Abstract:
A control system sets a set maximum speed of a utility vehicle, below the capable maximum speed of the vehicle, and recalibrates speed output signals corresponding to the set maximum speed. The system includes a microcontroller and a user-operated vehicle speed actuator, signal-connected to the microcontroller. The speed actuator is calibrated to actuate speeds, via the microcontroller, in a range from a minimum speed to a maximum speed. A user-operated speed set activator is used in conjunction with a ground speed sensor to set the set maximum speed. The microcontroller records the new maximum speed of the vehicle, and recalibrates the pedal position according to the new maximum speed.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to vehicles for industrial and agricultural use, such as utility tractors. Particularly, the invention relates to speed control of a utility vehicle that incorporates a hydrostatic transmission as an operator-controlled speed-adjusting component of the vehicle drive train. 
     BACKGROUND OF THE INVENTION 
     Agricultural or industrial utility vehicles typically utilize a drive train having an engine driving a hydrostatic transmission that drives a final drive transmission or range transmission of the vehicle. The final drive transmission drives at least one wheel. JOHN DEERE Series 4000 tractors, available from John Deere Commercial Products, include such drive trains. The vehicle speed is typically operator-modulated by changing the drive ratio of the hydrostatic transmission via movement of a foot pedal. The drive ratio is changed by changing the angle of a swashplate of a variable displacement pump of the hydrostatic transmission. 
     In the operation of tractors or other self-propelled machinery using hydrostatic transmissions, the maximum speed of operation is often limited to some value less than the full speed capacity of the vehicle. The operator modulates speed to any value less than the selected maximum speed. The operator performs the speed modulation control manually by adjusting foot pedal position. Given the entire pedal position range from a minimum speed up to the maximum speed, a precise modulation of the speed within a narrow speed tolerance between the minimum and maximum speeds is difficult to achieve. Attempting to modulate accurately at a precise speed between the minimum and maximum speeds is fatiguing to the operator, particularly to the operator&#39;s leg. 
     A partial solution to this problem is a kit, available from John Deere Commercial Products for current JOHN DEERE 4000 Series tractors, that provides an adjustable mechanical stop to limit foot pedal travel and thus limit the maximum speed of the tractor. This solution however, cannot adjust to changes in engine speed, nor is it easy to adjust for changes in working conditions. This technique is also not effective for ground speed control in systems using hydrostatic transmissions that lack swashplate position feedback control. 
     The present inventors have recognized the desirability of providing an adjustable maximum speed control which accurately sets a maximum speed, corresponding to a terminal pedal position, and which allows effective operator speed modulation st speeds less than the set maximum speed. 
     Additionally, the present inventors have recognized that when operating tractors or other self-propelled machines in which the speed is controlled by a foot pedal or other manual means, it would be desirable to improve the sensitivity of the speed control for better “inching” control, i.e., moving the machine very small distances and/or at very slow speeds. 
     SUMMARY OF THE INVENTION 
     A control system is provided for setting a set maximum speed of a utility vehicle, below the capable maximum speed of the vehicle, and recalibrating speed output signals corresponding to the set maximum speed. The system includes a microcontroller and a user-operated vehicle speed actuator, signal-connected to the microcontroller. The speed actuator moves over a limited mechanical range between a start end stop and a terminal end stop, and is calibrated to actuate speeds, via the microcontroller, in a range from a minimum speed to a maximum speed. The maximum speed corresponds to the terminal end stop. 
     A user-operated speed set activator is used in conjunction with a ground speed sensor to set the maximum speed. The ground speed sensor is signal-connected to the microcontroller, wherein, when initiated by the user, the activator causes the microcontroller to record the instantaneous ground speed signal from the sensor. The microcontroller sets a new maximum speed of the vehicle, the new maximum speed set to the instantaneous ground speed and the pedal position is recalibrated such that the new maximum speed corresponds to the pedal position at the terminal end stop. The microcontroller recalibrates the speed commands issued by the microcontroller to the servo control of the transmission, to be in a range from the minimum speed corresponding to the start end stop to the new maximum speed corresponding to the terminal end stop. 
     The speed set activator is operable to set a maximum speed for both forward and reverse vehicle operation. 
     The present invention uses speed control foot pedals with potentiometer sensors, a Hall effect sensor that measures the speed of a gear in the final drive of the transmission, on/off switches, a microcontroller and software. The invention provides a system and method for recalibrating the full pedal range of motion to provide for increased sensitivity of the speed control. 
     A method of controlling a ground speed of a utility vehicle is set forth. The method is adapted to control speed of a utility vehicle having an engine coupled to a transmission, the transmission coupled to a driven wheel, and a speed actuator movable over an actuator range to a terminal end stop, the actuator calibrated to drive the vehicle at speeds between a minimum speed and a maximum speed, the maximum speed corresponding to the terminal end stop. The method includes the steps of: during running out of the vehicle, selecting an instantaneous ground speed; setting a new maximum vehicle ground speed at the instantaneous ground speed; and recalibrating the actuator range from the minimum speed to the new maximum speed, the new maximum speed corresponding to the terminal end stop. 
