Patent Publication Number: US-2020276876-A1

Title: Self-propelled vehicles with suspension system control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/057,341, filed Aug. 7, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/544,139, filed Aug. 11, 2017 and U.S. Provisional Patent Application No. 62/586,453, filed Nov. 15, 2017, each of which being incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The field of the disclosure relates to self-propelled vehicles and, in particular, self-propelled vehicles that include a control system for adjusting the pitch of the vehicle during use. In some embodiments, the self-propelled vehicle may include an inclinometer for measuring the pitch of the terrain near the vehicle. 
     BACKGROUND 
     Self-propelled vehicles having a chassis with a front wheel support system and a rear wheel support system may be used in a variety of applications. As the vehicles are used, the loading on the apparatus (e.g., the carried load such as a formed bale) may cause variations in the weight carried by the front and rear support systems. The added weight may not be carried equally by the front and the rear support systems. Some support systems include a suspension system which allows relative movement between the chassis of the vehicle and the support system. Variations in the weight carried by the suspension system may change the pitch of the apparatus. 
     In addition, operation of the vehicle over hilly terrain will cause the weight distribution of the apparatus to shift. The shift in weight distribution causes the percentage of weight carried by the front wheel support system and by the rear wheel support system to vary. For vehicles having a suspension, this variation will cause the pitch of the chassis of the apparatus to vary relative to the terrain. In vehicles equipped with a sensor (e.g., inclinometer) for measuring the pitch of the terrain near the vehicle, changes in the relative position of the chassis to the terrain during operation of the vehicle may affect the measurements of the sensor and cause measurement error. 
     A need exists for systems to control the pitch of the chassis relative to a front or rear wheel support system of the vehicle in order to maintain a consistent relative pitch between the chassis and the terrain in order to allow for improved accuracy of onboard sensors such as inclinometers. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     SUMMARY 
     One aspect of the present disclosure is directed to a self-propelled vehicle. The vehicle includes a chassis and first and second rear wheels connected to the chassis. First and second front wheels are also connected to the chassis. The vehicle includes an inclinometer for sensing a ground pitch. A suspension system includes a suspension element that enables one of the first and second rear wheels and first and second front wheels to move relative to the chassis. A suspension system position sensor produces a signal based on the position of the suspension system relative to the chassis. A control unit receives signals from the suspension system position sensor. The control unit adjusts the suspension element based at least in part on the signal from the suspension system position sensor. 
     Another aspect of the present disclosure is directed to a self-propelled vehicle. The vehicle includes a chassis and first and second rear wheels connected to the chassis. First and second front wheels are connected to the chassis. A suspension system includes a suspension element connected to the subframe. The suspension element enables the subframe to move relative to the chassis. A suspension system position sensor produces a signal based on a distance of the suspension system from the chassis. A control unit receives signals from the suspension system position sensor. The control unit adjusts the suspension element at least in part based on the signal from the suspension system position sensor. 
     Yet a further aspect of the present disclosure is directed to a pitch-leveling hydraulic suspension system to enable a wheel to move relative to a chassis. The suspension system includes a hydraulic cylinder and a hydraulic pump for providing hydraulic fluid to the hydraulic cylinder. The system includes a suspension system position sensor. A control unit receives signals from the suspension system position sensor. The control unit adjusts the hydraulic cylinder at least in part based on the signal from the suspension system position sensor. 
     Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present disclosure may be incorporated into any of the above-described aspects of the present disclosure, alone or in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a self-propelled vehicle; 
         FIG. 2  is a side view of a self-propelled baling vehicle; 
         FIG. 3  is a perspective view of a self-propelled raking vehicle; 
         FIG. 4  is a perspective view of a self-propelled mower conditioner vehicle; 
         FIG. 5  is a perspective view of a self-propelled hay merger vehicle; 
         FIG. 6  is a front view of the self-propelled baling vehicle; 
         FIG. 7  is a perspective view of the self-propelled baling vehicle; 
         FIG. 8  is a perspective view of a caster assembly of the self-propelled baling vehicle; 
         FIG. 9  is cross-sectional side view of the self-propelled baling vehicle showing a baling chamber; 
         FIG. 10  is a schematic view of the vehicle showing a hydraulic suspension system; 
         FIG. 11  is perspective view of the self-propelled vehicle showing the engine mounting brackets; 
         FIG. 12  is a schematic view of the self-propelled vehicle showing the drive systems and the suspension system; 
         FIG. 13  is a side view of another embodiment of a self-propelled baling vehicle; 
         FIG. 14  is a front view the self-propelled baling vehicle of  FIG. 13 ; 
         FIG. 15  is a schematic of a self-propelled vehicle with a relatively light load without a pitch-leveling suspension system; 
         FIG. 16  is a schematic of a self-propelled vehicle with a relatively heavy load without a pitch-leveling suspension system; 
         FIG. 17  is a schematic of a self-propelled vehicle with a pitch-leveling suspension system; 
         FIG. 18  is a schematic of a self-propelled vehicle on a downward slope without a pitch-leveling suspension system; 
         FIG. 19  is a schematic of a self-propelled vehicle on a downward slope with a pitch-leveling suspension system; 
         FIG. 20  is a schematic of a self-propelled vehicle on an upward slope without a pitch-leveling suspension system; 
         FIG. 21  is a schematic of a self-propelled vehicle on an upward slope with a pitch-leveling suspension system; and 
         FIG. 22  is a schematic view of another embodiment of the vehicle showing a hydraulic suspension system. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION 
     A self-propelled vehicle is generally referred to as “ 1 ” in  FIG. 1 . The vehicle  1  is configured to carry a load and/or operate a device such as a baling device  5  ( FIG. 2 ) for forming a bale of crop or forage material. While the description and Figures below may show and/or reference a baling device, it should be noted that a baling device is shown as an exemplary device and the descriptions are applicable to the self-propelled vehicle itself, optionally with different devices attached thereto. While the device may, in some embodiments, be described as an agricultural device, in other embodiments the device may be suitable for use in other fields. The device  5  may be connected to the vehicle  1  in a fixed manner or may be removably connected to the vehicle  1  (e.g., modular). 
     The vehicle may be a self-propelled agricultural vehicle such as a rake ( FIG. 3 ), mower or mower conditioner ( FIG. 4 ), merger ( FIG. 5 ), sprayer, windrower, broadcast spreader, nut or fruit harvester or the like. Other vehicles include salt and aggregate spreaders, shipping vehicles (e.g., trash, commodities, household items or other goods), construction vehicles, trenchers, and concrete cutters. 
     The self-propelled vehicle  1  is controlled from an operator station  13  ( FIG. 2 ) and is powered by an engine  101 . The vehicle  1  includes a device  5  such as a baling device, cutting or mower head, sickle bar, spray tank and/or booms, harvesting devices (e.g., grape or nut harvesting devices), broadcast spreader, salt and aggregate spreader, shipping container (e.g., trash, commodities, household items or other goods), construction devices or trencher, or concrete cutter. The vehicle  1  may also power the device  5 . In some embodiments, the vehicle  1  is adapted to carry a load (e.g., bale, herbicide, fertilizer, or harvested crop such as nuts or fruits). 
     In the illustrated embodiment, the vehicle includes a baling device  5  and a pick-up device  11  ( FIG. 6 ) that rotates to feed crop or forage material to the baling device  5  to form a bale. Each of the operator station  13 , engine  101  and device  5  may be supported by a chassis  9  (i.e., the engine  101  is not part of a towing vehicle such as a tractor that releasably connects the device by a hitch assembly attached to an implement tongue). 
     The vehicle  1  includes first and second rear drive wheels  17  that are driven by first and second motors  23  ( FIG. 13 ) that are disposed within the drive wheels. The rear drive wheels  17  each have a rotational axis R 17  about which the drive wheels  17  rotate. In the illustrated embodiment, the wheels  17  have a common rotational axis R 17 . In other embodiments, the wheels  17  are offset from each other and have different axes of rotation. The drive wheels  17  are attached to the chassis  9 . In some embodiments, the drive wheels  17  have a diameter of at least about 4 feet, or even at least about 5 feet or at least about 6 feet (e.g., from about 4 feet to about 8 feet or from about 4 feet to about 6 feet). 
     The rear wheels  17  are fixed to the chassis  9  such that the wheels  17  maintain parallel alignment with a longitudinal axis A ( FIG. 7 ) of the vehicle  1  (i.e., do not pivot with respect to the chassis  9 ). In some embodiments, the rear drive wheels  17  are not suspended from the chassis  9 . In other embodiments, the rear drive wheels  17  are suspended. 
