Patent Publication Number: US-11391300-B2

Title: Externally regulated control for drive pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional patent application No. 62/951,131, filed Dec. 20, 2019, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     This disclosure is directed toward power machines. More particularly, this disclosure is directed toward a drive system of a power machine and control of a hydrostatic drive system. Power machines, for the purposes of this disclosure, include any type of machine that generates power to accomplish a particular task or a variety of tasks. One type of power machine is a work vehicle. Work vehicles are generally self-propelled vehicles that have a work device, such as a lift arm (although some work vehicles can have other work devices) that can be manipulated to perform a work function. Work vehicles include loaders, excavators, utility vehicles, tractors, and trenchers, to name a few examples. 
     Some power machines can convert power from a power source (e.g., an engine) into a form that can be used by a hydraulic drive system to move the machine (i.e., for traction control) or to operate work implements, such as a lift arm. For example, certain power machines can include a hydrostatic drive system in which one or more hydrostatic drive pumps selectively provide pressurized hydraulic fluid to one or more drive motors for moving the power machine over a support surface (e.g. ground). The drive pumps can be variable displacement pumps that are controlled by one or more control valves. One or more hydraulic charge pumps can be configured to charge the hydrostatic drive system, i.e., providing flow to replace leakage that typically occurs in components in a hydrostatic circuit or otherwise supplement a supply of hydraulic fluid in the hydrostatic drive system. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     SUMMARY 
     Some embodiments disclosed herein can include systems and related methods for improving operation of hydraulic drive systems, including through improved routing of control signals for variable displacement drive pumps. 
     Some of the disclosed embodiments provide a hydraulic charge circuit for providing pressurized hydraulic flow to a hydrostatic drive system for a power machine, the hydrostatic drive system including a hydrostatic drive circuit having a variable displacement drive pump operably coupled to a hydrostatic drive motor, and a control assembly configured to control displacement of the variable displacement drive pump. The hydraulic charge circuit can include a hydraulic charge pump, a supply hydraulic flow path, and a control flow path. The supply hydraulic flow path can extend from the hydraulic charge pump to a pressure relief valve that sets a hydraulic pressure for a charge flow of hydraulic fluid to be supplied to the hydrostatic circuit by the hydraulic charge pump. The control flow path can branch from the supply hydraulic flow path upstream of the pressure relief valve, and can be configured to provide a pressurized hydraulic control signal to the control assembly. 
     Some of the disclosed embodiments provide a power machine. A hydrostatic drive system of the power machine can have a variable displacement drive pump in communication with a hydrostatic drive motor via a hydrostatic drive circuit. A hydraulic charge circuit of the power machine can include a hydraulic charge pump that is configured to provide a hydraulic charge flow to the hydrostatic drive circuit via a hydraulic charge flow path. A control system of the power machine can include an actuator configured to control displacement of the variable displacement drive pump, a valve assembly configured to control the actuator, and one or more pilot supply valves. The one or more pilot supply valves can be configured to control hydraulic flow from the hydraulic charge pump to the valve assembly along one or more control flow paths that are separate from the hydraulic charge flow path. 
     Some of the disclosed embodiments provide a method of operating a hydrostatic drive circuit of a power machine. A hydraulic charge pump can be operated to provide a hydraulic flow along a supply hydraulic flow path of a hydraulic charge circuit. The hydraulic flow can be divided, within the hydraulic charge circuit, between a hydraulic charge flow path and a control flow path that branches from the supply hydraulic flow path downstream of the hydraulic charge pump and upstream of a hydraulic load included in the hydraulic charge circuit. The control flow path can provide a first flow to a control assembly that is configured to control the displacement of a variable displacement drive pump of the hydrostatic drive circuit. The hydraulic charge flow path can provide a second flow to charge the hydrostatic drive circuit. 
     This Summary and the Abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       DRAWINGS 
         FIG. 1  is a block diagram illustrating functional systems of a representative power machine on which embodiments of the present disclosure can be advantageously practiced. 
         FIGS. 2-3  illustrate perspective views of a representative power machine in the form of a skid-steer loader of the type on which the disclosed embodiments can be practiced. 
         FIG. 4  is a block diagram illustrating components of a power system of a loader such as the loader illustrated in  FIGS. 2-3 . 
         FIG. 5  is a simplified circuit diagram illustrating features of a hydrostatic drive system for a power machine, according to one of the disclosed embodiments. 
         FIG. 6  is a flowchart of a process for operating a hydrostatic drive system. 
     
    
    
     DETAILED DESCRIPTION 
     The concepts disclosed in this discussion are described and illustrated by referring to exemplary embodiments. These concepts, however, are not limited in their application to the details of construction and the arrangement of components in the illustrative embodiments and are capable of being practiced or being carried out in various other ways. The terminology in this document is used for the purpose of description and should not be regarded as limiting. Words such as “including,” “comprising,” and “having” and variations thereof as used herein are meant to encompass the items listed thereafter, equivalents thereof, as well as additional items. 
     In some configurations of power machines, hydrostatic drive systems can provide power to tractive elements, in order to move the power machines over terrain. For example, a variable displacement drive pump can be arranged to provide hydraulic flow to a hydraulic drive motor via a hydrostatic drive circuit. A control assembly, that in some embodiments includes a valve that provides pilot pressure to control a swash plate actuator, for example a servo valve and actuator combination (other combinations besides the servo valve and actuator combination can be employed), can be provided to control the displacement of the drive pump and thereby control hydraulic flow to the drive motor. 