     The operation of the maximum speed function according to the preferred embodiments is more particularly set forth as follows: an on/off switch is positioned to activate the use of the maximum speed function in the microcontroller software. Using the foot pedal controls on the vehicle, the operator drives the vehicle to the maximum speed limit desired for the specific task. Once at the desired maximum speed, a second switch is momentarily depressed and the microcontroller records the speed present in the final drive of the transmission from a Hall effect pulse pickup unit, as a new maximum speed limit. When the operator then releases the control pedal, the vehicle stops and the software becomes temporarily recalibrated so that the recorded new maximum speed limit is achieved with full foot pedal depression. The maximum speed limit can be increased or decreased incrementally during operation by momentarily depressing switches. To prevent the operator from disabling all vehicle motion, the algorithm requires the speed limit to be greater than zero. Moving the maximum speed switch to the off position disables the maximum speed limit function. 
     The vehicle response is scaled to the set maximum speed. In normal operation, the software in the microcontroller controls the ground speed from zero to the top vehicle speed as a preselected relationship to the pedal position input. When the maximum speed function is engaged, the relationship is rescaled to provide the new maximum set speed corresponding to a fully depressed pedal position. The new ground speed is thereafter controlled by a software control loop using the Hall effect pulse pickup signal for feedback control. 
     By basing the maximum speed limit on a measured transmission gear speed and using feedback control techniques, the speed limit control becomes insensitive to engine speed, terrain, attachment loading and gear selection. The driver can modulate the speed of the vehicle to any value less than the set maximum speed limit and return to the desired maximum speed limit simply by depressing the speed control pedal to its full travel. Preferably, the set maximum speed limit is the desired substantially constant operating speed. Holding the pedal in its fully depressed position is less fatiguing than having to hold the pedal partially depressed. 
     A second benefit to the maximum speed function is improved speed control sensitivity. The speed control software can be recalibrated according to a new maximum speed, substantially reduced from the actual maximum speed of the vehicle, so that each increment (inch or mm) of resulting pedal travel represents less speed change than when the maximum speed function is inactive. This permits the driver to optimize his control of the speed and position of the vehicle when attaching implements or working in close spaces. 
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is block diagram of the method of the utility vehicle speed control system of the present invention; 
     FIG. 2 is a schematic sectional view of the servo control system used in a hydrostatic transmission of FIG. 1; 
     FIG. 3 is an exploded, fragmentary perspective view of the servo control system of FIG. 2; 
     FIG. 3A is a schematic sectional view of a proportional pressure control valve of the system of FIG. 3; 
     FIG. 4 is a schematic sectional view of a hydrostatic transmission; 
     FIG. 5 is a block diagram of the speed control algorithm steps of the present invention; 
     FIG. 6 is a block diagram of a speed control algorithm routine incorporated into the present invention; and 
     FIG. 7 is a speed calibration diagram demonstrating the operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, a specific embodiment thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated. 
     FIG. 1 illustrates, in block diagram form, a vehicle  20  incorporating a preferred embodiment drive control system  24  of the present invention. The vehicle incorporates a hydrostatic transmission  26  and a range transmission  27 , such as a multi-speed gear transmission, for transmitting power through a differential (not shown) to one or more driven wheels  28 . 
     The hydrostatic transmission  26  includes a variable displacement pump  30 , and a hydraulic motor  34 . An engine drive  35  rotationally drives the variable displacement pump  30 . The hydraulic motor drives the multi-gear transmission drive  27  interposed between the hydraulic motor  34  and the driven wheel  28 . The range transmission  27  includes a transmission gear  40 . A transmission speed pickup  46 , such as a Hall effect sensor, is located in close proximity to the transmission gear  40 . 
     The control system  24  includes a controller  52 , such as a microprocessor-based microcontroller, in signal-communication with a speed control on/off switch  56  and an operator-operated speed set activator in the form of a maximum speed set switch  58 . The on/off switch  56  enables/disables the maximum speed control algorithm of the microcontroller  52 , and the set switch  58  is selectively activated to activate the maximum speed control algorithm in the microcontroller, as described hereinafter. The microcontroller  52  is also in signal-communication with an increase/decrease speed switch  64 , the operation of which will be described hereinafter. 
     The control system  24  includes an operator-operated vehicle speed actuator in the form of a forward pedal  72  and a reverse pedal  74 . The forward pedal  72  is operatively engaged with a potentiometer  82  to produce a forward pedal position signal, and a reverse pedal  74  is operatively engaged with a potentiometer  84  to produce a reverse pedal position signal. The potentiometers  82 ,  84  are signal-connected to the controller  52 . The foot pedal  72  is depressible to a terminal position or bottom end stop  73 . The foot pedal  74  is depressible to a terminal position or bottom end stop  75 . 
