Abstract:
A cruise control system for a utility vehicle, which is speed-controlled by a hydrostatic transmission, includes a controller having an input for receiving a selectable set speed signal, and an output for sending a speed control signal to at least one proportional valve of a hydrostatic transmission servo control system. The vehicle ground speed is continuously monitored by a speed sensor operatively associated with a rotating gear in the vehicle drive train that rotates in proportion to vehicle ground speed, to create a ground speed signal. The selectable set speed signal can be input to controller memory by driving the vehicle at a desired ground speed and then activating a speed set switch. The speed control algorithm of the controller thereafter compares the set speed to the ground speed signal from the speed sensor and corrects the control signal to the proportional valve to correct hydrostatic transmission speed output.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The invention relates to agricultural or industrial vehicles and particularly to speed control of such utility vehicles. 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  
         [0002]    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.  
           [0003]    Prior utility vehicles implement speed control or cruise control by various methods of setting and holding the swashplate angle of the variable displacement pump. In hydrostatic transmissions that are operator-modulated by mechanical displacement controls, cruise control devices have included electromagnets or friction plates to hold levers or pedals at a selected position corresponding to the desired speed. Electronically controlled hydrostatic transmissions usually hold the displacement of the variable displacement pump by maintaining a set electrical current to electro-hydraulic proportional pressure reducing valves which are used to control the swashplate angle.  
           [0004]    In some hydrostatic transmission speed control systems, the vehicle speed is controlled by maintaining a constant energizing current set point to the proportional valves of the hydrostatic transmission. These systems are referred to as “open loop” systems, i.e., there is no mechanical feedback of the swashplate position. Hydrostatic transmission pumps that lack mechanical feedback control of the swashplate position cannot maintain constant ground speed for all conditions, simply by maintaining a level of electrical energizing current to the proportional valves. For example, increased load on the vehicle will result in a reduced speed due to engine speed changes or transmission efficiency changes.  
           [0005]    In some other systems, swashplate mechanical position feedback control is provided. In these systems, the loads that tend to change the pump displacement by changing the swashplate angle are countered by swashplate mechanical position feedback control, so ground speed can be maintained by increasing or decreasing control current to the proportional valves in response to the mechanical position feedback control. However, this system provides a control loop that only maintains swashplate angle. The ground speed of the vehicle cannot be ensured by the control loop, as the control loop does not compensate for engine speed changes or transmission efficiency changes caused by load changes.  
           [0006]    The present inventors have recognized the desirability to implement speed control in a hydrostatic transmission whereby swashplate mechanical position feedback control is not require. The present inventors have recognized the desirability of providing a speed control system, which is minimally affected by engine speed changes, or transmission efficiency changes caused by load changes. The present inventors have also recognized the desirability of providing an effective method of increasing or decreasing the speed set point for both slow and fast vehicle speeds.  
         SUMMARY OF THE INVENTION  
         [0007]    This invention implements a speed control or cruise control system for a utility vehicle that is speed-modulated by a hydrostatic transmission. The speed control system includes a controller, such as a microcontroller having an input for receiving a selectable set speed signal, and an output for sending a speed control signal to at least one proportional valve of the hydrostatic transmission servo control system. The vehicle ground speed is continuously monitored by a speed sensor operatively associated with a rotating part in the vehicle drive train that rotates in proportion to vehicle ground speed, to create a ground speed signal. The selectable set speed signal can be input to microcontroller memory by driving the vehicle at a desired ground speed and then activating a speed set switch. By activating the speed set switch, the speed sensor inputs the current ground speed to the microcontroller memory. The speed control algorithm of the microcontroller thereafter compares the set speed to the ground speed signal from the speed sensor and corrects the control signal to the proportional valve to correct hydrostatic transmission speed output. Additionally, a speed increase/decrease switch is provided to manually increment the set speed, by an amount proportional to the set speed.  
           [0008]    The hydrostatic transmission includes a variable displacement pump providing variable volume flow rate of a pressurized hydraulic fluid. The pump has an angularly adjustable swashplate, pump displacement being set by the angle of the swashplate. At least one proportional control valve is operatively connected to the swashplate to change the angle of the swashplate, the control valve being signal-connected to the output of the microcontroller. A hydraulic motor receives the pressurized hydraulic fluid from the pump to rotate the hydraulic motor.  