     The longitudinal axis A ( FIG. 7 ) of the vehicle  1  extends from a front  55  ( FIG. 2 ) to a rear  57  of the vehicle  1 . As referenced herein, the “front” of the vehicle  1  refers to a leading portion or end of the vehicle  1  relative to the longitudinal axis during conventional use (e.g., during baling, harvesting, spraying and the like). The “rear” refers to the trailing portion or end relative to the longitudinal axis A during conventional operation. Similarly, the terms “front caster wheels” and “rear wheels” refer to the relative position of the wheels relative to the direction of travel of the vehicle  1  during conventional operation. The vehicle  1  also includes a lateral axis B ( FIG. 7 ) that extends from a first side  58  to a second side  59  of the vehicle  1  and that is transverse to the longitudinal axis A. The vehicle  1  also includes a vertical axis C ( FIG. 6 ). 
     With reference to  FIG. 12 , the first and second drive wheels  17  are each driven and controlled by separate drive systems  15 . Each drive system  15  has a drive motor  23  for rotating the drive wheel  17  forward or backward. The drive motors  23  may be hydraulic motors that are driven by a pump  20  that is powered by the engine  101 . Each drive wheel  17  may be controlled by a separate circuit (i.e., separate hydraulic pumps  20  with fluid lines  22  connected to the drive wheel motors  23 ). The first and second pumps  20  may be hydrostatic, variable displacement pumps. In some embodiments, fixed displacement or variable displacement motor(s) may be used. In this regard, while the drive systems  15  are shown and described herein as hydraulic systems, in other embodiments an electric drive system that operates as described may be used. Exemplary electric drive systems are disclosed in U.S. Pat. No. 8,657,041 which is incorporated herein by reference for all relevant and consistent purposes. 
     The wheels  17  are powered and rotated independently by the drive systems  15 . Accordingly, the wheels  17  can be rotated at different speeds by driving the motors at different speeds. In a drive wheel steering mode, the wheels  17  are driven at different speeds by the drive system  15 . For example, in this mode, the motors  23  receive different amounts of fluid from the respective pumps  20  to differentiate the speed of the wheels  17 . Separate fluid lines  22  extend between each pump  20  and drive motor  23  to independently rotate the wheels  17 . The direction of fluid flow may be forward or reverse to independently rotate the wheels forward or reverse to propel the vehicle forward, reverse, through an arc (e.g., as during steering) or about a vertical axis midway between the drive wheels  17  (e.g., as during zero turn steering). 
     The vehicle  1  includes a control system to control the drive wheels  17  and front caster wheels  27  based on inputs from an operator. The control system includes a control unit  80 , speed and direction control device  78 , a mode selector  79  and steering mechanism which is shown as a steering wheel  67 . The speed and direction control device  78 , mode selector  79  and steering wheel  67  may be controlled from the operator station  13 . 
     The mode selector  79  allows the operator to select a desired mode of operation (i.e., a drive wheel steering mode or a caster wheel steering mode). The control unit  80  receives the signal from the mode selector  79  and controls the mode of the steering system in response to the signal. The mode selector  79  may be, for example, part of a touch screen, a soft key, toggle switch, selection button or any other suitable interface for selecting the steering mode. 
     The speed and direction control device  78  is typically hand-operated and may be a sliding lever that causes an increase in forward speed as the lever is slid forward of a neutral position and an increase in reverse direction as the lever is slid rearward of the neutral position. The speed and direction control device  78  produces a signal in response to its position and the signal is transmitted to the control unit  80 . The control unit  80  produces an output signal transmitted to the hydraulic pumps  20  that drive the rear wheels  17 . The speed may also be controlled by a throttle that controls the engine speed. The vehicle  1  may be stopped by moving the speed and direction control device  78  to a zero-speed setting and/or by operating foot brake levers. 
     A suspension system  51  may be controlled by the control unit  80  at least in part by a signal from a suspension system position sensor  61  as further described below. The suspension system  51  may also be adjusted based on one or more other signals relayed to the control unit  80  including speed, mode and/or direction of travel. The control unit  80  regulates one or more valves  70 ,  71 ,  72  ( FIG. 10 ) within the suspension system  51  at least in part in response to sensor  61  and, optionally, in response to other signals relayed to the control unit  80 . 
     In the illustrated embodiment, steering may be performed by a steering mechanism shown as a steering wheel  67  which regulates the steering system. For example, in the drive wheel steering mode, a sensor  81  measures the direction and angle of the steering wheel  67  and sends signals to the control unit  80 . The control unit  80  produces a signal that is transmitted to the hydraulic pumps  20  to independently regulate the rotational speeds of the first and second drive wheels  17  (i.e., the speed and direction of rotation of each drive wheel  17 ). 