     Although hydrostatic drive circuits can provide efficient and effective delivery of tractive power, typical systems experience regular leakage of hydraulic fluid. Leakage from the hydrostatic drive circuit can be important, because it can provide a path for oil to leave the hydrostatic circuit, to be provided to an oil cooling circuit to maintain an acceptable temperature in the hydrostatic circuit. Many such systems also have a flushing valve to evacuate a given amount of hydraulic fluid from the circuit, with this fluid being directed to a heat exchanger for the purposes of cooling the hydraulic fluid. However, due to this constant removal of fluid from the hydrostatic drive circuit, pressurized hydraulic fluid needs to be continuously provided back into the hydrostatic circuit to replenish the hydrostatic drive circuit. Accordingly, a hydraulic charge pump (e.g., that is directly driven by an engine of a power machine) can be used to provide this make up oil to (i.e., to replenish) the hydrostatic drive circuit. The hydraulic charge circuit can further include a charge pressure relief valve (often located with a drive pump assembly) that sets pressure at which the hydraulic fluid is provided to the drive circuit. Thus, hydraulic flow can be provided to a hydrostatic drive circuit at a predetermined pressure to replace hydraulic leakage and ensure that an associated drive pump is appropriately primed. 
     In conventional arrangements, a hydraulic charge pump for a hydraulic charge circuit can be used for other purposes than simply to charge a hydrostatic drive circuit. For example, pressurized flow from the hydraulic charge circuit can be provided to a control valve on the one or more drive pumps (e.g., a servo control valve, as is well known in the art) to control of the displacement of an associated drive pump. Using a hydraulic charge pump to perform both of these functions can provide some efficiencies for a power machine. Notably, different pumps are not required to perform both of these functions, which provides cost and space benefits, and a reduced overall complexity of the machine. 
     Although the use of a hydraulic charge pump for multiple purposes can provide certain efficiencies for a power machine, some conventional arrangements may not be optimally arranged. For example, typical servo control valve assemblies use low flow rate, relatively high pressure signals to shift a servo control valve, whereas drive circuits can be efficiently charged using a relatively higher flow rate, relatively lower pressure flow (as compared to the pressure signal provided to the servo control assemblies). Thus, these two purposes require signals that conflict with each other. With conventional systems that rely on a common pressure setting device (such as a pressure relief valve) to set the pressure for both of these functions, systems can only be optimized for one of these functions or can instead select a pressure that is optimized for neither function. 
     In some embodiments, to address the issues noted above (or others), a second pressure setting device can be introduced into a system to provide a first path configured to provide high pressure flow to the servo control valves and a second path configured to provide low pressure flow to the hydrostatic drive circuit. Thus, using a single pump, a low volume, high pressure flow can be provided for control of the servo control valves and a high volume, low pressure flow can be provided to charge an associated hydrostatic drive circuit. Advantageously, the pressure provided to the servo control valves can be sufficiently large to control the spools and the pressure provided to the hydraulic drive circuit can be set at a lower level to improve the efficiency of the overall system. 
     These concepts can be practiced on various power machines, as will be described below. A representative power machine on which the embodiments can be practiced is illustrated in diagram form in  FIG. 1  and one example of such a power machine is illustrated in  FIGS. 2-3  and described below before any embodiments are disclosed. For the sake of brevity, only one power machine is illustrated and discussed as being a representative power machine. However, as mentioned above, the embodiments below can be practiced on any of a number of power machines, including power machines of different types from the representative power machine shown in  FIGS. 2-3 . Power machines, for the purposes of this discussion, include a frame, at least one work element, and a power source that can provide power to the work element to accomplish a work task. One type of power machine is a self-propelled work vehicle. Self-propelled work vehicles are a class of power machines that include a frame, work element, and a power source that can provide power to the work element. At least one of the work elements is a motive system for moving the power machine under power. 
       FIG. 1  is a block diagram that illustrates the basic systems of a power machine  100 , which can be any of a number of different types of power machines, upon which the embodiments discussed below can be advantageously incorporated. The block diagram of  FIG. 1  identifies various systems on power machine  100  and the relationship between various components and systems. As mentioned above, at the most basic level, power machines for the purposes of this discussion include a frame, a power source, and a work element. The power machine  100  has a frame  110 , a power source  120 , and a work element  130 . Because power machine  100  shown in  FIG. 1  is a self-propelled work vehicle, it also has tractive elements  140 , which are themselves work elements provided to move the power machine over a support surface and an operator station  150  that provides an operating position for controlling the work elements of the power machine. A control system  160  is provided to interact with the other systems to perform various work tasks at least in part in response to control signals provided by an operator. 
     Certain work vehicles have work elements that can perform a dedicated task. For example, some work vehicles have a lift arm to which an implement such as a bucket is attached such as by a pinning arrangement. The work element, i.e., the lift arm can be manipulated to position the implement to perform the task. The implement, in some instances can be positioned relative to the work element, such as by rotating a bucket relative to a lift arm, to further position the implement. Under normal operation of such a work vehicle, the bucket is intended to be attached and under use. Such work vehicles may be able to accept other implements by disassembling the implement/work element combination and reassembling another implement in place of the original bucket. Other work vehicles, however, are intended to be used with a wide variety of implements and have an implement interface such as implement interface  170  shown in  FIG. 1 . At its most basic, implement interface  170  is a connection mechanism between the frame  110  or a work element  130  and an implement, which can be as simple as a connection point for attaching an implement directly to the frame  110  or a work element  130  or more complex, as discussed below. 
