Abstract:
An electrohydrostatic actuator including an unbalanced area actuator whereby movement of an actuator piston causes a greater change in displacement on a first side of the actuator piston having a first area than on a second side of the actuator piston having a second area. The actuator includes a four-port, dual displacement hydraulic pump having a first pair of ports and a second pair of ports. If an axial piston pump is used, the pistons may be arranged in first and second rings of pistons arranged concentrically about a central axis. The pump has a port plate with a first pair of ports associated with the first ring of pistons and a second pair of ports associated with the second ring of pistons. At least one of the first pair of ports and at least one of the second pair of ports are connected to the first side of the actuator piston. At least one of the first pair of ports is connected to the second side of the actuator piston. At least one of the second pair of ports is connected to a reservoir of hydraulic fluid.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to electrohydrostatic actuators and more particularly to the use of a four-port, dual displacement pump with an unbalanced area actuator. 
       BACKGROUND 
       [0002]    An electrohydrostatic actuator (EHA) is an actuator that is directed and powered a variable speed electric motor that is used to drive a hydraulic pump. The hydraulic fluid pressurized by the pump drives a piston in a cylinder for moving an actuator shaft. The actuator shaft, in turn, is mechanically connected to a mechanism being controlled. 
         [0003]    Electrohydrostatic actuators may be configured several ways. Three of the ways are:
       1) double ended (balanced area) cylinders;   2) single ended (unbalanced area) cylinders with logic valves to control the flow between cylinder and reservoir; and   3) single ended (unbalanced area) cylinders with a dual displacement, three-port pump.
 
Each of these approaches are described below with their associated deficiencies.
       
