Abstract:
A control system for a utility vehicle transmission provides for operator selection of vehicle “aggressiveness,” or rates of acceleration in response to operator command. The aggressiveness of a vehicle&#39;s performance can be controlled by modulating control signals to proportional control valves, which determine the transmission acceleration, according to two or more electrical ramp-up (or ramp-down) profiles, in response to an operator&#39;s acceleration command (or deceleration command). The transmission control system includes a controller, directional switches and electro-hydraulic valves which control hydraulic pressure in the servo control system of a hydrostatic transmission. The operator is provided with a two-position set switch. With the set switch in the less aggressive position, in response to an operator&#39;s command, the software in the controller provides a relatively slow current ramp to energize the control valves. With the switch in the more aggressive position, the current ramps and resultant pressure ramps are faster, thus causing more aggressive transmission operation for the hydrostatic transmission.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The invention relates to utility vehicles for industrial and agricultural use, such as utility tractors. Particularly, the invention relates to transmission control systems for such vehicles.  
         BACKGROUND OF THE INVENTION  
         [0002]    Typical utility vehicles, such as compact tractors, utilize an engine operating substantially at a pre-selected speed that drives a transmission system or drive train that delivers power to one or more driven wheels. The transmission system includes a speed controllable transmission component, a gear selection component, and a differential component. The speed controllable transmission component can be, for example, a hydrostatic transmission, or a transmission that uses electro-hydraulically controlled forward and reverse clutch packs to initially accelerate the vehicle and to change vehicle direction (hereinafter referred to as a “reverser transmission”), such as a POWRREVERSER™ transmission incorporated in JOHN DEERE Series 4000 tractors.  
           [0003]    The present inventors have recognized that the desired “aggressiveness” of a vehicle&#39;s performance, or rates of acceleration and deceleration in response to operator commands, depends on operator experience, the operating conditions of the vehicle and the work being performed with the vehicle. For example, experienced operators performing material handling work tend to prefer a vehicle that accelerates and decelerates aggressively, and allows quick changes in direction. An operator that is using a vehicle for turf care work would prefer less aggressive accelerations and decelerations to prevent damage to the grass caused by slipping of the vehicle wheels.  
           [0004]    For hydrostatic transmissions and reverser transmissions, pre-selecting the vehicle performance is commonly done by sizing orifices to control the rate of fluid flow to the servo control system of the hydrostatic transmission or control the rate of fluid flow to clutch packs in the reverser transmission. With electronically controlled systems, the aggressiveness is commonly controlled by pre-selecting the rate of increase of the electrical control current to electro-hydraulic pressure reducing valves that control swashplate servo systems or clutch pack hydraulic pressures.  
           [0005]    However, compact utility tractors are commonly used for both material handling and turf care as well as many other operations. The present inventors have recognized the desirability of providing a utility tractor that would allow the driver to choose the aggressiveness of the tractor&#39;s performance according to the work being done. Such a selectable aggressiveness would lead to improved tractor productivity.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides for operator selection of vehicle “aggressiveness,” or rates of acceleration in response to operator command. The aggressiveness of a vehicle&#39;s performance can be controlled by modulating control signals to control valves, control valves which determine the acceleration of the vehicle transmission, according to two or more electrical ramp-up (or ramp-down) profiles, in response to an operator&#39;s acceleration command (or deceleration command).  
           [0007]    According to the preferred embodiment of the present invention, a vehicle transmission control system includes a controller, operator controlled potentiometers and electro-hydraulic control valves which control hydraulic pressure in the servo control system of the hydrostatic transmission.  
           [0008]    The operator is provided with a two-position set switch. With the set switch in the less aggressive position, in response to an operator&#39;s command, the software in the controller provides a relatively slow current ramp to energize the electro-hydraulic control valves that control the actuation of the servo system of a hydrostatic transmission. By ramping up the hydraulic pressure slowly, in response to the slow current ramps, acceleration is non-aggressive. Decelerations are also performed at a relatively non-aggressive rate.  
           [0009]    The selective aggressiveness function is operable for both forward and reverse operation.  
           [0010]    With the switch in the more aggressive position, the current ramps and resultant pressure ramps are faster, thus causing more aggressive transmission operation for the hydrostatic transmission. Decelerations for the hydrostatic transmission are also more aggressive.  
           [0011]    The two-position set switch could be replaced with a potentiometer, thus permitting an infinitely variable range in transmission aggressiveness control.  
           [0012]    By providing the tractor operator with selectable transmission aggressiveness, the operator can choose the acceleration/deceleration rates according to the operator&#39;s comfort or skill level and/or to the task being performed. The vehicle performance, controllability and productivity will be improved.  
           [0013]    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  
       [0014]    [0014]FIG. 1 is a block diagram of the control system of the present invention applied to a hydrostatic transmission system;  
         [0015]    [0015]FIG. 2 is a schematic sectional view of the servo control system used in a hydrostatic transmission of FIG. 1;  
         [0016]    [0016]FIG. 3 is an exploded, fragmentary perspective view of the servo control system of FIG. 2;  
         [0017]    [0017]FIG. 3A is a schematic sectional view of a control valve of the system of FIG. 3;  
         [0018]    [0018]FIG. 4 is a schematic sectional view of a hydrostatic transmission;  
         [0019]    [0019]FIG. 5 is a diagram demonstrating the two aggressiveness settings and the time response of hydrostatic transmission servo system hydraulic pressure as a percentage of the total drive command hydraulic pressure, for accelerations; and  
         [0020]    [0020]FIG. 6 is a diagram demonstrating the two aggressiveness settings and the time response of hydrostatic transmission servo system hydraulic pressure as a percentage of the total drive command hydraulic pressure, for decelerations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments 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 embodiments illustrated.  