     In other embodiments, speed and/or steering may be controlled by different operator controls such as wheel levers, digital inputs, joysticks, dual sticks, and headsets. 
     In some embodiments, the self-propelled vehicle  1  is configured to optionally operate autonomously. The vehicle  1  may include sensors (e.g., cameras, GPS sensors and the like) that sense the position of a crop (e.g., windrow) and/or that may sense the position of the vehicle. The vehicle  1  may also include a controller that sends signals to the first and second rear wheel pumps or to various actuators to independently control the first and second rear drive wheels. In some embodiments, the area in which the vehicle is propelled is mapped and the area map is used to autonomously control the operation of the vehicle in the field. In such embodiments, the vehicle may include a riding station to carry an operator or the operator station may be eliminated. 
     The self-propelled vehicle  1  includes first and second front caster wheels  27  that are pivotally connected to the chassis  9  about a vertical pivot axis (which may be offset from the vertical axis at a caster angle). The first and second caster wheels  27  swing below a portion of the chassis  9 . The front caster wheels  27  may be spaced to allow a windrow of crop or forage material to pass between the front caster wheels  27  to, for example, engage a pick-up device  11 . 
     Each front caster wheel  27  has a rotational axis R 27  ( FIG. 2 ) about which the front caster wheels  27  rotate. Description herein to the position of the caster wheels  27  and/or their rotational axes R 27  relative to other portions and components of the vehicle  1  refers to the position of the caster wheels  27  when the vehicle is being propelled forward with the caster wheels generally aligned with the longitudinal axis A, unless stated otherwise. 
     As shown in  FIG. 2 , the rear wheels  17  have a diameter larger than the front caster wheels  27 . In some embodiments, the ratio of the diameter of the rear wheels  17  to the diameter of the front caster wheels  27  is at least about 1.25:1 or at least about 1.5:1 or even at least about 3:1. In some embodiments, the vehicle  1  may have a wheelbase of at least about 80 inches, at least about 100 inches or at least about 125 inches (e.g., from about 80 inches to about 200 inches, from about 100 inches to about 180 inches or from about 110 inches to about 150 inches). 
     The vehicle  1  includes an engine  101  (e.g., gas or diesel powered engine) that drives one or more hydraulic pumps that in turn power the various hydraulic motors and cylinders (e.g., first and second drive wheel motors and any device motors). The engine  101  also provides power for the electrical systems of the vehicle  1 . As shown in  FIG. 2 , the engine  101  is between the rotational axes R 17  of the rear drive wheels  17  and the rotational axes R 27  of the front caster wheels  27 . In other embodiments, the engine  101  is disposed to a side of the cab  121  and is above the front caster wheels  27 . The engine  101  may be arranged transverse to the longitudinal axis A of the vehicle  1 . 
     The engine  101  is disposed between the rotational axis R 17  of the rear drive wheels  17  and the operator station  13  ( FIG. 2 ) and cab  121 . In some embodiments, the “operator station” comprises the seat and controls for steering and controlling the speed of the vehicle  1 . The operator station  13  is enclosed in the cab  121 . As shown in  FIG. 2 , the operator station  13  is forward of the rear drive wheels  17  and is also forward to the engine  101 . At least a portion of the operator station  13  and/or cab  121  are disposed above the caster wheels  27  (i.e., above the caster wheels  27  when generally aligned with the longitudinal axis A as the vehicle is propelled forward.) Stated otherwise, at least a portion of the operation station  13  and/or cab  121  overlap the front caster wheels  27  relative to the longitudinal axis A (e.g., overlap a trailing portion of the caster wheel, overlap the caster wheel axle or overlap the entire caster wheel when the caster wheels  27  is generally aligned with the longitudinal axis A as the vehicle is propelled forward). Generally, the cab  121  and operation station  13  (e.g., operator seat) are not reversible. 
     The front caster wheels  27  are connected to the chassis independent from each other which allows the caster wheels to be independently suspended to absorb forces transmitted during travel over uneven terrain. The front caster wheels  27  may be part of first and second swivel caster assemblies  31 . Generally the first and second swivel caster assemblies  31  and subframes  41  described below are symmetric and description herein of an assembly or subframe also applies to the second assembly or subframe (e.g., description of a hub of the assembly indicates that the first assembly has a first hub and that the second assembly has a second hub). Each assembly  31  includes a hub  35  ( FIG. 8 ) and a caster shaft  37  (which may be referred to as a “kingpin”) that rotates within the hub  35 . The swivel caster assemblies  31  may include bushings or bearings within the hub  35  that allow for rotation of the shaft  37  within the hub  35 . Each caster shaft  37  is connected to a leg assembly  42  that connects to the front caster wheel axle. In the illustrated embodiment, the leg assembly  42  includes a single leg that attaches to an inner side of the wheel axle. In other embodiments, the leg assembly includes two legs that connect to the axle of the front caster wheel on each side of the wheel (as with a caster fork). 
