Abstract:
An apparatus and method for controlling the engagement rate of a clutch in a partially or fully automated mechanical transmission in response to an indication provided by the driver, such as the depression of an accelerator pedal of the vehicle. The apparatus includes an electronic controller which initially sets a desired speed for the vehicle engine during the shifting process, determines a rate of engagement movement of a release bearing of the clutch, and actuates appropriate valves to initiate the gradual engagement of the clutch. The electronic controller is responsive to the position of the accelerator pedal of the vehicle for adjusting the rate of engagement of the release bearing of the clutch. In a first embodiment, the electronic controller is responsive to movement of the accelerator pedal in a first direction (depressed for further acceleration) for incrementing the rate of engagement of the release bearing. Similarly, the electronic controller is responsive to movement of the accelerator pedal in a second direction (released for further deceleration) for decrementing the rate of engagement of the release bearing. If the accelerator pedal is maintained in a constant position, the rate of engagement of the release bearing is unchanged. In a second embodiment, the electronic controller is responsive to rate or amount of movement of the accelerator pedal in the first direction for incrementing the rate of engagement of the release bearing, similarly, the rate or amount of movement of the accelerator pedal in the second direction for decrementing the rate of engagement of the release bearing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to vehicle transmissions and in particular to a method and apparatus for automatically controlling the operation of a clutch for use with an automated mechanical transmission in a vehicle drive train assembly. 
     In virtually all land vehicles in use today, a transmission is provided in a drive train between a source of rotational power, such as an internal combustion or diesel engine, and the driven axle and wheels of the vehicle. A typical transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner. 
     To facilitate the operation of the transmission, it is well known to provide a clutch between the vehicle engine and the transmission. When the clutch is engaged, the transmission is driven by the vehicle engine to operate the vehicle at a selected gear ratio. To shift the transmission from a first gear ratio to a second gear ratio, the clutch is initially disengaged such that power is not transmitted from the vehicle engine to the transmission. This allows the gear shifting operation to occur within the transmission under a non-torque loading condition to prevent undesirable clashing of the meshing gear teeth. Thereafter, the clutch is re-engaged such that power is transmitted from the vehicle engine to the transmission to operate the vehicle at the second gear ratio. 
     A typical structure for a vehicle clutch includes a cover which is connected to a flywheel secured to the end of the output shaft of the vehicle engine for rotation therewith. A pressure plate is disposed within the clutch between the cover and the flywheel. The pressure plate is connected for rotation with the flywheel and the cover, but is permitted to move axially relative thereto. Thus, the flywheel, the cover, and the pressure plate are all constantly rotatably driven by the vehicle engine. Between the flywheel and the pressure plate, a driven disc assembly is disposed. The driven disc assembly is supported on the input shaft of the transmission for rotation therewith, but is permitted to move axially relative thereto. To engage the clutch, the pressure plate is moved axially toward the flywheel to an engaged position, wherein the driven disc assembly is frictionally engaged between the flywheel and the pressure plate. As a result, the driven disc assembly (and the input shaft of the transmission upon which it is supported) are driven to rotate with the flywheel, the cover, and the pressure plate. To disengage the clutch, the pressure plate is moved axially away from the flywheel to a disengaged position. When the pressure plate is moved axially to this disengaged position, the driven disc assembly is not frictionally engaged between the flywheel and the pressure plate. As a result, the driven disc assembly (and the input shaft of the transmission upon which it is supported) are not driven to rotate with the flywheel, the cover, and the pressure plate. 
     To effect such axial movement of the pressure plate between the engaged and disengaged positions, most vehicle clutches are provided with a release assembly including a generally hollow cylindrical release sleeve which is disposed about the output shaft of the clutch. The forward end of the release sleeve extends within the clutch and is connected through a plurality of levers or other mechanical mechanism to the pressure plate. In this manner, axial movement of the release sleeve causes corresponding axial movement of the pressure plate between the engaged and disengaged positions. Usually, one or more engagement springs are provided within the clutch to urge the pressure plate toward the engaged position. The engagement springs typically react between the release sleeve and the cover to normally maintain the clutch in the engaged condition. The rearward end of the release sleeve extends outwardly from the clutch through a central opening formed through the cover. Because the release sleeve is connected to the cover and the pressure plate of the clutch, it is also constantly driven to rotate whenever the vehicle engine is operating. Thus, an annular release bearing is usually mounted on the rearward end of the release sleeve. The release bearing is axially fixed on the release sleeve and includes an inner race which rotates with release sleeve, an outer race which is restrained from rotation, and a plurality of bearings disposed between the inner race and the outer race to accommodate such relative rotation. The non-rotating outer race of the release bearing is typically engaged by an actuating mechanism for moving the release sleeve (and, therefore, the pressure plate) between the engaged and disengaged positions to operate the clutch. 
     In a conventional mechanical transmission, both the operation of the clutch and the gear shifting operation in the transmission are performed manually by an operator of the vehicle. For example, the clutch can be disengaged by depressing a clutch pedal located in the driver compartment of the vehicle. The clutch pedal is connected through a mechanical linkage to the outer race of the release bearing of the clutch such that when the clutch pedal is depressed, the pressure plate of the clutch is moved from the engaged position to the disengaged position. When the clutch pedal is released, the engagement springs provided within the clutch return the pressure plate from the disengaged position to the engaged position. Similarly, the gear shifting operation in the transmission can be performed when the clutch is disengaged by manually moving a shift lever which extends from the transmission into the driver compartment of the vehicle. Manually operated clutch/transmission assemblies of this general type are well known in the art and are relatively simple, inexpensive, and lightweight in structure and operation. Because of this, the majority of medium and heavy duty truck clutch/transmission assemblies in common use today are manually operated. 
