Patent Publication Number: US-10759476-B2

Title: System and method for executing straight tracking control of a work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/979,675, filed May 15, 2018, the disclosure of which is hereby incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present subject matter relates generally to work vehicles and, more particularly, to a system and method for executing straight tracking control of a work vehicle having a dual-path electronically controlled hydrostatic transmission to allow the work vehicle to be driven along a straight path when straight travel is commanded by the operator. 
     BACKGROUND OF THE INVENTION 
     Certain types of work vehicles, such as skid steer loaders, bulldozers, and other crawler-type vehicles, include a dual-path electronically controlled hydrostatic transmission that is used for propulsion and steering of the vehicle. Specifically, the hydrostatic transmission typically includes an independent hydraulic pump and motor to drive each of the vehicle&#39;s left and right side drive components (e.g., left and right side wheels/tires or left and right side drive wheels/tracks). For example, a first hydraulic pump and motor may be provided for rotationally driving the vehicle&#39;s left side wheels while a separate hydraulic pump and motor may be provided for rotationally driving the vehicle&#39;s right side wheels. As such, the rotational speed of the left and right side wheels may be independently controlled, thereby allowing the vehicle to be steered. For example, by rotationally driving the left side wheels faster than the right side wheels, the work vehicle may be steered to the right. Similarly, by rotationally driving the right side wheels faster than the left side wheels, the work vehicle may be steered to the left. 
     Additionally, the operator of the work vehicle is typically provided with one or more input devices within the vehicle&#39;s cab, such as a left joystick and a right joystick, to provide independent control commands for regulating the drive speed of the left and right side drive components. For example, the operator may utilize the left joystick to provide suitable joystick commands for controlling the drive speed of the left side wheels and may utilize the right joystick to provide suitable joystick commands for controlling the drive speed of the right side wheels. In this regard, the operator may steer the work vehicle as it is being driven by adjusting the relative positioning of the left and right joysticks, thereby adjusting the relative drive speed of the left and right side wheels, respectively. Similarly, to drive the work vehicle along a straight path, the operator may move the left and right joysticks from their neutral position to the same forward or reverse position to provide equal joystick commands associated with driving the left and right side wheels at the same speed. 
     However, even when the operator commands that the work vehicle be driven along a straight path (e.g., by providing equal inputs via the left and right joysticks), the vehicle may veer to the left or right due to one or more factors, such as manufacturing tolerances, hydraulic driveline inefficiencies, “wear and tear” of the pump and motor components, loading conditions, differing tire sizes, etc. For instance, differences in the operating efficiencies between the left and right side pumps/motors may result in differing left/right side drive outputs despite the operator providing equal inputs into the system, thereby causing the vehicle to veer to the left or right. Similarly, even with equal drive outputs from the left and right side pumps/motors, the work vehicle may still veer to the left or right due to varying tire diameters between the left and right side tires (e.g., due to tire manufacturing tolerances and/or due to differing tire pressures). In this regard, prior art systems have been proposed that attempt to address this issue by allowing the drive output to the left and/or right side wheels to be adjusted, as necessary, to permit the work vehicle to track straight when so commanded by the operator. For example, known prior art systems allow an operator to manually input a trim adjustment that is directly applied to the joystick commands provided by the operator. However, despite such prior art systems, a need still exists for more efficient and/or effective systems for providing straight tracking control for a work vehicle having a dual-path, electronically controlled hydrostatic transmission. 
     Accordingly, an improved system and method for executing straight tracking control of a work vehicle to allow the vehicle to be driven along a straight path when so commanded by the operator would be welcomed in the technology. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one aspect, the present subject matter is directed to a computer-implemented method for executing straight tracking control of a work vehicle having an electronically controlled hydrostatic transmission. The method may generally include receiving, with a computing device, at least one operator input command associated with controlling an operation of at least one of a first-side drive system or a second-side drive system of the hydrostatic transmission, with the at least one operator input command being associated with commanding the work vehicle to be driven along a straight path. The method may also include receiving, with the computing device, a first speed signal associated with a first output speed of the first-side drive system and a second speed signal associated with a second output speed of the second-side drive system, and modifying, with the computing device, one of the first speed signal or the second speed signal based on a speed scaling factor to generate a corrected speed signal. In addition, the method may include determining, with the computing device, an adjusted control command for controlling the operation of the hydrostatic transmission as a function of the at least one operator input command and a control output determined based at least in part on a differential between the corrected speed signal and the other of the first speed signal or the second speed signal. Further, the method may include controlling, with the computing device, the operation of the hydrostatic transmission based at least in part on the adjusted control command to drive the work vehicle along the straight path. 
     In another aspect, the present subject matter is directed to a system for executing straight tracking control of a work vehicle. The system may generally include a hydrostatic transmission having a first-side drive system and a second-side drive system, with the first side-drive system being configured to rotationally drive at least one corresponding drive output component located on a first side of the work vehicle, and the second-side drive system being configured to rotationally drive at least one corresponding drive output component located on a second side of the work vehicle. The system may also include a first speed sensor configured to detect a first output speed associated with the first-side drive system, and a second speed sensor configured to detect a second output speed associated with the second-side drive system. In addition, the system may include a controller communicatively coupled to the first and second speed sensors, with the controller including a processor and related memory. The memory may store instructions that, when implemented by the processor, configure the controller to receive at least one operator input command associated with controlling the operation of the first-side drive system or the second-side drive system, with the at least one operator input command being associated with commanding the work vehicle to be driven along a straight path. The controller may also be configured to receive a first speed signal from the first speed sensor associated with the first output speed of the first-side drive system and a second speed signal from the second speed sensor associated with the second output speed of the second-side drive system. In addition, the controller may be configured to modify one of the first speed signal or the second speed signal based on a speed scaling factor to generate a corrected speed signal. Moreover, the controller may be configured to determine an adjusted control command for controlling the operation of the hydrostatic transmission as a function of the at least one operator input command and a control output determined based at least in part on a differential between the corrected speed signal and the other of the first speed signal or the second speed signal. Furthermore, the controller may be configured to control the operation of the hydrostatic transmission based at least in part on the adjusted control command to drive the work vehicle along the straight path. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  illustrates a side view of one embodiment of a work vehicle in accordance with aspects of the present subject matter; 
         FIG. 2  illustrates a schematic view of various components of the work vehicle shown in  FIG. 1 , particularly illustrating one embodiment of a hydrostatic transmission of the work vehicle in accordance with aspects of the present subject matter; 
         FIG. 3  illustrates a schematic view of one embodiment of a suitable control system for controlling various components of a work vehicle in accordance with aspects of the present subject matter, particularly illustrating the control system configured for controlling components of the hydrostatic transmission of the work vehicle; 
         FIG. 4  illustrates a schematic view of a specific implementation of the control system described above with reference to  FIG. 3  in accordance with aspects of the present subject matter, particularly illustrating an exemplary embodiment of the system configured as a straight tracking control system for electronically controlling the operation of the vehicle&#39;s hydrostatic transmission when the operator has commanded that the work vehicle be driven along a straight path; 
         FIG. 5  illustrates a flow diagram providing control logic for executing one embodiment of the straight tracking control described above with reference to  FIG. 4  in accordance with aspects of the present subject matter; 
         FIG. 6  illustrates a flow diagram of one embodiment of a methodology for executing a calibration routine to calculate a speed scaling factor for use during straight tracking control of a work vehicle in accordance with aspects of the present subject matter; 
         FIG. 7  illustrates a flow diagram of another embodiment of a methodology for executing a calibration routine to calculate a speed scaling factor for use during straight tracking control of a work vehicle in accordance with aspects of the present subject matter; 
         FIG. 8  illustrates a flow diagram of a further embodiment of a methodology for executing a calibration routine to calculate a speed scaling factor for use during straight tracking control of a work vehicle in accordance with aspects of the present subject matter; 
         FIG. 9  illustrates a flow diagram of one embodiment of a methodology for allowing an operator to manually adjust a speed scaling factor used during straight tracking control of a work vehicle in accordance with aspects of the present subject matter; and 
         FIG. 10  illustrates a flow diagram of one embodiment of a method for executing straight tracking control of a work vehicle having an electronically controlled hydrostatic transmission in accordance with aspects of the present subject matter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     In general, the present subject matter is directed to improved systems and methods for executing straight tracking control of a work vehicle to allow the vehicle to be driven along a straight path when so commanded by the operator. Specifically, in several embodiments, the present subject matter is applicable to work vehicles having independently controlled drive systems for separately controlling the drive speed of the vehicle along each of its sides, such as any vehicle including a dual-path hydrostatic transmission. As will be described below, a controller of the disclosed system may be configured to utilize a speed scaling factor to modify a corresponding speed signal associated with the output speed of either the left-side drive system or the right-side drive system of the vehicle. In such instance, the speed scaling factor may be selected or set to account for any differences between the left-side and right-side drive systems that may otherwise result in the work vehicle veering to the left or right when straight travel is commanded, such as manufacturing tolerances for the vehicle&#39;s tires, differing tire pressures, misalignments between the wheel assemblies, load imbalances, and/or the like. The modified or corrected speed signal determined based on the speed scaling factor may then be used to determine a speed error between the left-side and right-side drive systems, which may then be input into a closed-loop control algorithm to determine a control output for modifying the input command(s) received from the operator (e.g., the joystick command(s)) in a manner that allows the work vehicle to track straight when commanded. 