     On some power machines, implement interface  170  can include an implement carrier, which is a physical structure movably attached to a work element. The implement carrier has engagement features and locking features to accept and secure any of a number of different implements to the work element. One characteristic of such an implement carrier is that once an implement is attached to it, it is fixed to the implement (i.e. not movable with respect to the implement) and when the implement carrier is moved with respect to the work element, the implement moves with the implement carrier. The term implement carrier as used herein is not merely a pivotal connection point, but rather a dedicated device specifically intended to accept and be secured to various different implements. The implement carrier itself is mountable to a work element  130  such as a lift arm or the frame  110 . Implement interface  170  can also include one or more power sources for providing power to one or more work elements on an implement. Some power machines can have a plurality of work element with implement interfaces, each of which may, but need not, have an implement carrier for receiving implements. Some other power machines can have a work element with a plurality of implement interfaces so that a single work element can accept a plurality of implements simultaneously. Each of these implement interfaces can, but need not, have an implement carrier. 
     Frame  110  includes a physical structure that can support various other components that are attached thereto or positioned thereon. The frame  110  can include any number of individual components. Some power machines have frames that are rigid. That is, no part of the frame is movable with respect to another part of the frame. Other power machines have at least one portion that can move with respect to another portion of the frame. For example, excavators can have an upper frame portion that rotates with respect to a lower frame portion. Other work vehicles have articulated frames such that one portion of the frame pivots with respect to another portion for accomplishing steering functions. 
     Frame  110  supports the power source  120 , which is configured to provide power to one or more work elements  130  including the one or more tractive elements  140 , as well as, in some instances, providing power for use by an attached implement via implement interface  170 . Power from the power source  120  can be provided directly to any of the work elements  130 , tractive elements  140 , and implement interfaces  170 . Alternatively, power from the power source  120  can be provided to a control system  160 , which in turn selectively provides power to the elements that capable of using it to perform a work function. Power sources for power machines typically include an engine such as an internal combustion engine and a power conversion system such as a mechanical transmission or a hydraulic system that is configured to convert the output from an engine into a form of power that is usable by a work element. Other types of power sources can be incorporated into power machines, including electrical sources or a combination of power sources, known generally as hybrid power sources. 
       FIG. 1  shows a single work element designated as work element  130 , but various power machines can have any number of work elements. Work elements are typically attached to the frame of the power machine and movable with respect to the frame when performing a work task. In addition, tractive elements  140  are a special case of work element in that their work function is generally to move the power machine  100  over a support surface. Tractive elements  140  are shown separate from the work element  130  because many power machines have additional work elements besides tractive elements, although that is not always the case. Power machines can have any number of tractive elements, some or all of which can receive power from the power source  120  to propel the power machine  100 . Tractive elements can be, for example, track assemblies, wheels attached to an axle, and the like. Tractive elements can be mounted to the frame such that movement of the tractive element is limited to rotation about an axle (so that steering is accomplished by a skidding action) or, alternatively, pivotally mounted to the frame to accomplish steering by pivoting the tractive element with respect to the frame. 
     Power machine  100  includes an operator station  150  that includes an operating position from which an operator can control operation of the power machine. In some power machines, the operator station  150  is defined by an enclosed or partially enclosed cab. Some power machines on which the disclosed embodiments may be practiced may not have a cab or an operator compartment of the type described above. For example, a walk behind loader may not have a cab or an operator compartment, but rather an operating position that serves as an operator station from which the power machine is properly operated. More broadly, power machines other than work vehicles may have operator stations that are not necessarily similar to the operating positions and operator compartments referenced above. Further, some power machines such as power machine  100  and others, whether or not they have operator compartments or operator positions, may be capable of being operated remotely (i.e. from a remotely located operator station) instead of or in addition to an operator station adjacent or on the power machine. This can include applications where at least some of the operator-controlled functions of the power machine can be operated from an operating position associated with an implement that is coupled to the power machine. Alternatively, with some power machines, a remote-control device can be provided (i.e. remote from both the power machine and any implement to which it is coupled) that is capable of controlling at least some of the operator-controlled functions on the power machine. 
       FIGS. 2-3  illustrate a loader  200 , which is one particular example of a power machine of the type illustrated in  FIG. 1  where the embodiments discussed below can be advantageously employed. Loader  200  is a skid-steer loader, which is a loader that has tractive elements (in this case, four wheels) that are mounted to the frame of the loader via rigid axles. Here the phrase “rigid axles” refers to the fact that the skid-steer loader  200  does not have any tractive elements that can be rotated or steered to help the loader accomplish a turn. Instead, a skid-steer loader has a drive system that independently powers one or more tractive elements on each side of the loader so that by providing differing tractive signals to each side, the machine will tend to skid over a support surface. These varying signals can even include powering tractive element(s) on one side of the loader to move the loader in a forward direction and powering tractive element(s) on another side of the loader to mode the loader in a reverse direction so that the loader will turn about a radius centered within the footprint of the loader itself. The term “skid-steer” has traditionally referred to loaders that have skid steering as described above with wheels as tractive elements. However, it should be noted that many track loaders also accomplish turns via skidding and are technically skid-steer loaders, even though they do not have wheels. For the purposes of this discussion, unless noted otherwise, the term skid-steer should not be seen as limiting the scope of the discussion to those loaders with wheels as tractive elements. 