     More recently, however, in order to improve the convenience of use of manually operated clutch/transmission assemblies, various structures have been proposed for partially or fully automating the shifting of an otherwise manually operated transmission. In a partially or fully automated manual transmission, the driver-manipulated clutch pedal may be replaced by an automatic clutch actuator, such as a hydraulic or pneumatic actuator. The operation of the automatic clutch actuator can be controlled by an electronic controller or other control mechanism to selectively engage and disengage the clutch without manual effort by the driver. Similarly, the driver-manipulated shift lever may also be replaced by an automatic transmission actuator, such as a hydraulic or pneumatic actuator which is controlled by an electronic controller or other control mechanism to select and engage desired gear ratios for use. 
     In both manually operated transmissions and in partially or fully automated manual transmissions, one of the most difficult operations to perform is to initially launch the vehicle from at or near a stand-still. This is because the force required to overcome the inertia of the vehicle is the greatest when attempting to initially accelerate the vehicle from at or near zero velocity. This relatively large amount of inertial force results in a relatively large load being placed on the vehicle engine when the clutch is engaged during a vehicle launch. Thus, the movement of the release bearing from the disengaged position to the engaged position must be carefully controlled during the initial launch of the vehicle to prevent the engine from stalling and to avoid undesirable sudden jerking movement of the vehicle. Although the same considerations are generally applicable when re-engaging the clutch during subsequent shifting operations in the higher gear ratios of the transmissions, the control of the movement of the release bearing from the disengaged position to the engaged position has been found to be less critical when shifting among such higher gear ratios because a much lesser force is required to overcome the inertia of the vehicle when the vehicle is already moving. 
     To address these considerations, the total movement of the release bearing from the disengaged position to the engaged position can be divided into three ranges of movement. The first range of movement is from the disengaged position to a first intermediate position (referred to as the transition point). The transition point is selected to be relatively near, but spaced apart from, the position of the release bearing at which the driven disc assembly of the clutch is initially engaged by the flywheel and the pressure plate. Thus, during this first range of movement (referred to as the transition movement), the clutch is completely disengaged, and no torque is transmitted through the clutch to the transmission. The second range of movement is from the transition point to a second intermediate position (referred to as the kiss point). The kiss point is the position of the release bearing at which the driven disc assembly is initially engaged by the flywheel and the pressure plate. Thus, during this second range of movement (referred to as the approach movement) from the transition point to the kiss point, the clutch is disengaged until the release bearing reaches the kiss point, at which point the first measurable amount of torque is transmitted through the clutch to the transmission. The third range of movement of the release bearing is from the kiss point to the engaged position. The engaged position is the position of the release bearing at which the driven disc assembly is completely engaged by the flywheel and the pressure plate. Thus, during this third range of movement (referred to as the engagement movement), the clutch is gradually engaged so as to increase the amount of torque which is transmitted through the clutch to the transmission from the first measurable amount at the kiss point to the full capacity of the clutch at the engaged position. 
     As mentioned above, during the engagement movement of the release bearing from the kiss point to the engaged position, the clutch is gradually engaged so as to increase the amount of torque which is transmitted through the clutch to the transmission from the first measurable amount at the kiss point to the full capacity of the clutch at the engaged position. Thus, although it is desirable that this engagement movement of the release bearing be accomplished as quickly as possible, it is still important to engage the clutch smoothly to prevent the engine from stalling and to avoid undesirable sudden jerking movement of the vehicle. In the past, the rate of engagement movement of the release bearing (referred to as the engagement rate) has been determined as a function of the difference between the rotational speeds of the input member and the output member of the clutch. However, it has been found that such a comparison of input and output member rotational speeds may not be well suited for all of the varying conditions under which the vehicle may be operated. For example, if the driver rapidly depresses the accelerator pedal of the vehicle, it can be inferred that a more aggressive acceleration of the vehicle is desired than if the accelerator pedal is depressed in a more leisurely manner. Thus, it would be desirable to provide an apparatus and method for controlling the operation of a clutch in a partially or fully automated mechanical transmission which is responsive to an indication provided by the driver for varying the engagement rate of release bearing during re-engagement of the clutch. 
     SUMMARY OF THE INVENTION 
     This invention relates to an apparatus and method for controlling the engagement rate of a clutch in a partially or fully automated mechanical transmission in response to an indication provided by the driver, such as the depression of an accelerator pedal of the vehicle. The apparatus includes an electronic controller which initially sets a desired speed for the vehicle engine during the shifting process, determines a rate of engagement movement of a release bearing of the clutch, and actuates appropriate valves to initiate the gradual engagement of the clutch. The electronic controller is responsive to the position of the accelerator pedal of the vehicle for adjusting the rate of engagement of the release bearing of the clutch. In a first embodiment, the electronic controller is responsive to movement of the accelerator pedal in a first direction (depressed for further acceleration) for incrementing the rate of engagement of the release bearing. Similarly, the electronic controller is responsive to movement of the accelerator pedal in a second direction (released for further deceleration) for decrementing the rate of engagement of the release bearing. If the accelerator pedal is maintained in a constant position, the rate of engagement of the release bearing is unchanged. In a second embodiment, the electronic controller is responsive to rate or amount of movement of the accelerator pedal in the first direction for incrementing the rate of engagement of the release bearing, similarly, the rate or amount of movement of the accelerator pedal in the second direction for decrementing the rate of engagement of the release bearing. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a vehicle drive train assembly including an electronic controller in accordance with this invention. 
