust flow and the inlet flow over the at least one projection. 
     A first diaphragm plate exposed to one of the first cooling chamber and the first process chamber; and a membrane extending radially relative to the first diaphragm plate; wherein the first diaphragm plate includes at least one first heat sink formed on the first diaphragm plate. 
     A fastener connects the first diaphragm plate to a screw, the screw receiving the rotational output from the rotor and providing the linear input to the fluid displacement member. 
     A second diaphragm plate exposed to the other one of the first cooling chamber and the first process chamber, wherein an inner portion of the membrane is captured between the first diaphragm plate and the second diaphragm plate. 
     The second diaphragm plate includes at least one second heat sink formed on the second diaphragm plate. 
     The first fluid displacement member reciprocates in a first direction and a second direction; the first fluid displacement member simultaneously performs a pumping stroke of the process fluid and a suction stroke of the air as the first fluid displacement member moves in the first direction; and the first fluid displacement member simultaneously performs a pumping stroke of the air and a suction stroke of the process fluid as the first fluid displacement member moves in the second direction. 
     The air pumped by the first fluid displacement member is forced through the electric motor to remove heat from the electric motor. 
     A drive mechanism connected to the rotor and the first fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the first fluid displacement member; wherein the air pumped by the first fluid displacement member is forced to contact the drive mechanism and remove heat from the drive mechanism. 
     The drive mechanism includes a screw connected to the fluid displacement member and disposed coaxially with the rotor. 
     A double diaphragm pump having an electric motor includes a housing; an electric motor comprising a stator and a rotor, the rotor configured to rotate to generate rotational input; a first diaphragm connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first diaphragm; a second diaphragm connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the second diaphragm; wherein the first diaphragm fluidly separates a first process fluid chamber disposed on a first side of the first diaphragm from a first cooling chamber disposed on a second side of the first diaphragm; wherein the second diaphragm fluidly separates a second process fluid chamber disposed on a first side of the second diaphragm from a second cooling chamber disposed on a second side of the second diaphragm; wherein the first diaphragm and the second diaphragm reciprocate in a first direction and a second direction, wherein the first diaphragm simultaneously performs a pumping stroke of the process fluid and a suction stroke of the air as the first diaphragm moves in the first direction; wherein the second diaphragm simultaneously performs a suction stroke of the process fluid and a pumping stroke of the air as the second diaphragm moves in the first direction; wherein the first diaphragm simultaneously performs a pumping stroke of the air and a suction stroke of the process fluid as the first diaphragm moves in the second direction; and wherein the second diaphragm simultaneously performs a pumping stroke of the process fluid and a suction stroke of the air as the second diaphragm moves in the second direction. 
     The double diaphragm pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The air pumped by the first diaphragm and the second diaphragm is forced through the electric motor to remove heat from the electric motor. 
     A drive mechanism connected to the rotor, the first diaphragm, and the second diaphragm, wherein the drive mechanism is configured to convert a rotational output from the rotor into a linear input to the first diaphragm and the second diaphragm; wherein the air pumped by the first diaphragm is forced to contact the drive mechanism and remove heat from the drive mechanism. 
     The air pumped from the first cooling chamber is pumped to the second cooling chamber. 
     A method of cooling an electrically operated pump includes driving reciprocation of a first fluid displacement member and a second fluid displacement member by an electric motor having a rotor configured to rotate about a pump axis, wherein the first fluid displacement member and the second fluid displacement member are disposed coaxially with the rotor and connected to the rotor via a drive mechanism; drawing air into a first cooling chamber of a cooling circuit of the pump by the first fluid displacement member, the first cooling chamber disposed between the first fluid displacement member and the rotor; pumping the air from first cooling chamber to a second cooling chamber disposed between the second fluid displacement member and the rotor; and driving the air out of the second cooling chamber by the second fluid displacement member to exhaust the air from the cooling circuit. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Directing an external airflow outside of a pump housing within which the electric motor is disposed such that the external airflow flows over at least one heat sink formed on the pump housing. 