     Loader  200  is one particular example of the power machine  100  illustrated broadly in  FIG. 1  and discussed above. To that end, features of loader  200  described below include reference numbers that are generally similar to those used in  FIG. 1 . For example, loader  200  is described as having a frame  210 , just as power machine  100  has a frame  110 . Skid-steer loader  200  is described herein to provide a reference for understanding one environment on which the embodiments described below related to track assemblies and mounting elements for mounting the track assemblies to a power machine may be practiced. The loader  200  should not be considered limiting especially as to the description of features that loader  200  may have described herein that are not essential to the disclosed embodiments and thus may or may not be included in power machines other than loader  200  upon which the embodiments disclosed below may be advantageously practiced. Unless specifically noted otherwise, embodiments disclosed below can be practiced on a variety of power machines, with the loader  200  being only one of those power machines. For example, some or all of the concepts discussed below can be practiced on many other types of work vehicles such as various other loaders, excavators, trenchers, and dozers, to name but a few examples. 
     Loader  200  includes frame  210  that supports a power system  220 , the power system being capable of generating or otherwise providing power for operating various functions on the power machine. Power system  220  is shown in block diagram form but is located within the frame  210 . Frame  210  also supports a work element in the form of a lift arm assembly  230  that is powered by the power system  220  and that can perform various work tasks. As loader  200  is a work vehicle, frame  210  also supports a traction system  240 , which is also powered by power system  220  and can propel the power machine over a support surface. The lift arm assembly  230  in turn supports an implement interface  270 , which includes an implement carrier  272  that can receive and secure various implements to the loader  200  for performing various work tasks and power couplers  274 , to which an implement can be coupled for selectively providing power to an implement that might be connected to the loader. Power couplers  274  can provide sources of hydraulic or electric power or both. The loader  200  includes a cab  250  that defines an operator station  255  from which an operator can manipulate various control devices  260  to cause the power machine to perform various work functions. Cab  250  can be pivoted back about an axis that extends through mounts  254  to provide access to power system components as needed for maintenance and repair. 
     The operator station  255  includes an operator seat  258  and a plurality of operation input devices, including control levers  260  that an operator can manipulate to control various machine functions. Operator input devices can include buttons, switches, levers, sliders, pedals and the like that can be stand-alone devices such as hand operated levers or foot pedals or incorporated into hand grips or display panels, including programmable input devices. Actuation of operator input devices can generate signals in the form of electrical signals, hydraulic signals, and/or mechanical signals. Signals generated in response to operator input devices are provided to various components on the power machine for controlling various functions on the power machine. Among the functions that are controlled via operator input devices on power machine  100  include control of the tractive elements  219 , the lift arm assembly  230 , the implement carrier  272 , and providing signals to any implement that may be operably coupled to the implement. 
     Loaders can include human-machine interfaces including display devices that are provided in the cab  250  to give indications of information relatable to the operation of the power machines in a form that can be sensed by an operator, such as, for example audible and/or visual indications. Audible indications can be made in the form of buzzers, bells, and the like or via verbal communication. Visual indications can be made in the form of graphs, lights, icons, gauges, alphanumeric characters, and the like. Displays can be dedicated to providing dedicated indications, such as warning lights or gauges, or dynamic to provide programmable information, including programmable display devices such as monitors of various sizes and capabilities. Display devices can provide diagnostic information, troubleshooting information, instructional information, and various other types of information that assists an operator with operation of the power machine or an implement coupled to the power machine. Other information that may be useful for an operator can also be provided. Other power machines, such walk behind loaders may not have a cab nor an operator compartment, nor a seat. The operator position on such loaders is generally defined relative to a position where an operator is best suited to manipulate operator input devices. 
     Various power machines that can include and/or interacting with the embodiments discussed below can have various different frame components that support various work elements. The elements of frame  210  discussed herein are provided for illustrative purposes and frame  210  is not the only type of frame that a power machine on which the embodiments can be practiced can employ. Frame  210  of loader  200  includes an undercarriage or lower portion  211  of the frame and a mainframe or upper portion  212  of the frame that is supported by the undercarriage. The mainframe  212  of loader  200 , in some embodiments is attached to the undercarriage  211  such as with fasteners or by welding the undercarriage to the mainframe. Alternatively, the mainframe and undercarriage can be integrally formed. Mainframe  212  includes a pair of upright portions  214 A and  214 B located on either side and toward the rear of the mainframe that support lift arm assembly  230  and to which the lift arm assembly  230  is pivotally attached. The lift arm assembly  230  is illustratively pinned to each of the upright portions  214 A and  214 B. The combination of mounting features on the upright portions  214 A and  214 B and the lift arm assembly  230  and mounting hardware (including pins used to pin the lift arm assembly to the mainframe  212 ) are collectively referred to as joints  216 A and  216 B (one is located on each of the upright portions  214 ) for the purposes of this discussion. Joints  216 A and  216 B are aligned along an axis  218  so that the lift arm assembly is capable of pivoting, as discussed below, with respect to the frame  210  about axis  218 . Other power machines may not include upright portions on either side of the frame or may not have a lift arm assembly that is mountable to upright portions on either side and toward the rear of the frame. For example, some power machines may have a single arm, mounted to a single side of the power machine or to a front or rear end of the power machine. Other machines can have a plurality of work elements, including a plurality of lift arms, each of which is mounted to the machine in its own configuration. Frame  210  also supports a pair of tractive elements in the form of wheels  219 A-D on either side of the loader  200 . 