     FIG. 2 is sectional elevational view of the clutch actuator and portions of the clutch and transmission illustrated in FIG. 1 showing the clutch actuator and the clutch in a disengaged position, together with a block diagram of the valves and related control circuitry for operating the clutch actuator and the clutch. 
     FIG. 3 is a flow chart of a first portion of an algorithm for controlling the movement of the release bearing of the clutch in its engagement movement from the kiss point to the engaged position. 
     FIG. 4 is a flow chart of a second portion of the algorithm illustrated in FIG. 3 showing a first embodiment of a pedal position adjustment routine in accordance with this invention. 
     FIG. 5 is a flow chart of the second portion of the algorithm illustrated in FIG. 3 showing a embodiment of a pedal position adjustment routine in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is illustrated in FIG. 1 a block diagram of a vehicle drive train assembly, indicated generally at  10 . The drive train assembly  10  includes a conventional engine  11  or other source of rotational power. The engine  11  is connected through an output shaft  11   a,  such as a crankshaft of the engine  11 , to a clutch  12 . The clutch  12  is also conventional in the art and functions to selectively connect the output shaft  11   a  of the engine  11  to an input shaft  13   a  of a transmission  13 . The transmission  13  contains a plurality of meshing gears (not shown) which are selectively connected between the input shaft  13   a  and an output shaft  13   b.  The meshing gears contained within the transmission  13  are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, a desired speed reduction gear ratio can be provided between the input shaft  13   a  and the output shaft  13   b.  Consequently, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner. The output shaft  13   b  is connected to a conventional axle assembly  14 . The axle assembly  14  includes one or more wheels which are rotatably driven by the engine  11  whenever the clutch  12  and the transmission  13  are engaged. This general structure for the drive train assembly  10  is well known in the art. 
     The illustrated transmission  13  may be either a partially or fully automated mechanical transmission. In a typical partially automated manual transmission, a driver-manipulated shift lever (not shown) engages and moves certain ones of a plurality of shift rails contained within the transmission to engage a first set of gear ratios for use. However, an automatically shifting transmission actuator  15  is provided on the transmission  13  to engage and move the remaining shift rails to engage a second set of gear ratios for use. For example, it is known to provide a partially automated manual transmission wherein the lower gear ratios are manually selected and engaged by the vehicle driver using the shift lever, while the higher gear ratios are automatically selected and engaged by the transmission actuator  15 . One example of a typical partially automated manual transmission of this general structure is disclosed in detail in U.S. Pat. No. 5,450,767, owned by the assigned of this application. The disclosure of that patent is incorporated herein by reference. In a fully automated manual transmission, the driver-operated shift lever is usually replaced by the transmission actuator  15 . The transmission actuator  15  functions to shift all of the shift rails contained within the transmission so as to select and engage all of the available gear ratios. The above-referenced patent discusses the adaptability of the disclosed partially automated transmission actuator  15  to fully automate the shifting of the transmission disclosed therein. 
     To facilitate the automatic shifting of the transmission  15 , the clutch  12  is provided with a clutch actuator  16 . The structure and operation of the clutch actuator  16  will be discussed further below. Briefly, however, the clutch actuator  16  is provided to replace a driver-manipulated clutch pedal so as to partially or fully automate the operation of the clutch  12 . The clutch actuator  16  is effective to operate the clutch  12  in either an engaged or disengaged mode. When the clutch  12  is engaged, the transmission  13  is driven by the vehicle engine  11  to operate the vehicle at a selected gear ratio. To shift the transmission  13  from a first gear ratio to a second gear ratio, the clutch  12  is initially disengaged such that power is not transmitted from the vehicle engine  11  to the transmission  13 . This allows the transmission actuator  15  to effect a gear shifting operation within the transmission  13  under a non-torque loading condition to prevent undesirable clashing of the meshing gear teeth. Thereafter, the clutch  12  is re-engaged such that power is transmitted from the vehicle engine  11  to the transmission  13  to operate the vehicle at the second gear ratio. 
     The operation of the clutch actuator  16  and the transmission actuator  15  are controlled by an electronic controller  20 . The electronic controller  20  can be embodied as any conventional microprocessor or similar computing apparatus which can be programmed to operate the clutch actuator  16  (to effect automatic disengagement and engagement of the clutch  12 ) and the transmission actuator  15  (to effect automatic shifting of the transmission  13  when the clutch  12  is disengaged) as described above. The operation of the electronic controller  20  will be described in detail below. A transmission output shaft speed sensor  21  provides an input signal to the electronic controller  20 . The transmission output shaft speed sensor  21  is conventional in the art and is adapted to generate an electrical signal which is representative of the actual rotational speed of the output shaft  13   b  of the transmission  13 . A clutch position sensor  22  also provides an input signal to the electronic controller  20 . The structure and operation of the clutch position sensor  22  will be described below. 