     Pumping the air from first cooling chamber to a second cooling chamber disposed between the second fluid displacement member and the rotor includes flowing the air through at least one passage extending between the first cooling chamber and the second cooling chamber. 
     Flowing the air through at least one passage extending between the first cooling chamber and the second cooling chamber includes flowing the air through a stator air passage, the stator air passage remaining stationary relative to the stator during pumping. 
     Flowing the air through at least one passage extending between the first cooling chamber and the second cooling chamber includes flowing the air through an air passage formed at least partially by a rotor passage rotating about the pump axis with the rotor. 
     Preventing air disposed within the second cooling chamber from backflowing into the at least one passage by an internal check valve disposed between the at least one passage and the second cooling chamber. 
     Controlling airflow into the first cooling chamber with a first check valve; and controlling airflow out of the second cooling chamber with a second check valve. 
     A displacement pump for pumping a fluid includes an electric motor including a rotor and a stator, the rotor located within the stator; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the fluid displacement member; and a position sensor including a sensing component disposed radially inside the rotor, the position sensor configured to sense rotation of the rotor and to provide data to a controller. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A permanent magnet array of the rotor includes a plurality of back irons and a plurality of permanent magnets. 
     The sensing component is disposed radially inward of a radially inner edge of a permanent magnet array of the rotor. 
     The rotor includes an axial extension projecting from an axial end of the rotor, and wherein at least a portion of the sensing component extends below the axial extension such that the axial extension is disposed between the position sensor and the permanent magnet array. 
     The position sensor is disposed radially outward from a bearing supporting the rotor. 
     The position sensor includes an array of Hall-effect sensors. 
     The position sensor is mounted to the stator. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the fluid displacement member; and a controller configured to: regulate current flow to the electric motor such that the rotor applies torque to the drive mechanism with the pump in both a pumping state and a stalled state; wherein in the pumping state, the rotor applies torque to the drive mechanism and rotates about the pump axis causing the fluid displacement member to apply force to a process fluid and displace axially along the pump axis; and wherein in the stalled state, the rotor applies torque to the drive mechanism and does not rotate about the pump axis such that the fluid displacement member applies force to the process fluid and does not displace axially due to the force being insufficient to overcome the downstream pressure of the process fluid. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The controller is further configured to regulate the current flow to the electric motor with the pump in the stalled state such that the current provided is a maximum current. 
     The maximum current is a maximum operating current. 
     The maximum current is a target operating current. 
     The controller is further configured to pulse the current to the electric motor with the pump in the stalled state. 
     The pump does not include a working fluid for causing the fluid displacement member to apply force to the process fluid. 
     A dual pump for pumping a fluid includes an electric motor comprising a stator and a rotor, the rotor configured to generate rotational output; a controller configured to regulate current flow to the electric motor; a drive mechanism comprising a screw, the screw extending within the rotor, the screw configured to receive the rotational output and convert the rotational output into linearly reciprocating motion of the screw, wherein rotation of the rotor in a first direction drives the screws to linearly move in a first direction along an axis, and rotation of the rotor in a second direction drives the screws to linearly move in a second direction along the axis; a first fluid displacement member and a second fluid displacement member, the screw located between the first and the second fluid displacement members, the screw translating the first and the second fluid displacement members in the first direction along the axis when the rotor rotates in the first direction and in the second direction along the axis when the rotor rotates in the second direction; wherein: the first fluid displacement performs a pumping stroke of the process fluid and the second fluid displacement performs a suction stroke of the process fluid as the screw moves in the first direction, the first fluid displacement performs a suction stroke of the process fluid and the second fluid displacement performs a pumping stroke of the process fluid as the screw moves in the second direction, the controller regulates output pressure of the process fluid by regulating current flow to the motor such that the rotor rotates to cause the first and the second fluid displacement members to reciprocate to pump the process fluid until pressure of the process fluid stalls the rotor while the first fluid displacement member is in the pump stroke and the second fluid displacement member is in the suction stroke even while current continues to be supplied to the motor by the controller, the first and the second fluid displacement members resuming pumping when the pressure of the process fluid drops enough for the rotor to overcome the stall and resume rotating. 
