Abstract:
A controller adjust a clutch actuator position is response to movement of a clutch pedal. During an engagement or a disengagement, the controller monitors sensor signals to determine the actuator position corresponding to the touch point. The sensors may directly indicate clutch torque or may respond indirectly. A Giant Magneto Resistive (GMR) sensor provides a precise shaft rotational position signal which can be twice numerically differentiated to yield an accurate and stable acceleration signal. The controller updates the touch point based on a change in the sensed acceleration or torque. The controller then adjusts the relationship of actuator pedal position to clutch pedal position, making mechanical wear adjustment unnecessary.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of transmission systems. More particularly, the disclosure pertains to a manual transmission with a clutch controlled by a controller in response to movement of a clutch pedal. 
     BACKGROUND 
     A typical manual powertrain is illustrated in  FIG. 1 . Solid lines represent mechanical power flow through rotating shafts. Dashed lines represent control connections, which may be implemented using mechanical linkages. Engine  10  generates power at crankshaft  12  by burning fuel. The engine responds to changes in the position of accelerator pedal  14  to generate more power when the pedal is depressed further by the driver. Transmission  16  transmits power from crankshaft  12  to output shaft  18 . Transmission  16  includes a friction clutch  20  and a gearbox  22  connected by input shaft  24 . Gearbox  22  is capable of establishing a variety of forward speed ratios and at least one reverse speed ratio in response to driver manipulation of shifter  26 . The driver controls the torque capacity of clutch  20  by manipulation of clutch pedal  28 . Differential  30  splits power from output shaft  18  between a left axle  32  driving a left wheel  34  and a right axle  36  driving a right wheel  38  while permitting slight speed differences between the axles as the vehicle turns a corner. In a typical rear wheel drive powertrain, the transmission output shaft is a driveshaft that extends to the differential. In a typical front wheel drive powertrain, the output shaft  16  may be driveably connected to the differential by a final drive gear. The transmission and differential of a front wheel drive powertrain are frequently combined into a single housing and called a transaxle. 
     For internal combustion engine  10  to generate power, crankshaft  12  must rotate at sufficient speed. However, when the vehicle is stationary with gearbox  22  establishing a speed ratio, input shaft  24  is also stationary. In order to start the vehicle moving, the driver controls the torque capacity of clutch  20  to transmit power from moving crankshaft  12  to stationary input shaft  24 . As the vehicle accelerates the speed of input shaft  24  gradually increases until it is equal to the speed of crankshaft  12 , at which point clutch  20  can be fully engaged. With clutch  20  fully engaged, the speed of crankshaft  12  is proportional to vehicle speed. As the vehicle accelerates in 1st gear, the speed of crankshaft  12  becomes excessive, necessitating a shift to 2nd gear. Gearbox  22  is not capable of changing ratios while transmitting power. Therefore, the driver shifts by disengaging clutch  20 , then manipulating shifter  26  to change the gearbox ratio, then re-engaging clutch  20 . Re-engagement of clutch  20  forces the crankshaft speed to become equal to input shaft speed, predominantly by changing the speed of the crankshaft. 
     Whenever clutch  20  transits torque between shafts rotating at different speeds, as during a vehicle launch event, some power must be dissipated. Power is the product of speed and torque. During a launch event, the torque exerted by the crankshaft and the torque exerted on the input shaft are both equal to the clutch torque capacity. The power flowing into the clutch is the torque capacity multiplied by the crankshaft speed. The power flowing out of the clutch mechanically is the torque capacity multiplied by the input shaft speed. The difference between the power inflow and the mechanical power outflow is dissipated by conversion into heat. Initially, the heat is absorbed into clutch components causing the temperature of those components to increase. Then, the heat is gradually transferred to the environment through convection, conduction, and radiation, gradually reducing the temperature of the clutch components. 
     The amount of energy dissipated by the clutch in a time interval is equal to the integral of the power dissipation over time. If an excessive amount of energy is dissipated in a short amount of time, the clutch temperature will rise excessively. When the clutch temperature is elevated, the rate of wear of the clutch facing material increased dramatically. At sufficiently high temperatures, the friction coefficient of the material decreases and the clutch may be incapable of achieving sufficient torque capacity. Driver technique in manipulating the accelerator pedal, clutch pedal, and shifter strongly influences energy dissipation. 