     The hub  35  and shaft  37  form a swivel joint  43 . The first and second front caster wheels  27  of the caster assemblies  31  are each connected to a subframe  41  by the swivel joint  43 . The subframes  41  are suspended from the chassis  9  by a suspension system  51  having a suspension element  49 , shown as a hydraulic cylinder in the illustrated embodiment. With reference to  FIG. 10 , each cylinder  49  may be connected to an accumulator  50  in the suspension system with the hydraulic fluid being provided from a source  54  by a hydraulic pump  52 . At least one valve  70 ,  71 ,  72  regulates the hydraulic fluid in the suspension system  51 . Other suspension elements such as airbags and shock absorbers may be used. In some embodiments, a suspension mechanism such as single pivot joint of sliding/telescoping mechanism may connect the caster assemblies to the chassis (e.g., non-shock absorbing suspension mechanisms). 
     Each subframe  41  is also pivotally attached to the chassis  9  at an outer pivot point P 1  and an inner pivot point P 2 . In this arrangement, the chassis  9  is supported by the subframes  41  and the chassis  9  and components carried by the chassis (e.g., operator station and cab) may move up and down relative to the subframes  41  as the vehicle  1  travels over uneven terrain. 
     As shown in  FIG. 8 , the subframe  41  has a longitudinal arm  45  (or “first arm”) and lateral arm  47  (or “second arm”) that each extend from the chassis  9 . The swivel joint  43  is at the point at which the arms  45 ,  47  meet and is forward of the inner and outer pivot points P 1 , P 2  relative to a longitudinal axis A ( FIG. 7 ) of the vehicle  1 . The swivel joint  43  is also outward to both the inner and outer pivot points P 1 , P 2  relative to the lateral axis B ( FIG. 7 ) of the vehicle  1  (i.e., the outer pivot point P 1  of each subframe  41  is positioned between the inner pivot point P 2  and the point of attachment of the suspension element  49  relative to the lateral axis B). 
     In the illustrated embodiment, the first arm  45  is generally parallel to the longitudinal axis A ( FIG. 7 ) and the second arm  47  is generally parallel to the lateral axis B. In other embodiments ( FIGS. 13-14 ), the first arm  145  is angled upward toward the swivel joint  143  with respect to the longitudinal axis A. In the embodiment illustrated in  FIGS. 13-14 , the second arm  147  is generally parallel to the lateral axis B. 
     As shown in  FIG. 2 , the first and second front caster wheels  27  (i.e., the axes of rotation R 27  of each wheel) are offset from the swivel joint  43  relative to the longitudinal axis A ( FIG. 7 ) of the vehicle. The offset allows the first and second front caster wheels  27  to self-align with the direction of travel of the vehicle  1  as the vehicle is steered by differences between the speeds of the rear wheels  17 . In some embodiments, the offset of the caster wheels (i.e., distance between the axis of rotation R 27  of the wheel and the swivel joint  43 ) is at least about 4 inches, at least about 8 inches or from about 8 to about 20 inches. These ranges are exemplary and other ranges may be used unless stated otherwise. 
     In other embodiments and/or in different modes of operation the front caster wheels  27  are steered. In such embodiments, the offset may be eliminated. 
     In a drive wheel steering mode, the vehicle  1  is steered by creating a differential speed between the first and second rear drive wheels  17  (i.e., by creating a difference between the first drive wheel rotational speed and the second drive wheel rotational speed). In this mode, each drive wheel  17  is capable of being driven forward or in reverse independent of the speed and direction of the other wheel (i.e., the drive wheels may be operated in counter-rotation). As an operator controls a steering mechanism (e.g., steering wheel), the rear drive wheels  17  rotate at different speeds to steer the vehicle  1  through an arc or deviation in the travel pathway. The speed and direction of travel (forward or rearward) may be controlled by a separate operator control. In the drive wheel steering mode, the vehicle  1  may be turned within its own footprint. In this mode, the caster wheels  27  self-align with the direction in which the drive wheels propel the vehicle, i.e., the caster wheels  27  follow the direction of travel of the rear drive wheels  17 . 