     The dual pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The controller is configured to receive a pressure output setting for the pump from a user, the pressure output setting corresponding to a current level at which the controller supplies the current to the motor. 
     The dual pump does not include a pressure transducer that influences the level of power supplied by the controller to the motor. 
     The controller is configured to regulate the current flow to the motor based on data other than pressure information from a pressure transducer. 
     A method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; regulating, by a controller, a flow of current to a stator of the electric motor such that the rotational force is applied to the rotor during both a pumping state and a stalled state; wherein in the pumping state, the rotor applies torque to the drive mechanism and rotates about the pump axis causing the fluid displacement member to apply force to a process fluid and displace axially along the pump axis; and wherein in the stalled state, the rotor applies torque to the drive mechanism and does not rotate about the pump axis such that the fluid displacement member applies force to the process fluid and does not displace axially. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The drive mechanism is at least partially disposed within the rotor. 
     Applying, by the drive mechanism, axial force to the fluid displacement member includes applying, by a drive nut of the drive mechanism connected to the rotor to rotate with the rotor, axial force to a screw of the drive mechanism, the screw disposed coaxially with the fluid displacement member; and applying, by the screw, the axial force to the fluid displacement member. 
     Applying, by the rotor, torque to the drive mechanism includes applying, by the rotor, torque to a drive nut connected to the rotor to rotate with the rotor, the drive nut disposed coaxially with a screw and configured to drive axial displacement of the screw. 
     Applying force to the screw by a rolling element disposed between the drive nut and the screw. 
     Regulating, by the controller, the flow of current to the stator includes pulsing the current in the stalled state such that the rotor applies varying amounts of torque to the drive mechanism when in the stalled state. 
     Pulsing the current between a first current and a second current, the first current being a maximum operating current, and the second current being a current less than the maximum operating current. 
     Pulsing the current between first current and a second current, the first current being a set point current less than a maximum operating current, and the second current being a current less than the set point current. 
     The set point current is a target operating current for the pump. 
     A method of operating a reciprocating pump includes providing electric current to an electric motor disposed on a pump axis and connected to a fluid displacement member configured to reciprocate along the pump axis; and regulating, by a controller, current flow to the electric motor to control a pressure output by the pump to a target pressure. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Regulating, by the controller, current flow to the electric motor when the pump is in a pumping state, such that the current is maintained at or below a maximum current; regulating, by the controller, current flow to the electric motor when the pump is in a stalled state, such that the fluid displacement member applies force to a process fluid with the pump in the stalled state. 
     Determining, by the controller, that the pump is in the pumping state based on a rotor of the electric motor rotating about the pump axis. 
     Regulating, by the controller, the current flow to the electric motor when the pump is in the stalled state includes pulsing the current provided to the electric motor. 
     Regulating, by the controller, the current flow to the electric motor when the pump is in the stalled state includes maintaining the current at the maximum current. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism configured to convert a rotational output from the rotor into a linear input to the fluid displacement member; and a controller configured to: cause current to be provided to the stator to drive rotation of the rotor, thereby driving reciprocation of the fluid displacement member; and regulate the current flow to the electric motor to control a pressure output by the pump to a target pressure. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The controller regulates the current flow to the electric motor without pressure feedback from a pressure sensor. 
     The controller is configured to regulate the current flow such that the actual current does not exceed a maximum current for the target pressure, and wherein the controller is further configured to regulate a rotational speed of the rotor such that an actual rotational speed does not exceed a maximum speed. 
     The controller is configured to set both the maximum current and the maximum speed based on a single parameter input received by the controller. 
     The fluid displacement member includes a variable working surface area, and wherein the controller is configured to vary the current throughout a stroke of the fluid displacement member to control the pressure output to the target pressure. 
     A method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a fluid displacement member along a pump axis, the fluid displacement member disposed coaxially with a rotor of the electric motor; regulating, by a controller, a rotational speed of the rotor thereby directly controlling an axial speed of the fluid displacement member such that the rotational speed is at or below a maximum speed; and regulating, by the controller, current provided to the electric motor such that the current provided is at or below a maximum current. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The fluid displacement member includes a variable working surface area. 
     Varying, by the controller, current provided to the electric motor such that a first current is provided to the electric motor at a beginning of a pumping stroke of the fluid displacement member and a second current is provided to the electric motor at an end of the pumping stroke. 
     A method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a fluid displacement member along a pump axis, the fluid displacement member disposed coaxially with a rotor of the electric motor, wherein the fluid displacement member includes a variable working surface area; and varying, by a controller, current provided to the electric motor such that a first current is provided to the electric motor at a beginning of a pumping stroke of the fluid displacement member and a second current is provided to the electric motor at an end of the pumping stroke, the second current less than the first current. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the fluid displacement member, the drive mechanism comprising a screw and configured to convert a rotational output from the rotor into a linear input to the fluid displacement member; and a controller configured to operate the pump in a start-up mode and a pumping mode, wherein during the start-up mode the controller is configured to: cause the motor to drive the fluid displacement member in a first axial direction; and determine an axial location of the fluid displacement member based on the controller detecting a first current spike when the fluid displacement member encounters a first stop. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The controller is further configured to determine whether the first stop is a mechanical stop. 
     The mechanical stop corresponds with a travel limit of the fluid displacement member. 
     The controller is configured to cause the motor drive the fluid displacement member in a second axial direction opposite the first axial direction; detect a second stop; measure a stroke length between the first stop and the second stop; and compare the measured stroke length to a reference stroke length to determine a stop type of the first stop. 
     The controller is configured to classify at least one of the first stop and the second stop as a fluid stop based on the measured stroke length being less than the reference stroke length. 
     The controller is configured to determine a stop type of the first stop based on a comparison of a plurality of stop locations. 
     The controller is configured to determine that the first stop is a mechanical stop based on the comparison indicating that differences between the plurality of stop locations are less than a threshold difference. 
     The mechanical stop corresponds with a travel limit of the fluid displacement member. 
     The controller is configured to determine that the first stop is a fluid stop based on the comparison indicating at least one difference between the plurality of stop locations exceeds a threshold difference. 
     The fluid stop is due to downstream fluid pressure acting on the fluid displacement member. 
     The controller is configured to determine a stop type of the first stop based on a slope of a current profile of the first current spike. 
     The axial location is determined based on rotations of the rotor. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a first fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a second fluid displacement member configured to pump fluid and disposed coaxially with the rotor; a drive mechanism connected to the rotor and the first and second fluid displacement members, the drive mechanism comprising a screw and configured to convert a rotational output from the rotor into a linear input to the first and second fluid displacement members; and a controller configured to operate the pump in a start-up mode and a pumping mode. During the start-up mode the controller is configured to cause the motor to drive the first and second fluid displacement members in a first axial direction; and determine an axial location of at least one of the first and second fluid displacement members based on the controller detecting a first current spike when the at least one of the first and second fluid displacement members encounters a first stop. Moving the first and second fluid displacement members in the first axial direction moves one of the first and second fluid displacement members through a pumping stroke and moves the other of the first and second fluid displacement members through a suction stroke. Moving the first and second fluid displacement members in a second axial direction opposite the first axial direction moves the one of the first and second fluid displacement members through a suction stroke and moves the other of the first and second fluid displacement members through a pumping stroke. 
     A method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member in a first axial direction on a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and determining, by a controller, an axial location of the first fluid displacement member based on the controller detecting a first current spike due to the first fluid displacement member encountering a first stop and the rotor stopping rotation. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Driving the first fluid displacement member in the first axial direction a plurality of times to generate a plurality of stop locations; and determining, by the controller, a stop type of the first stop based on axial locations of each of the plurality of stop locations. 
     Comparing the plurality of stop locations to determine the stop type; and classifying the first stop as a mechanical stop based on differences between the stop locations being less than a threshold difference. 
     Comparing the plurality of stop locations to determine the stop type; and determining that the first stop is a fluid stop based on the comparison indicating differences between any two of the plurality of stop locations exceeding a threshold difference. 
     Driving, by the electric motor, a second fluid displacement member in a second axial direction opposite the first axial direction along the pump axis, the second fluid displacement member disposed coaxially with the rotor; detecting a second current spike due to the second fluid displacement member encountering a second stop and the rotor stopping rotation; and determining, by a controller, a measured stroke length based on an axial location of the first current spike and an axial location of the second current spike. 
     Comparing the measured stroke length to a reference stroke length; and classifying at least one of the first stop and the second stop as one of a mechanical stop and a fluid stop based on the comparison of the measured stroke length and the reference stroke length. 
     Classifying the first stop as one of a mechanical stop and a fluid stop based on a current profile generated by the first current spike. 
     Driving, by the electric motor, a second fluid displacement member in a second axial direction opposite the first axial direction along the pump axis, the second fluid displacement member disposed coaxially with the rotor; and determining, by the controller, an axial location of the second fluid displacement member based on the controller detecting a second current spike due to the second fluid displacement member encountering a second stop and the rotor stopping rotation. 
     Recording the locations of the first stop and the second stop as travel limits for the first fluid displacement member and the second fluid displacement member, such that a distance between the first stop and the second stop defines a maximum stroke length. 
     A method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member through a pumping stroke in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; initiating, by a controller, deceleration of the rotor when the first fluid displacement member is at a first deceleration point disposed a first axial distance from a first target point along the pump axis; determining, by the controller, a first adjustment factor based on a first axial distance between a first stopping point and the first target point, wherein the first stopping point is an axial location where the first fluid displacement member stops displacing in the first axial direction; and managing, by the controller, a stroke length based on the first adjustment factor. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Managing, by the controller, the stroke length includes altering an axial location of the first deceleration point based on the first adjustment factor. 
     Shifting a location of the first deceleration point axially closer to the target point based on the stopping point undershooting the target point. 
     Shifting a location of the first deceleration point axially further from the target point based on the stopping point overshooting the target point. 
     Adjusting an axial location of a second deceleration point for a second fluid displacement member configured to shift through a second pumping stroke in a second axial direction opposite the first axial direction based on the first adjustment factor. 
     Managing, by the controller, the stroke length includes controlling a second stroke length in a second axial direction opposite the first axial direction based on the first adjustment factor. 
     Generating a second adjustment factor based on a second axial distance between a second stopping point, where a second fluid displacement member stops displacing in the second axial direction, relative to the second target point; 
     Adjusting a first stroke length in the first axial direction based on the second adjustment factor. 
     A rotor assembly for an electric motor includes a rotor body formed from a first body component and a second body component; a drive component disposed within a chamber defined by the first body component and the second body component; and a permanent magnet array disposed on an outer surface of the rotor body; wherein the first body component and the second body component form a clamshell receiving the drive component. 
     The rotor assembly of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A first bearing assembly mounted to the first body component; and a second bearing assembly mounted to the second body component. 
     The drive component is a drive nut of a drive mechanism configured to convert a rotary motion of rotor body to linear motion of a linear displacement member. 
     The linear displacement member is a screw. 
     The drive component includes a shaft extending axially beyond an outer axial end of the first body component. 
     The drive component defines a bore configured to receive a shaft, the bore interfacing with the shaft to drive rotation of the shaft. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor; a fluid displacement member connected to the rotor such that a rotational output from the rotor provides a linear reciprocating input to the first fluid displacement member; and a controller configured to regulate current flow to the electric motor based on a current limit to thereby regulate an output pressure of the fluid pumped by the fluid displacement member; regulate a rotational speed of the rotor based on a speed limit to thereby regulate an output flowrate of the fluid pumped by the fluid displacement member; set a current limit and a speed limit based on a single parameter command received by the controller. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A user interface operatively connected to the controller, the user interface including a parameter input configured to provide the single parameter command to the controller. 
     The parameter input is one of a knob, a dial, a button, and a slider. 
     A method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; regulating, by a controller, a flow of current to a stator of the electric motor based on a current limit; regulating, by the controller, a speed of the rotor based on a speed limit; generating the single parameter command based on a single input from a user; and setting, by the controller, both the current limit and the speed limit based on the single parameter command received by the controller. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Setting, by the controller, both the current limit and the speed limit based on the single parameter command received by the controller includes proportionally adjusting the current limit and the speed limit based on the single parameter command. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member operatively connected to the rotor to be reciprocated to pump fluid; a controller configured to operate the motor in a start-up mode and a pumping mode, wherein during the pumping mode the controller is configured to operate the electric motor based on a target current and a target speed, and wherein during the start-up mode the controller is configured to operate the electric motor based on a maximum priming speed that less than the target speed. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The controller is further configured to exit the start-up mode and enter the pumping mode based on an operating parameter reaching a threshold. 
     The operating parameter is one of a time of operation, a number of pump cycles of the fluid displacement member, a number of pump strokes of the fluid displacement member, a count of rotations of the rotor, and a current draw of the electric motor. 
     The controller is configured to operate the pump in the start-up mode on power up. 
     A method of operating a reciprocating pump includes electromagnetically applying a rotational force to a rotor of an electric motor; applying, by the rotor, torque to a drive mechanism; applying, by the drive mechanism, axial force to a fluid displacement member configured to reciprocate on a pump axis to pump process fluid; regulating, by a controller, power to the electric motor to control an actual speed of the rotor during a start-up mode such that the actual speed is less than a maximum priming speed; regulating, by a controller, the power to the electric motor to control an actual speed of the rotor during a pumping mode such that the actual speed is less than a target speed; wherein the maximum priming speed is less than the target speed. 
     A method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member through a pumping stroke in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and managing, by the controller, a stroke length of the first fluid displacement member during a first operating mode and a second operating mode such that the stroke length during the second operating mode is shorter than the stoke length during the first operating mode. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Increasing a number of changeovers between stroke directions for the first fluid displacement member while in the second operating mode relative to the first operating mode. 
     Regulating, by the controller, an actual speed of the first fluid displacement member during the first operating mode based on a maximum speed; and regulating, by the controller, an actual speed of the first fluid displacement member during the second operating mode based on the maximum speed. 
     Regulating, by the controller, an actual speed of the first fluid displacement member during the first operating mode based on a first maximum speed; and regulating, by the controller, an actual speed of the first fluid displacement member during the second operating mode based on a second maximum speed greater than the first maximum speed. 
     A method of operating a reciprocating pump includes driving, by an electric motor, a first fluid displacement member through a pumping stroke in a first axial direction along a pump axis, the first fluid displacement member disposed coaxially with a rotor of the electric motor; and managing, by the controller, a stroke of the first fluid displacement member during a first operating mode such that a pump stroke occurs in a first displacement range along the pump axis; and managing, by the controller, a stroke of the first fluid displacement member during a first operating mode such that the pump stroke occurs in a second displacement range along the pump axis, wherein the second displacement range is a subset of the first displacement range. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a fluid displacement member operatively connected to the rotor to be reciprocated along the pump axis to pump fluid; a controller configured to operate the motor in a first operating mode and a second operating mode. During the first operating mode the controller is configured to manage a stroke length of the fluid displacement member such that a pump stroke of the fluid displacement member occurs in a first displacement range along the pump axis. During the second operating mode the controller is configured to manage the stroke length of the fluid displacement member such that the pump stroke of the fluid displacement member occurs in a second displacement range along the pump axis. The second displacement range has a smaller axial extent than the first displacement range. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     The second displacement range is a subset of the first displacement range. 
     A second fluid displacement member configured to pump fluid and disposed coaxially with the rotor. 
     A drive mechanism connected to the rotor and the first and second fluid displacement members, the drive mechanism comprising a screw and configured to convert a rotational output from the rotor into a linear input to the first fluid displacement member and the second fluid displacement member. 
     A method of operating a reciprocating pump includes driving, by an electric motor, reciprocation of a first fluid displacement member and a second fluid displacement member to pump fluid; and monitoring, by a controller, an actual operating parameter of the electric motor; and determining, by the controller, that an error has occurred based on the actual operating parameter differing from an expected operating parameter during a particular phase of a pump cycle. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     Monitoring, by the controller, the actual operating parameter of the electric motor includes monitoring, by the controller, the actual current draw of the electric motor; and determining, by the controller, that the error has occurred based on the actual operating parameter differing from the expected operating parameter during the particular phase of the pump cycle includes determining, by the controller, that the error has occurred based on the actual current draw differing from the expected current draw. 
     Monitoring, by the controller, the actual operating parameter of the electric motor includes monitoring, by the controller, the actual speed of the electric motor; and determining, by the controller, that the error has occurred based on the actual operating parameter differing from the expected operating parameter during the particular phase of the pump cycle includes determining, by the controller, that the error has occurred based on the actual speed differing from the expected speed. 
     Determining, by the controller, that the error has occurred based on the actual operating parameter differing from the expected operating parameter during the particular phase of the pump cycle includes comparing a first value of the actual operating parameter during a pumping stroke of the first fluid displacement member to a second value of the actual operating parameter during a pumping stroke of the second fluid displacement member; and determining, by the controller, that the error has occurred based on the comparison of the first value and the second value indicating a variation between the first value and the second value. 
     Determining, by the controller, that the error has occurred based on the comparison of the first value and the second value indicating the variation between the first value and the second value includes determining that the error has occurred based on the variation exceeding a threshold. 
     Determining, by the controller, the first value of the actual operating parameter at a beginning of the pumping stroke of the first fluid displacement member; and determining, by the controller, the second value of the actual operating parameter at a beginning of the pumping stroke of the second fluid displacement member. 
     Displacing, by the electric motor, the first fluid displacement member through a pumping stroke in a first axial direction along a pump axis; displacing, by the electric motor, the second fluid displacement member through a pumping stroke in a second axial direction along the pump axis, the second axial direction being opposite the first axial direction. 
     Driving rotation of a rotor of the electric motor about the pump axis, such that the rotor, the first fluid displacement member, and the second fluid displacement member are disposed coaxially on the pump axis. 
     Generating, by the controller, an error code for the error. 
     Providing, by the controller, the error code to a user interface; and providing, by the user interface, the error code to a user. 
     A displacement pump for pumping a fluid includes an electric motor including a stator and a rotor configured to rotate about a pump axis; a drive connected to the rotor, the drive configured to convert a rotational output from the rotor into a linear input; a first fluid displacement member connected to the drive to be driven by the linear input; a controller configured to: cause current to be provided to the stator to drive rotation of the rotor, thereby driving reciprocation of the fluid displacement member; and monitor an actual operating parameter of the electric motor; and determine that an error has occurred based on the actual operating parameter differing from an expected operating parameter during a particular phase of a pump cycle. 
     The displacement pump of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A second fluid displacement member connected to the drive to be driven by the linear input. 
     The controller is further configured to compare a first value of the actual operating parameter during a pumping stroke of the first fluid displacement member to a second value of the actual operating parameter during a pumping stroke of the second fluid displacement member; and determine that the error has occurred based on the comparison of the first value and the second value indicating a variation between the first value and the second value. 
     The controller is further configured to monitor an actual current draw of the electric motor, the actual current draw forming the actual operating parameter; and determine that the error has occurred based on the actual current draw differing from an expected current draw. 
     The controller is further configured to monitor an actual speed of the electric motor, the actual speed forming the actual operating parameter; and determine that the error has occurred based on the actual speed differing from an expected speed. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.