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 clutch packs in a reverser 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 reverser transmission.

Description:
TECHNICAL FIELD OF THE INVENTION 
     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 
     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. 
     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. 
     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. 
     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 
     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). 
     The selection of vehicle aggressiveness controls the acceleration rates of the vehicle in both forward and reverse operation. 
     According to the preferred embodiment of the present invention, a vehicle transmission control system includes a controller, directional switches or potentiometers and electro-hydraulic control valves which control hydraulic pressure in the clutch pack hydraulics in the reverser 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 electro-hydraulic control valves that control the actuation of transmission clutch packs in a reverser transmission. By ramping up the hydraulic pressure slowly, in response to the slow current ramps, acceleration is non-aggressive. 
     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 reverser transmissions. 
     The two-position set switch could be replaced with a potentiometer, thus permitting an infinitely variable range in transmission aggressiveness control. 
     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. 
     The invention also provides an interlock system. The system is used for a reverser transmission to prevent unanticipated motion. In order to start the engine on the tractor, the following must be sensed at the controller: 
     1. The forward-neutral-reverse lever must be sensed by the controller to be in neutral. Neutral is defined as having a neutral signal in combination with no forward signal and no reverse signal. 
     2. The main transmission shift lever must be in neutral. 
     3. The tractor power-take-off must be turned off. 
     When the forward-neutral-reverse lever is in the neutral position, no clutch-actuating hydraulic pressure must be sensed in either clutch pack or the tractor engine will be shut off. When the forward-neutral-reverse lever is in forward, then clutch-actuating hydraulic pressure must be present in the forward clutch pack, but not in the reverse clutch pack or the engine will be shut off. Reverse drive also has similar logic. Because clutch pack hydraulic pressures will be in transition when the forward-neutral-reverse lever is moved from one position to the other, time delays are provided to permit the ramping up or ramping down of pressure signals before the interlock logic is applied. 
     If hydraulic pressures in the forward or reverse clutch packs are not realized in approximately one second after the respective signal is received from the forward-neutral-reverse lever, then the controller will disable motion in that direction until another neutral signal from the forward-neutral-reverse lever is sensed. 
     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 a block diagram of an alternate control system of the present invention applied to a reverser transmission utilizing forward and reverse clutch packs; 
     FIG. 2A is a front view of a clutch pack; 
     FIG. 2B is a schematic sectional view of a clutch pack; 
     FIG. 2C is a schematic sectional view of a reverser transmission utilizing clutch packs for forward or reverse drive initiation, shown in a forward mode of operation; 
     FIG. 2D is a schematic view of a control valve of the system of FIG. 1; 
     FIG. 3 is a diagram showing the relationship between clutch pressure control valve energizing current and time after an operator&#39;s command is made, via the vehicle direction selector; and 
     FIG. 4 is a diagram demonstrating the two aggressiveness settings and the time response of clutch pack hydraulic pressure as a percentage of the total drive command hydraulic pressure, for accelerations. 
    
    
     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, 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 Reverser Transmission 
     FIG. 1 illustrates a control system  150  for use with a reverser transmission  152  of the type that uses electro-hydraulic control of clutch packs to engage forward or reverse tractor driving direction, for example as embodied in the POWRREVERSER™ transmission used in John Deere Series 4500 PRT and 4600 PRT tractors. 
     The reverser transmission  152  is driven by an engine drive  153 . The reverser transmission  152  drives a range transmission  154  that drives one or more wheels via a differential (not shown). The reverser transmission can be used in a drive train such as disclosed in U.S. Pat. No. 6,002,976, herein incorporated by reference. 
     According to the invention, a controller  156 , such as a microprocessor-based microcontroller, is signal-connected to forward and reverse proportional pressure control valves  162 , 166 , which in turn are pressure-fluid-connected to forward and reverse clutch packs  172 , 176 . The controller can be a microprocessor based electronic control module. 
     Pressure sensors or switches  182 , 186  are connected between the proportional valves  162 , 166  and the clutch packs  172 , 176 . The pressure sensors  182 ,  186  act as feedback control sensors to the controller  156 . A forward-neutral-reverse lever  202  is connected to switches  208 , which are signal-connected to the controller  156 . A seat switch  212  is also signal-connected to the controller  156 . A main shift lever  220  includes a neutral position switch  222  that is also signal-connected to the controller. 
     A selectable aggressive/non-aggressive switch  230  is signal-connected to the controller. A clutch switch  234  and pedal position sensor, such as a potentiometer  238 , are mounted to a clutch or “inching” pedal and signal-connected to the controller  156 . 
     The forward and reverse clutch packs  172 , 178  are hydraulically-engaged and spring-released. The torque capacity of the clutch packs is thus a function of the hydraulic pressure supplied to the clutch packs. The proportional valves  162 , 166  produce a downstream hydraulic pressure that is proportional to the current applied to the respective valve&#39;s solenoid, and thus control the hydraulic pressure supplied to the respective forward or reverse clutch pack. 
     The controller  156  controls the current to the correct valve  162 , 166  and at the correct level to satisfy the clutch torque being commanded by the operator. Based on the operator&#39;s command via the vehicle direction selector  202 , the forward, reverse, or neither control valve  162 , 166  is energized. 
     Based on the signal from a potentiometer  238 , preferably a 5K ohm potentiometer, connected to a clutch pedal or “inching pedal”  239 , the current to the respective control valve  162 , 166  is also modulated as a function of clutch pedal position. The current ramp commanded by the direction control selector  202 , and the switch  230 , is compared with the current commanded by the inching pedal potentiometer  238 , and the lesser of the two currents is applied to the valve. When transitioning from fully depressed clutch pedal to any other pedal position, a fast fill time T fill  is applied. 
     The inching pedal  239  includes a bottom of travel (BOT) switch  241 . The BOT switch  241  is open only in the full pedal down position. Both switches  238 ,  241  switch system voltage as signals into the controller  156 . A 5V reference and ground can be supplied to the potentiometer  238  and the potentiometer  238  returns a voltage to the controller  156  proportional to inching pedal position. Maximum voltage can be returned at the pedal up position with decreasing voltage as the pedal is depressed. 
     The control valves  162 , 166  include solenoids  162   a ,  166   a . Both solenoids are driven via a capture compare function through the controller  156 . The controller  156  modulates a component (FET/TRIAC) (not shown) that supplies the requested current to the proportional valve. 
     The control valve solenoids  162   a ,  166   a  are preferably pulse width modulated type drivers that are opened proportionally to the width of step pulses of current applied to the solenoids. The pulses are applied at a substantially constant frequency, and the pulse widths are controlled in order to modulate the opening of the valves  162 , 166 . 
     When either control valve solenoid  162   a , 166   a  transitions from off to on there are three energizing phases. The first phase is a fast fill phase FP, the second phase is the ramp-on or modulation phase ROP, and the third phase is the static phase SP. These phases are shown graphically in FIG.  3 . 
     As illustrated in FIG. 3, for a fill time T fill , the respective control valve solenoid  162   a , 166   a  is supplied maximum current I max , which represents the current at which the valve is full on or fully energized. I max , can have a magnitude of 660 mA. The time T fill  can be determined experimentally, preferably in a range of 0 to 250 ms. The purpose of the fill time is to quickly eliminate the clearance between the clutch friction plates and the clutch stationary plates in preparation for the ramp-on phase. 
     After T fill , the control valve current from the controller is reduced to I base , which represents the current required to produce the pressure to maintain the clutch piston displacement at a zero clutch friction plates-separator plates clearance, the net rotary force being approximately equal to zero. From T fill  to T ramp  the current rises from I base  at the rate di/dt. Under steady-state conditions, the selected clutch pressure control valve can be driven at a current I max  equal to 660 mA. 
     Forward and reverse shifts will commence after a new shift lever position has been indicated. Separate fill times and hold levels will be used for each direction. The shift preferrably will involve the following in chronological order: 
     1. a period of fast fill at a maximum drive current 660 mA to bring the clutch to a position where it is just ready to transmit torque; 
     2. a hold period at a value that does not allow the clutch position to change, this value will be held just long enough to allow the transient movements of the valve to stabilize when coming off of the fast fill; 
     3. a series of multiple ramps, such as three ramps, for the remainder of the shift, from the hold level to the maximum drive current. 
     Use of Switches for Operational Control 
     The pressure transducers or pressure switches  182 , 186  are connected downstream of the forward and reverse valves  162 , 166 , respectively. The transducers send an analog voltage to the controller  156 . When pressure sensed by the switch is  182 , 186  passes a predefined threshold, a shift is considered underway. 
     1. If both pressure sensors are closed for more than one second, then a terminate signal is sent to a fuel cut off solenoid supplying fuel to the vehicle engine, and an error message is displayed. 
     2. If a neutral state is requested after one second and either of the pressure transducers are above their threshold values, then a terminate signal is sent to the fuel cut off solenoid and an error message is displayed. 
     3. If a “shuttle” (moving from forward directly to reverse, and vice versa) has been requested and after one second both pressure transducers have not changed state, then a terminate signal is sent to the fuel cut off solenoid and an error message is displayed. 
     4. If a shift is requested and the pressure transducer for the direction requested does not pass its threshold value in one second, and the inching pedal is greater than 25 percent of its travel, then the control defaults to neutral and an error message is displayed. The neutral position is that position wherein the valves  162 , 166  prevent pressurized fluid from actuating the clutch packs. 
     The transmission direction selector  202  comprises three switches  208  corresponding to forward, neutral and reverse. The switches cause system voltage to be switched into the controller  156 . 
     The BOT switch  241  and the analog input of inching pedal position potentiometer  238  control the output level of the controller signal to the valves  162 , 166 . When the BOT switch  241  is open at the bottom of clutch pedal travel, if after one second the pressure switches  182 , 186  are closed, then the fuel cut off solenoid is activated to discontinue engine fuel supply, and an error message is displayed. When the BOT switch  241  is open, the analog voltage from the inching pedal potentiometer  238  must be within a certain range. If outside this range then the signal from the controller  156  to the valves  162 , 166  defaults to neutral, and an error message is displayed. 
     If the inching pedal analog voltage from the potentiometer  238  is within a certain range consistent with being at the bottom of clutch pedal travel, and the BOT switch  241  closed switch signal is still present at the controller  156 , then the signal from the controller  156  to the valves  162 , 166  defaults to neutral and an error message is displayed. 
     A shift can only take place if there is a forward or reverse signal from the direction control  202 . If there are two or more signals (forward, reverse, neutral) at the controller  156  at the same time, or no signal for longer than one second, then the signal from the controller  156  to the valves  162 , 166  defaults to neutral, and an error message is displayed. 
     A shift can only take place if: after starting the tractor, the shift handle has been in neutral, the forward and reverse pressure switches  182 , 186  are open, the clutch BOT switch  241  signal changes state in either direction, and the inching pedal position analog signal is at a voltage that is appropriate for being at bottom of clutch pedal travel position. If the seat switch  212  is open from more than one second while the shift handled is in forward or reverse, the signal from the controller  156  to the valves  162 , 166  defaults to neutral, and an error message is displayed. 
     The invention also provides an interlock system. In order to start the engine on the vehicle, the following must be sensed at the controller: 
     1. The forward-neutral-reverse lever  202  must be sensed by the controller  156  to be in neutral. Neutral is defined as having a neutral signal in combination with no forward signal and no reverse signal. 
     2. The main transmission shift lever  220  must be in neutral as sensed by the controller  156  from the neutral position switch  222 . 
     3. The tractor power-take-off must be turned off. 
     When the forward-neutral-reverse lever  202  is in the neutral position, no clutch-actuating hydraulic pressure must be sensed in either clutch pack  172 ,  176  or the tractor engine drive  153  will be shut off. When the forward-neutral-reverse lever  202  is in forward, then clutch-actuating hydraulic pressure must be present in the forward clutch pack  172 , but not in the reverse clutch pack  176 . Reverse drive also has similar logic. Because clutch pack hydraulic pressures will be in transition when the forward-neutral-reverse lever  202  is moved from one position to the other, time delays are provided to permit the ramping up or ramping down of pressure signals before the interlock logic is applied. 
     If hydraulic pressures in the forward or reverse clutch packs  172 ,  176  are not realized in approximately one second after the respective signal is received from the forward-neutral-reverse lever  202 , then the controller  156  will disable motion in that direction until another neutral signal from the forward-neutral-reverse lever is sensed. 
     Reverser Transmission 
     FIGS. 2A and 2B illustrate one of the identical clutch packs  172 ,  176 . The clutch pack  172  includes a central cylinder  270  and a hub  272  connected thereto. A clutch piston  274  is located within the hub  272 . A backup plate  276  and an end plate  278  are on opposite sides of a series of separator disks  280  and interposed friction disks  282 . A clutch gear  286  includes a plurality of gear teeth  287  around its circumference and a central cylinder portion  288 . The central cylinder portion  288  carries the friction disks  282  fixed to rotate therewith, and the hub  272  carries the backup plate  276 , the end plate  278 , and the separator disks  280 . The separator disks  280  are fixed for rotation with the hub  272 . A return spring  290  is compressed between the clutch piston  274  and the clutch gear  286  to maintain the friction disks  282  out of engagement with the separator disks  280  absent sufficient hydraulic pressure to engage the clutch pack  172 . When sufficient pressurized hydraulic fluid is delivered to the clutch pack  172 , the clutch gear  286  and the clutch piston  274  are drawn together to engage the friction disks  282  with the separator disks  280 , between the backup plate  276  and the end plate  278 . The disks  280 ,  282  engage for mutual rotation, causing the clutch gear  286  and the hub  272  and cylinder  270  to mutually rotate. 
     A radial aperture  294  is provided to feed pressurized fluid into or out of a space  296  between the clutch piston  274  and the cylinder  270 . Pressurized hydraulic fluid forces the clutch piston  274  from right to left in the FIG.  6 . Lubricating fluid flows through radial channels  297 . 
     FIG. 2C Illustrates the reverser transmission  152  in more detail. The clutch packs  172 ,  176  are arranged side by side, and only the forward clutch pack  172  is observable in FIG. 2C. A clutch output shaft  300  penetrates through the cylinder  270 , and is splined, or otherwise fixed thereto. The control valves  162 , 166  route hydraulic fluid to the forward and reverse clutch packs  172 ,  176 , and control operation of the reverser transmission  152 . 
     For forward operation, power is transferred from the engine (not shown), through a clutch input shaft  302  and then to an input gear  304  and to the forward clutch pack  172 . Particularly, the input gear  304  rotates the clutch gear  286 . When the tractor is placed in forward and the clutch pedal is released, pressurized hydraulic fluid is routed through the valve  162 , at a modulated pressure, through channels  308  formed in the transmission housing, through an aperture  309   a  in a seal ring assembly  309 , and through channels  310  within the output shaft  300 , through the cylinder  270 , and through the aperture  294  bored through the cylinder  270 , and to the space  296  of the clutch pack  172 . The hydraulic fluid forces the clutch piston  274  to engage the clutch friction plates  282  together with the separator plates  280 . When the friction and separator plates are engaged together, power is transferred from the clutch gear  286  to the clutch hub  272 , to the clutch cylinder  270  and then to the output shaft  300 . The output shaft  300  is connected to an output gear  314  and thereafter to the range transmission. 
     A further channel  316  through the shaft  300  provides fluid to the channels  297  for lubrication purposes. 
     The output shaft can have four gears (not shown) splined to it that are the input gears for the range transmission  154  such as a 4-speed transmission or gearbox (shown schematically in FIG.  5 ). Forward engagement of the clutch pack  172  causes the output shaft to rotate in the same direction as the engine flywheel. 
     Reverse operation occurs substantially in the same manner with the exception that the reverse control valve  166  modulates pressurized hydraulic fluid pressure that actuates the reverse clutch pack  176 . The reverse clutch pack  176  drives a reverse output shaft  318  that is geared to the output gear  314  in a manner to reverse the relative rotation of the output shaft  300 . During reverse operation, the forward clutch pack  172  is disengaged and the hub  270  can spin freely with the shaft  300 . 
     Control Valves 
     An exemplary example of a control valve, such as the control valve  162 , is illustrated in FIG.  2 D. The solenoid  162  includes a plunger  420  (shown schematically) driven by the solenoid coil  421  (shown schematically). The plunger  420  drives a valve spool  422  within a housing  423 . The housing provides a pressurized hydraulic fluid inlet  426 , in the form of plural openings, and an outlet  424 , in the form of plural openings, to the hydraulic fluid reservoir. A control pressure outlet  425  communicates hydraulic fluid at a modulated pressure to the clutch pack  172  as shown in FIG.  1 . The solenoid coil  421  drives the plunger  420  downward (in FIG. 3A) to open the inlet  426  to the outlet  425  through an annular channel  422   a.    
     The channel  422   a  is open to an oblong orifice  422   b  through the spool  422  to communicate fluid into an interior  422   c  of the spool. The interior of the spool is open to the outlet  425 . The pressure of the hydraulic fluid at the control outlet  425  is substantially proportional to the force applied to the spool by the plunger, ranging between reservoir pressure, the pressure at the outlet  425  with the inlet  426  closed, to pressurized hydraulic fluid supply pressure, the spool  422  moved down to close the outlet  424  and open the inlet  426 . 
     An annular screen  426   a  and a circular screen  425   a  can be supplied to the inlet  426  and to the outlet  425  respectively. 
     The control valve  166  can be identically configured as described above for the control valve  162 . 
     Hydraulic Pressure Ramp Profiles 
     FIG. 4 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  230 , a 100 percent drive command corresponding to direction actuation by the selector  202 , either forward or reverse, (clutch 100% engaged i.e., no clutch pedal  239  modulation) results in a proportional hydraulic pressure, controlled by the software of the controller  156  and the respective control valve  162 , 166  in the respective clutch pack of the reverser 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  156  and the respective control valve  162 , 166 , in the respective clutch pack of the reverser transmission, within two seconds. 
     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.