     Referring now to the drawings,  FIGS. 1 and 2  illustrate different views of one embodiment of a work vehicle  10 . Specifically,  FIG. 1  illustrates a side view of the work vehicle  10  and  FIG. 2  illustrates a schematic view of various components of the work vehicle  10  shown in  FIG. 1 . As shown, the work vehicle  10  is configured as a skid steer loader. However, in other embodiments, the work vehicle  10  may be configured as any other suitable work vehicle known in the art that includes independently controlled drive systems for separately controlling the drive speed of the vehicle along each of its sides, such as any vehicle including a dual-path hydrostatic transmission. 
     As shown, the work vehicle  10  includes a pair of front wheels  12 ,  14  (and associated front tires  15 ), a pair of rear wheels  16 ,  18  (and associated rear tires  19 ), and a chassis  20  coupled to and supported by the wheels  12 ,  14 ,  16 ,  18 . An operator&#39;s cab  22  may be supported by a portion of the chassis  20  and may house various input devices, such as one or more speed/steering control levers or joysticks  24  and one or more lift/tilt levers or joysticks  25 , for permitting an operator to control the operation of the work vehicle  10 . In addition, the work vehicle  10  may include an engine  26  and a dual-path hydrostatic transmission  28  coupled to or otherwise supported by the chassis  20 . 
     Moreover, as shown in  FIG. 1 , the work vehicle  10  may include a pair of loader arms  30  (one of which is shown) coupled between the chassis  20  and a suitable implement  32  (e.g., a bucket, fork, blade and/or the like). Hydraulic cylinders  34 ,  35  may also be coupled between the chassis  20  and the loader arms  30  and between the loader arms  30  and the implement  32  to allow the implement  32  to be raised/lowered and/or pivoted relative to the ground. For example, a lift cylinder  34  may be coupled between the chassis  20  and each loader arm  30  for raising and lowering the loader arms  30 , thereby controlling the height of the implement  32  relative to the ground. Additionally, a tilt cylinder  35  may be coupled between each loader arm  30  and the implement  32  for pivoting the implement  32  relative to the loader arms  30 , thereby controlling the tilt or pivot angle of the implement  32  relative to the ground. 
     As particularly shown in  FIG. 2 , the hydrostatic drive unit  28  of the work vehicle  10  may include a pair of independently controlled hydraulic drive systems (e.g., a first or left-side drive system  36  and a second or right-side drive system  38 ), with each drive system  36 ,  38  being configured to separately drive either the left-side  12 ,  16  or the right-side wheels  14 ,  18  of the work vehicle  10  via an associated hydraulic pump/motor. For example, as shown in the illustrated embodiment, the left-side drive system  36  includes a first or left hydraulic motor  40  configured to drive the left-side wheels  12 ,  16  via front and rear axles  41 ,  43 , respectively, while the right-side drive system  38  includes a second or right hydraulic motor  42  configured to drive the right-side wheels  14 ,  18  via front and rear axles  41 ,  43 , respectively. Alternatively, the motors  40 ,  42  may be configured to drive the wheels  12 ,  14 ,  16 ,  18  using any other suitable means known in the art, such as by coupling each motor  40 ,  42  to its respective pair of wheels via a suitable sprocket/chain arrangement (not shown). Additionally, each drive system  36 ,  38  may also include a separate hydraulic pump driven by the engine  26 , which, in turn, supplies pressurized fluid to its respective motor. For example, in the illustrated embodiment, the left-side drive system  36  includes a first or left hydraulic pump  44  fluidly connected to the left motor  40  (e.g., via a suitable hydraulic hose or other fluid coupling  48 ) while the right-side drive system  38  includes a second or right hydraulic pump  46  fluidly connected to the right motor  40  (e.g., via a suitable hydraulic hose or other fluid coupling  48 ). 
     In the illustrated embodiment, by individually controlling the operation of each pump  44 ,  46 , the speed of the left-side wheels  12 ,  16  may be regulated independent of the right-side wheels  14 ,  18 . For example, each pump  44 ,  46  may include or be associated with one or more electronically controlled actuators (e.g., forward and reverse solenoids coupled to a control piston) configured to adjust the position of its associated swashplate, thereby allowing the displacement of the pump  44 ,  46  to be automatically adjusted. As such, by adjusting the displacement of the left pump  44 , the flow of the hydraulic fluid supplied to the left motor  40  may be varied, thereby adjusting the output speed of the motor  40  and, thus, the associated drive speed of the left-side wheels  12 ,  16 . Similarly, by adjusting the displacement of the right pump  46 , the flow of the hydraulic fluid supplied to the right motor  42  may be varied, thereby adjusting the output speed of the motor  42  and, thus, the associated drive speed of the right-side wheels  14 ,  18 . 
     It should be appreciated that the configuration of the work vehicle  10  described above and shown in  FIGS. 1 and 2  is provided only to place the present subject matter in an exemplary field of use. Thus, it should be appreciated that the present subject matter may be readily adaptable to any manner of work vehicle configuration. For example, in an alternative embodiment, the work vehicle  10  may be configured as a tracked vehicle and, thus, may include tracks as opposed to wheels/tires. In such an embodiment, each motor  40 ,  42  may be configured to rotationally drive an associated drive wheel of the tracked vehicle that, in turn, is coupled to and drives an endless track. Accordingly, it should be appreciated that any suitable drive output component(s) (including wheels/tires or a drive wheel/track) may be included within or otherwise associated with each of the left-side and right-side drive systems  36 ,  38  for propelling and/or steering the work vehicle  10 . 
     Referring now to  FIG. 3 , one embodiment of a control system  100  suitable for controlling the various components of a work vehicle is illustrated in accordance with aspects of the present subject matter. In general, the control system  100  will be described herein with reference to the work vehicle  10  described above with reference to  FIGS. 1 and 2 . However, it should be appreciated by those of ordinary skill in the art that the disclosed system  100  may generally be utilized to control one or more components of any suitable work vehicle. 
     As shown, the control system  100  includes a controller  102  configured to electronically control the operation of one or more components of the work vehicle  10 , such as the various hydraulic components of the work vehicle  10  (e.g., the hydrostatic transmission  28 ). In general, the controller  102  may comprise any suitable processor-based device known in the art, such a computing device or any suitable combination of computing devices. Thus, in several embodiments, the controller  102  may include one or more processor(s)  104  and associated memory device(s)  106  configured to perform a variety of computer-implemented functions. As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. Additionally, the memory device(s)  106  of the controller  102  may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)  106  may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)  104 , configure the controller  102  to perform various computer-implemented functions, such as the various aspects of the calibration routines and/or other control methodologies described herein. In addition, the controller  102  may also include various other suitable components, such as a communications circuit or module, one or more input/output channels, a data/control bus and/or the like. 