     An engine controller  23  is provided to control the operation of the vehicle engine  11 . The engine controller  23  can also be embodied as any conventional microprocessor or similar computing apparatus which can be programmed to operate the engine  11  in a desired manner. Primarily, the engine controller  23  controls the operation of the engine  11  in response to an input signal generated by an accelerator pedal position sensor  24 . The accelerator pedal position sensor  24  is conventional in the art and is adapted to generate an electrical signal which is representative of the actual position of the accelerator pedal (not shown) of the vehicle. As is well known, the accelerator pedal is physically manipulated by the foot of the driver of the vehicle to control the operation thereof. The accelerator pedal is depressed by the driver when it is desired to increase the speed of the engine  11  and move the vehicle. Conversely, the accelerator pedal is released when it is desired to decrease the speed of the engine  11  to slow or stop such movement of the vehicle. Thus, the engine controller  23  controls the speed of the engine  11  in response to the signal from the accelerator pedal position sensor  24  so as to operate the vehicle as desired by the driver. The accelerator pedal position sensor  24  may, if desired, be replaced by a throttle position sensor (not shown) or other driver-responsive sensor which generates a signal which is representative of the desired speed or mode of operation of the vehicle. A second input to the engine controller  23  is an engine output shaft speed sensor  25 . The engine output shaft speed sensor  25  is conventional in the art and is adapted to generate an electrical signal which is representative of the actual rotational speed of the output shaft  11   a  of the engine  11 . 
     The electronic controller  20  and the engine controller  23  communicate with one another over a data bus line  26  extending therebetween. In a manner which is generally conventional in the art, the electronic controller  20  and the engine controller  23  are programmed to communicate and cooperate with one another to so as to control the operation of the vehicle in a manner desired by the driver of the vehicle. Specifically, the electronic controller  20  and the engine controller  23  are effective to control the operation of the engine  11 , the clutch  12 , and the transmission  13  in such a manner that the vehicle can be started and stopped solely by physical manipulation of the accelerator and brake pedals, similar to a conventional automatic transmission in a passenger car. To accomplish this, the signals from the accelerator pedal position sensor  24  and the engine output shaft speed sensor  25  are available to the electronic controller  20  over the data bus line  26 . Alternatively, the signals from the accelerator pedal position sensor  24  and the engine output shaft speed sensor  25  can be fed directly to the electronic controller  20 . 
     Referring now to FIG. 2, the clutch actuator  16  and portions of the clutch  12  and the transmission  13  are illustrated in detail. The structure and operation of the clutch actuator  16  are disclosed and illustrated in detail in U.S. Pat. No. 5,794,752, issued Aug. 18, 1998 (owned by the assigned of this invention), the disclosure of which is incorporated herein by reference. Briefly, however, the clutch actuator  16  includes an outer cylinder housing  30 , a hollow cylindrical piston  31 , and an inner cylinder housing  32 . The piston  31  has at least one, and preferably a plurality, of axially forwardly projecting protrusions  31   a,  each of which has a circumferentially extending groove  31   b  formed therein. To assemble the clutch actuator  16 , the piston  31  is initially disposed concentrically within the outer cylinder housing  30 , and the inner cylinder housing  32  is disposed concentrically within the piston  31 . Then, the outer cylinder housing  30  is secured to a forwardly facing surface of a case of the transmission  13  by threaded fasteners (not illustrated) or other means. When this is done, a forwardly facing surface  32   a  of the inner cylinder housing  32  abuts a complementary shaped, rearwardly facing annular surface  30   a  formed within the outer cylinder housing  30 . At the same time, a rearwardly facing surface  30   b  of the outer cylinder housing  30  abuts portions of the case of the transmission  13 . Thus, the inner cylinder housing  32  is captured between the case of the transmission  13  and the outer cylinder housing  30  so as to be fixed in position relative thereto. At the same time, a circumferential rim portion  31   c  of the piston  31  is received in an undercut  30   c  formed in the interior of the outer cylinder housing  30 . Thus, the piston  31  is capable of limited axial movement relative to the outer cylinder housing  30  and the inner cylinder housing  32 . 
     The clutch  12  is a conventional pull-to-release type clutch and includes a cover  12   a  which is connected to a flywheel (not illustrated) which, in turn, is connected to the output shaft  11   a  of the engine  11 . The flywheel and the cover  12   a  are thus rotatably driven by the engine  11  of the vehicle for rotation about an axis. The cover  12   a  has a central opening formed therethrough which receives a hollow, generally cylindrical release sleeve  12   b.  The release sleeve  12   b  is disposed concentrically about the transmission input shaft  13   a.  A driven disc assembly (not shown) is mounted within the clutch  12  on the forward end of the transmission input shaft  13   a  for rotation therewith and for axial movement relative thereto. When the clutch  12  is engaged, torque is transmitted from the driven disc assembly to the transmission input shaft  13   a  in a known manner. When the clutch  12  is disengaged, no torque is transmitted from the driven disc assembly to the transmission input shaft  13   a.    
     A forward end of the release sleeve  12   b  has an annular groove formed thereabout which receives the radially innermost ends of a plurality of clutch operating levers  12   c  therein. Thus, axial movement of the release sleeve  12   b  causes pivoting movement of the clutch operating levers  12   c  which, in turn, causes engagement and disengagement of the clutch  12  in a known manner. A plurality of clutch engagement springs  12   d  (only one of which is illustrated) reacts between the cover  12   a  and the forward end of the release sleeve  12   b.  The ends of the clutch engagement springs  12   d  are preferably supported on respective seats provided on the release sleeve  12   b  and the cover  12   a.  The springs  12   d  urge the release sleeve  12   b  axially forwardly (toward the left when viewing FIG. 2) toward an engaged position, wherein the components of the clutch  12  are frictionally engaged so as to cause the transmission input shaft  13   a  to be rotatably driven by the engine  11 . When the release sleeve  12   b  is moved axially rearwardly (toward the right when viewing FIG. 2) against the urging of the engagement springs  12   d  toward a disengaged position, the components of the clutch  12  are frictionally disengaged so as to prevent the transmission input shaft  13   a  from being rotatably driven by the engine  11 . 