     In a caster wheel steering mode, the swivel position of the caster wheels  27  may be controlled to steer the vehicle. As used herein, the “swivel position” of the caster wheels generally refers to the angular position of the caster wheels relative to the longitudinal axis A ( FIG. 7 ) of the vehicle. The caster wheels  27  are connected to a steering system that selectively controls the swivel position of the caster wheels  27 . 
     In some embodiments, the vehicle  1  includes a sensor  90  ( FIG. 8 ) to determine the pitch of the ground near the vehicle (e.g., forward or behind the vehicle). The sensor  90  (i.e., inclinometer) may be mounted to or supported by the chassis  9  of the vehicle  1 . Any suitable sensor for measuring the pitch or slope of the terrain may be used unless stated otherwise. The inclinometer may be configured as a mercury level. The inclinometer may generate a signal based on the incline of the vehicle itself relative to an absolute position. In other embodiments, the sensor measures the pitch of the ground near the vehicle  1  (e.g., by directing signals towards the terrain). The pitch of the ground refers to a difference in the height of the ground along the line of travel of the vehicle. 
     In some embodiments, the sensor  90  provides a signal related to the angle of inclination during or after adjustment of the steering system as described below (e.g., adjustment of one or more suspension elements) to improve the accuracy of the measurement. The signal from the sensor  90  (e.g., inclinometer) may be relayed to a controller (e.g., such as the control unit  80 ) to adjust the vehicle based on the signal after the suspension has been adjusted. In embodiments in which the self-propelled vehicle  1  is a self-propelled baler, the vehicle  1  may include a controller that presents a suggested bale position based on the pitch or the pitch and roll of the ground around the vehicle  1  as disclosed in commonly-owned PCT Patent Application No. PCT/US2017/033641, filed, May 19, 2017, entitled “Baling Vehicle with Automated Round Bale Ejection”, which is incorporated herein by reference for all relevant and consistent purposes. 
     Referring now to  FIG. 10 , the vehicle  1  includes a suspension system position sensor  61  that measures the position and/or distance of the suspension system  51  (e.g., suspension element  49  or subframe  41  suspended from suspension element  49 ) relative to the chassis  9  of the vehicle  1 . Any sensor that measures the position of the suspension system  51  relative to the chassis and/or the inclination or pitch of the chassis may be used unless stated otherwise. 
     In the embodiment illustrated in  FIG. 8 , the sensor  61  is a rotary potentiometer that is connected to the subframe  41  and the chassis  9  (e.g., through a linkage  62 ). In other embodiments, the position sensor  61  is an integral position sensor such as an integral position sensor within a hydraulic cylinder. The sensor  61  may operate by magnetostriction, by variable resistance (potentiometer-type) or by linear variable inductance. 
     The sensor  61  produces a signal based on the position of the suspension system  51  relative to the chassis  9  of the vehicle  1 . The sensor  61  is communicatively connected to the control unit  80  which in turn is communicatively coupled to first valve  70 , second valve  71 , and third valve  72 . In the illustrated embodiment, the control unit  80  is the same unit that controls operation of the drive systems. In other embodiments, the control unit  80  which adjusts the suspension system  51  is a different control unit. The control unit  80  and valves  70 ,  71 ,  72  regulate the hydraulic fluid in the hydraulic system with changes in the amount of hydraulic fluid altering the pressure supplied to suspension elements  49  (e.g., through action of accumulator  50  and pump  52 ). The valves  70 ,  71 ,  72  are controlled, at least in part, based on a signal produced by the control unit  80 . In other embodiments, the pump  52  is controlled, at least in part, based on a signal produced by the control unit  80 . 
     To adjust fluid pressure to suspension elements  49 , a valve (e.g., the first valve  70 ) may be used to control the pressure in the system downstream of the valve. For example, the first valve  70  may be a proportional relieving valve that is adjusted by variable electric input (e.g., variable input from controller  80 ). The first valve  70  is used to regulate the amount of hydraulic fluid downstream of the first valve  70  (e.g., by adding or removing fluid downstream of the valve) to control the pressure in suspension elements  49  and accumulator  50 . The illustrated arrangement of valves (e.g., first valve  70 , second valve  71 , and third valve  72 ) is an example and any valve or combination of valves which allows the suspension elements  49  to be controlled to adjust the pitch of the chassis  9  of the vehicle  1  may be used unless stated otherwise. In other embodiments, the pitch of the vehicle chassis  9  is controlled by devices other than valves (e.g., by controlling the hydraulic output of a dedicated hydraulic pump). 