       Transmission Control of a Hydrostatic Transmission  
       [0022]    [0022]FIG. 1 illustrates, in block diagram form, a vehicle  12  incorporating a first embodiment drive control system  16  of the present invention. The vehicle incorporates a hydrostatic transmission  26  and a range gear drive, such as a multi-speed gear transmission  27 , for transmitting power through a differential (not shown) to one or more driven wheels  28 .  
         [0023]    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 .  
         [0024]    The control system  16  includes a controller  52 , such as a microprocessor-based microcontroller, in signal-communication with an acceleration mode or “aggressiveness” set switch  56 . The set switch  56  is selectively activated to trigger a more aggressive or a less-aggressive acceleration mode in the controller software, as described hereinafter.  
         [0025]    The control system  16  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 .  
         [0026]    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 pressure control valves  106 ,  108  is substantially proportional to the corresponding pedal position signal. An adjustable profile can be used to give the pedal a non-linear response to increase vehicle drivability.  
         [0027]    The selectable ramps of the output current from the controller  52  control the rate of acceleration and deceleration of the vehicle. Two different programmed parameter sets in the controller software provide for a more aggressive operating mode and a less aggressive operating mode. The operator-activated set switch  56  is used to select between the two operating modes. The parameter sets can become effective by switching to the desired operating mode without returning the foot pedals  72 ,  74  to neutral.  
         [0028]    [0028]FIGS. 2 and 3 illustrate the hydrostatic transmission servo control in more detail. Given an engine drive speed and a range transmission or gear transmission 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 at a reduced pressure. Preferably, the return channel R recirculates hydraulic fluid back to the vehicle&#39;s hydraulic system reservoir.  
         [0029]    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, having a current vs. time profile selected by position of the set switch  56 , to energize the solenoid driver  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  to pressurize 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.  
         [0030]    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 swashplate  118  attains a range of forward positions selected by the foot pedal  72 .  
         [0031]    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, having a current vs. time profile selected by position of the set switch  56 , 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  to pressurize 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.  
         [0032]    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.  
         [0033]    When either the forward or reverse pedals  72 ,  74  are released, the controller  52  modulates the deceleration command according to a preselected current output ramps to the respective control valve solenoids  106   a ,  108   a  in a similar, but reversed, fashion as described for acceleration, based on the respective pedal position signal from the respective potentiometers  82 ,  84  and the selected current ramp profile from the set switch  56 .  
         [0034]    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 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&#39;s hydraulic system.  
         [0035]    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.    
         [0036]    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 .  
         [0037]    An annular screen  107   a  and a circular screen  125   a  can be supplied to the inlet  107  and to the outlet  125  respectively.  
         [0038]    The control valve  108  can be identically configured as described above for the control valve  106 .  
         [0039]    Hydrostatic Transmission  
         [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 servo piston  112  controls this angle. Moving the respective directional pedal  72 ,  74  controls the valves  106 ,  108  via the controller software to provide a hydraulic assist to the double acting piston  112  which controls the position of the swashplate  118 .  
         [0042]    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 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. determines the transmission speed.  
         [0043]    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.  
         [0044]    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.  
         [0045]    Hydraulic Pressure Ramp Profiles  
         [0046]    [0046]FIG. 5 presents a comparison between a less aggressive power control and a more aggressive power control. As an example, for the more aggressive setting of the set switch  56 , a 100 percent drive command corresponding to full pedal depression, either forward pedal  72  or reverse pedal  74 , results in a proportional hydraulic pressure, controlled by the software of the controller  52  and the respective control valve  106 , 108 , in the servo cylinder  114 , of the hydrostatic transmission, within one second. For the less aggressive setting, 100 percent of the drive command results in a corresponding hydraulic pressure, controlled by the software of the controller  52  and the respective control valve  106 , 108 , in the servo cylinder  114  of the hydrostatic transmission, within two seconds.  
         [0047]    [0047]FIG. 6 illustrates that for the hydrostatic transmission control described in FIG. 1, the controller  52  and the respective control valve  106 , 108  also modulate decelerations for both forward and reverse operation. For a more aggressive modulation setting of the set switch  56 , the software of the controller  52  and the respective control valve  106 , 108  cause a 100 percent deceleration command by the foot pedal position signal, to be realized in a corresponding hydraulic pressure reduction in the servo cylinder  114  of the hydrostatic transmission, within one second. For a less aggressive modulation setting of the set switch  56 , the software of the controller  52 , and the respective control valve  106 , 108 , cause a 100 percent deceleration command by the foot pedal position signal to be realized in a corresponding hydraulic pressure reduction in the servo cylinder  114  of the hydrostatic transmission, within two seconds.  
         [0048]    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.