     The rearward end of the release sleeve  12   b  extends axially rearwardly through the central opening in the cover  12   a.  An annular release bearing  33  is disposed about the rearward end of the release sleeve  12   b  and is retained on one side by a snap ring  34  disposed within an annular groove. A retaining ring  35  is also disposed about the rearward end of the release sleeve  12   b  adjacent to the forward side of the release bearing  33 . A snap ring  36  is disposed in an annular groove in the release sleeve  12   b  to retain the retaining ring  35  on the release sleeve  12   b.  Thus, the release bearing  33  and the retaining ring  35  are secured to the release sleeve  12   b  for axial movement therewith. A snap ring  37  is disposed within the groove formed in the outer surface of the retaining ring  35 . The snap ring  37  connects the piston  31  with the retaining ring  35  such that axial movement of the piston  31  causes corresponding axial movement of the retaining ring  35 , the release bearing  33 , and the release sleeve  12   b.    
     An annular chamber  38  is defined between the outer surface of the body of the piston  31 , the enlarged rim portion  31   c  formed at the rearward end of the piston  31 , and the undercut  30   c  formed in the inner surface of the outer cylinder housing  30 . The chamber  38  is sealed to form a fluid-tight chamber by sealing elements, such as O-rings. A radially extending port  39  is formed through the outer cylinder housing  30 . As will be explained in detail below, pressurized fluid (hydraulic or pneumatic, as desired) is supplied through the port  39  used to effect axial movement of the piston  31  in one direction relative to the outer cylinder housing  30  and the inner cylinder housing  31 . 
     The clutch position sensor  22  is mounted on the outer cylinder housing  30  for generating an electrical signal which is representative of the axial position of the piston  31  relative to the outer and inner cylinder housings  30  and  32 . Such an electrical position signal is used by an electronic controller  20  for automatically operating the clutch actuator  16  in a manner described in detail below. The clutch position sensor  22  is conventional in the art. 
     The port  39  communicates through a conduit  40  with an engage valve  41  and a disengage valve  42 . The engage valve  41  communicates with a reservoir (in hydraulic systems) or the atmosphere (in pneumatic systems), while the disengage valve  42  communicates with a source of pressurized fluid  43 , either hydraulic or pneumatic as desired. The operation of the engage valve  41  is controlled by an engage solenoid  44 , while the operation of the disengage valve  42  is controlled by a disengage solenoid  45 . The engage solenoid  44  and the disengage solenoid  45  are, in turn, connected to the electronic controller  23  so as to be selectively operated thereby. 
     The clutch  12  is normally maintained in the engaged position under the influence of the engagement springs  12   d.  When it is desired to disengage the clutch  12 , the engage solenoid  44  is actuated by the electronic controller  20  to close the engage valve  41 , and the disengage solenoid  45  is actuated by the electronic controller  20  to open the disengage valve  42 . As a result, pressurized fluid from the source  43  is supplied to the chamber  38 , causing the piston  31  to move rearwardly (toward the right when viewing FIG. 2) against the urging of the engagement springs  12   d.  As discussed above, such rearward movement of the piston  31  causes the clutch  12  to be disengaged. For several reasons which are well known in the art, the disengage valve  42  is operated by the electronic controller  20  in an on-off manner, i.e., either wide open or completely closed. 
     When it is desired to subsequently re-engage the clutch  12 , the engage solenoid  44  is actuated by the electronic controller  20  to open the engage valve  41 , and the disengage solenoid  45  is actuated by the electronic controller  20  to close the disengage valve  42 . As a result, the chamber  38  is vented to the reservoir, causing the piston  31  to move forwardly (toward the left when viewing FIG. 2) under the influence of the engagement springs  12   d.  As discussed above, such forward movement of the piston  31  causes the clutch  12  to be engaged. For several reasons which are well known in the art, the engage valve  44  is operated using pulse width modulation techniques to control the engagement of the clutch  12 . The electronic controller  20  varies the duty cycle of the pulse width modulation of the engage valve  41  so as to adjust the rate at which the pressurized fluid in the chamber  38  is vented to the reservoir. By adjusting the rate of venting of the chamber  38  in this manner, the speed at which the release bearing  33  is moved from the disengaged position to the engaged position can be precisely controlled. Precise control of the speed of movement of the release bearing from the disengaged position to the engaged position is important to engage the clutch  12  smoothly and avoid undesirable sudden jerking movement of the vehicle. 
     As discussed above, the total movement of the release bearing  33  from the disengaged position to the engaged position can be divided into three ranges of movement. The first range of movement of the release bearing  33  is from the disengaged position to a first intermediate position (referred to as the transition point). The transition point is selected to be relatively near, but spaced apart from, the position of the release bearing  33  at which the driven disc assembly of the clutch  12  is initially engaged by the flywheel and the pressure plate. Thus, during this first range of movement (referred to as the transition movement), the clutch  12  is completely disengaged, and no torque is transmitted through the clutch  12  to the transmission  13 . The second range of movement of the release bearing  33  is from the transition point to a second intermediate position (referred to as the kiss point). The kiss point is the position of the release bearing  33  at which the driven disc assembly is initially engaged by the flywheel and the pressure plate. Thus, during this second range of movement (referred to as the approach movement) from the transition point to the kiss point, the clutch  12  is disengaged until the release bearing  33  reaches the kiss point, at which point the first measurable amount of torque is transmitted through the clutch  12  to the transmission  13 . The third range of movement of the release bearing  33  is from the kiss point to the engaged position. The engaged position is the position of the release bearing  33  at which the driven disc assembly is completely engaged by the flywheel and the pressure plate. Thus, during this third range of movement (referred to as the engagement movement), the clutch  12  is gradually engaged so as to increase the amount of torque which is transmitted through the clutch  12  to the transmission  13  from the first measurable amount at the kiss point to the full capacity of the clutch  12  at the engaged position. 
     Movement of the release bearing  33  through the first and second ranges of movement can be accomplished in any known manner. As suggested above, the initial movement of the release bearing  33  from the disengaged position to the transition point can be accomplished by pulse width modulating the engage valve  41  at a predetermined duty cycle so as to cause rapid movement of the release bearing  33  from the disengaged position to the transition point. To accomplish this, the engage valve  41  may be pulse width modulated at a constant rate throughout the transition movement of the release bearing  33 . Alternatively, the engage valve  41  may be pulse width modulated at a rate which varies with the current position of the release bearing  33  relative to the transition point so as to decelerate the release bearing  33  somewhat as it approaches the transition point. The electronic controller  20  can be programmed to monitor the clutch position signal from the clutch position sensor  22  to determine when the release bearing  33  has reached the transition point. Regardless of the specific transition rate which is used, it is desirable that the initial transition movement of the release bearing  33  be performed as rapidly as possible because the clutch  12  is completely disengaged throughout. Therefore, no sudden and undesirable engagement of the clutch  12  will occur during this initial transition movement of the release bearing  33 . 
     Similarly, the approach movement of the release bearing  33  from the transition point to the kiss point can be accomplished by pulse width modulation of the engage valve  41  at a duty cycle which is initially relatively long (to initially maintain the rapid movement of the release bearing  33 ), but subsequently is shortened to decelerate the release bearing  33  as it approaches the kiss point. By slowing the movement of the release bearing  33  as it approaches the kiss point, the clutch  12  will be engaged smoothly so as to prevent the engine from stalling and avoid undesirable sudden jerking movement of the vehicle. The electronic controller  20  can be programmed to automatically alter the duty cycle of the engage valve during this approach movement of the release bearing  33  in response to sensed operating conditions. For example, the electronic controller  20  can be responsive to the amount of depression of the accelerator pedal from the pedal position sensor  24  for adjusting the duty cycle of the engage valve. However, any known algorithms may be used to control the movement of the release bearing  33  in its initial transition movement from the disengaged position to the transition point, and in its subsequent approach movement from the transition point to the kiss point. 
     The algorithm of this invention relates to the control of the movement of the release bearing  33  in its engagement movement from the kiss point to the engaged position. As discussed above, during the engagement movement of the release bearing  33  from the kiss point to the engaged position, the clutch  12  is gradually engaged so as to increase the amount of torque which is transmitted through the clutch  12  to the transmission  13  from the first measurable amount at the kiss point to the full capacity of the clutch  12  at the engaged position. Thus, although it is desirable that this engagement movement of the release bearing  33  be accomplished as quickly as possible, it is still important to engage the clutch  12  smoothly to prevent the engine from stalling and avoid undesirable sudden jerking movement of the vehicle. 
     Referring now to FIG. 3, there is illustrated is a first portion of a flow chart of an algorithm  50  for controlling the movement of the release bearing  33  of the clutch  12  in its engagement movement from the kiss point to the engaged position. In the first step  51  of the algorithm  50 , the electronic controller  20  issues a command to the engine controller  23  setting a desired engine speed signal V ENG . The desired engine speed signal V ENG  is selected to be sufficiently high such that the engine  11  is capable of overcoming the inertia of the vehicle as the clutch  12  is engaged and thereby avoid stalling during the engagement process. The desired engine speed signal V ENG  can, therefore, vary with the specific structure of the engine  11 , the transmission  13  used in conjunction with the engine  11 , and other factors. The second step  52  of the algorithm  50  is to determine the engagement rate of the release bearing  33  of the clutch  12 . For the purposes of this invention, the engagement rate can be determined in any conventional manner in response to a number of operating conditions of the vehicle. For example, the engagement rate can be selected to be a constant rate or may vary with the movement of the release bearing  33  from the kiss point to the engaged position in the manner discussed above. As will become apparent below, the algorithm  50  of this invention monitors the status of clutch engagement and alters the predetermined engagement rate under certain circumstances. 
     Next, the third step  53  of the algorithm  50  causes the electronic controller  20  to read the accelerator pedal position signal PED POS  from the engine controller  23 . As discussed above, the accelerator pedal position sensor  24  generates the accelerator pedal position signal PED POS  to the engine controller  23  which is representative of the actual position of the accelerator pedal of the vehicle. That information is available to the electronic controller  20  from the engine controller  23  over the data bus line  26 . Then, as shown in the fourth step  54  of the algorithm  50 , the engage valve  41  and the disengage valve  42  are actuated (by means of the respective solenoids  44  and  45 ) to effect movement of the release bearing  33  of the clutch  12  according to the selected engagement rate. Thus, the clutch engagement process is initiated. 