     Another embodiment of a hydraulic suspension system  51  is shown in  FIG. 22 . The system includes two suspension position sensors  61  (i.e., two sensors  61  that measure the position of each suspension element  49 ). Each suspension position sensor  61  communicates with the control unit  80 . The third valve  72  is a normally closed valve. 
     The suspension system  51  may also be configured to absorb shock during operation of the vehicle  1 . The absorber  50  allows fluid to flow to and from the suspension elements  49  when the vehicle travels over uneven terrain. The suspension system  51  may include a check valve  82  and/or restrictor  84  disposed between the second valve  71  and the third valve  72 . The check valve  82  is configured to allow hydraulic fluid to flow to the accumulator  50  (e.g., to flow relatively quickly to the accumulator  50  for shock absorption when vehicle  1  encounters a change in terrain). The restrictor  84  is configured to allow hydraulic fluid to flow back to the suspension elements  49  (e.g., to dampen the flow of fluid to the suspension elements  49  to smooth the ride of an operator). 
     By varying the pressure of the hydraulic fluid supplied to the suspension elements  49 , the position of the suspension elements  49  may be adjusted to allow the pitch of the chassis  9  of the vehicle  1  to maintain a desired distance relative to the subframe  41 . Any control system in which the suspension system  51  (e.g., suspension elements  49 ) is adjusted based at least in part on a signal from the suspension system position sensor  61  may generally be used unless stated otherwise. 
     In the illustrated embodiment, the vehicle  1  includes a single suspension system position sensor  61 . In other embodiments, the vehicle  1  includes a first suspension system position sensor  61  that measures the position of the suspension relative to the chassis on the first side  58  ( FIG. 6 ) of the vehicle  1  and includes a second suspension system position sensor  61  that measures the position of a second suspension system on the second side  59  of the vehicle  1  as illustrated in the embodiment of  FIG. 22 . Alternatively or in addition, the suspension system position sensor  61  may generally be positioned at any position along the chassis  9  and/or subframe  41  to operatively measure the pitch of the vehicle relative to the suspension. 
     The control unit  80  that controls the position of the suspension elements  49  based at least in part on a signal from the suspension system position sensor  61  includes a processor and a memory. The processor processes the signals received from various sensors, selectors and control devices of the system. The memory stores instructions that are executed by the processor. 
     Control unit  80  may be a computer system. Computer systems, as described herein, refer to any known computing device and computer system. As described herein, all such computer systems include a processor and a memory. However, any processor in a computer system referred to herein may also refer to one or more processors wherein the processor may be in one computing device or a plurality of computing devices acting in parallel. Additionally, any memory in a computer device referred to herein may also refer to one or more memories wherein the memories may be in one computing device or a plurality of computing devices acting in parallel. 
     The term processor, as used herein, refers to central processing units, microprocessors, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuit or processor capable of executing the functions described herein. The above are examples only, and are thus not intended to limit in any way the definition and/or meaning of the term “processor.” 
     In one embodiment, a computer program is provided to enable control unit  80 , and this program is embodied on a computer readable medium. In an example embodiment, the computer system is executed on a single computer system, without requiring a connection to a server computer. In a further embodiment, the computer system is run in a Windows® environment (Windows is a registered trademark of Microsoft Corporation, Redmond, Wash.). In yet another embodiment, the computer system is run on a mainframe environment and a UNIX® server environment (UNIX is a registered trademark of X/Open Company Limited located in Reading, Berkshire, United Kingdom). Alternatively, the computer system is run in any suitable operating system environment. The computer program is flexible and designed to run in various different environments without compromising any major functionality. In some embodiments, the computer system includes multiple components distributed among a plurality of computing devices. One or more components may be in the form of computer-executable instructions embodied in a computer-readable medium. 
     The computer systems and processes are not limited to the specific embodiments described herein. In addition, components of each computer system and each process can be practiced independent and separate from other components and processes described herein. Each component and process also can be used in combination with other assembly packages and processes. 