     It should be appreciated that the controller  102  may correspond to an existing controller of the work vehicle  10  or the controller  102  may correspond to a separate processing device. For instance, in one embodiment, the controller  102  may form all or part of a separate plug-in module that may be installed within the work vehicle  10  to allow for the disclosed system and method to be implemented without requiring additional software to be uploaded onto existing control devices of the vehicle  10 . 
     As shown in  FIG. 3 , the controller  102  may be communicatively coupled to various components for controlling the operation of the hydraulic pumps  44 ,  46  (and, thus, the hydraulic motors  40 ,  42 ) of the left-side and right-side drive systems  36 ,  38  of the hydrostatic transmission  28 . For purposes of description, the various components for controlling the operation of the left hydraulic pump  44  and the associated left hydraulic motor  40  will be described below in detail with reference to  FIG. 3 . However, it should be appreciated that the components for controlling the operation of the right hydraulic pump  46  and the right hydraulic motor  42  may function the same or similar as those for the left/pump/motor to provide the desired operation. 
     As indicated above, the hydraulic pump  44  may be driven by the engine  26  and may be fluidly connected to the hydraulic motor  40  via suitable fluid couplings  48  (e.g., hydraulic hoses). The hydraulic motor  40  may, in turn, drive the left-side wheels  12 ,  16  of the vehicle  10 . In several embodiments, the motor  40  may be configured as a fixed displacement motor while the hydraulic pump  44  may be configured as a variable displacement pump. Accordingly, to change the rotational speed of the motor  40  (and, thus, the rotational speed of the wheels  12 ,  16 ), the displacement of the hydraulic pump  44  may be varied by adjusting the position or angle of a swashplate (indicated by the arrow  108 ) of the pump  44 , thereby adjusting the flow of hydraulic fluid to the motor  40 . 
     To electronically control the displacement of the swashplate  108 , the controller  102  may be commutatively coupled to suitable pressurize regulating valves  110 ,  112  (PRVs) (e.g., solenoid-activated valves) configured to regulate the pressure of hydraulic fluid supplied to a control piston  114  of the pump  44 . Specifically, as shown schematically in  FIG. 3 , the controller  102  may be coupled to both a forward PRV  110  configured to regulate the pressure of the hydraulic fluid supplied to a forward chamber  116  of the control piston  114  and a reverse PRV  112  configured to regulate the pressure of the hydraulic fluid supplied to a reverse chamber  118  of the control piston  114 . By pressurizing the forward chamber  116 , the swashplate  108  of the pump  44  may be displaced such that hydraulic fluid flows through the fluid loop defined between the pump/motor  44 ,  40  in a manner that causes the motor  40  to drive the wheels  12 ,  16  in the forward direction. Similarly, by pressurizing the reverse chamber  118 , the swashplate  108  may be displaced such that hydraulic fluid flows through the fluid loop in a manner that causes the motor  40  to drive the wheels  12 ,  16  in the reverse direction. 
     As is generally understood, the current supplied to each PRV  110 ,  112  is directly proportional to the pressure supplied to its corresponding chamber  116 ,  118 , the pressure difference of which is, in turn, directly proportional to the displacement of the swashplate  108 . Thus, for example, by increasing the current command to the forward PRV  110  by a given amount, the pressure within the forward chamber  116  and, thus, the angle of the swashplate  108  may be increased by a proportional amount(s). As the angle of the swashplate  108  is increased, the flow of hydraulic fluid supplied to motor  40  is similarly increased, thereby resulting in an increase in the rotational speed of the wheels  12 ,  16  in the forward direction. A similar control strategy may be used to increase the rotational speed of the wheels  12 ,  16  in the reverse direction by increasing the current command supplied to the reverse PRV  112 . 
     As indicated above, the various components associated with the right-side pump/motor  46 ,  42  may function or operate the same as or similar to the corresponding components associated with the left-side pump/motor  44 ,  40 . For example, as shown in  FIG. 3 , the controller  102  may be communicatively coupled to respective forward and reverse PRVs  120 ,  122  (e.g., solenoid-activated valves) for regulating the pressure of hydraulic fluid supplied to forward and reverse chambers  124 ,  126 , respectively, of an associated control piston  128  of the right hydraulic pump  46 . By pressurizing the forward chamber  124  (or the reverse chamber  126 ), a swashplate (indicated by arrow  129 ) of the right hydraulic pump  46  may be displaced such that hydraulic fluid flows through the fluid loop defined between the pump  46  and associated motor  42  in a manner that causes the motor  42  to drive the right-side wheels  14 ,  18  in the forward direction (or the reverse direction). As such, by adjusting the current command to the forward PRV  120  (or the reverse PRV  122 ) by a given amount, the pressure within the forward chamber  124  (or the reverse chamber  126 ) and, thus, the angle of the swashplate  129  may be adjusted by a proportional amount(s), which may, in turn, adjust the flow of hydraulic fluid supplied to motor  42 , thereby resulting in the rotational speed of the wheels  14 ,  18  being varied. 
     Additionally, as shown in  FIG. 3 , the controller  102  may be communicatively coupled to one or more input devices  130  for receiving input commands from the operator. For example, as indicated above, the controller  102  may be communicatively coupled to one or more joysticks  104  (e.g., speed/steering joystick(s)  24  shown in  FIG. 1 ) to allow the operator to provide speed/steering inputs to the controller  102  for controlling the operation of the left-side and right-side drive systems  36 ,  38 . Specifically, in one embodiment, the work vehicle  10  may include both a left joystick for allowing the operator to provide input commands (e.g., left joystick commands) for controlling the operation of the left-side drive system  36  and a right joystick (or other suitable input device) for allowing the operator to provide input commands (e.g., right joystick commands) for controlling the operation of the right-side drive system  38 . Alternatively, the work vehicle  10  may include a single speed/steering joystick for providing input commands (e.g., both left and right joystick commands) for controlling the operation of the left-side and right-side drive systems  36 ,  38 . Regardless, based on the left and right joystick commands received from the operator, the controller  102  may, in turn, be configured to provide suitable pump commands (e.g., in the form of current commands to the associated PRVs  110 ,  112 ,  120 ,  122 ) for controlling the operation of each pump  44 ,  46 . For example, if the left and right joystick commands received from the operator indicate that the operator desires to travel along a straight path (e.g., when equal inputs are received from the left and right joysticks or when the single joystick is pushed straight forward or backwards), the controller  102  may be configured to transmit suitable pump commands for controlling the left-side and right-side drive systems  36 ,  38  in a manner that results in straight travel of the work vehicle  10 . Similarly, if the left and right joystick commands received from the operator indicate that the operator desires to steer left or right (e.g., when the inputs received from the left and right joysticks differ or when the single joystick is moved to the left or the right), the controller  102  may be configured to transmit suitable pump commands for controlling the left-side and right-side drive systems  36 ,  38  in a manner that results in the work vehicle  10  being steered in the desired direction. 
     In addition, the work vehicle  10  may also include various other input devices  134  for allowing the operator to provide inputs to the controller  102 . For example, various other input devices  134  may be located within the vehicle&#39;s cab  22 , such as buttons, knobs, levers, display panels (e.g., touch screen panels), and/or the like provided along the vehicle&#39;s control panel or otherwise positioned at an accessible location with the cab  22 . Such input devices  134  may also include buttons or other interface elements provided in association with other input devices, such as when the joystick(s)  132  is configured as a multi-function joystick or lever and includes various different buttons or elements associated therewith. In addition, the input devices  134  may also include auxiliary input devices that allow the operator (including service technicians) to interface with the controller  102 , such as laptops, service tools, and/or other devices that may be coupled to the controller  102  via a suitable port or other I/O channel or means associated with the work vehicle  10 . 
     In addition, the controller  102  may also be communicatively coupled to one or more sensors for monitoring one or more operating parameters of the work vehicle  10 , such as one or more sensors for monitoring the operation of the hydrostatic transmission  28 . For example, as shown in  FIG. 3 , the controller  102  may be communicatively coupled to one or more speed sensors  136 ,  138  for monitoring an output speed of each of the drive systems  36 ,  38 . Specifically, in the illustrated embodiment, the controller  102  is coupled to a left-side speed sensor  136  for monitoring the output speed of the left-side drive system  36 , such as by monitoring the output speed of the left hydraulic motor  40 . Additionally, the controller  102  is also coupled to a right-side speed sensor  138  for monitoring the output speed of the right-side drive system  38 , such as by monitoring the output speed of the right hydraulic motor  42 . As will be described below, the speed signals received from the left-side and right-side speed sensors  136 ,  138  may be utilized by the controller to execute a straight tracking control system to allow the work vehicle  10  to be driven in a straight line when so commanded by the operator, such as by using the speed signals as feedback into a closed-loop control algorithm for calculating a control output for modifying one or both of the joystick commands received from the operator. 