     SUMMARY OF THE DISCLOSURE 
     A vehicle includes an engine, a clutch, a gearbox, and a controller. The gearbox establishes one of a plurality of power flow paths between an input shaft and an output shaft in response to movement of a shift lever. The clutch transmits torque from the engine to the input shaft with a torque capacity that varies in response to a position of a clutch actuator. The torque capacity is negligible when the actuator position is on a released side of a touch point and increases monotonically with respect to actuator positions on an engaged side of the touch point. The controller adjusts the actuator position according to a function of a clutch pedal position. During a first launch event, the controller monitors a sensor array and modifies the function in response to the received signals. The sensor array may include a torque sensor, a rotational speed sensor such as a Giant Magneto Resistive (GMR) sensor, or an acceleration sensor. During a second launch, the controller adjusts the actuator position according to the modified function of the pedal position. The function modified such that, as the touch point changes due to wear or other effects, the touch point corresponds to a constant, predetermined clutch pedal position. Under certain conditions, the controller may adjust the actuator to the released side of the touch point while the clutch pedal is on an engaged side of the constant, predetermined position. These conditions include coasting with the accelerator pedal released and the vehicle being stationary with the transmission engaged in a forward drive power flow path. In some circumstances, the controller may shut the engine off while the clutch actuator is in a released position and may maintain the actuator in a released position, independent of the clutch pedal position, until the engine is restarted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is schematic illustration of a vehicle powertrain having a manual transmission. 
         FIG. 2  is a schematic illustration of a vehicle powertrain having a manual transmission with an electronically actuated clutch. 
         FIG. 3  is a schematic diagram of a gearing arrangement for a manual transmission. 
         FIG. 4  is a cross section of an electronically actuated manual transmission clutch in a disengaged position. 
         FIG. 5  is a cross section of an electronically actuated manual transmission clutch at a touch point position. 
         FIG. 6  is a cross section of an electronically actuated manual transmission clutch in an engaged position. 
         FIG. 7  is a graph of clutch torque capacity as a function of clutch actuator position. 
         FIG. 8  is a flow chart for a method of adaptively updating a touch point estimate using a clutch torque measurement. 
         FIG. 9  is a flow chart for a method of adaptively updating a touch point estimate using an acceleration measurement. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
       FIG. 2  illustrates a powertrain that utilizes a controller to add features not practical in the purely manual powertrain of  FIG. 1 . In the powertrain of  FIG. 2 , engine  10  and clutch  20  do not respond directly to movement of the accelerator pedal  14  and the clutch pedal  28  respectively. Instead, controller  40  senses the position of the pedals and sends commands to the engine and clutch. In some instances, the commands may not correspond directly to driver commands as indicated by manipulation of the pedals. To assist in determining the proper commands, the controller may receive additional signals, including a signal indicating the position of shifter  26  and signals from an array of sensors  42  in gearbox  22 . 
       FIG. 3  illustrates an exemplary configuration of a front wheel drive transmission. Transmission output  18  is a final drive ring gear fixed to a carrier of a differential. Two countershafts,  50  and  52  are parallel to transmission input shaft  24 . Final drive pinion gears  54  and  56  are fixed to countershafts  50  and  52  respectively and mesh continuously with output gear  18 . Therefore, the speeds of the countershafts are related to the speed of output  18  by fixed ratios. A number of gears fixed to input shaft  24  mesh with corresponding gears that are supported for rotation about one of the countershafts. Particular gear ratios are engaged by moving one of the synchronizers  58 ,  60 , or  62 , to selectively couple one of these gears to one of the countershafts. A shift mechanism (not shown) moves the synchronizers in response to driver manipulation of the shifter  26 . 
     The transmission also includes a number of sensors which respond in various ways to clutch torque. These sensors collectively make up sensor array  42 . Several methods of determining torque are described below, although only one method of determining torque is required. Particular embodiments may determine torque using a subset of the sensors shown in  FIG. 3  or may utilize different sensors that respond to clutch torque. Torque sensor  64  directly senses the torque on input shaft  24  which, when clutch  20  is slipping, is equal to the clutch torque capacity. For example, torque sensor  64  may measure the shear strain in the shaft. 
     Two Giant Magneto Resistive (GMR) sensors  66  and  68  are located at opposite ends of countershaft  50 . GMR sensors generate a voltage that varies sinusoidally based on the rotational position of a magnet fixed to the end of a shaft. GMR sensors provide a rotational position measurement accurate to a fraction of a degree at intervals of around 50 micro-seconds. Unlike the speed sensors commonly used in transmissions, GMR sensors can provide a usable signal even when the shaft is at zero speed. One limitation of GMR sensors is that they must be mounted at the end of a shaft. However, in this application, that is not a problem. 
     When either 1st or 2nd gear is selected, at least a portion of countershaft  50  will transmit torque that is proportional to the clutch torque. The portion of the shaft transmitting the torque is twisted as a result of the torque. The amount of twist can be measured by measuring rotational position of each end of the shaft and taking the difference. The clutch torque is proportional to this difference. The coefficient of proportionality differs between 1st and 2nd gear. 
     Due to the accuracy of GMR position signals, an accurate and stable speed signal can be obtained by differentiating the position signal. In turn, an accurate rotational acceleration signal can be obtained by differentiating the rotational speed signal. The acceleration of countershaft shaft  50  is proportional to vehicle acceleration, independent of which gear ratio, if any, is selected. Vehicle acceleration is related to proportional to clutch torque, the selected gear ratio, and inversely proportional to vehicle mass. Since the selected gear ratio is known to the controller and the vehicle mass typically changes relatively slowly within a narrow range, vehicle acceleration can be used as a surrogate for clutch torque for some purposes. Vehicle acceleration also responds to additional factors such as road grade, wind, and road resistance. These factors also tend to change relatively slowly such that the controller can compensate for them. Deriving a surrogate torque signal based on vehicle acceleration requires only one of the GMR sensors  66  or  68 . 
       FIG. 4  shows a cross sectional view of a clutch  20 . A clutch disk  70  is splined to gearbox input shaft  24 . In some embodiments, clutch disk  70  may include a damper to provide torsional isolation between the engine and driveline when the clutch is engaged. Friction material  72  is attached to the front and back sides of clutch disk  70 . A flywheel  74  is fixed to crankshaft  12 . In some embodiments, the flywheel may include provisions for torsional isolation. A pressure plate  76  is supported for rotation with the flywheel, but allowed to move axially. A clutch cover  78  is fixed to the flywheel. A diaphragm spring  80  is attached to the clutch cover  78 . In its natural state, diaphragm spring  80  is conical is shape such that it tends to push the pressure plate to compress the clutch disk between the pressure plate and the flywheel. However, in the disengaged state shown in  FIG. 4 , an actuator  82  has pushed the center of diaphragm spring inward such that the spring assumes a more flat shape, allowing for slight separation between the friction material  72  and the flywheel and pressure plate. Since actuator  82  does not rotate and diaphragm spring  80  rotates with the flywheel, they are separated by a bearing  84 . 
       FIG. 5  shows the clutch with the actuator moved to the touch point. At the touch point, the pressure plate has moved toward the flywheel enough to eliminate the space such that the friction material  72  is in contact with the flywheel and pressure plate, but the normal force compressing the friction material is zero. Since the normal force is zero, the torque capacity of the clutch is zero at the touch point.  FIG. 6  shows the clutch with the actuator moved beyond the touch point. As the actuator position moves beyond the touch point, the normal force compressing the friction material increases. Consequently, the torque capacity of the clutch also increases. 
       FIG. 7  illustrates the relationship between actuator position and torque capacity. Line  90  indicates the relationship for a new clutch. The torque capacity is zero when the actuator position is less than the touch point  92  and the increases steadily as actuator position increases. Controller  40  determines a desired torque capacity based on clutch pedal position, among other inputs, and uses information about the relationship between actuator position and torque capacity to determine what actuator position to command. As the clutch is used, the friction material  72  gradually wears away. Line  94  illustrates the relationship for the same clutch after significant friction material wear has occurred. The touch point  96  increases as the clutch wears. In addition to clutch wear, other noise factors such as temperature may shift the touch point. If the controller uses an incorrect estimate of the touch point, the torque capacity may differ substantially from the desired torque capacity. 
     The clutches of some manual transmissions are equipped with mechanical wear compensators that shift the touch point back such that it corresponds to roughly the same clutch pedal position. However, mechanical wear compensators tend to make the adjustments in discrete steps. These discrete steps are small enough that drivers typically do not notice. However, to a controller capable of finer control, unpredictable adjustments create an additional noise factor. Therefore, it is desirable to replace the mechanical wear compensation devices of manual transmissions with algorithmic wear compensation. 
       FIG. 8  is a flow chart for adaptively determining the actuator position corresponding to the clutch touch point when a clutch torque signal is available. The method is executed, beginning at  100 , at regular intervals. If the clutch is not slipping, as determined at  102 , the method proceeds to  104  to update the current actuator position without updating the touch point. For example, the actuator position may be set according to a function of a sensed clutch pedal position such that a predetermined pedal position places the actuator at the estimated clutch touch point. If the clutch is slipping, the method proceeds to  106  to measure the clutch torque using one of the methods described above or some other method. Equivalently, the method may measure a quantity that is directly proportional to clutch torque, such as a transmission shaft torque. If the measured clutch torque is less than a threshold at  108 , the method branches based on a conclusion that the current actuator position is less than the actual touch point. The threshold is set near zero, but large enough that sensor variation does not falsely conclude that the clutch is in the transmitting torque. At  110 , the current estimate of the touch point is compared to the current actuator position. If the estimate of the touch point is less than the current position, then the estimate of the touch point is updated at  112  to be equal to the current position. Then, the current position is updated based on the function of pedal position and the revised estimate of the touch point. Alternatively, updating the function based on the revised clutch point may be delayed until after the current event, such as a vehicle launch event or a shift is completed. If the measured clutch torque exceeds the threshold at  108 , on the other hand, the method branches based on the conclusion that the current actuator position is past the touch point such that the clutch torque is responding linearly to changes in actuator position. If that conclusion is inconsistent with the current estimate of the touch point, as determined at  114 , then the estimate is updated at  112  and the current position is updated at  104 . 
       FIG. 9  is a flow chart for adaptively determining the actuator position corresponding to the clutch touch point when a vehicle acceleration signal is available. As with the method of  FIG. 8 , the method is executed, beginning at  100 , at regular intervals. If the clutch is not slipping, as determined at  102 , the method proceeds to  104  to update the current actuator position without updating the touch point. If the clutch is slipping, the method proceeds to  120  where the method records the acceleration measurement of the previous execution of the method, corresponding to a previous actuator position. At  122  the method measures vehicle acceleration or a quantity that is directly proportional to vehicle acceleration such as the acceleration of a transmission shaft. At  124 , the method calculates the slope of the relationship between acceleration and actuator position between the previous execution of the method and the current execution. If the clutch position is less than the touch point, this slope would nominally be zero. If the clutch position is greater than the touch point, the slope will be related to the gear ratio. If the computed slope is less than a threshold at  126 , the method branches based on a conclusion that the current actuator position is less than the actual touch point. Different thresholds may be used when different gear ratios are selected. Furthermore, an absolute value may be used when reverse gear is selected. At  128  and  130 , the current estimate of the touch point is updated if it is less than both the current and the previous position. If the computed slope is greater than the threshold at  126 , the method branches based on a conclusion that the current actuator position is greater than the actual touch point. At  132  and  134 , the current estimate of the touch point is updated if it is greater than both the current and the previous position. At  136 , the current actuator position is recorded for use in the next execution. Then, the current position is updated based on the pedal position. 
     If the actual touch point changes, the methods of either  FIG. 8  or  FIG. 9  will result in a change in the estimate of the touch point following a clutch engagement event. As a result, the controller will modify the function that relates commanded actuator position to sensed clutch pedal position. Then, during a subsequent launch or shift event, the actuator position will be different for a given clutch pedal position. As the clutch wears, or as other noise factors influence the actual touch point, the function may be modified such that the pedal position at the clutch touch point remains nearly constant. 
     Although the nominal behavior of the controller is to position the actuator based solely on the position of the clutch pedal, the controller may depart from this behavior is some circumstances. The ability to over-ride the driver clutch pedal movement is one of the advantages of an electronically actuated clutch. One such circumstance occurs when the vehicle is coasting (neither accelerator pedal nor brake pedal depressed) and the driver leaves the transmission in gear with the clutch pedal released (which corresponds to the clutch being engaged). In this circumstance, vehicle inertia causes the engine to rotate. This causes the engine to exert drag torque which may be substantial if the engine speed is relatively high. To prevent the vehicle from decelerating unnecessarily, the controller may move the clutch actuator to a released position and then control the engine to rotate at idle speed. Alternatively, the controller may shut the engine off to reduce fuel consumption further. When the driver depresses the accelerator, the controller must quickly bring the engine back to synchronous speed and then re-engage the clutch. In order to be able to re-engage quickly, the controller positions the actuator close to the touch point, but on the released side of the touch point. For the controller to accomplish this, it must have accurate information about the location of the touch point. 
     When the vehicle is stopped, the controller may shut the engine off to save fuel. The controller must then quickly restart the engine when the driver releases the brake pedal and depresses the accelerator pedal. Some manual transmission drivers waiting at a stop light disengage the transmission with shifter  26  and release clutch pedal  28 . When they are ready to drive away, they depress the clutch pedal  28 , engage 1st gear with shifter  26 , and then step on the accelerator pedal  14  and gradually release clutch pedal  28 . These sequential steps give the controller sufficient time to restart the engine before the driver begins releasing the clutch pedal. However, other drivers leave the transmission in 1st gear and depress clutch pedal  28  while waiting at a stop light. If the controller stops the engine in this circumstance, the driver may begin releasing the clutch pedal before the controller has started the engine. If the driver engages the clutch before the engine is started, that will prevent a proper engine start. With the electronically actuated clutch described herein, the controller can prevent engagement of the clutch until the engine has restarted. Therefore, the controller can stop the engine in more conditions than otherwise, reducing fuel consumption. Specifically, the controller moves the actuator to a released position near the touch point while the engine is shut down and maintains the actuator in that position, regardless of clutch pedal position, until the engine has restarted. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.