     In the fifth step  55  of the algorithm  50 , the electronic controller  20  reads the clutch input shaft speed signal V IN  from the engine controller  23 . As discussed above, the engine output shaft speed sensor  25  generates the clutch input shaft speed signal V IN  to the engine controller  23  which is representative of the actual rotational speed of the output shaft  11   a  of the engine  11 . That information is also available to the electronic controller  20  from the engine controller  23  over the data bus line  26 . In the sixth step  56  of the algorithm  50 , the electronic controller  20  reads the transmission output shaft speed signal directly from the speed sensor  21 . The seventh step  57  in the algorithm  50  is to calculate the clutch output shaft speed signal V OUT . The clutch output shaft speed signal V OUT  can be calculated by multiplying the transmission output shaft speed with the gear ratio of the transmission  13  selected by the electronic controller  20  and implemented by the transmission actuator  15 . 
     The algorithm  50  next enters a first decision point  58 , wherein the clutch input shaft speed signal V IN  is compared with the clutch output shaft speed signal V OUT . In this step, the magnitude of the difference between the clutch input shaft speed signal V IN  and the clutch output shaft speed signal V OUT  is compared against a first constant value K 1 . The first constant value K 1  is selected to be relatively small, typically about fifty revolutions per minute. If the magnitude of the difference between the clutch input shaft speed signal V IN  and the clutch output shaft speed signal V OUT  is greater than the first constant value K 1 , then the clutch  12  is not close to full engagement. In this instance, the algorithm  50  branches to a pedal position adjustment routine  70 , which will be described in detail below. Following the execution of the pedal position routine  70 , the algorithm returns to the fourth step  54 , wherein the electronic controller  20  the engage valve  41  and the disengage valve  42  are actuated by the electronic controller  20 . This loop of the algorithm  50  is repeated until the magnitude of the difference between the clutch input shaft speed signal V IN  and the clutch output shaft speed signal V OUT  is less than or equal to the first constant value K 1 . 
     If the magnitude of the difference between the clutch input shaft speed signal V IN  and the clutch output shaft speed signal V OUT  is less than or equal to the first constant value K 1 , it can be inferred that the clutch  12  is sufficiently close to full engagement as to warrant the interruption the gradual engagement process and immediately move the release bearing  33  from its current position to the fully engaged position. This interruption is desirable because it decreases the overall time required to complete the engagement process, while preventing the engine from stalling and avoiding undesirable sudden jerking movement of the vehicle. In practice, however, it has been found that during the engagement of the clutch  12 , the driven disc assembly is not always frictionally engaged between the flywheel and the pressure plate in a smooth manner. Rather, in some instances, the driven disc assembly is frictionally engaged in a somewhat stuttering or hesitating manner. If the samplings of the clutch input shaft speed signal V IN  and the clutch output shaft speed signal V OUT  are made during this stuttering engagement of the clutch  12 , a false inference of full engagement of the clutch  12  may be generated when, in fact, the clutch  12  is not yet sufficiently close to full engagement as to warrant the interruption of the gradual engagement process. 
     To address this, the algorithm  50  includes a second decision point  59 , wherein the clutch input shaft speed signal V IN  is compared with the desired engine speed signal V ENG . Specifically, the magnitude of the difference between the clutch input shaft speed signal V IN  and the desired engine speed signal V ENG  is compared against a second constant value K 2 . Alternatively, the magnitude of the difference between the clutch output shaft speed signal V OUT  and the desired engine speed signal V ENG  could be compared against the second constant value K 2 . In either event, the second constant value K 2  is selected to be relatively small, typically about twenty revolutions per minute. When the magnitude of the difference between the clutch input shaft speed signal V IN  and the desired engine speed signal V ENG  is greater than the second constant value K 2 , then it can be inferred that the clutch  12  is not close to full engagement. Thus, the gradual engagement process is continued, and the algorithm  50  again branches to the pedal position adjustment routine  70 , then back to the fourth step  54  as described above. This loop of the algorithm  50  is repeated until the magnitude of the difference between the clutch input shaft speed signal V IN  and the desired engine speed signal V ENG  is less than or equal to the second constant value K 2 . When the magnitude of the difference between the clutch input shaft speed signal V IN  and the desired engine speed signal V ENG  is less than or equal to the second constant value K 2 , then the inference that the clutch  12  is sufficiently close to full engagement is confirmed. Thus, the algorithm  50  enters the step  60  wherein the engage valve  41  is actuated to interrupt the gradual engagement process and immediately move the release bearing  33  from its current position to the fully engaged position. 
     Referring now to FIG. 4, there is illustrated a flow chart of a first embodiment of the pedal position adjustment routine  70 . As shown therein, the first step  71  of the pedal position adjustment routine  70  causes the electronic controller  20  to read an updated accelerator pedal position signal PED POS+1  from the engine controller  23 . The pedal position adjustment routine  70  next enters a first decision point  72  wherein the updated accelerator pedal position signal PED POS+1  is compared with the prior accelerator pedal position signal PED POS . If the updated accelerator pedal position signal PED POS+1  is greater than the prior accelerator pedal position signal PED POS , then it can be inferred that the accelerator pedal is being further depressed by the operator of the vehicle. Such further depression is indicative of a desire to move the vehicle at a speed which is faster than the current speed. When this occurs, the pedal position adjustment routine  70  branches to a step  73  wherein the current engagement rate being implemented by the electronic controller  20  is incremented. As a result, the engagement of the clutch  12  is pursued more aggressively, thereby decreasing the overall time duration required to complete the engagement process. Thereafter, the pedal position adjustment routine  70  enters a step  74  wherein the prior accelerator pedal position signal PED POS  is re-defined as the updated accelerator pedal position signal PED POS+1 . The pedal position adjustment routine  70  then returns to the third step  53  in the algorithm  60 . 