     It should be appreciated that the system configuration shown in  FIG. 3  is simply illustrated to provide one example of a suitable system configuration in accordance with aspects of the present subject matter. In other embodiments, the system configuration may have any other suitable configuration. For instance, in one embodiment, the hydraulic motors may also correspond to electronically controlled, variable displacement motors. In such an embodiment, the output speed of each drive system  36 ,  38  may be controlled by adjusting the displacement of the pumps and/or the motors. Similarly, in another embodiment, the hydraulic pumps may include swash angle sensors to provide closed-loop feedback to the controller for controlling the PRVs. In such an embodiment, the controller may configured to regulate the operation of the PRVs to achieve a desired swash angle(s) based on the feedback provided by the sensors. 
     Referring now to  FIG. 4 , a schematic view of a specific implementation of the control system  100  described above with reference to  FIG. 3  is illustrated in accordance with aspects of the present subject matter, particularly illustrating an exemplary embodiment of the system  100  configured as a straight tracking control system  200  for electronically controlling the operation of the vehicle&#39;s hydrostatic transmission  28  when the operator has commanded that the work vehicle  10  be driven along a straight path. It should be appreciated that, although aspects of the embodiment of the straight tracking control system  200  shown in  FIG. 4  will be described as being implemented by or forming part of the controller  102  described above with reference to  FIG. 4 , such aspects may, instead, by implemented by or form part of a separate controller or control module that may be communicatively coupled to controller  102  (e.g., as part of distributed computing system or network) or that is configured as a “stand-alone” control system or sub-system. 
     In general, the control system  200  may be used to provide pump commands (e.g., in the form of current commands or signals) to the forward and reverse PRVs  110 ,  112 ,  120 ,  122  used to control the displacement of both the left and right hydraulic pumps  44 ,  46  of the hydrostatic transmission  28 . As indicated above, the left pump  44  may be configured to provide power to the left hydraulic motor  40 , which, in turn, drives the left-side drive output component(s)  201  of the left-side drive system  36  (e.g., the left-side wheels/tires of a wheeled vehicle or the left-side drive wheel/track of a tracked vehicle). Similarly, the right pump  46  may be configured to provide power to the right hydraulic motor  42 , which, in turn, drives the right-side drive output component(s)  203  of the right-side drive system  38  (e.g., the left-side wheels/tires of a wheeled vehicle or the left-side drive wheel/track of a tracked vehicle). Moreover, the left-side and right-side speed sensors  136 ,  138  may be utilized to monitor the output speed of the left and right hydraulic motors  40 ,  42 , respectively, as each motor  40 ,  42  is being operated to rotationally drive its associated drive output component(s)  201 ,  203 . It should be appreciated that, in addition to the speed sensors  136 ,  138  (or an alternative thereto), the system may also include suitable speed sensors for monitoring the rotational speed of the drive output components  201 ,  203  of each drive system  36 ,  38 , such as speed sensors configured to monitor the rotational speed of the left-side wheels  12 ,  16  and the right-side wheels  12 ,  18  of the work vehicle  10  (or the left-side drive wheel and the right-side drive wheel of a tracked vehicle). 
     In general, the speed signals from the left-side and right-side speed sensors  136 ,  138  may be utilized to calculate a speed error  202  between the left-side and right-side drive systems  36 ,  38  (e.g., at the subtractor block  204  shown in  FIG. 4 ) that is used as feedback into a closed-loop control algorithm  206 . However, prior to calculating the speed error  202 , the speed signal(s) output from either the left-side speed sensor  136  or the right-side speed sensor  138  may be modified by a speed scaling factor  208 . For example, as shown in the illustrated embodiment, the right speed signal(s) from the right-side speed sensor  138  may be modified by the speed scaling factor  208  (e.g., at multiplier block  210 ) to generate a modified or corrected right speed signal (indicated by arrow  212  in  FIG. 4 ). The corrected right speed signal  212  may then be input into the subtractor block  204  together with the left speed signal to calculate the speed error  202  between the speed signals. It should be appreciated that, in one embodiment, each speed signal may be filtered (e.g., via filters  214 ) as an initial processing step prior to being modified by the speed scaling factor  208  and/or prior to being used to calculate the speed error  202 . 
     As will be described in greater detail below, the speed scaling factor  208  may generally correspond to a speed modifier that is calculated or selected to allow either the left-side speed signals or the right-side speed signals to be adjusted in a manner that accounts for differences between the left-side and right-side drive systems  36 ,  38  that may otherwise result in the vehicle  10  veering to the left or right when straight travel is commanded, such as manufacturing tolerances for the vehicle&#39;s tires, differing tire pressures, misalignment between the wheel assemblies, load imbalances, etc. Specifically, the speed scaling factor  208  may be used to scale-up or scale-down the associated speed signal(s), as needed, such that the final pump commands provided to the associated PRVs  110 ,  112 ,  120 ,  122  of the hydraulic pumps  44 ,  46  result in straight travel of the work vehicle  10 . For example, if the work vehicle  10  is veering to the right when the vehicle  10  is commanded to travel straight due to the right-side tires having a smaller tire diameter than the left-side tires (e.g., due to tire manufacturing tolerances or due to differing tire pressures), the speed scaling factor  208  may be selected to decrease the right-side speed signal (e.g., by selecting a speed scaling factor  208  that is less than one) such that the speed error  202  being input into the closed-loop control algorithm  206  is similarly adjusted, which may, in turn, result in the pump command to the left pump  44  being adjusted (e.g., decreased) in a manner that accounts for the differing tire diameters. 
     Additionally, as will be described below, the speed scaling factor  208  may, in several embodiments, be calculated automatically by the controller  102  and/or set manually by the operator. For example, in one embodiment, the controller  102  may be configured to execute aspects of a calibration routine that allows for a speed scaling factor  208  to be calculated as the work vehicle  10  is being driven along a straight path. In another embodiment, the operator may be allowed to provide manual inputs (e.g., incremental adjustments) to account for any left or right veering of the work vehicle  10  when the operator has commanded straight travel. It should also be appreciated that the speed scaling factor  208  may be adjusted, as is desired or necessary, to account for changing wheel alignment/misalignment, changing tire diameters, and/or the like. For instance, when the work vehicle  10  is being initially operated for the very first time, the speed scaling factor  208  may be set (e.g., via execution of an appropriate calibration routine and/or via manual inputs from the operator) so as to allow the work vehicle  10  to track straight when so commanded. Thereafter, if the work vehicle  10  subsequently begins to veer left or right due to a change in a parameter associated with one of the drive systems  36 ,  38 , the speed scaling factor  208  may be re-calculated or adjusted (e.g., via re-executing the calibration routine or via manual inputs from the operator) to provide for straight tracking. 
     As indicated above, the speed error  202  calculated based on the speed signals (e.g., at subtractor block  204 ) may be input into a closed-loop control algorithm  206  for generating a control output (indicated by arrows  216  in  FIG. 4 ) for adjusting one or both of the joystick commands received from the operator (e.g., left joystick command  218  and right joystick command  220  shown in  FIG. 4 ). In one embodiment, the controller  202  may be configured as or may include a closed-loop controller (e.g., a Proportional-Integral (PI) controller) that executes one or more closed-loop control algorithms  206  for generating or calculating an appropriate control output  216  based on the speed error  202 . In one embodiment, the control gains applied within the closed-loop control algorithm  206  may be dependent on one or more parameters of the work vehicle  10 , such as the temperature of the hydraulic fluid being supplied to the pumps  44 ,  45 . As is generally understood, the control gains may be stored within the controller&#39;s memory  106  so to be readily accessible by the controller  102 . For example, in one embodiment, the control gains may be stored in the form of a look-up table or mathematical expression that correlates the gain values to the speed error  202  and/or the relevant operating parameters of the work vehicle  10  (e.g., hydraulic fluid temperature). 