     The lift arm assembly  230  shown in  FIGS. 2-3  is one example of many different types of lift arm assemblies that can be attached to a power machine such as loader  200  or other power machines on which embodiments of the present discussion can be practiced. The lift arm assembly  230  is what is known as a vertical lift arm, meaning that the lift arm assembly  230  is moveable (i.e. the lift arm assembly can be raised and lowered) under control of the loader  200  with respect to the frame  210  along a lift path  237  that forms a generally vertical path. Other lift arm assemblies can have different geometries and can be coupled to the frame of a loader in various ways to provide lift paths that differ from the radial path of lift arm assembly  230 . For example, some lift paths on other loaders provide a radial lift path. Other lift arm assemblies can have an extendable or telescoping portion. Other power machines can have a plurality of lift arm assemblies attached to their frames, with each lift arm assembly being independent of the other(s). Unless specifically stated otherwise, none of the inventive concepts set forth in this discussion are limited by the type or number of lift arm assemblies that are coupled to a particular power machine. 
     The lift arm assembly  230  has a pair of lift arms  234  that are disposed on opposing sides of the frame  210 . A first end of each of the lift arms  234  is pivotally coupled to the power machine at joints  216  and a second end  232 B of each of the lift arms is positioned forward of the frame  210  when in a lowered position as shown in  FIG. 2 . Joints  216  are located toward a rear of the loader  200  so that the lift arms extend along the sides of the frame  210 . The lift path  237  is defined by the path of travel of the second end  232 B of the lift arms  234  as the lift arm assembly  230  is moved between a minimum and maximum height. 
     Each of the lift arms  234  has a first portion  234 A of each lift arm  234  is pivotally coupled to the frame  210  at one of the joints  216  and the second portion  234 B extends from its connection to the first portion  234 A to the second end  232 B of the lift arm assembly  230 . The lift arms  234  are each coupled to a cross member  236  that is attached to the first portions  234 A. Cross member  236  provides increased structural stability to the lift arm assembly  230 . A pair of actuators  238 , which on loader  200  are hydraulic cylinders configured to receive pressurized fluid from power system  220 , are pivotally coupled to both the frame  210  and the lift arms  234  at pivotable joints  238 A and  238 B, respectively, on either side of the loader  200 . The actuators  238  are sometimes referred to individually and collectively as lift cylinders. Actuation (i.e., extension and retraction) of the actuators  238  cause the lift arm assembly  230  to pivot about joints  216  and thereby be raised and lowered along a fixed path illustrated by arrow  237 . Each of a pair of control links  217  are pivotally mounted to the frame  210  and one of the lift arms  232  on either side of the frame  210 . The control links  217  help to define the fixed lift path of the lift arm assembly  230 . 
     Some lift arms, most notably lift arms on excavators but also possible on loaders, may have portions that are controllable to pivot with respect to another segment instead of moving in concert (i.e. along a pre-determined path) as is the case in the lift arm assembly  230  shown in  FIG. 2 . Some power machines have lift arm assemblies with a single lift arm, such as is known in excavators or even some loaders and other power machines. Other power machines can have a plurality of lift arm assemblies, each being independent of the other(s). 
     An implement interface  270  is provided proximal to a second end  232 B of the lift arm assembly  234 . The implement interface  270  includes an implement carrier  272  that is capable of accepting and securing a variety of different implements to the lift arm  230 . Such implements have a complementary machine interface that is configured to be engaged with the implement carrier  272 . The implement carrier  272  is pivotally mounted at the second end  232 B of the arm  234 . Implement carrier actuators  235  are operably coupled the lift arm assembly  230  and the implement carrier  272  and are operable to rotate the implement carrier with respect to the lift arm assembly. Implement carrier actuators  235  are illustratively hydraulic cylinders and often known as tilt cylinders. 
     By having an implement carrier capable of being attached to a plurality of different implements, changing from one implement to another can be accomplished with relative ease. For example, machines with implement carriers can provide an actuator between the implement carrier and the lift arm assembly, so that removing or attaching an implement does not involve removing or attaching an actuator from the implement or removing or attaching the implement from the lift arm assembly. The implement carrier  272  provides a mounting structure for easily attaching an implement to the lift arm (or other portion of a power machine) that a lift arm assembly without an implement carrier does not have. 
     Some power machines can have implements or implement like devices attached to it such as by being pinned to a lift arm with a tilt actuator also coupled directly to the implement or implement type structure. A common example of such an implement that is rotatably pinned to a lift arm is a bucket, with one or more tilt cylinders being attached to a bracket that is fixed directly onto the bucket such as by welding or with fasteners. Such a power machine does not have an implement carrier, but rather has a direct connection between a lift arm and an implement. 