           [0009]    The hydraulic motor is operatively connected to the rotating part of the drive train, preferably a gear, in order to rotate the part by rotary power from the hydraulic motor. The speed sensor is arranged to sense rotation of the part, the sensor being signal-connected to the microcontroller to send a rotation speed signal, or a stream of pulses, to the microcontroller. The microcontroller controls the proportional control valve to change the angular position of the swashplate in response to a difference, calculated by the microcontroller, between the rotation speed signal and the set speed.  
           [0010]    A method of controlling the speed of a utility vehicle is set forth. The method includes the steps of: continuously sensing the rotational speed of a rotating part in a transmission of the vehicle; operating the vehicle at a desired ground speed; at the desired ground speed, selecting the rotational speed as a set speed; and if the rotational speed of the rotating part differs from the set speed, changing the transmission output to diminish the difference.  
           [0011]    The preferred embodiment speed control system of the 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 and selector switches, and a programmed microcontroller having a speed control algorithm and memory means.  
           [0012]    The operation of the preferred embodiment cruise control function is as follows: an on/off switch is positioned to activate the use of the cruise control function in the microcontroller software. Using the foot pedal controls on the vehicle, the operator drives the vehicle to the speed desired for the specific task. Once at the desired ground speed, a second switch is momentarily depressed and the microcontroller records the speed of a rotary part, e.g., a gear, present in the transmission that rotates proportionally to ground speed, as a set speed. The set speed is sensed by the Hall effect pulse pickup unit located adjacent to the rotating part, and is recorded in memory in the microcontroller. The current being supplied to proportional valves used to control the swashplate angle of the variable displacement pump is recorded in the memory of the microcontroller at the same time. Using the recorded current as a starting point, the control current to the proportional valves that control transmission output is modulated via a control algorithm to maintain the set speed of the vehicle. The algorithm monitors the speed signal from the Hall effect pulse pickup unit and increases or decreases the control current to the proportional valves using the error between the set speed and the actual speed signal from the pulse pickup unit.  
           [0013]    While in cruise control mode, the set speed can be incremented up and down in steps by depressing momentary switches. For each depression of the switch, the microcontroller responds by changing the set speed by a percentage of the current set speed. This percentage is adjustable in the microcontroller software. Changing the speed by a percentage provides small speed changes when the vehicle set speed is slow and larger speed changes at higher vehicle set speeds. This is an advantageous feature of the invention. Slow operations generally require small speed adjustments, but larger speed adjustments are typically desired while transporting at higher speeds on the road.  
           [0014]    The invention provides a cruise control for hydrostatic transmissions that does not require swashplate position feedback. By using a transmission speed signal to represent ground speed, and the speed signal to create a set point speed, and using feedback control, the set speed and feedback control are independent of engine speed changes and transmission efficiency changes caused by vehicle load changes.  
           [0015]    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  
       [0016]    [0016]FIG. 1 is a block diagram of a utility vehicle cruise control system of the present invention;  
         [0017]    [0017]FIG. 2 is a schematic sectional view of the servo control system used in a hydrostatic transmission of FIG. 1;  
         [0018]    [0018]FIG. 3 is an exploded, fragmentary perspective view of the servo control system of FIG. 2;  
         [0019]    [0019]FIG. 3A is a schematic sectional view of a proportional pressure control valve of the system of FIG. 3;  
         [0020]    [0020]FIG. 4 is a schematic sectional view of a hydrostatic transmission;  
         [0021]    [0021]FIG. 5 is a block diagram of the cruise control algorithm steps of the present invention;  
         [0022]    [0022]FIG. 6 is a block diagram of a speed control algorithm routine incorporated into the present invention; and  
         [0023]    [0023]FIG. 7 is a speed change diagram demonstrating the operation of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    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.  
         [0025]    [0025]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 to, for transmitting power through a differential (not shown) to one or more driven wheels  28 .  
         [0026]    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 .  
         [0027]    The control system  24  includes a controller, such as a microprocessor-based microcontroller  52 , in signal-communication with a cruise control on/off switch  56  and a set switch  58 . The on/off switch  56  enables/disables the cruise control algorithm of the microcontroller  52 , and the set switch  58  is selectively activated to activate the cruise 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.  