     In general, the control output  216  from the closed-loop control algorithm  206  may be utilized to modify one of the joystick commands  218 ,  220  received from the operator. For instance, in the illustrated embodiment, the control output  216  is use to modify the left joystick command  218  (e.g., at subtractor block  222 ), with the resultant adjusted control command (indicated by arrow  224  in  FIG. 4 ) being provided to the appropriate PRV  110 ,  112  of the left hydraulic pump  44  as a final pump command. In such an embodiment, the right joystick command  220  received from the operator may correspond to an open loop command and, thus, may be provided directly to the appropriate PRV  120 ,  122  of the right hydraulic pump  46  as a final pump command. However, it should be appreciated that, in other embodiments, the control output  216  may be used to modify the right joystick command  220 . For instance, as will be described below with reference to  FIG. 5 , the specific joystick command  218 ,  220  to which the control output  216  is applied may, in one embodiment, vary depending on whether the control output  216  is a positive value or a negative value. In addition, as will be described below with reference to  FIG. 5 , the control output  216  may, in one embodiment, be added to or subtracted from the appropriate joystick command  218 ,  220  depending on the commanded direction of travel (e.g., forward or reverse). 
     Moreover, as shown in  FIG. 4 , in one embodiment, the control output  216  from the closed-loop control algorithm  206  may optionally be input into a latch  226  or other memory element of the controller&#39;s memory  106 . In such an embodiment, the latch  226  may be configured to store or freeze the control output  216  from the closed-loop control algorithm  206  when the operator commands that the vehicle  10  be turned (and, thus, ceases to command straight travel). Specifically, at the initiation of a turn by the operator, the latch  226  may store or freeze the last control output  216  received from the closed-loop control algorithm  206  until after the turn has been completed, with the latched control output value being applied during the turn. In such instance, when the operator subsequently commands straight travel of the work vehicle  10 , the stored control output  216  may continue to be utilized as the current control output  216  for modifying the relevant joystick command (e.g., at second subtractor block  222 ). After a short time delay following the re-initiation of straight travel, the control output  216  may then be unlatched or unfrozen and new control outputs from the closed-loop control algorithm  206  may be used to modify the relevant joystick command. Similarly, as long as the operator continues to command straight travel, the latch  226  will not store or freeze the control output  216  (i.e., the input to the latch  226  and the output of the latch  226  will be the same value) and, thus, the current control output  216  from the closed-loop control algorithm  206  will be used to modify the relevant joystick command. 
     It should be appreciated that, in one embodiment, prior to utilizing the control output  216  to modify one of the joystick commands  218 ,  220  received from the operator, the control output  216  may be scaled by the magnitude of the associated joystick command(s)  218 ,  220  so that small (or zero) commands from the operator result in small (or zero) control outputs being applied thereto. As such, if a large value is latched while the vehicle is turning (e.g., when the vehicle was traveling at a high speed prior to the initiation of a turn) and the operator then commands a low speed as the vehicle as he/she resumes straight travel, the latched value can be scaled down appropriately. 
     Additionally, as shown in  FIG. 4 , in one embodiment, a straight tracking reserve  228  may be used to provide additional flow to the left and right pumps  44 ,  46 , as needed, during execution of straight tracking control to ensure that the work vehicle  10  can maintain a straight path. For instance, in one embodiment, the straight tracking reserve  228  can be used to provide additional fluid to one of the pumps (e.g., the left pump  44 ) to avoid saturation of the pump flow when such pump receives a command signal from the straight tracking control algorithm to operate at a speed greater than the maximum speed of the machine. In another embodiment, the straight tracking reserve  228  may be used to provide additional flow to the motors  40 ,  42  by increasing pump displacement beyond the nominal maximum pump displacement during maximum vehicle speed while not executing a turning operation. 
     Referring now to  FIG. 5 , a flow diagram providing control logic  300  for executing one embodiment of the straight tracking control described above with reference to  FIG. 4  is illustrated in accordance with aspects of the present subject matter. Specifically,  FIG. 5  illustrates an exemplary process flow when executing straight tracking control that varies the joystick command to which the control output is being applied based on the sign of the control output and also varies the specific application of the control output (e.g., as an added value or a subtracted value) based on the commanded direction of travel of the work vehicle (e.g., forward or reverse). It should be appreciated that, for purposes of discussion, the control logic  300  shown in  FIG. 5  will be described herein as being executed by the controller  102  described above with reference to  FIG. 4  and in association with the straight tracking control system  200  described above with reference to  FIG. 4 . However, in other embodiments, the control logic  300  may be executed by any suitable processor-based device, such as a separate controller or control module that may be communicatively coupled to the controller  102  (e.g., as part of distributed computing system or network) or that is configured as a “stand-alone” control system or sub-system, and/or may be executed in association with any other suitable straight tracking control system. 
     As shown in  FIG. 5 , at ( 302 ), the controller  102  may be configured to receive the speed signals from the left-side and right-side speed sensors  136 ,  138 . As indicated above with reference to  FIG. 4 , as an initial processing step, the controller  102  may filter one or both of the speed signals (e.g., via filters  214 ). Additionally, at ( 304 ), the controller  102  may be configured to modify one of the speed signals based on an associated speed scaling factor stored within the controller&#39;s memory to generate a corrected speed signal. For instance, as described above with reference to  FIG. 4 , in one embodiment, the right speed signal may be modified using the speed scaling factor  208  (e.g., as a multiplier) to generate a corrected right speed signal  212 . In such an embodiment, the left speed signal may remain unmodified (except for any adjustments made during filtering). However, in an alternative embodiment, the left speed signal may be modified using the speed scaling factor (e.g., as a multiplier) to generate a corrected left speed signal. In such an embodiment, the right speed signal may remain unmodified (except for any adjustments made during filtering). 
     Moreover, as shown in  FIG. 5 , at ( 306 ), the controller  102  may be configured to calculate a speed error (e.g., speed error  202  of  FIG. 4 ) between the corrected speed signal and the unmodified speed signal received from the other speed sensor. For instance, when the speed scaling factor is being applied to the right speed signal, the speed error may be equal to the difference between the left speed signal (e.g., as received from the left-side speed sensor with or without filtering) and the corrected right speed signal. Similarly, when the speed scaling factor is being applied to the left speed signal, the speed error may be equal to the difference between the corrected left speed signal and the right speed signal (e.g., as received from the right-side speed sensor with or without filtering). 
     Upon calculation of the speed error, at ( 308 ), the controller  102  may be configured to execute a closed-loop control algorithm (e.g., control algorithm  206  of  FIG. 4 ) to calculate a control output based on the speed error. Thereafter, at ( 310 ), a determination may be made as to whether the control output from the closed-loop control algorithm corresponds to a positive value or a negative value. Specifically, if the control output is greater than zero (i.e., a positive value), the controller  102  may be configured to apply the control output to the left joystick command. However, if the control output is less than zero (i.e., a negative value), the controller  102  may be configured to apply the control output to the right joystick command. 
     As shown in  FIG. 5 , in the event that the control output is a positive value, the controller may, at ( 312 ), determine whether the operator is currently commanding forward travel or reverse travel. If the operator is commanding forward travel, the control output may be subtracted from the left joystick command to calculate the final pump command for the left hydraulic pump  44  while the final pump command for the right hydraulic pump  46  may correspond to the open-loop, right joystick command (e.g., as indicated by box ( 314 ) in  FIG. 5 ). In contrast, if the operator is commanding reverse travel, the control output may be added to the left joystick command to calculate the final pump command for the left hydraulic pump  44  while the final pump command for the right hydraulic pump  46  may correspond to the open-loop, right joystick command (e.g., as indicated by box ( 316 ) in  FIG. 5 ). 
     Moreover, as shown in  FIG. 5 , in the event that the control output is a negative value, the controller  102  may, at ( 318 ), determine whether the operator is currently commanding forward travel or reverse travel. If the operator is commanding forward travel, the control output may be added to the right joystick command to calculate the final pump command for the right hydraulic pump  46  while the final pump command for the left hydraulic pump  44  may correspond to the open-loop, left joystick command (e.g., as indicated by box ( 320 ) in  FIG. 5 ). In contrast, if the operator is commanding reverse travel, the control output may be subtracted from the right joystick command to calculate the final pump command for the right hydraulic pump  46  while the final pump command for the left hydraulic pump  44  may correspond to the open-loop, left joystick command (e.g., as indicated by box ( 322 ) in  FIG. 5 ). 
     It should be appreciated that the control logic  300  shown in  FIG. 5  simply provides one example of suitable logic for executing straight tracking control in accordance with aspects of the present subject matter. For instance, the control logic  300  of  FIG. 5  is based on certain assumptions associated with the operating and/or control parameters of the system, such as the range of values associated with the joystick commands and/or the sign of the speed signals. Thus, one of ordinary skill in the art would readily appreciate that the associated control logic may vary using different assumptions. 
     As indicated above, the speed scaling factor used to modify the right or left speed signals may generally correspond to a speed modifier or scaling value that is selected or set to account for any differences between the left-side and right-side drive systems  36 ,  38  that may otherwise result in a work vehicle veering to the left or right when straight travel is commanded, such as manufacturing tolerances for the vehicle&#39;s tires, differing tire pressures, misalignments between the wheel assemblies, load imbalances, etc. For example, in several embodiments, the controller  102  may be configured to execute aspects of a calibration routine to calculate an appropriate speed scaling factor for ensuring straight travel of the work vehicle when so commanded by the operator. In doing so, the controller  102  may be configured to analyze the speed signals received from the left-side and right-side speed sensors  136 ,  138  as the work vehicle  10  is actually being driven along a straight path to calculate the speed scaling factor to be used when executing straight tracking control. 
     For example,  FIG. 6  illustrates a flow diagram of one embodiment of a methodology for executing a calibration routine  400  to calculate a speed scaling factor for use during straight tracking control of a work vehicle in accordance with aspects of the present subject matter. As shown, the flow diagram includes both steps performed by the operator (e.g., along the right-side column of  FIG. 6 ) and processing steps performed by the controller  102  (e.g., along the left-side column of  FIG. 6 ) for executing the calibration routine  400 . 
     As shown in  FIG. 6 , at ( 402 ), the operator may instruct the controller  102  to enter into a calibration mode (e.g., via a suitable input device, such as an input device located or accessible within the vehicle&#39;s cab and/or via an auxiliary input device (e.g., a separate service tool)). For example, the operator may be initially setting-up the machine and may be required to run the calibration routine to calibrate the vehicle&#39;s straight tracking control system. Alternatively, the operator may have noticed that the vehicle is not tracking straight and, thus, may desire to re-calibrate the system. As another example, the operator may correspond to a service technician that desires to perform the calibration routine following the performance of a service or maintenance activity (e.g., following the replacement of the vehicle tires and/or the replacement of one of the other drive system components, such as one or both of the vehicle&#39;s hydraulic pumps and/or motors). 
     At ( 404 ), the controller  102  may receive the operator&#39;s request. 
     Thereafter, upon receipt of the request, the controller may, at ( 406 ), disable its straight tracking control system. For example, the controller  102  may disable the closed-loop control algorithm described above with reference to  FIG. 4  such that the speed feedback signals received from the left-side and right-side speed sensors  136 ,  138  are not considered. Rather, during the calibration sequence, the controller  102  may be configured to simply implement open-loop control based on the joystick commands received from the operator. 
     As shown in  FIG. 6 , at ( 408 ), the operator may then command straight travel of the work vehicle  10  and observe whether the vehicle  10  is veering to the left or right. As indicated above, to command straight travel when using separate left and right joysticks, the operator may actuate or move both joysticks to the same position relative to their neutral position to provide equal inputs to the controller  102 , such as by moving both joysticks to command 50% of the maximum speed for both the left-side and right-side drive systems  36 ,  38 . Alternatively, when using a single joystick, the operator may push the joystick straight forward to command straight travel in the forward direction. In response such user input command(s), the controller  102  may, at ( 410 ), be configured to transmit corresponding pump commands to the left and right pumps  44 ,  46  for controlling the operation of the left-side and right-side drive systems  36 ,  38 . For example, if the joystick command(s) received from the operator is associated with driving the work vehicle  10  straight in the forward direction at 40% of its maximum speed, the controller  102  may transmit suitable current commands to the associated PRVs  110 ,  120  of the pumps  44 ,  46  for achieving the commanded speed. In one embodiment, the controller  102  may be configured to limit the speed of the work vehicle  10  during the performance of the calibration routine (e.g., to 50% of the maximum speed), in which case the controller  102  may scale the pump commands associated with each joystick command(s) based on the speed limit. Thereafter, as the hydrostatic transmission  28  is being controlled based on the joystick command(s), the operator can view the travel path of the work vehicle  10  to determine if the vehicle  10  veers to the left or to the right. In the event the work vehicle travels  10  straight as commanded, the calibration routine may be exited or aborted. 
     However, if the operator observes the work vehicle  10  veering to the left or right, the operator may, at ( 412 ), adjust the joystick position(s) in the direction of the required travel correction. For instance, to correct for veering to the left when using separate left and right joysticks, the operator may increase the left joystick command and/or reduce the right joystick command. Similarly, to correct for veering to the left when using a single joystick, the operator may move the joystick to the right to counteract such veering. As long the operator holds the joystick(s) at its adjusted position(s), the controller  102  may, at ( 414 ), be configured to incrementally adjust one or both of the pump commands to achieve the requesting travel correction, such as by incrementally increasing or decreasing the pump command to one side or by incrementally increasing the pump command to one side while incrementally decreasing the pump command to the other side. As a result, while the operator holds the joystick(s) at the adjusted position, the controller  102  may slowly or incrementally adjust the travel direction of the work vehicle  10  in the direction requested by the operator. Once the operator observes that the work vehicle  10  is traveling straight, the operator may move the joystick(s) back to the original position to which it was initially when commanding straight travel. Such movement of the joystick(s) back to the original straight travel position may trigger the controller  102  to stop further incremental adjustments of the pump command(s) and freeze or lock the relative pump command(s) at such point to allow the vehicle  10  to continue to be driven straight. 
     Following the relative left and right pump commands being frozen, the operator may again view the travel path of the work vehicle  10  and confirm that it is actually traveling straight. Thereafter, at ( 416 ), assuming the vehicle is traveling straight, the operator may instruct the controller  102  to determine or “learn” a corresponding speed scaling factor associated with achieving such straight travel. As shown in  FIG. 6 , at ( 418 ), the controller  102  may receive the operator&#39;s “learn” request and may subsequently, at ( 420 ), begin to analyze the speed signals received from the speed sensors  136 ,  138  as the work vehicle  10  is traveling straight to calculate an associated speed scaling factor. Specifically, in one embodiment, the controller  102  may be configured to calculate a speed ratio between the left speed signal and the right speed signal (or between the right speed signal and the left speed signal—depending on which speed signal the scaling factor is to be applied) as the work vehicle  10  is traveling straight. In such an embodiment, the speed ratio may then be set as the speed scaling factor, which may then be stored within the controller&#39;s memory (e.g., at ( 422 )) for subsequent use during execution of the straight tracking control system. Alternatively, the controller  102  may be configured to integrate the left and right output speeds associated with the speed signals received from the speed sensors  136 ,  138  over a predetermined time period to determine the total wheel travel for both the left-side and right-side wheels across such time period as the work vehicle is traveling straight. In such an embodiment, the controller  102  may be configured to calculate a wheel travel ratio between the left-side wheel travel and the right-side wheel travel (or between the right-side wheel travel and the left-side wheel travel—depending on which speed signal the scaling factor is to be applied). The wheel travel ratio may then be set as the speed scaling factor and stored within the controller&#39;s memory (e.g., at ( 422 )) for subsequent use. 
     Referring now to  FIG. 7 , a flow diagram of another embodiment of a methodology for executing a calibration routine  500  to calculate a speed scaling factor for use during straight tracking control of a work vehicle is illustrated in accordance with aspects of the present subject matter. As shown, the flow diagram includes both steps performed by the operator (e.g., along the right-side column of  FIG. 7 ) and processing steps performed by the controller  102  (e.g., along the left-side column of  FIG. 7 ) for executing the calibration routine  500 . 
     As shown in  FIG. 7 , at ( 502 ), the operator may instruct the controller  102  to enter into a calibration mode (e.g., via a suitable input device, such as an input device located or accessible within the vehicle&#39;s cab and/or via an auxiliary input device (e.g., a separate service tool)). Thereafter, similar to the calibration routine  400  described above with reference to  FIG. 6 , the controller  102  may, at ( 504 ) and ( 506 ), receive the operator&#39;s request and disable its straight tracking control system. As such, during the calibration sequence, the controller  102  may be configured to simply implement open-loop control based on the joystick commands received from the operator. 
     As shown in  FIG. 7 , at ( 508 ), the operator may then command vehicle movement and adjust the joystick position(s) until straight travel is achieved. For example, the operator may initially command straight travel of the work vehicle and observe whether the vehicle is veering to the left or right. Upon determining that the vehicle is veering to one side, the operator may adjust the joystick position(s) in the direction of the required travel correction. For instance, to correct for veering to the left when using separate left and right joysticks, the operator may increase the right joystick command and/or reduce the left joystick command. Similarly, to correct for veering to the left when using a single joystick, the operator may move the joystick to the right to counteract such veering. In response such user input command(s), the controller, at ( 510 ), may be configured to transmit corresponding pump commands to the left and right pumps for controlling the operation of the left-side and right-side drive systems. In one embodiment, the controller  102  may be configured to limit the speed of the work vehicle during the performance of the calibration routine (e.g., to 50% of the maximum speed), in which case the controller  102  may scale the pump commands associated with each joystick command(s) based on the speed limit. 
     Once the operator is able to get the vehicle traveling straight, the operator may, at ( 512 ), instruct the controller  102  to determine or “learn” a corresponding speed scaling factor associated with achieving such straight travel. As shown in  FIG. 7 , at ( 514 ), the controller  102  may receive the operator&#39;s “learn” request and may subsequently, at ( 516 ), begin to analyze the speed signals received from the speed sensors  136 ,  138  as the work vehicle is traveling straight to calculate an associated speed scaling factor. Specifically, as indicated above, the controller  102  may, for example, be configured to set the speed scaling factor as the speed ratio between the left speed signal and the right speed signal (or between the right speed signal and the left speed signal—depending on which speed signal the scaling factor is to be applied) as the work vehicle is traveling straight. Alternatively, the controller  102  may be configured to set the speed scaling factor as the wheel travel ratio between the left-side wheel travel and the right-side wheel travel (or between the right-side wheel travel and the left-side wheel travel—depending on which speed signal the scaling factor is to be applied) determined based on integrating the left and right wheels speeds over a predetermined time period as the vehicle is traveling straight. Regardless, once the speed scaling factor has been calculated, the controller  102  may be configured to store the scaling factor within its memory (e.g., at ( 518 )) for subsequent use. 
     Referring now to  FIG. 8 , a flow diagram of a further embodiment of a methodology for executing a calibration routine  600  to calculate a speed scaling factor for use during straight tracking control of a work vehicle is illustrated in accordance with aspects of the present subject matter. As shown, the flow diagram includes both steps performed by the operator (e.g., along the right-side column of  FIG. 8 ) and processing steps performed by the controller  102  (e.g., along the left-side column of  FIG. 8 ) for executing the calibration routine  600 . 
     As shown in  FIG. 8 , at ( 602 ), the operator may instruct the controller  102  to enter into a calibration mode (e.g., via a suitable input device, such as an input device located or accessible within the vehicle&#39;s cab and/or via an auxiliary input device (e.g., a separate service tool)). Thereafter, similar to the calibration routines  400 ,  500  described above with reference to  FIGS. 6 and 7 , the controller  102  may, at ( 604 ) and ( 606 ), receive the operator&#39;s request and disable its straight tracking control system. As such, during the calibration sequence, the controller  102  may be configured to simply implement open-loop control based on the joystick commands received from the operator. 
     As shown in  FIG. 8 , at ( 608 ), the operator may then command straight travel of the work vehicle and observe whether the vehicle is veering to the left or right. As indicated above, to command straight travel when using separate left and right joysticks, the operator may actuate or move both joysticks to the same position relative to their neutral position to provide equal inputs to the controller  102 . Alternatively, when using a single joystick, the operator may push the joystick straight forward to command straight travel in the forward direction. In response such user input command(s), the controller  102  may, at ( 610 ), be configured to transmit corresponding pump commands to the left and right pumps for controlling the operation of the left-side and right-side drive systems. In one embodiment, the controller  102  may be configured to limit the speed of the work vehicle during the performance of the calibration routine (e.g., to 50% of the maximum speed), in which case the controller  102  may scale the pump commands associated with each joystick command(s) based on the speed limit. As the hydrostatic transmission  28  is being controlled based on the joystick command(s), the operator can view the travel path of the work vehicle  10  to determine if the vehicle  10  veers to the left or to the right. In the event the work vehicle travels straight as commanded, the calibration routine may be exited or aborted. 
     However, if the operator observes the work vehicle veering to the left or right, the operator may, at ( 612 ), provide operator inputs to incrementally adjust the vehicle&#39;s steering until the work vehicle  10  is traveling straight. Specifically, in one embodiment, the operator may be allowed to use one or more of the vehicle&#39;s input devices (e.g., separate buttons or other interface elements located with the cab) to provide steering inputs to the controller  102  in order to correct the observed left or right veering of the vehicle. For example, if the work vehicle is veering to the left as straight travel is being commanded, the operator may press an associated “right steering” button to incrementally adjust the vehicle steering to the right. In such instance, each actuation of the “right steering” button by the operator may correspond to a predetermined incremental adjustment of the joystick command(s) and/or a predetermined incremental adjustment of one or both of the left and right pump command(s). For example, in one embodiment, each steering input provided by the operator via the “right steering” button or the opposite “left steering” button may be mapped to a given percent change in the joystick command or a given percent change in the pump command, which is then applied to either increase or decrease the left pump command and/or the right pump command. Accordingly, as the operator views the travel path of the work vehicle, the operator may continue to provide steering inputs via the appropriate steering button to incrementally adjust the vehicle&#39;s steering until the vehicle is traveling along a straight path. Additionally, as shown in  FIG. 8 , at ( 614 ), in response to the operator providing such steering input(s), the controller  102  may be configured to incrementally adjust the left pump command and/or the right pump command to adjust the vehicle&#39;s steering left or right based on the received steering input(s). 
     Once the operator is satisfied that the vehicle&#39;s steering has been corrected and the work vehicle is traveling straight, the operator may, at ( 616 ), instruct the controller to determine or “learn” a corresponding speed scaling factor associated with achieving such straight travel. As shown in  FIG. 8 , at ( 618 ), the controller  102  may receive the operator&#39;s “learn” request and may subsequently, at ( 620 ), begin to analyze the speed signals received from the speed sensors  136 ,  138  as the work vehicle is traveling straight to calculate an associated speed scaling factor. Specifically, as indicated above, the controller  102  may, for example, be configured to set the speed scaling factor as the speed ratio between the left speed signal and the right speed signal (or between the right speed signal and the left speed signal—depending on which speed signal the scaling factor is to be applied) as the work vehicle is traveling straight. Alternatively, the controller  102  may be configured to set the speed scaling factor as the wheel travel ratio between the left-side wheel travel and the right-side wheel travel (or between the right-side wheel travel and the left-side wheel travel—depending on which speed signal the scaling factor is to be applied) determined based on integrating the left and right wheels speeds over a predetermined time period as the vehicle is traveling straight. Regardless, once the speed scaling factor has been calculated, the controller  102  may be configured to store the scaling factor within its memory (e.g., at ( 622 )) for subsequent use. 