     The controller  52  is signal-connected, through appropriate signal conditioning or amplifying circuitry (not shown), to a solenoid  106   a  of a forward drive proportional pressure control valve  106  and to a solenoid  108   a  of a reverse drive proportional pressure control valve  108 . The output current to energize the forward or reverse control valve solenoids  106   a ,  108   a  is substantially proportional to the corresponding pedal position signal. 
     FIGS. 2 and 3 illustrate the hydrostatic transmission servo control in more detail. Given an engine drive speed and a range transmission or final drive gear selection, the hydrostatic transmission provides infinitely variable speed control, forward and reverse, by operation of the foot pedals  72 ,  74 . Each valve  106 ,  108  is connected to a source of pressurized hydraulic fluid S and a return channel R that is at a reduced hydraulic pressure. Preferably, the return channel R recirculates hydraulic fluid back to the reservoir of the vehicle&#39;s hydraulic system. 
     Depressing the forward foot pedal  72  causes an electrical output signal or voltage of the potentiometer  82  to be transmitted to the controller  52 . The controller  52 , through software, generates a pre-selected current ramp output, to energize the solenoid  106   a  of the forward drive proportional valve  106 . The proportional valve  106  is opened according to the ramp output, allowing pressurized hydraulic fluid, fed from the source S into the inlet  107  of the valve  106 , to flow through the valve  106 . The pressurized hydraulic fluid is communicated into, and pressurizes, a servo cylinder  114  on one side of a servo piston  112  that is slidably housed in the cylinder  114 . The other valve  108  allows fluid to flow from within the cylinder  114 , from an opposite side of the servo piston  112 , to the return channel R. 
     The piston  112  has a notch  115  that holds a piston follower  116  (FIG.  3 ). The piston follower  116  controls movement of a variable displacement pump cam plate or swashplate  118 . Movement of the piston  112  causes the swashplate  118  in the hydraulic pump to rotate out of the neutral position. Maximum displacement of the pump  30  is attained when the servo piston  112  is moved to its extreme position. The swashplate  118  can be positioned in a range of forward positions selected by the foot pedal  72 . 
     When the reverse pedal  74  is pressed, the potentiometer  84  sends an electrical output signal or voltage to the controller  52 . The controller  52 , through software, generates a pre-selected current output ramp to energize the solenoid  108   a  of the reverse drive proportional valve  108 . The reverse drive proportional valve  108  is opened, according to the ramp output, to allow pressurized hydraulic fluid, fed into an inlet  119  of the valve  108  from the source S, to flow through the valve  108 . The pressurized hydraulic fluid is communicated into, and pressurizes the servo cylinder  114  on an opposite side of the servo piston  112  within the cylinder  114 . The other valve  106  allows fluid to flow from within the cylinder  114 , from the one side of the servo piston  112 , to the return channel R. 
     Preferably, the valve solenoids  106   a ,  108   a  are driven by pulse width modulation type currents and causes pressure to be modulated at the outlet proportionally according to the controlled width of step pulses of current applied. While the frequency of the pulses remains substantially the same, the pulse widths are changed to modulate the valves. 
     The hydrostatic system is preferably a closed loop fluid power system that consists of a charge pump (not shown), and the variable displacement pump  30 , which is driven by a flex plate/dampener assembly (not shown) connected to the engine flywheel. The charge pump provides pressurized fluid to the proportional valve inlets  107 ,  119 . Return fluid from the servo control unit is routed to the reservoir of the vehicles hydraulic system. 
     An exemplary example of a control valve, such as the control valve  106 , is illustrated in FIG.  3 A. The solenoid  106   a  includes a plunger  120  (shown schematically) driven by the solenoid coil  121  (shown schematically). The plunger  120  drives a valve spool  122  within a housing  123 . The housing provides the pressurized hydraulic fluid inlet  107 , in the form of plural openings, and an outlet  124 , in the form of plural openings, to the hydraulic fluid reservoir. A control pressure outlet  125  communicates hydraulic fluid at a modulated pressure to the servo cylinder  114  as shown in FIG.  2 . The solenoid coil  121  drives the plunger  120  downward (in FIG. 3A) to open the inlet  107  to the outlet  125  through an annular channel  122   a.    
     The channel  122   a  is open to an oblong orifice  122   b  through the spool  122  to communicate fluid into an interior  122   c  of the spool. The interior of the spool is open to the outlet  125 . The pressure of the hydraulic fluid at the control outlet  125  is substantially proportional to the force applied to the spool by the plunger, ranging between reservoir pressure, the pressure at the outlet  125  with the inlet  107  closed, as shown in FIG. 3A, to pressurized hydraulic fluid supply pressure, the spool  122  moved down to close the outlet  124  and open the inlet  107 . 