         [0028]    The control system  24  includes 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 .  
         [0029]    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.  
         [0030]    [0030]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 directs the hydraulic fluid back to the reservoir of the vehicle.  
         [0031]    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.  
         [0032]    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 cam plate  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 cam plate  118  can be positioned in a range of forward positions selected by the foot pedal  72 .  
         [0033]    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 driver  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  is controlled to allow fluid to flow from within the cylinder  114 , from the one side of the servo piston  112 , to the return channel R.  
         [0034]    Preferably, the valve solenoids  106   a ,  108   a  are driven by pulse width modulation type current signals and produce controlling pressure proportional to the controlled width of step pulses of current applied to the solenoid drivers. While the frequency of the pulses remains substantially the same, the pulse widths are changed to modulate the valves.  
         [0035]    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 vehicle.  
         [0036]    An exemplary example of a control valve, such as the control valve  106 , is illustrated in FIG. 3A. 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.    
         [0037]    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 .  
         [0038]    An annular screen  107   a  and a circular screen  125   a  can be supplied to the inlet  107  and to the outlet  125  respectively.  
         [0039]    The control valve  108  can be identically configured as described above for the control valve  106 .  
         [0040]    [0040]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).  
         [0041]    Fluid flow through the pump  30  is controlled by changing the angle of the swashplate  118 . The location, off center, of the cam plate controls the distance the pistons  130  travel inside the piston bores  132  of the rotating assembly. The direction that the cam plate is rotated from center determines the direction of fluid flow (forward or reverse). The number of degrees the cam plate is deflected determines how much fluid will be displaced, i.e., controlling transmission speed.  
         [0042]    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.  
         [0043]    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.  
         [0044]    The cruise 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 set speed. The speed control algorithm of the microcontroller  52  thereafter will control the 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 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.  
         [0045]    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  322 , monitoring the rotational speed of the rotating part; step  324 , comparing the rotational speed to the set speed; step  326 , determining a difference between the rotational speed and the set speed; and step  328 , if the rotational speed of the rotating part differs from the set speed, 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.  
         [0046]    [0046]FIG. 6 illustrates a control algorithm routine of the software of the microcontroller  52  which compares the ground speed as sensed by the Hall effect pickup unit  46  to the set speed recorded in memory of the microcontroller and which uses PID (proportional, integral, derivatives) 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.  
         [0047]    The cruise control function can be turned off by: 1. depressing the brakes; 2. depressing the reverse foot pedal; 3. turning off the switch that activates cruise control; 4. repeatedly depressing the switch to decrement the speed set point below the lowest set point allowed by the software.  
         [0048]    To resume to the previously set speed, the operator must first initiate some forward motion using the normal foot pedal controls. The increase/decrease switch  64  also functions as the resume switch when the cruise control function is in the turned off mode. Once moving, the operator can then momentarily depress the switch  64  and the microcontroller will control the vehicle speed to increase or decrease to the previous set speed using the cruise control algorithm in the microcontroller.  
         [0049]    [0049]FIG. 7 illustrates the proportional relationship between the vehicle speed and the vehicle speed change for manually selected increment/decrement speed change steps. Using the speed selector switch  64 , the cruise control set speed can be manually increased or decreased incrementally by a percentage of the current set speed. For slower speeds, the incremental speed change caused by activating the momentary switch will be a preselected percentage of such lower speed, in effect, a small speed change. For greater speeds the incremental speed change caused by activating the momentary switch will also be the preselected percentage applied to the greater set speed, in effect, a relatively large speed change. This relationship is important because at low speeds, such as for inching control, the speed change required are correspondingly small. For highway travel, the speed change is required are correspondingly a larger.  
         [0050]    For example, as shown in FIG. 7, with the set speed set at 60 percent of full tractor speed, an activation of the switch  64  for either increase or decrease in the set speed, results in a speed change, plus or minus, of about 3 kph. With the set speed set at 100 percent of full tractors speed, an activation of the switch  64  for either increase or decrease in the set speed, results in a speed change, plus or minus, of about 5 kph.  
         [0051]    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.