 
         [0007]      FIG. 1  shows a balanced actuator  1  with an ordinary 2-port pump  2 . Piston areas  3  and  4  are equalized by adding a tailstock  5  to a cylinder  6 . This approach simplifies the pump and the associated hydraulic circuitry, but at an increased cost. This approach also adds extra length and weight to the actuator  1 . Moreover, reliability may be reduced because of an incremental leak path related to an incremental rod seal  7 . If configured as a tandem actuator, the unit becomes even longer, having two balanced cylinders plus the added tailstock length. 
         [0008]      FIG. 2  shows one way to use valves to control an unbalanced flow between a cylinder and a reservoir. The valve shown responds to four modes of operation: 
         [0009]    1) retracting motion with opposing load; 
         [0010]    2) retracting motion with aiding load; 
         [0011]    3) extending motion with opposing load; and 
         [0012]    4) extending motion with aiding load. 
         [0000]    While the advantages of an unbalanced area cylinder and simple 2-port pump are realized, this scheme has several drawbacks. One problem is that the switching valve is costly due to the fast response and low leakage required. Other problems include: reduced dynamic actuator stiffness, the potential for causing instability in control loops, the potential for adverse impact to EHA performance (e.g., threshold, frequency response, heat rejection), and the fact that the ratio of actuator shaft speed to pump RPM is dependent on the direction of motion. 
         [0013]      FIG. 3  shows an actuator  10  that uses a 3-port pump  11  with a single set of pistons. This configuration allows use of unbalanced areas  12  and  13  with a relatively simple pump. The location of a split  14  between a pair of ports, C 2  and C 3 , determines the flow ratio. A portion of the pump piston stroke equal to that of the flow ratio (area  12 /area  13 ) is used on port C 2 , while the remainder is used on port C 3 . Another port C 1  uses the entire stroke. 
         [0014]    Splitting a port into two ports, C 2  and C 3 , as shown in  FIG. 3  has many undesirable ramifications. One problem is that at the transition or split  14  between ports C 2  and C 3 , the pump piston speed may retain 70% to 80% of its maximum velocity (depending on the design flow ratio). As a cylinder barrel port commutates across the C 2 -C 3  split  14 , it is instantaneously blocked off. If that particular piston is on an intake stroke, a momentary vacuum is drawn on the fluid causing vapor and gas bubbles to form. After the transition is crossed, the bubbles collapse and may cause cavitation damage to internal pump components such as the barrel porting. On the other hand, if that particular piston is on a discharge stroke, extreme over-pressure can occur inside the barrel because the flow is momentarily blocked from exiting the cylinder barrel. Therefore, the barrel must be designed to withstand the resulting stresses for the designed fatigue life of the pump components. Such designs may impose a weight penalty. Both of these problems are aggravated with increasing RPM and become the limiting factors for maximum pump speed. Accordingly, a larger, heavier, slower turning pump may be required. 
         [0015]    The pressure extremes discussed above can cause another problem in that they carry over into the next port, thus causing the actual flow ratio of the pump to drift. For example, assume port C 2  is currently acting as an inlet. Because the porting is temporarily blocked near the transition to port C 3 , not all the flow returning from the actuator cylinder makes it back through the pump, causing an effect called “pressure pump-up” in the actuator. Once at port C 3 , fluid rushes in to fill the void of vapor bubbles. During opposite rotation, port C 3  will be acting as an outlet, but because of the port blockage, the fluid is over-compressed. Once at the C 2  port, the high pressure fluid expands causing an excess of flow going to the actuator, and once again, the “pump-up” effect occurs. However, by using a mirror image pump cam, the “pump-up” effect can be transformed into a “pump-down” effect. These effects necessitate the use of anti-cavitation check valves  15 . The pressure spikes have been known to noticeably reduce pump efficiency because of increased loading between internal components. 
         [0016]    To help alleviate these problems, porting under-lap is incorporated at the C 2 -C 3  transition zone. The under-lap allows some leakage between the two ports. Although the under-lap helps with the aforementioned problems, it imposes a penalty on pump volumetric efficiency, which in turn aggravates actuator heat rejection. Designing a numerically low flow ratio into the port plate makes these issues worse. The issues worsen because the piston velocity at the transition zone increases when the flow ratio decreases. Therefore, pressure ripple from the C 3  port may be quite high and cause fatigue and component damage in the actuator manifold. With typical flow ratios, only one piston is connected with this port at a time, causing a highly pulsating flow. 
       SUMMARY OF THE INVENTION 
       [0017]    The invention solves these problems by using a four-port, dual displacement hydraulic pump with unbalanced area EHA&#39;s (Electro-Hydrostatic-Actuators) and EBHA&#39;s (Electro-Backup-Hydrostatic-Actuators). The pump may be a 4-port pump that utilizes dual rows or rings of pistons to achieve the dual displacement characteristic desirable for unbalanced area actuators. The 4-port pump eliminates many of the previously mentioned problems because all port transitions occur at bottom and top dead center of piston travel, where piston velocity is zero. With such a pump it is possible to design for a wider range of flow ratios, including low ratios. Additionally, it may be possible to operate the pump at higher speeds, resulting in a weight savings not just in the pump, but also manifested in a smaller, lower torque, higher speed electric drive motor. 
         [0018]    One aspect of the invention provides an electrohydrostatic actuator including an actuator having a cylinder and a piston movable in the cylinder. The actuator is an unbalanced actuator whereby movement of the actuator piston causes a greater change in displacement on a first side of the actuator piston having a first area than on a second side of the actuator piston having a second area. The electrohydrostatic actuator also includes a hydraulic pump having a first pair of ports and a second pair of ports. At least one of the first pair of ports and at least one of the second pair of ports are fluidly connected to the first side of the actuator piston. At least one of the first pair of ports is fluidly connected to the second side of the actuator piston. At least one of the second pair of ports is fluidly connected to a reservoir of hydraulic fluid. 
         [0019]    Another aspect of the invention provides an electrohydrostatic actuator wherein the hydraulic pump is an axial piston hydraulic pump having two pluralities of pistons arranged about a central axis at two different radii. The first pair of ports is associated with the first plurality of pistons and the second pair of ports is associated with the second plurality of pistons. 
         [0020]    Another aspect of the invention provides an electrohydrostatic actuator wherein the ratio of the displacement of the port fluidly connected to the second side of the actuator piston to the displacement of the ports fluidly connected to the first side of the actuator piston is generally equivalent to the ratio of the area of the second side of the piston to the area of the first side of the piston. 
         [0021]    Another aspect of the invention provides an electrohydrostatic actuator including an actuator including a cylinder and a piston movable in the cylinder, the actuator being an unbalanced actuator whereby movement of the piston causes a greater change in volume on a first side of the piston than on a second side of the piston. The electrohydrostatic actuator also includes a pump having two pluralities of pistons arranged about a central axis at different diameters. Two ports are associated with the first plurality of pistons and two ports are associated with the second plurality of pistons. Three conduits provide fluid communication between the ports and the two sides of the pistons and a reservoir. 
         [0022]    Another aspect of the invention provides an electrohydrostatic actuator wherein the pump is drivable in one direction to pump hydraulic fluid from the first side of the piston through the first conduit and through the second conduit to the second side of the actuator piston and from the first side of the actuator piston through the first conduit and through the third conduit to the reservoir. Moreover, the pump is drivable in an opposite direction to pump hydraulic fluid from the second side of the actuator piston through the second conduit and through the first conduit to the first side of the actuator piston and from the reservoir through the third conduit and through the first conduit to the first side of the actuator piston. 
         [0023]    Another aspect of the invention provides an electrohydrostatic actuator including a cylinder and a 4-port pump. The cylinder includes a piston slidably disposed within the cylinder having a first side and a second side and a ram secured to the piston for extending from the cylinder. The pump includes a cylinder barrel having a first ring of cylinders having pistons slidably disposed therein and a second ring of cylinders having pistons slidably disposed therein wherein the first ring of cylinders has a first diameter and the second ring of cylinders has a second diameter. The pump also includes a port plate having a first plurality of ports in communication with the first ring of cylinders and a second plurality of ports in communication with the second ring of cylinders. Additionally, the ports of the pump are associated with specific portions of the actuator. One of the first plurality of ports and one of the second plurality of ports are in communication with the first side of the actuator piston. One of the first plurality of ports is in communication with the second side of the actuator piston. One of the second plurality of ports is in communication with a reservoir. 
         [0024]    The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description, and the annexed drawings setting forth in detail one or more illustrative embodiments of the invention, such being indicative, however, of but one or a few of the various ways in which the principles of the invention may be employed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a schematic view of a prior art balanced area actuator with a tailstock. 
           [0026]      FIG. 2  is a schematic view of a prior art unbalanced area actuator with additional ports for four modes of operation. 
           [0027]      FIG. 3  is a schematic view of a prior art unbalanced area actuator with a 3-port pump. 