     In some embodiments, pitch adjustment of the vehicle  1  may be suspended during one or more operations of the vehicle  1 . For example, pitch-adjustment (and, as in some embodiments, suspension system shock absorption) may be suspended during (1) counter-steering of the vehicle (i.e., when the rear wheels  17  rotate in opposite directions), (2) during zero-turn steering (i.e., when one rear wheel  17  is stationary while the other rear wheel  17  rotates), (3) during operation at vehicle speeds less than a threshold (e.g., less than 3 mph), (4) while the vehicle travels in reverse (i.e., both drive wheels rotate in reverse), and/or (5) during ejection of a load from the vehicle (e.g., in embodiments in which the self-propelled vehicle is a baler, during ejection of a bale). Pitch-leveling may be suspended by closing the third valve  72  to isolate the suspension elements  49  from the accumulator  50 . The third valve  72  may be a proportional relieving valve. In other embodiments, two or more valves block flow to the accumulator  50  and/or block flow between suspension elements  49 . In some embodiments, control unit  80  receives signals from the speed and direction control device  78  ( FIG. 12 ) and/or mode selector  79  and suspends pitch-adjustment and/or shock absorption based on signals received from the speed and direction control device  78  and/or mode selector  79 . 
     The control unit  80  may be configured such that the suspension elements  49  are adjusted to a targeted compression length (or within a range of compression lengths). The targeted compression length may be a compression length which allows for a full range of suspension, such as near the center of the range of motion of the suspension element  49 . By moving the suspension elements (e.g., hydraulic cylinders) to a compressed length at which a substantially full range of suspension may be achieved, the suspension elements are less limited in the amount of suspension they provide even with changing vehicle loads and/or travel over terrain in which the pitch of the ground changes. In some embodiments, the suspension elements are not centered within their effective range of motion. 
     In some embodiments, the control unit  80  and/or sensor  61  are calibrated to match the pitch of the chassis  9  with the ground pitch. Calibration may be performed when the ground pitch is substantially zero. Calibration may also be performed under load (e.g., with a bale in the baling chamber to account for pitch change caused by tire squat or deflection). 
     Leveling of the pitch of a vehicle having a front-wheel suspension is shown schematically in  FIGS. 15-21 . Without pitch-leveling, removing a load from the vehicle  1  ( FIG. 15 ) causes the pitch of the chassis of the vehicle  1  to not match the ground pitch. Adding a load ( FIG. 16 ) without pitch-leveling also causes the chassis pitch to mismatch the ground pitch. By adjusting the suspension elements  49  based on output from the suspension system position sensor  61 , the pitch of the vehicle chassis may more closely match the pitch of the ground ( FIG. 17 ) which improves the measurement of an inclinometer onboard the vehicle. 
     Similarly, traveling over ground that is downward pitched ( FIG. 18 ) or upward pitched ( FIG. 20 ) may cause the pitch of the chassis to not match the pitch of the ground. This may be corrected ( FIGS. 19 and 21 ) by adjusting the suspension elements  49  based on output from the suspension system position sensor  61 . 
     In the illustrated embodiment, the front wheels  27  are suspended by suspension elements  49  ( FIG. 8 ). In other embodiments, the rear wheels  17  are suspended by suspension elements  49  or both the front wheels  27  and rear wheels  17  are suspended by suspension elements  49 . The control unit  80  may adjust the suspension of any wheel  17 ,  27  which includes a suspension element  49 . For example, the pitch of the chassis  9  may be adjusted in embodiments in which only the front wheels  27  are suspended, only the rear wheels  17  are suspended, or both the front  27  and rear wheels  17  are suspended. 
     Compared to conventional vehicles, embodiments of the self-propelled vehicle of the present disclosure have several advantages. By adjusting the suspension elements at least in part based on the signal from the suspension system position sensor, the pitch of the chassis of the vehicle may be made generally level with the pitch of the ground as the load of the vehicle changes and/or the pitch of the ground changes during travel of the vehicle. Adjusting the suspension elements allows the suspension elements to be adjusted toward a target compression length such a compression length with a larger increase in the suspension range (e.g., toward the center of the range of motion of the suspension elements). By maintaining the pitch of the vehicle chassis closer to the pitch of the ground during operation of the vehicle, the caster wheels are more maneuverable during vehicle turning and the vehicle operator does not sense misalignment of the pitch of the vehicle chassis with the pitch of the ground (i.e., sense a “nose-up” or “nose-down” condition). Further, in vehicles that include an inclinometer onboard the vehicle to measure the pitch of the ground, the accuracy of the inclinometer may be improved by leveling the vehicle chassis pitch with the pitch of the ground. 
     As used herein, the terms “about,” “substantially,” “essentially” and “approximately” when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation. 
     When introducing elements of the present disclosure or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “containing” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience of description and does not require any particular orientation of the item described. 
     As various changes could be made in the above constructions and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense.