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, it is desirable to insure that the actual load on the vehicle engine does not drop below a predetermined level which might result in a stall condition. 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 the actual load on the vehicle engine for varying the engagement rate of release bearing during re-engagement of the clutch.