     As indicated above, as an alternative to executing a calibration routine to allow the controller  102  to determine the speed scaling factor, the operator may, instead, be allowed to manually adjust the speed scaling factor up or down to correct for left/right veering when straight travel is otherwise commanded. In such instance, the manual adjustments to speed scaling factor may be made at any time, such as when the work vehicle is moving or when the vehicle is stationary. Specifically, assuming that the operator has observed that the work vehicle is veering to the left or right when straight travel is commanded, the operator may be allowed to provide steering inputs (e.g., via separate “steering” buttons) that are used directly adjust the speed scaling factor, with each steering input being associated with a corresponding incremental adjustment of the speed scaling factor. 
     For example,  FIG. 9  illustrates a flow diagram of one embodiment of a methodology for allowing an operator to manually adjust a speed scaling factor used during straight tracking control of a work vehicle in accordance with aspects of the present subject matter. As shown, the flow diagram includes both steps performed by the operator (e.g., along the right-side column of  FIG. 9 ) and processing steps performed by the controller  102  (e.g., along the left-side column of  FIG. 9 ). 
     As shown in  FIG. 9 , at ( 702 ), the operator may indicate his/her desire to make manual steering adjustments to allow the work vehicle to be driven straight. For example, the operator may have observed that the work vehicle is veering to the left or right when straight travel is command and, thus, may desire to correct the vehicle&#39;s steering. In such instance, the operator may be allowed to provide a suitable input (e.g., via a button or other input device) instructing the controller  102  to enter into a manual straight tracking adjustment mode to allow manual adjustments to be made to the speed scaling factor being applied within the vehicle&#39;s straight tracking control system. 
     At ( 704 ), the controller  102  may receive the operator&#39;s request. Thereafter, upon receipt of the request, the controller  102  may, at ( 706 ), maintain its straight tracking control system active. Specifically, unlike the embodiments of the calibration routines  400 ,  500 ,  600  described above with reference to  FIGS. 6-8 , the controller  102  may be configured to continue to implementing straight tracking control during the manual straight tracking adjustment mode. As such, as the operator commands movement of the work vehicle, the controller  102  may continue to execute closed-loop control based on the feedback received from the left-side and right-side speed sensors  136 ,  138 . 
     Additionally, as shown in  FIG. 9 , at ( 708 ), the operator may then command straight travel of the work vehicle and observe whether the vehicle is veering to the left or right. As indicated above, to command straight travel when using separate left and right joysticks, the operator may actuate or move both joysticks to the same position relative to their neutral position to provide equal inputs to the controller  102 . Alternatively, when using a single joystick, the operator may push the joystick straight forward to command straight travel in the forward direction. In response such user input command(s), the controller  102 , at ( 710 ), may be configured to transmit corresponding pump commands to the left and right pumps for controlling the operation of the left-side and right-side drive systems. In one embodiment, the controller  102  may be configured to limit the speed of the work vehicle during the performance of the calibration routine (e.g., to 50% of the maximum speed), in which case the controller  102  may scale the pump commands associated with each joystick command(s) based on the speed limit. As the hydrostatic transmission  28  is being controlled based on the joystick command(s), the operator can view the travel path of the work vehicle  10  to determine if the vehicle  10  veers to the left or to the right. In the event the work vehicle travels straight as commanded, the calibration routine may be exited or aborted. 
     However, if the operator observes the work vehicle veering to the left or right, the operator may, at ( 712 ), may provide operator inputs to incrementally adjust the vehicle&#39;s steering until the work vehicle is traveling straight. Specifically, in one embodiment, the operator may be allowed to use one or more of the vehicle&#39;s input devices (e.g., separate buttons or other interface elements located with the cab) to provide discrete steering inputs to the controller  102  in order to correct the observed left or right veering of the vehicle. For example, if the work vehicle is veering to the left as straight travel is being commanded, the operator may press an associated “right steering” button to incrementally adjust the vehicle steering to the right. In such instance, each actuation of the “right steering” button by the operator may correspond to a predetermined incremental adjustment of the speed scaling factor being applied within the vehicle&#39;s straight tracking control system. For example, in one embodiment, each steering input provided by the operator via the “right steering” button or the opposite “left steering” button may be mapped to a corresponding percent change in the speed scaling factor (e.g., a 5% or 10% change). The change in the speed scaling factor with each steering input provided by the operator may then be applied within the straight tracking control system to result in an adjustment to the speed error being input into the closed loop control algorithm, which, in turn, varies the control output being applied to the left/right joystick command so as to adjust the vehicle&#39;s steering. Accordingly, as the operator views the travel path of the work vehicle, the operator may continue to provide steering inputs via the appropriate steering button to incrementally adjust the speed scaling factor until the vehicle is traveling along a straight path. Additionally, as shown in  FIG. 9 , at ( 714 ), in response to the operator providing such steering input(s), the controller may be configured to incrementally adjust the speed scaling factor to adjust the vehicle&#39;s steering left or right based on the received steering input(s). 
     Once the operator is satisfied that the vehicle&#39;s steering has been corrected and the work vehicle is traveling straight, the operator may, at ( 716 ), instruct the controller  102  to store the current speed scaling factor. Thereafter, at ( 718 ), the controller  102  may store the speed scaling factor within its memory for subsequent use within the straight tracking control system. 
     Referring now to  FIG. 10 , a flow diagram of one embodiment of a method  800  for executing straight tracking control of a work vehicle having an electronically controlled hydrostatic transmission is illustrated in accordance with aspects of the present subject matter. In general, the method  800  will be described with reference to the work vehicle  10  and the embodiments of the control system  100 ,  200  described above with reference to  FIGS. 1-4 . However, it should be appreciated by those of ordinary skill in the art that the disclosed method  800  may generally be utilized with any manner of work vehicle and/or associated control system. In addition, although  FIG. 10  depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     As shown in  FIG. 10 , at ( 802 ), the method  800  may include receiving at least one operator input command associated with controlling an operation of at least one of a first-side drive system or a second-side drive system of a hydrostatic transmission to drive a work vehicle along a straight path. For example, as indicated above, the operator may command straight travel of the work vehicle by providing a suitable input command(s) to the vehicle&#39;s controller  102 , such as a suitable joystick command(s) via separate left and right joysticks or via a single joystick. 
     Additionally, at ( 804 ), the method  800  may include receiving a first speed signal associated with a first output speed of the first-side drive system and a second speed signal associated with a second output speed of the second-side drive system. Specifically, as indicated above, the controller  102  may be communicatively coupled to separate left-side and right-side speed sensors  136 ,  138  configured to detect the output speeds associated with the first-side and second-side drive systems  36 ,  38 , respectively. 
     Moreover, at ( 806 ), the method  800  may include modifying one of the first speed signal or the second speed signal based on a speed scaling factor to generate a corrected speed signal. For example, as indicated above, the controller  102  may be configured to modify the right speed signal or the left speed signal based on a speed scaling factor that is selected or set to account for any differences between the left-side and right-side drive systems  36 ,  38  that may otherwise result in the work vehicle veering to the left or right when straight travel is commanded, such as manufacturing tolerances for the vehicle&#39;s tires, differing tire pressures, misalignments between the wheel assemblies, load imbalances, and/or the like. Specifically, in one embodiment, the speed scaling factor may correspond to a multiplier that is directly applied to one of the speed signals to generate the corrected speed signal. 
     Referring still to  FIG. 10 , at ( 808 ), the method  800  may include determining an adjusted control command for controlling the operation of the hydrostatic transmission as a function of the at least one operator input command and a control output determined based at least in part on a differential between the corrected speed signal and the other of the first speed signal or the second speed signal. Specifically, as indicated above, the controller  102  may be configured to determine a speed error or differential between the corrected speed signal and the other, unmodified speed signal. The speed error may then be input into a closed-loop control algorithm to generate a control output that is used to modify at least one of the joystick commands received from the operator to generate an adjusted pump command for at least one of the hydraulic pumps of the vehicle&#39;s hydrostatic transmission. 
     Additionally, at ( 810 ), the method  800  may include controlling the operation of the hydrostatic transmission based at least in part on the adjusted control command to drive the work vehicle along the straight path. For example, as indicated above, the controller  102  may, in one embodiment, be configured to transmit the adjusted control or pump command to the PRVs associated with one of the hydraulic pumps to control the operation thereof, thereby allowing the work vehicle to be drive along a straight path. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.