     If the updated accelerator pedal position signal PED POS+1  is not greater than the prior accelerator pedal position signal PED POS , then it can be inferred that the accelerator pedal is not being further depressed by the operator of the vehicle. The pedal position adjustment routine  70  next enters a second decision point  75  wherein the updated accelerator pedal position signal PED POS+1  is again compared with the prior accelerator pedal position signal PED POS . If the updated accelerator pedal position signal PED POS+1  is less than the prior accelerator pedal position signal PED POS , then it can be inferred that the accelerator pedal is being released by the operator of the vehicle. Such release is indicative of a desire to move the vehicle at a speed which is slower than the current speed. When this occurs, the pedal position adjustment routine  70  branches to a step  76  wherein the current engagement rate being implemented by the electronic controller  20  is decremented. As a result, the engagement of the clutch  12  is pursued less aggressively, thereby increasing the overall time duration require to complete the engagement process. Thereafter, the pedal position adjustment routine  70  enters the step  74  wherein the prior accelerator pedal position signal PED POS  is re-defined as the updated accelerator pedal position signal PED POS+1 , and the pedal position adjustment routine  70  returns to the third step  53  in the algorithm  60 . 
     Lastly, if the updated accelerator pedal position signal PED POS+1  is neither greater than nor less than the prior accelerator pedal position signal PED POS , then it can be inferred that the accelerator pedal is being held at a constant position by the operator of the vehicle. When this occurs, it can be inferred that the vehicle is moving at or near the speed desired by the operator of the vehicle. Thus, no change is made in the engagement rate, and the pedal position adjustment routine  70  enters the step  74  to re-define the prior accelerator pedal position signal PED POS  as the updated accelerator pedal position signal PED POS+1  before returning to the third step  53  in the algorithm  60 . 
     Referring now to FIG. 5, there is illustrated a flow chart of a second embodiment of the pedal position adjustment routine  70 . As shown therein, the first step  81  of the pedal position adjustment routine  80  causes the electronic controller  20  to read an updated accelerator pedal position signal PED POS+1  from the engine controller  23 . Then, as shown in the second step  82 , the pedal position adjustment routine  80  calculates the rate of change of pedal position ΔPED POS . The rate of change of pedal position ΔPED POS  can be calculated by subtracting the prior accelerator pedal position signal PED POS  from the updated accelerator pedal position signal PED POS+1 , then dividing by the time duration therebetween. The pedal position adjustment routine  80  next enters a first decision point  83  wherein rate of change of pedal position ΔPED POS  is compared with a third constant K 3 . If the rate of change of pedal position ΔPED POS  is greater than the third constant K 3 , then it can be inferred that the accelerator pedal is being further depressed by the operator of the vehicle. Such further depression is indicative of a desire to move the vehicle at a speed which is faster than the current speed. When this occurs, the pedal position adjustment routine  80  branches to a step  84  wherein the current engagement rate being implemented by the electronic controller  20  is incremented. As a result, the engagement of the clutch  12  is pursued more aggressively, thereby decreasing the overall time duration require to complete the engagement process. Thereafter, the pedal position adjustment routine  80  enters a step  85  wherein the prior accelerator pedal position signal PED POS  is re-defined as the updated accelerator pedal position signal PED POS+1 . The pedal position adjustment routine  80  then returns to the third step  53  in the algorithm  60 . 
     If the rate of change of pedal position ΔPED POS  is not greater than the prior accelerator pedal position signal PED POS , then it can be inferred that the accelerator pedal is not being further depressed by the operator of the vehicle. The pedal position adjustment routine  80  next enters a second decision point  86  wherein the rate of change of pedal position ΔPED POS  is compared with a fourth constant value K 4 . If the rate of change of pedal position ΔPED POS  is less than the fourth constant value K 4 , then it can be inferred that the accelerator pedal is being released (or at least increased at a slower rate) by the operator of the vehicle. Such release is indicative of a desire to move the vehicle at a speed which is slower than the current speed. When this occurs, the pedal position adjustment routine  80  branches to a step  87  wherein the current engagement rate being implemented by the electronic controller  20  is decremented. As a result, the engagement of the clutch  12  is pursued less aggressively, thereby increasing the overall time duration require to complete the engagement process. Thereafter, the pedal position adjustment routine  80  enters the step  86  wherein the prior accelerator pedal position signal PED POS  is re-defined as the updated accelerator pedal position signal PED POS+1 , and the pedal position adjustment routine  80  returns to the third step  53  in the algorithm  60 . 
     Lastly, if the rate of change of pedal position ΔPED POS  is neither greater than the third constant value K 3  nor less than the fourth constant value K 4  (K 3  and K 4  may be equal if desired), then it can be inferred that the vehicle is moving at or near the speed desired by the operator of the vehicle. Thus, no change is made in the engagement rate, and the pedal position adjustment routine  80  enters the step  85  to re-define the prior accelerator pedal position signal PED POS  as the updated accelerator pedal position signal PED POS+1  before returning to the third step  53  in the algorithm  60 . 
     This invention has been described and illustrated in the context of controlling the engagement rate of the clutch  12  in response to the depression of the accelerator pedal of the vehicle. However, it will be appreciated that this invention may be applied to the other ranges of movement of the release bearing  33  of the clutch  12  from the disengaged position to the engaged position. Thus, the electronic controller  20  may be programmed to alter the rate of transition movement of the release bearing  33  from the disengaged position to the transition point in response to the depression of the accelerator pedal if desired. Similarly, the electronic controller  20  may be programmed to alter the rate of approach movement of the release bearing  33  from the transition point to the kiss point in response to the depression of the accelerator pedal if desired. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.