           [0028]      FIG. 4  is a cross-sectional view of a prior art 2-port pump configured to provide variable displacement by adjusting the swashplate angle. 
           [0029]      FIG. 5  is a schematic view of an unbalanced area actuator with a 4-port pump in accordance with the present invention. 
           [0030]      FIGS. 6A and 6B  show perspective views of a port plate configuration of the 4-port pump shown in  FIG. 5 . 
           [0031]      FIGS. 7A and 7B  show perspective views of a cylinder barrel of the 4-port pump shown in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    As shown in  FIG. 4  of the drawings, a conventional 2-port variable displacement piston pump  20  includes a housing  21  composed of a housing body  22  closed by an end wall block  23  secured thereto in a fluid-tight manner to form therein a cavity  24  to be filled with hydraulic fluid. Disposed within the housing cavity  24  is a rotary cylinder barrel  25  that is splined to a drive shaft  26  rotatably mounted within the housing  21 . The cylinder barrel  25  is formed with a plurality of circumferentially equally spaced cylinders  25 A in which a corresponding number of pistons  25 B are axially slidably disposed for reciprocation. The cylinder barrel  25  is axially movable on the drive shaft  26  and has a forward end face in slidable contact with a port plate  27  under the load of a compression coil spring supported thereon. The port plate  27  is secured in place to an internal surface of the end wall block  23 , the port plate being formed with a semi-circular intake slot  27 A and a semi-circular discharge slot  27 B respectively in open communication with inlet and outlet passages  28 A and  28 B in the end wall block  23 . The intake and discharge slots  27 A and  27 B are arranged to communicate with the barrel cylinders  25 A for intake and discharge operation of hydraulic fluid. An inclined swash plate  29  is tiltably supported at its opposite sides on the housing body  22  for frictional engagement with shoes coupled with each spherical head of the plurality of pistons  25 B. During rotation of the cylinder barrel  25 , frictional engagement of the piston shoes on the inclined swash plate  29  causes pumping action by reciprocating the pistons  25 B in the barrel cylinders  25 A. In the variable displacement pump of  FIG. 4 , the angle of the swashplate  29  may be adjusted to provide a different pump displacement at the same rotation speed. 
         [0033]      FIG. 5  shows a simplified schematic representation of a dual-tandem actuator  30  in accordance with the invention. The actuator  30  has a right hand cylinder  31  and a left hand cylinder  32 . The right-hand cylinder  31  includes a piston  33  having equal areas A 2  on both of its sides. The actuator  30  includes a ram  34  extending from the right hand side. A simple 2-port pump  35  with equal displacement in either direction suffices to transfer fluid from one side of the piston  33  to the other to force the actuator to move. There is no net transfer of fluid between the cylinder chambers and a reservoir  36 . 
         [0034]    The left hand cylinder  32 , however, has differential piston areas A 1  and A 2  on each side of a piston  37 . If the ram  34  is extending, fluid must be transferred from a reservoir  38  into the cylinder  32  and vice versa. This fluid transfer is normally across a pressure difference, so a simple connection to the reservoir  38  is not sufficient. A dual-displacement pump  39  described herein performs this function. 
         [0035]    In  FIG. 5 , the pumps  35  and  39  are depicted as representations of their fluid commutation ports (typically called port plates). For the 2-port pump  35  there are two ports  40  and  41 . For a given direction of shaft rotation, one port will be an outlet and the other an inlet. Reversed shaft rotation reverses the direction of the flow. The dual displacement pump  39  has four ports  42 ,  43 ,  44  and  45 , two ports for each of two concentric rows of pistons.  FIGS. 6A ,  6 B,  7 A and  7 B further illustrate the geometry. As  FIG. 5  shows, the two ports on the left,  42  and  43 , are plumbed together and connect to an A 1  side of the actuator piston  37 . The outer right-hand port  45  connects to an A 2  side of the piston  37 , and the inner right-hand port  44  is connected to the reservoir  38 . 
         [0036]    During cylinder extension, fluid from both left-hand ports  42  and  43  supply oil to the A 1  side of the actuator piston  37 . The outer right-hand port  45  receives flow returning from the A 2  side of the actuator, and the inner right-hand port  44  receives inlet flow from the reservoir  38 . Thus, the pump  39  causes a net transfer of fluid from the reservoir  38  into the left hand actuator cylinder  32 . 
         [0037]    During cylinder retraction, the pump  39  rotates in the opposite direction and the ports function in a reverse manner. The pump  39  passes a portion of the cylinder return flow back to the reservoir  38 . 
         [0038]    For proper operation, the ratio of port displacements should approximate the ratio of actuator piston areas A 1  and A 2 . This ratio, A 2  divided by A 1 , is defined as the pump&#39;s “flow ratio” and is generally in the range of 0.8 to 0.9. In the following equation, D 42 , D 43 , D 44 , and D 45  represent the displacements (e.g. cc per revolution) associated with each respective port: 
         [0000]    
       
         
           
             
               
                 D 
                 45 
               
               
                 
                   D 
                   42 
                 
                 + 
                 
                   D 
                   43 
                 
               
             
             = 
             
               
                 A 
                 2 
               
               
                 A 
                 1 
               
             
           
         
       
     
         [0000]    Since D 42 +D 43 =D 45 +D 44 , the displacement associated with the reservoir may be written as: 
         [0000]    
       
         
           
             
               D 
               44 
             
             = 
             
               
                 ( 
                 
                   1 
                   - 
                   
                     
                       A 
                       2 
                     
                     / 
                     
                       A 
                       1 
                     
                   
                 
                 ) 
               
                
               
                 ( 
                 
                   
                     D 
                     42 
                   
                   + 
                   
                     D 
                     43 
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    The port plate and barrel cylinders shown in  FIGS. 6A ,  6 B,  7 A, and  7 B are based on a flow ratio of 0.85, which is typical for an actuator area ratio. This configuration allows for clearance between adjacent piston shoes (not shown). 
         [0039]    Turning to  FIGS. 6A and 6B , an exemplary port plate  60  in accordance with the invention is shown. The port plate  60  has faces  60 A and  60 B. Face  60 A is shown in  FIG. 6A . Face  60 B is shown in  FIG. 6B . Turning to  FIG. 6A , the left portion of face  60 A is broken by two semi-circular ports: inner port  62  and outer port  64 . When port plate  60  is used in the dual-displacement pump  39  of  FIG. 5 , the inner port  62  of  FIG. 6A  corresponds to the inner port  43  of  FIG. 5 . The outer port  64  of  FIG. 6A  corresponds to the outer port  42  of  FIG. 5 . As shown in  FIG. 5 , the inner and outer ports are plumbed together to supply oil to and receive oil from the left hand cylinder  32  in communication with the A 1  side of the piston  37 . Turning back to  FIG. 6A , the right portion of face  60 A is broken by two semi-circular ports: inner port  66  and outer port  68 . When port plate  60  is used in the dual-displacement pump  39  of  FIG. 5 , the inner port  66  of  FIG. 6A  corresponds to the inner port  44  of  FIG. 5 . The outer port  68  of  FIG. 6A  corresponds to the outer port  45  of  FIG. 5 . As shown in  FIG. 5 , the inner port  44  (and corresponding inner port  66  of  FIG. 6A ) is plumbed to the reservoir  38 . The outer port  45  (and corresponding outer port  68  of  FIG. 6A ) is plumbed to supply oil to and receive oil from the cylinder  31  in communication with the A 2  side of the piston  37 . 
         [0040]    Turning to  FIG. 6B , the opposite face  60 B of the port plate  60  is shown with the reverse side ports  62 ,  64 ,  66 ,  68  routed or plumbed as noted above. On the right hand portion of face  60 B (which corresponds to the left hand portion of the face  60 A), two circular plumbing ports  63  are shown that are both in communication with inner and outer ports  62  and  64 . On the left hand portion of face  60 B (which corresponds to the right hand portion of the face  60 A), one inner circular plumbing port  67  is shown in communication with inner port  66 . Two outer circular plumbing ports  69  are in communication with outer port  68 . 
         [0041]    Typical materials for the port plate  60  include hardened steel. 
         [0042]    Turning now to  FIGS. 7A and 7B , an exemplary cylinder barrel  70  in accordance with the invention is shown. The cylinder barrel  70  has faces  70 A and  70 B. Face  70 A is shown in  FIG. 7A . Face  70 B is shown in  FIG. 7B . Turning to  FIG. 7A , face  70 A is shown with two rings of cylinders: an inner ring  72  comprising nine cylinders  74  and an outer ring  76  comprising cylinders  78 . 
         [0043]    Turning to  FIG. 7B , the opposite face  70 B of the cylinder barrel  70  is shown with the inner and outer rings of cylinders  72  and  76  in corresponding communication with an inner ring of ports  73  comprising elongated ports  75  and an outer ring of ports  77  comprising elongated ports  79 . 
         [0044]    Typical materials for the cylinder barrel  70  include bronze, bronze plated steel, and cast iron. Additionally, it is noted that the invention is in no way limited to the number of cylinder bores noted in the example herein. Any number of cylinders per ring may be used, depending on the size of the pump and the application. 
         [0045]    When the cylinder barrel  70  and the port plate  60  are assembled in a pump assembly (such as that shown in  FIG. 4 ), face  60 A of port plate  60  shown in  FIG. 6A  is mated with face  70 B of the cylinder barrel  70  shown in  FIG. 7B . When so configured, the inner ring of elongated ports  73  is in communication with the inner semi-circular ports  62  and  66  of face  60 A. The outer ring of elongated ports  77  is in communication with outer semi-circular ports  64  and  68  of face  60 A. When the pistons (not shown) are in sliding reciprocal communication with the cylinders  74  and  78 , the pistons displace oil into or receive oil from elongated ports  75  and  79  respectively. Elongated ports  75  and  79 , in turn, supply oil to or receive oil from their respective mating semi-circular inner ports  62  and  66  or outer semi-circular ports  64  and  68  of the port plate  60 . The inner and outer ports are in communication for supplying or receiving oil from the cylinder or the reservoir as discussed above with respect to  FIG. 5 . 
         [0046]    Referring back to  FIG. 4 , the port plate  60  and cylinder barrel  70  may be assembled into a pump in the manner shown in the 2-port pump  20  of  FIG. 4 . Cylinder barrel  70  replaces original cylinder barrel  25  and port plate  60  replaces original port plate  27 . When assembled with the proper number of pistons for the two concentric rings of cylinders and properly plumbed with new inlet and outlet passages (to replace  28 A and  28 B), the newly configured 4-port pump forms an example of pump  39  of  FIG. 5 . Please note that the variable angle swashplate of  FIG. 4  is not essential to the invention. 
         [0047]    Additionally, it is noted that the invention is not limited to the axial piston pump of the example herein. Any dual displacement pump having four outlets and inlets may be used. Examples of such alternative pumps include: a gear pump with one large gear pair and one small gear pair or a vane pump having two adjacent chambers of different sizes. 
         [0048]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.