     The implement interface  270  also includes an implement power source  274  available for connection to an implement on the lift arm assembly  230 . The implement power source  274  includes pressurized hydraulic fluid port to which an implement can be removably coupled. The pressurized hydraulic fluid port selectively provides pressurized hydraulic fluid for powering one or more functions or actuators on an implement. The implement power source can also include an electrical power source for powering electrical actuators and/or an electronic controller on an implement. The implement power source  274  also exemplarily includes electrical conduits that are in communication with a data bus on the excavator  200  to allow communication between a controller on an implement and electronic devices on the loader  200 . 
     Frame  210  supports and generally encloses the power system  220  so that the various components of the power system  220  are not visible in  FIGS. 2-3 .  FIG. 4  includes, among other things, a diagram of various components of the power system  220 . Power system  220  includes one or more power sources  222  that are capable of generating and/or storing power for use on various machine functions. On power machine  200 , the power system  220  includes an internal combustion engine. Other power machines can include electric generators, rechargeable batteries, various other power sources or any combination of power sources that can provide power for given power machine components. The power system  220  also includes a power conversion system  224 , which is operably coupled to the power source  222 . Power conversion system  224  is, in turn, coupled to one or more actuators  226 , which can perform a function on the power machine. Power conversion systems in various power machines can include various components, including mechanical transmissions, hydraulic systems, and the like. The power conversion system  224  of power machine  200  includes a pair of hydrostatic drive pumps  224 A and  224 B, which are selectively controllable to provide a power signal to drive motors  226 A and  226 B. The drive motors  226 A and  226 B in turn are each operably coupled to axles, with drive motor  226 A being coupled to axles  228 A and  228 B and drive motor  226 B being coupled to axles  228 C and  228 D. The axles  228 A-D are in turn coupled to tractive elements  219 A-D, respectively. The drive pumps  224 A and  224 B can be mechanically, hydraulic, and/or electrically coupled to operator input devices to receive actuation signals for controlling the drive pumps. Although not shown in  FIG. 4 , some machines have a hydraulic charge pump that provides flow for various hydraulic functions, including providing makeup flow to the hydrostatic drive circuits. 
     The arrangement of drive pumps, motors, and axles in power machine  200  is but one example of an arrangement of these components. As discussed above, power machine  200  is a skid-steer loader and thus tractive elements on each side of the power machine are controlled together via the output of a single hydraulic pump, either through a single drive motor as in power machine  200  or with individual drive motors. Various other configurations and combinations of drive pumps and motors can be employed as may be advantageous. 
     The power conversion system  224  of power machine  200  also includes a hydraulic implement pump  224 C, which is also operably coupled to the power source  222 . The hydraulic implement pump  224 C is operably coupled to work actuator circuit  238 C. Work actuator circuit  238 C includes lift cylinders  238  and tilt cylinders  235  as well as control logic to control actuation thereof. The control logic selectively allows, in response to operator inputs, for actuation of the lift cylinders and/or tilt cylinders. In some machines, the work actuator circuit also includes control logic to selectively provide a pressurized hydraulic fluid to an attached implement. The control logic of power machine  200  includes an open center, 3-spool valve in a series arrangement. The spools are arranged to give priority to the lift cylinders, then the tilt cylinders, and then pressurized fluid to an attached implement. 
     The description of power machine  100  and loader  200  above is provided for illustrative purposes, to provide illustrative environments on which the embodiments discussed below can be practiced. While the embodiments discussed can be practiced on a power machine such as is generally described by the power machine  100  shown in the block diagram of  FIG. 1  and more particularly on a loader such as track loader  200 , unless otherwise noted or recited, the concepts discussed below are not intended to be limited in their application to the environments specifically described above. 
       FIG. 5  illustrates aspects of a hydraulic drive system that can be used for traction control of a power machine, including as a configuration for a hydraulic drive system  246  of the power machine  200  of  FIGS. 2 and 3  (see, e.g.,  FIG. 4 ). As illustrated in  FIG. 5 , solid connecting lines indicate relatively high pressure hydraulic flow lines, dashed connecting lines represent relatively low pressure hydraulic flow lines, and dash-dot-dash lines indicate electronic signal lines. In some configurations, other connecting line arrangements are possible. For example, in some configurations, electronic control can be replaced with hydraulic control, and vice versa. 
     In the illustrated embodiment, the hydraulic drive system  346  includes a set of variable displacement hydrostatic drive pumps  324 A,  324 B, which are located within respective hydrostatic drive circuits  338 A,  338 B along with hydrostatic drive motors  326 A,  326 B. In some embodiments, the hydrostatic drive pumps  324 A,  324 B can be contained within a single housing, although other configurations are possible. The displacement of the drive pumps  324 A,  324 B can be controlled via respective swash plate actuators  362 A,  3626 , of any of a variety of known types, which can themselves be hydraulically actuated in order to move respective swash plates (not shown) of the drive pumps  324 A,  324 B. The actuators  362 A,  362 B in turn can be controlled via respective control valve assemblies  364 A,  364 B that can regulate relatively high pressure and low volume hydraulic flows and can be controlled by a control device  340 . In some embodiments, the illustrated system is configured so that high pressure, low volume hydraulic flows are flows at between 20 bar and 30 bar and between 5 L/min. and 15 L/min. (inclusive), with optimal performance in some cases at 25 bar and 10 L/min. 
     In some embodiments, the control device  340  is an electronic device. In other embodiments, the control device can be a mechanical device, an electromechanical device, an electrohydraulic device or any other suitable device. In some embodiments, the control valve assemblies  364 A and  364 B are servo control valves of any variety of known configurations, although other control valves can be used in other cases. 
     A hydraulic charge pump  348  is arranged to pump hydraulic fluid from a reservoir  356  along a supply flow path  330  of a hydraulic charge circuit  342  that also includes a hydraulic charge flow path  332 , to charge the hydrostatic drive circuits  338 A,  338 B. In particular, the hydraulic charge pump  348  provides an initial high pressure, high volume flow to a hydraulic load  358 , which can perform work using power from the flow and thereby reduce hydraulic pressure. In some embodiments, the hydraulic load  358  can be a fan motor for thermal management of the power machine, although other hydraulic loads (or no hydraulic loads) can be provided in other cases. 
     Downstream of the hydraulic load  358 , flow is then directed to a charge pressure relief valve  350  that establishes a predetermined minimum set pressure for supply of a charging flow to the drive circuits  338 A,  338 B. In the illustrated embodiment, drive system relief valves  352 A,  352 B are also provided, to set a maximum pressure in the hydrostatic circuits, such that high loads on a drive motor do not raise the pressure of the hydrostatic drive circuits  338 A,  338 B above the settings of the relief valves  352 A,  352 B. In some embodiments, the charge pressure relief valve  350  can set a pressure of between 5 bar and 15 bar (inclusive), with some optimal configurations having a set pressure of 10 bar. 
     In some embodiments, inclusion of a hydraulic load upstream of a charge pressure relief valve may be important because of the pressure drop imposed on the charge hydraulic flow by the hydraulic load. Due to this pressure drop, hydraulic pressure in a hydraulic charge circuit at a location upstream of the hydraulic load may be substantially higher (e.g., two or more times higher) than pressure at a location downstream of the hydraulic load (e.g., as set by the charge pressure relief valve). As further discussed below, this higher pressure can then be appropriately diverted for a higher pressure, lower flow signal for control of a hydrostatic drive pump. 
     In conventional systems, pressurized hydraulic flow for control of the displacement of the drive pumps  324 A,  324 B would be provided from the hydraulic charge circuit  342  downstream of the hydraulic load  358 , with the pressure level being set by a relief valve included in hydrostatic pump assembly. As also discussed above, to ensure appropriately high pressure for control of the drive-pump displacement, this type of conventional configuration also would provide the same high pressure flow to the hydrostatic circuit in the form of make-up fluid. As also noted above, because pressure at a level that may be required for control of drive-pump displacement is typically substantially larger than pressure required for make-up flow into a hydrostatic circuit, this conventional arrangement can result in notable inefficiency. 
     In contrast, in the illustrated embodiment, hydraulic flow for control of the displacement of the drive pumps  324 A,  324 B branches away from the hydraulic charge circuit  342  upstream of the hydraulic load  358  (and upstream of the hydraulic charge flow path  332 ). In particular, pressurized flow for operation of the swash plate actuators  362 A,  362 B is diverted from the hydraulic charge circuit  342  along a control flow path  344 . The control flow path  344  branches from hydraulic charge circuit  342  between the hydraulic load  358  and the outflow from the hydraulic charge pump  348  and routes the flow through a pilot supply valve  354  (e.g., a pressure reducing valve of a variety of known types). Accordingly, a relatively high pressure, low volume flow for the control valve assemblies  364 A,  364 B can be diverted from the hydraulic charge circuit  342  before the substantial pressure reduction imposed by the hydraulic load  358  (or another load, if present). Further, a relatively low pressure, high volume flow for charging the hydrostatic drive circuits  338 A,  338 B may continue from downstream of the hydraulic load  358 . 
     In some embodiments, a flow for charging a hydrostatic drive circuit can be controlled to be at substantially lower pressure (i.e., at a pressure that is reduced by 50% or more) relative to a flow for control of a variable displacement drive pump. As also noted above, in some embodiments, the illustrated system is configured so that the high pressure, low volume hydraulic flows along the control flow path  344  are flows at between 20 bar and 30 bar and between 5 L/min. and 15 L/min. (inclusive), with optimal performance in some cases at about 25 bar and 10 L/min. In contrast, in some embodiments, the illustrated system is configured so that the low pressure, high volume hydraulic flows to charge the hydrostatic drive circuits  338 A,  338 B are flows at between 5 bar and 15 bar and between 25 L/min. and 35 L/min. (inclusive), with optimal performance in some cases at about 10 bar and 30 L/min. However, other pressures and flow rates or combinations of pressures and flow rates are possible in other embodiments. 
     In some embodiments, the control flow path  344  may branch from the hydraulic charge circuit  342  within the physical package of the hydraulic charge pump  348  or the pilot supply valve  354  may be within the physical package of the hydraulic charge pump  38 . In some embodiments, the branch for the control flow path  344  or the pilot supply valve  354  may not be included as part of the physical package that houses the hydraulic charge pump  348  (i.e., may be external to the hydraulic charge pump). 