     An annular screen  107   a  and a circular screen  125   a  can be supplied to the inlet  107  and to the outlet  125  respectively. 
     The control valve  108  can be identically configured as described above for the control valve  106 . 
     FIG. 4 illustrates the hydrostatic transmission  26  in more detail. The hydrostatic pump  30  illustrated is an axial piston, servo controlled, variable displacement piston pump. Input shaft splines  126  are driven via a flex plate (not shown) bolted onto the engine flywheel (not shown). 
     Fluid flow through the pump  30  is controlled by changing the angle of the swashplate  118 . The location, off center, of the swashplate controls the distance the pistons  130  travel inside the piston bores  132  of the rotating assembly. The direction that the swashplate is rotated from center determines the direction of fluid flow (forward or reverse). The number of degrees the swashplate is deflected determines how much fluid will be displaced, i.e., controlling the transmission speed. 
     The hydrostatic pump  30  provides hydraulic fluid to the hydrostatic motor  34  through the back plate  138 . Hydraulic fluid in the power train circulates in a closed loop. Fluid leaves the hydrostatic pump  30 , flows through the hydrostatic motor  34 , and is returned to the hydrostatic pump. Fluid that leaves this closed loop circuit, such as to the case drain, is replenished by fluid from the charge pump. 
     The hydrostatic motor  34  is a high torque axial piston motor. The motor is located on the rear of the back plate. The hydrostatic motor drives an output shaft coupled to the range transmission  27  that transfers power to the wheels. The range transmission  27  can be a multi-speed range gear transmission, such as a three-speed or four-speed gearbox. 
     The speed control system  24  of the invention can be activated by pushing the on/off switch  56  and then operating the vehicle to the desired ground speed and then activating the set switch  58  to select the desired maximum set speed that corresponds to the ground speed at that instant. The operator then releases the respective control pedal  72  or  74 , the vehicle stops, and the controller  52  recalibrates the pedal travel up to the new maximum set speed corresponding to the foot pedal  72  located against the end stop  73  or the foot pedal  74  located against the end stop  75 . The speed control algorithm of the controller  52  thereafter will control the speed throughout pedal travel in a range up to the maximum set speed using a PID routine as described below with regard to FIGS. 5 and 6. If it is desired to increase or decrease the set maximum speed by a preselected percentage, an increase/decrease speed switch  64  can be activated to incrementally increase or decrease the set speed as described below with regard to FIG.  7 . The maximum speed control function can be turned off by the switch  56  that activates maximum speed control. 
     A method of controlling the speed of a utility vehicle is set forth in FIG.  5 . The method includes the steps of: step  300 , continuously sensing the rotational speed of a rotating part in a range transmission of the vehicle; step  304 , operating the vehicle at a desired ground speed; step  308 , at the desired ground speed, selecting the rotational speed as a set speed; step  312 , recording the rotational speed of the part as a set speed; step  316 , recording the energizing current to the proportional control valves of the hydrostatic transmission as an initial energizing current; step  320 , recalibrating the relationship between the pedal travel and the microcontroller output to the proportional valves such that the set maximum speed corresponds to the maximum pedal travel; step  322 , monitoring the rotational speed of the rotating part; step  324 , comparing the rotational speed to the speed commanded by the pedal position; step  326 , determining a difference between the rotational speed and the speed commanded by the pedal position; and step  328 , if the rotational speed of the rotating part differs from the speed commanded by the pedal position, using a correction algorithm routine to change the energizing current to the proportional control valves of the hydrostatic transmission to change the transmission speed output to diminish the difference. 
     FIG. 6 illustrates a control algorithm routine of the software of the microcontroller  52  which compares the vehicle ground speed as sensed by the Hall effect pickup unit  46  to the speed commanded by the pedal position sensor, and which uses PID (proportional, integral, derivative) feedback control mathematics to diminish the difference by controlling the speed output of the hydrostatic transmission. The routine changes the output signal from the microcontroller to the proportional control valves to reduce or increase the hydrostatic transmission speed output. 
     FIG. 7 illustrates the proportional relationship between the vehicle speed and the and pedal stroke, or position, for two speed profiles, a normal speed profile NS wherein the maximum speed is approximately the maximum speed of the vehicle, and a set maximum speed profile SP wherein the maximum speed, corresponding to the foot pedal terminal position, is a reduced maximum speed. The speed profiles shown in FIG. 7 illustrate a maximum speed of 30 kph for the normal speed profile NS, and a maximum speed of 15 kph for the maximum set speed profile SP. Both speed profiles include an inflection at about 50 percent of pedal stroke for increased vehicle drivability. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.