Abstract:
A hydrostatic transmission system includes a pump including a rotatable swash plate, wherein rotation of the swashplate effects a change in volumetric displacement of the pump. An actuator is coupled to the swashplate, the actuator operative to effect rotation of the swashplate to change the volumetric displacement of the pump. An electronic controller is operatively coupled to the actuator, the electronic controller configured to command the actuator to effect rotation of the swashplate.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims priority of U.S. Provisional Application No. 61/979,291 filed on Apr. 14, 2013, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to hydrostatic transmissions, and more particularly to pump control systems for hydrostatic transmissions. 
       BACKGROUND 
       [0003]    Hydrostatic transmissions are well known and generally include a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor may be arranged as separate components or may be combined together in an integral unit. Axial swashplate type hydraulic piston pumps are frequently used in many such hydrostatic transmissions. Such pumps generate a pump action by causing pistons to reciprocate within a piston bore, with reciprocation of the pistons being caused by a swashplate that the pistons act against as a cylinder barrel containing the pistons rotates. Pump fluid output flow or displacement for each revolution of the barrel depends on the bore size and the piston stroke as well as the number of pistons that are utilized. The swashplate can pivot about a swashplate pivot center or axis, and the swashplate pivot angle determines the length of the piston stroke. By changing the swashplate angle, the pump displacement can be changed as is known in the art. 
         [0004]    With the swashplate at its extreme pivot angle relative to the axis of rotation of the barrel, a maximum fluid displacement is achieved. When the swashplate is centered at a right angle relative to the axis of rotation of the barrel, the pistons will not reciprocate and the displacement of the pump will be substantially zero. In some axial swashplate type piston pump designs, the swashplate has the capability of crossing over center which results in the pump displacement being generated at opposite ports. In an over center swashplate axial piston pump, each system port can be either an inlet or an outlet port depending on the pivot angle of the swashplate. Over center axial swashplate piston pumps are widely used in hydrostatic transmissions, to provide driving in both forward and reverse directions. 
         [0005]    One use for hydrostatic transmissions is zero turn vehicles such as zero turn lawn mowers. A separate over center swashplate axial piston pump may drive a hydraulic motor and wheel on each side of the vehicle. When the swash plate angles of the two pumps are equal and the output flow rotates the wheels in the same direction at the same speed, the vehicle travels in a substantially straight line path in either the forward or the reverse direction. When the swash plate angles of the two pumps are not equal and the output flow rotates the wheels in the same direction but at different speeds, the output flow rotates one wheel faster than the other so that the vehicle will turn. When one of the pumps is rotating its associated wheel in one direction and the other pump is rotating its associated wheel in the other direction, the vehicle will make a zero radius turn. An operator interface allows the vehicle operator to control the swashplate angles of the separate over center swashplate axial piston pumps, to control straight line or turning or zero radius turns for the vehicle. 
       SUMMARY OF INVENTION 
       [0006]    The present disclosure provides a system and method for controlling a hydraulic pump system. A swashplate type axial piston hydraulic pump may have a swashplate tiltable about a swashplate tilt axis, a barrel with axial pistons disposed in the barrel, the barrel and pistons being rotatable about a barrel rotation axis relative to the swashplate, the pistons each being moveable relative to the barrel along a straight line piston path, and the pistons having a stroke determined by the position of the swashplate. A fluid-powered actuator may be drivingly connected to the swashplate for displacing the swashplate about the swashplate tilt axis in response to fluid power provided thereto. An electrical controller may generate electrical command signals in response to controller inputs, and communicate such control signals to a fluid power control device. The fluid power control device is responsive to the control signals to vary fluid power provided to the actuator and thus change a tilt angle of the swashplate. 
         [0007]    According to one aspect of the invention, a pump control system includes: a pump including a swash plate tiltable about a swashplate tilt axis, wherein rotation of the swashplate changes the title angle and effects a change in volumetric displacement of the pump; an actuator drivingly coupled to the swashplate, the actuator operative to displace the swashplate about the tilt axis to change the volumetric displacement of the pump; and a fluid power control device operative to vary fluid power provided to the actuator in response to a control signal; and a controller operatively coupled to the fluid power control device, the controller configured to generate the control signal to modulate the fluid power provided by the fluid power control device to the actuator to effect rotation of the swashplate. 
         [0008]    According to one aspect of the invention, the system includes an input device operatively coupled to the controller, the input device operative to provide an input command corresponding to an output characteristic of a hydrostatic transmission, wherein the controller is configured to control an angular orientation of the swashplate based on the input command. 
         [0009]    According to one aspect of the invention, the system includes a sensor communicatively coupled to the controller, the sensor operative to detect an angular position of the swashplate and to provide the detected angular position to the controller. 
         [0010]    According to one aspect of the invention, the controller is configured to effect rotation of the swashplate independent of a user supplied command. 
         [0011]    According to one aspect of the invention, the fluid power control device comprises an electronically-operated valve. 
         [0012]    According to one aspect of the invention, the electronically-operated valve comprises a pressure control valve. 
         [0013]    According to one aspect of the invention, the actuator comprises a hydraulic actuator. 
         [0014]    According to one aspect of the invention, the hydraulic actuator comprises a linear actuator or a rotary actuator. 
         [0015]    According to one aspect of the invention, the actuator is directly coupled to the swashplate. 
         [0016]    According to one aspect of the invention, the actuator is indirectly coupled to the swashplate. 
         [0017]    According to one aspect of the invention, the actuator comprises a ball-screw actuator. 
         [0018]    According to one aspect of the invention, the system includes a prime-mover coupled to the pump and operative to provide mechanical power to the pump module. 
         [0019]    According to one aspect of the invention, the system includes a hydrostatic transmission. 
         [0020]    According to one aspect of the invention, a zero-turn lawn mower includes a prime mover, and a hydrostatic transmission having a swashplate control system as described herein. 
         [0021]    According to one aspect of the invention, the system includes a method for controlling volumetric displacement of a hydraulic pump having a swash plate tiltable about a swashplate tilt axis, wherein rotation of the swashplate changes the title angle and effects a change in volumetric displacement of the pump. An actuator is drivingly coupled to the swashplate, the actuator operative to displace the swashplate about the tilt axis to change the volumetric displacement of the pump. The method includes using an electronic controller to modulating hydraulic power provided to the actuator to effect rotation of the swashplate. 
         [0022]    According to one aspect of the invention, modulating hydraulic power includes using a fluid power control device to modulate fluid power to the actuator. 
         [0023]    According to one aspect of the invention, the method includes: receiving at the controller a user-initiated command corresponding to an output characteristic of the hydrostatic transmission; and controlling an angular orientation of the swashplate based on the user-initiated command. 
         [0024]    According to one aspect of the invention, the method includes: receiving at the controller position data corresponding to an angular orientation of the swashplate; and controlling an angular orientation of the swashplate based on the position data. 
         [0025]    According to one aspect of the invention, using the electronic controller to modulate hydraulic power provided to the actuator includes modulating pressure independent of a user supplied command. 
         [0026]    The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a perspective view of an exemplary zero-turn-radius mower employing a hydrostatic transmission to which the principles of the invention can be applied, as discussed below. 
           [0028]      FIG. 2  is a block diagram of an exemplary control system in accordance with aspects of the present invention. 
           [0029]      FIG. 3  is a schematic diagram of an exemplary control system in accordance with an embodiment of the present invention. 
           [0030]      FIG. 4  is a schematic diagram illustrating a fluid-powered rotary actuator that may be used in a control system in accordance with the present invention. 
           [0031]      FIG. 5  is a schematic diagram of another exemplary control system in accordance with an embodiment of the present invention. 
           [0032]      FIG. 6  is a block diagram illustrating an exemplary regulator that can be used to control swashplate position in accordance with the present invention. 
           [0033]      FIG. 7  is a perspective view of certain components of an exemplary pump that may be used in accordance with the present invention. 
           [0034]      FIG. 8  is an enlarged perspective view of certain other components of an exemplary pump that may be used in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Aspects of the present invention will now be described in the context of a hydrostatic transmission of a zero-turn-radius mower. It should be appreciated, however, that aspects of the invention are applicable to other applications in which a hydrostatic transmission is utilized. 
         [0036]    Referring now to the drawings in detail, and initially to  FIG. 1 , an exemplary zero-turn-radius mower  10  is illustrated. It is noted that the design of the illustrated mower  10  is merely exemplary in nature, and it will be appreciated that other mower designs and vehicle types can be used in accordance with the invention. 
         [0037]    The mower  10  includes a frame  12 , a mower deck  14  supported by the frame  12  for mowing grass, an operator seat  16 , and a plurality of controls  18  for operating the mower  10 . A rear mounted engine attached to the frame  12  behind the seat  16  provides power to left and right hydrostatic transmissions also mounted to the frame  12  (the engine and hydrostatic transmissions are not shown in  FIG. 1 ). As will be described in more detail below, each hydrostatic transmission includes a hydraulic pump having a swashplate, the swashplate operative to vary a volumetric displacement of the respective hydraulic pump. 
         [0038]    A controller  20  is attached to the frame  12  and preferably located in an enclosure or other protected area. In the embodiment shown in  FIG. 1  the controller  20  is located under the seat  16 , although other locations are contemplated. As will be described in more detail below, the controller  20  is operatively coupled to the plurality of controls  18  and to the swashplate of each hydrostatic transmission. Based on commands received from the controls  18 , the controller  20  can control the hydrostatic transmissions to independently drive respective rear wheels  22  to propel the mower and provide zero-turn-radius functionality. 
         [0039]    With reference to  FIG. 2 , a block diagram is provided illustrating the general architecture of a control system  30  in accordance with the present invention. More specifically, the system  30  includes the aforementioned controller  20 , which can include a processor for executing instructions and a storage device, such as memory, for storing instructions executable by the processor. Alternatively, the controller  20  may be in the form of a dedicated circuit, such as an application-specific integrated circuit (ASIC) or other custom circuit. 
         [0040]    The controller  20  is operatively coupled to a user interface module  32  (also referred to as an input device) to receive inputs for operating the mower  10 . Generally, the user interface module  32  converts operator commands into signals that can be read by the controller  20 . Thus, for example, the user interface module  32  can include the plurality of operator controls  18  and sensing devices operatively coupled thereto, the sensing devices operative to convert, for example, linear or rotary motion into signals readable by the controller  20  (e.g., analog voltage or current signals, digital signals, etc.). The signals provided to the controller  20  may correspond to a desired output characteristic of the hydrostatic transmission (e.g., speed, power, torque, swashplate position, etc.). 
         [0041]    Exemplary operator controls include a steering wheel, pedals, lap bars, joysticks and the like, while exemplary sensors include potentiometers, encoders, resolvers, and the like. The operator controls  18  may also include devices that provide binary on/off data, e.g., selector switches, pushbuttons and the like. Based on data received by the controller  20  from the user interface module  32 , the controller  20  generates a control signal for regulating a position of a swashplate of the hydrostatic transmission. 
         [0042]    A power module  34  provides fluid or electric power to the system. In some embodiments the power module  34  may be fluid power provided by a pump (e.g., pneumatic or hydraulic power). In other embodiments the power module  34  may provide electric power. Power provided by the power module  34  is provided to a regulator module  36 . 
         [0043]    The regulator module  36  receives the power provided from the power module  34  and the control signal from the controller  20 . Based on the control signal from the controller  20 , the regulator module  36  modulates the power (e.g., pressure or voltage) at its output and provides the modulated power to an actuator module  38 . The actuator module  38  includes an actuator, such as a pneumatic, hydraulic or electric actuator, which may be in the form of a linear or rotary actuator. Modulation of the power provided to the actuator module  38  produces a desired displacement of the actuator. 
         [0044]    A pump module  40  includes a hydraulic pump having a rotatable swashplate to vary displacement of the pump, the swashplate being operatively coupled to the actuator of the actuator module  38 . By virtue of the coupling between the actuator and the swashplate, displacement of the actuator also effects angular displacement of the swashplate. 
         [0045]    Accordingly, pump displacement (and thus power output by each hydrostatic transmission) is electronically controlled by the controller  20 . Such control by the controller  20  is advantageous in that it enables rotation of the swashplate independent of a user-supplied command. Independent control can be useful for implementing custom control modes for the mower  10 , such as cruise control, optimal implement speed control, four-wheel steering control, etc. 
         [0046]    With additional reference to  FIG. 3 , a schematic representation of a control system  50  in accordance with  FIG. 2  is shown for a system using hydraulically actuated swashplate. While a hydraulic system is illustrated in  FIG. 3 , it should be appreciated that other types of fluid power may be utilized without departing from the scope of the invention. For example, instead of hydraulic power the system may utilize pneumatic power. 
         [0047]    As shown in  FIG. 3  a hydrostatic transmission  52  includes a variable displacement hydraulic pump  54  for generating hydraulic power used by the hydrostatic transmission  52 . The hydraulic pump  54  may be driven by a prime mover  56 , such as an internal combustion engine, an electric motor or the like via drive system  58  (e.g., belt drive, chain drive, gear drive, etc.). Hydraulic power generated by the pump  54  is provided to a hydraulic motor  60  of the hydrostatic transmission  52  via ports, conduits and/or lines (not shown) within the hydrostatic transmission  52 . The hydraulic motor  60  converts the hydraulic power received from the pump  54  into rotational power, which is provided at the output shaft  62  for driving wheels  22 . 
         [0048]    The hydraulic pump  54  includes a rotatable swashplate  64 , where variation of the angular position of the swashplate  64  varies its tilt angle and thus displacement of the pump  54  (e.g., between a minimum displacement (e.g., approximately 0%) and a maximum displacement (e.g., 100%)). An angle sensor  66  monitors the swashplate  64  to detect an angular position of the swashplate  64 . The sensor  66  may be in the form of an encoder, a resolver, or other suitable sensor for detecting angular position or displacement. The sensor may directly monitor position of the swashplate  66 , or indirectly monitor the position of the swashplate (e.g., via a trunnion shaft). 
         [0049]    Operatively coupled to the swashplate  64  are first and second hydraulic cylinders  68  and  70 . The cylinders  68  and  70  may be indirectly coupled to the swashplate  64 . For example, the swashplate  64  may include a trunnion shaft  73  that effects rotation of the swashplate, the trunnion shaft being coupled to the cylinders  68  and  70  via arms  68   a  and  70   a . Alternatively, the cylinders  68  and  70  may be directly coupled to the swashplate  64 . Linear displacement of the first cylinder  68  effects rotation of the swashplate  64  in a first direction, and linear displacement of the second cylinder  70  effects rotation of the swashplate  64  in a second direction opposite from the first direction. 
         [0050]    The first and second cylinders  68  and  70  are in fluid communication with first and second fluid power control devices  72  and  74 , respectively. First and second fluid power control devices  72  and  74 , which in the present example are two-way valves, receive hydraulic power from a hydraulic power source  76 , such as a fixed-displacement pump driven by the prime mover  56 . While the exemplary embodiment utilizes two-way valves, other devices may be used, e.g., three-way valves. 
         [0051]    While linear actuators are described in the present embodiment, other types of actuators may be used without departing from the scope of the invention. For example, instead of linear actuators, rotary actuators may be utilized. Briefly,  FIG. 4  illustrates use of rotary actuators in a hydraulic system. The system is similar to the hydraulic portion of  FIG. 3 , except the first and second actuators  68  and  70  are replaced with a rotary hydraulic actuator  67 . In response to hydraulic power provided by the fluid power control devices  72  and  74  to the rotary actuator  67 , rotation of an output shaft  67   a  in a forward or reverse direction is achieved. The output shaft  67   a  may be directly coupled to the trunnion shaft  73  of the swashplate  64 , or optionally a gearbox  67   b  may be arranged between the output shaft  67   a  and the trunnion shaft  73 . 
         [0052]    Additionally, while not shown in  FIG. 3  the system can include an adjustment device to set a neutral position for the hydraulic actuators. The adjustment device is manipulated during a calibration procedure to return the cylinders  68  and  70  (or rotary actuator  67 ) to a neutral position and remain in that position during power loss. 
         [0053]    The controller  20  includes one or more outputs for providing control signals, status signals, etc. to other devices, such as the fluid power control devices  72  and  74 . For example, first and second outputs  78  and  80  of the controller are operatively coupled to the first and second fluid power control devices  72  and  74 , respectively, to provide first and second control signals (e.g., analog signals such as 0-10 VDC or 4-20 mA signals) to the respective fluid power control devices  72  and  74  that are proportional to a desired fluid flow through the fluid power control devices, or proportional to a desired fluid pressure at the output of the fluid power control devices. In this regard, 0 VDC (or 4 mA) may correspond to no fluid flow or no pressure, while 10 VDC (or 20 mA) may correspond to 100% fluid flow or 100% pressure. In this manner, the controller  20  can control the delivery of fluid power to the actuators  68  and  70 . While analog signals are described in the present example, other signal types may be utilized without departing from the scope of the invention. For example, instead of using outputs embodied as analog outputs, control signals may be communicated to the valves  72  and  74  (or other devices) via a communication bus (e.g., a network). The controller may include additional outputs that may be used by the system, such as wheel speed reference signals, implement speed reference signals, or any other parameter that may be controlled by the controller  20 . Such outputs may be used to provide enhanced control functions. 
         [0054]    The controller  20  includes one or more inputs for receiving data from other devices, such as the operator controls  18 . For example, the controller  20  includes a first input  82  for receiving an input command from a user-operated device, such as a speed command, a power command, a direction command, etc. For sake of clarity only one input is shown for the operator controls. It will be appreciated, however, that the controller  20  may have a plurality of inputs as needed for the respective operator controls. As discussed above, the user operated device may be coupled to a sensor  86  so as to convert linear or rotary motion into a signal readable by the controller  20 . The controller  20  also includes a second input  84  communicatively coupled to the angle sensor  66  for receiving data corresponding to an angular position of the swashplate  64 . The controller  20  may optionally include other inputs for detecting various parameters, such as, for example, power take off engaged/disengaged, prime mover speed, implement speed, wheel speed, or any other parameter that may be used by the controller  20 . The inputs may be analog inputs (e.g., 0-10 VDC, 4-20 mA, etc.), digital inputs, optical inputs, networks, or other conventional means for providing data to the controller  20 . 
         [0055]    Referring to  FIG. 5 , another embodiment of a control system  50 ′ in accordance with the present invention is illustrated. The system  50 ′ is similar to the system  50  of  FIG. 3 , except that electrically-operated actuators are used instead of hydraulically operated actuators. More particularly, the hydrostatic transmission  52  and its subcomponents (hydraulic pump  54  and swashplate  64 , hydraulic motor  60 ), the prime mover  56 , drive system  58 , angle sensor  66 , controller  20  and associated I/O are the same as those in the system of  FIG. 3 . Therefore, discussion of these components will be omitted for  FIG. 5 . 
         [0056]    The system  50 ′ includes first and second electrically-operated actuators  69  and  71  operatively coupled to the swashplate  64 . Stepper motors, servo motors, shape memory alloys and piezoelectric actuators are examples of electrically-operated actuators that may be used in accordance with the present invention. The electrically-operated actuators  69  and  71  may be indirectly coupled to the swashplate  64 . For example, the swashplate  64  may include a trunnion shaft  73  that effects rotation of the swashplate  64 , and the trunnion shaft may be coupled to the electrically-operated actuators  69  and  71  via arms  68   a  and  70   a . Alternatively, the electrically-operated actuators  69  and  71  may be directly coupled to the swashplate  64 . Linear displacement of the first electrically-operated actuator  69  effects rotation of the swashplate  64  in a first direction, and linear displacement of the second electrically-operated actuator  71  effects rotation of the swashplate  64  in a second direction opposite from the first direction. 
         [0057]    While linear electrically-operated actuators are described in the present embodiment, other types of electrically-operated actuators may be used without departing from the scope of the invention. For example, instead of linear actuators, rotary actuators may be utilized. In one embodiment, the linear actuator is a motor-driven ball-screw arrangement. 
         [0058]    The electrically-operated actuators  69  and  71  receive power from an electrical power source  77 . The electrical power source  77 , for example, may be an alternator or generator driven by the prime mover  56 . Alternatively, the electrical power source  77  may be a battery. 
         [0059]    The electrically-operated actuators  69  and  71  are operatively coupled to the controller  20  via outputs  78  and  80 . The outputs may be analog outputs that provide a voltage or current control signal as described with respect to the embodiment of  FIG. 3 , a communication network that provides digital control signals to the actuators, or any other means for communicating the control signals to the actuators  69  and  71 . Based on the control signals, the electrically operated actuators  69  and  71  rotate the swashplate  64  into any one of a number of different positions, and may be considered infinitely variable. 
         [0060]    Regardless of the form of the actuators (i.e., hydraulic or electric), the controller  20  includes logic configured to position the swashplate  64  so as to produce a desired characteristic from the hydrostatic transmission  52  (e.g., output power, output speed, output torque, etc.). The logic may be stored in memory of the controller  20  and executable by a processor of the controller  20 . The logic stored in the controller  20  may be configured to control the position of the swashplate  64  based on a user-command provided by the plurality of controls  18 . For example, the plurality of user-operated controls  18 , such as a foot-operated pedal, a hand-operated lever, or the like can be operatively coupled to a respective sensor  86  to provide a signal corresponding to displacement of the pedal or lever (or other device). The signal generated by the sensor  86  can be provided to the controller  20  via the first input  82 . The controller  20  can equate a low end of the signal range (e.g., 0 VDC, 4 mA) to a first angular position of the swashplate  64  corresponding to minimum pump displacement, and a high end of the signal range (e.g., 10 VDC, 20 mA) to a second angular position of the swashplate  64  corresponding to a maximum pump displacement. The user-input signal may be filtered and scaled as is conventional. 
         [0061]    The logic executed by the controller  20  may include a position regulator for controlling a position of the swashplate  64 . In this regard, the signal generated from the sensor  86  can be a “reference” position for the swashplate  64 , and the signal provided by the angle sensor  66  can be the “actual” position of the swashplate  64 . Based on a difference between the reference position and the actual position, the position regulator may generate a control signal, which may be filtered and scaled as is conventional. The control signal may be provided to one of the fluid power control device  72  and  74  (or to the electrically-operated actuators  69  and  71 ) via the outputs  78  and  80  of the controller  20 . In response to the control signal, the fluid power control devices  72  or  74  will alter the fluid flow and/or fluid pressure provided to the actuators  68  or  70 , thereby causing actuator displacement and effecting rotation of the swashplate  64 . Alternatively, in response to the control signal the electrically-operated actuators  69  and  71  will utilize the electrical power from the power source  77  to produce actuator displacement, thus effecting rotation of the swashplate  64 . 
         [0062]    With reference to  FIG. 6 , an exemplary position regulator  100  is illustrated in block form, the position regulator  100  being executable by the controller  20  to control an angular orientation of the swashplate  64 . Beginning at block  102 , the controller  20  receives the user input signal for controlling a feature of the hydrostatic transmission, e.g., output velocity. The user input signal may be a signal obtained from the user interface module  32 . For example, and as described herein, the user may manipulate an operator control  18 , which in turn causes a sensor  86  coupled to the operator control  18  to generate a signal. The signal, which may be an analog signal, a digital signal, an optical signal or any other signal readable by the controller  20 , preferably is proportional displacement of the respective operator control. The generated signal is read by the controller  20  via an input module corresponding to the type of signal (e.g., an analog voltage signal would be input via an analog voltage input). Next at block  104  the user input signal is optionally scaled and filtered to produce a signal corresponding to the regulated parameter. In the example shown in  FIG. 6 , the user input signal may be scaled to correspond to the feedback device coupled to the swashplate (i.e., sensor  66 ). In this regard, the user input signal could be scaled to correspond to swashplate angular orientation. Based on such scaling, the output of block  104  is a position reference signal and is provided to a positive input of summing junction  106 . 
         [0063]    As described herein, an angular position of the swashplate  64  is detected by sensor  66  and is provided to the controller  20  at block  108 . The sensor signal may be analog, digital, optical or any other signal type readable by the controller  20 . Next at block  110 , the position feedback signal is optionally scaled and filtered to correspond to the position reference signal, and the position feedback signal then is provided to a negative input of summing junction  106 . The output of the summing junction is an error signal indicative of the error between the desired position of the swashplate  64  and the actual position of the swashplate  64 . The error signal is provided to an input of controller  112 , which is shown as a proportional-plus-integral-plus-derivative (PID) controller, although other controllers may be used (e.g., a proportional controller, a proportional-plus-integral controller, etc.). 
         [0064]    Based on the error signal the controller  112  generates a control signal, which is output by the controller at block  114  and provided to the actuator (e.g., to one of the fluid power control devices  68  or  70  in  FIG. 3  or to one of the electrical actuators  69  and  71  in  FIG. 5 ). In response to the control signal, displacement of the actuator and thus of the swashplate  64  is effected. 
         [0065]    While the exemplary embodiment is described in the context of a position regulator, it should be appreciated that other regulation schemes may be employed without departing from the scope of the invention. For example, a speed regulator, torque regulator, power regulator, etc. may be used instead of or in conjunction with the position regulator. 
         [0066]    Referring now to  FIGS. 7-8 , each hydrostatic transmission  52  includes a conventional over center swashplate type axial piston hydraulic pump  54 . Pump  54  includes an input  120  that is drivingly connected to prime mover  56  to rotate a conventional pump barrel  122 . A plurality of axial pistons  124  are disposed within the pump barrel  122  and rotate with the pump barrel  122  about a barrel axis  126 . Pump  54  also includes a conventional over center swashplate  64  which is tiltable about a swashplate tilt axis  128 . The pistons  124  are each moveable relative to the barrel along a straight line piston path  130  that is substantially parallel to the barrel rotation axis  126 , and the pistons  124  have a stroke determined by the position of the swashplate  64 . When the swashplate  64  is in a neutral or center position perpendicular to the barrel axis 126 , the stroke of the pistons  124  is substantially zero and the output fluid flow displacement from the pump  54  is substantially zero. When the swashplate  64  begins to be displaced or titled in either direction about its tilt axis  128 , the stroke of pistons  124  begins to increase and output fluid flow displacement from the pump  54  begins. As the tilt angle of the swashplate  64  increases, the stroke of pistons 124  increases and the output fluid flow displacement from the pump  54  increases in a known manner. The output fluid flow displacement from pump  54  will be in one direction when the swashplate  64  is tilted in one direction from its neutral position and will be in the other direction when the swashplate  64  is tilted in the opposite direction. The output fluid flow from each pump  54  of each hydrostatic transmission flows through conduits (not shown) to a hydraulic motor  60  ( FIG. 3 ) of each hydrostatic transmission  52 , and such output flow rotates its associated hydraulic motor  60  to rotate its associated wheel  22  in the forward or reverse direction in a known manner. A reservoir  132  provides hydraulic fluid to the pump  54 , and a lever  134  opens and closes a fluid by-pass route (not shown) to enable pushing vehicle  10  when required. 
         [0067]    Although the invention has been shown and described with respect to a certain 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.