     Although a particularly useful configuration is illustrated in  FIG. 5 , a variety of other configurations can be used to provide similar benefits for power machines. For example, other configurations can include any variety of different types of supply valves, actuators for control of drive pump displacement, and control valve assemblies for control of those actuators. Further, although the illustrated embodiment provides unified control of flow from the hydraulic charge circuit  342  to the control valve assemblies  364 A,  364 B via the single control flow path  344  and the single supply valve  354 , some embodiments can include separate control flow paths or separate supply valves for each relevant control valve assembly. Similarly, the details of the hydrostatic drive circuits  338 A,  338 B and the components along the hydraulic charge circuit  342  for charging the circuits  338 A,  338 B (e.g., the relief valves  352 A,  352 B, the relief valve  350 , etc.) are provided as examples only, and the principles discussed above can be readily applied to power machines exhibiting differently arranged hydraulic drive circuits. 
     In some implementations, devices or systems disclosed herein can be implemented as methods embodying aspects of the invention. Correspondingly, description herein of particular features or capabilities of a device or system is generally intended to inherently include disclosure of a method of using such features for intended purposes and of implementing such capabilities. Similarly, express discussion of any method of using a particular device or system, unless otherwise indicated or limited, is intended to inherently include disclosure, as embodiments of the invention, of the utilized features and implemented capabilities of such device or system. 
     In this regard, methods of operation of the hydraulic drive system  346  and of the hydraulic charge circuit  342  for various power machines has been generally described above. However, a flowchart of a process for operating a hydrostatic drive circuit of a power machine (e.g., the loader  200 ) has been included to provide further details for certain embodiments. For example,  FIG. 6  shows an example of a flowchart of a method  400  for operating a hydrostatic drive circuit (e.g., the hydrostatic drive circuits  338 A,  338 B) of a hydraulic drive system (e.g., the hydraulic drive system  346 ) of a power machine, such as may be useful for hydraulic charging one or more hydrostatic drive circuits and controlling displacement of one or more drive pumps in communication with the hydrostatic drive circuit(s). 
     In particular, the method  400  can include operating  402  a hydraulic charge pump (e.g., the hydraulic charge pump  348 ) to provide hydraulic flow along a supply hydraulic flow path (e.g., the hydraulic flow path  342 ) of a hydraulic charge circuit. The method  400  can further include dividing  404  the hydraulic flow along the supply hydraulic flow path between at least two paths—e.g., a control flow path and a hydraulic charge flow path. The control flow path, which can branch away from the supply hydraulic flow path downstream of the hydraulic charge pump, is configured to control the displacement of a variable displacement drive pump. In contrast, the hydraulic charge flow path is configured to direct hydraulic flow to charge a hydrostatic drive circuit. In some cases, flow can be passively divided  404  by providing hydraulic flow lines to route flow. In some cases, flow can be more actively divided  404 , including through active control of one or more valves. 
     As generally noted above, the method  400  can also include, via the dividing  404  of hydraulic flow from the hydraulic charge pump, providing  406  a first hydraulic flow along a first flow path to control displacement of one or more variable displacement drive pumps. For example, the provided  406  first hydraulic flow can flow along a control flow path to a control assembly (e.g., the control valve assemblies  364 A,  364 B) that can be configured to control displacement of a variable displacement drive pump in a variety of known ways. In other words, the properties of the first hydraulic flow (e.g., the flow rate, the pressure, etc.), as controlled by one or more relevant valve assemblies, can be used to adjust the displacement of the variable displacement drive pump (e.g., by adjusting an orientation of a swash plate). In some cases, a control flow path can include a pilot supply valve (e.g., the single supply valve  354 ), which can be, for example, a pressure reducing value. In some cases, the control flow path can be directed to one or more valve and actuator assemblies (e.g., the control valve assemblies  364 A,  364 B and the swash plate actuators  362 A,  362 B) for control of pump displacement. 
     Continuing, the method  400  can also include, via the dividing  404  of hydraulic flow from the hydraulic charge pump, providing  408  a second hydraulic flow along a second flow path to hydraulically charge one or more hydrostatic drive circuits (e.g., the hydrostatic drive circuits  338 A,  338 B). For example, the provided  408  second hydraulic flow can flow along a hydraulic charge flow path to one or more inlets into one or more corresponding hydrostatic drive circuits. In some cases, the hydraulic charge flow path can include a hydraulic load (e.g., a fan motor), which can provide a substantial pressure drop for of the second hydraulic flow. In some configurations, the hydraulic charge flow path can include a charge pressure relief valve (e.g., the charge pressure relief valve  350 ) that can regulate the pressure at which the second hydraulic flow charges the hydrostatic drive circuit. 
     In some embodiments, the provided  406  first hydraulic flow can have a first pressure and a first flow rate, and the provided  408  second hydraulic flow can have a second pressure and a second flow rate. For example, as also discussed above, the first pressure of the first hydraulic flow can be higher than the second pressure of the second hydraulic flow, while the first flow rate of the first hydraulic flow rate can be lower than the second flow rate of the second hydraulic flow. This difference in pressures and flow rates can provide a higher efficiency for the hydraulic drive system. For example, optimized control of pump displacement may require relatively high pressure but relatively low flow, whereas optimized hydraulic charging of a hydrostatic drive circuit may require a relatively high flow rate but a relatively low pressure. In some embodiments, the first pressure can be in a range from 20 bar to 30 bar (e.g., 25 bar), the second pressure can be in a range from 5 bar to 15 bar (e.g., 10 bar), the first flow rate can be in a range from 5 L/min. to 15 L/min. (e.g., 10 L/min.), and the second flow rate can be in a range from 25 L/min. to 35 L/min (e.g., 30 L/min.). 
     Although the present invention has been described by referring preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the discussion.