Abstract:
A clutch control process for a motor vehicle transmission during brake lift-out monitors a time rate of change of a brake pedal position and torque to detect a lift-out condition with rapidly changing torque. When the process detects a brake lift-out condition, the process limits the minimum scheduled clutch pressure as well as modifies the clutch pressure to torque relationship.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/476,606, filed Apr. 18, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present invention relates to a clutch control strategy for a motor vehicle transmission. More specifically, the present invention relates to a clutch control strategy for a motor vehicle transmission during brake lift-out. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
         [0004]    In a motor vehicle transmission, closed throttle downshifts can be thought of as an inverted power-on upshift. During such situations, if a lift-out of torque (i.e. a reduction in torque), typical clutch control strategies detect lift-out conditions and modify the control strategy to address the rapidly decreasing torque. The same condition can occur during negative torque regenerative braking closed throttle downshifts when the driver removes pressure from the brake (i.e. brake lift-out) during the clutch control process. Specifically, in particular hybrid arrangements, the coast regenerative torque can vary through the shift process depending on the driver brake input and a desire to maintain constant power to the wheels of the motor vehicle. While braking at high regenerative levels, when the driver lifts out of the brake, it is expected that the level of deceleration returns to a normal level. Thus the torque must be ramped out a predetermined level. Current clutch control strategies, however, are unable to handle situations in which the torque changes rapidly. In particular, current clutch control strategies cannot predict and control the proper clutch pressure to torque relationship. 
         [0005]    Accordingly, there is a need for an improved clutch control strategy during brake lift-out. 
       SUMMARY 
       [0006]    A clutch control process for a motor vehicle transmission during brake lift-out monitors a time rate of change of a brake pedal position and torque to detect a lift-out condition with rapidly changing torque. When the process detects a brake lift-out condition, the process limits the minimum scheduled clutch pressure as well as modifies the clutch pressure to torque relationship. 
         [0007]    The clutch control process eliminates the possibility of losing control of the transmission&#39;s clutch. 
         [0008]    Further features, advantages, and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0009]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings: 
           [0010]      FIG. 1  is a graph of simulation data for uncontrolled command of clutch torque during brake lift-out conditions; 
           [0011]      FIG. 2  is a flow diagram for a process to control clutch torque during brake lift-out in accordance with an embodiment of the present invention; and 
           [0012]      FIG. 3  is a graph of simulation data for controlled command of clutch torque during brake lift-out in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0014]    Referring now to the drawings, a process embodying the principles of the present invention is illustrated therein and designated as  10  ( FIG. 2 ). Turning in particular to  FIG. 1 , a graph of simulation data illustrates conditions that may occur during closed throttle downshifts of a motor vehicle transmission when the driver removes pressure from the brake (i.e. brake lift-out). Specifically, brake lift-out occurs at a time t 1 , as indicated by a sudden drop in brake pressure (a). When this occurs, there is a rapid rise in the possible torque (b) applied to the transmission&#39;s clutch. As such, the possible torque (b) exceeds the target clutch torque value (c), which is the desired torque value for the present conditions. As expected, the clutch torque value (d) is proportional to the possible torque value (b). The rapid rise in the possible torque (b) occurs because of an uncontrolled situation in which the motor vehicle&#39;s engine is attempting to return to an idle condition, as indicated by the uncontrolled oncoming command (f) signal to the clutch. That is, in this situation, the change in clutch torque, and hence the change in clutch pressure, results in the rapid change in the possible torque (b). 
         [0015]    After the clutch torque changes rapidly, the control device responds in a proportional manner so that the clutch pressure corresponds to the new scheduled clutch torque. The control in this case, however, overshoots the desired value because of the control device&#39;s hysteresis, which causes the oncoming clutch engagement to exhaust for a brief period of time. This results in an uncontrolled loss of transmission input speed control. 
         [0016]    When the process  10  is implemented, the brake lift-out during downshift conditions becomes controllable. As such, any shift condition that was uncontrollable prior to the use of the process  10  is eliminated by the process  10  so that brake lift-out downshifts become unperceivable to the operator of the motor vehicle. 
         [0017]    The process  10  begins with a function call (step  12 ). Next, the process  10  proceeds to a decision step  14 . Here, the process determines (i) if the vehicle is in a coast down situation. (ii) if the position of the accelerator pedal is less than a desired calibrated value for the accelerator position, and (iii) if the features associated with the process  10  are desired. If these three conditions are not met, then the process  10  resets in a step  16  and then ends in a step  18 . If the three conditions are satisfied, then the initial position of the vehicle&#39;s brake and the initial possible torque are stored in a step  20  as input values for a subsequent calculation. 
         [0018]    Next, in a decision step  22 , the process  10  determines (i) if the actual brake pedal position is not equal to the initial brake pedal position, (ii) if the difference between the actual clutch torque and the initial clutch torque is greater than or equal to set calibration value, and (iii) if the change in clutch torque divided by a loop time (i.e. the rate of change in clutch torque) is greater than or equal to a set value. If these conditions are not satisfied, then the process  10  proceeds to step  16  where the process  10  resets. The process  10  ends in step  18 . If the three conditions in step  22  are satisfied, then the process  10  proceeds to step  24 . In step  24 , the process  10  sets the scheduled oncoming limit to a calibrated value and sets the coast down clutch pressure to torque gain to a calibrated value for the remainder of the shift. Specifically, the step  24  swaps the actual increasing clutch gain output with a decreasing clutch gain to compensate for the control device&#39;s hysteresis associated with the vehicle&#39;s transmission during such downshifts. 
         [0019]    The results of the implementation of the process  10  are illustrated in  FIG. 3 . After brake lift-out, which occurs at time t 1 , clutch torque value (d) is attenuated in comparison to the clutch torque value shown in  FIG. 1 . Also note, that the rise and fall of the uncontrolled oncoming command signal to the clutch shown in  FIG. 1  are damped out with the implementation of the process  10 , as shown in  FIG. 3 . 
         [0020]    As a result of the implementation of the process  10 , when the clutch torque decreases, the control process attempts to decrease the clutch pressure. This pressure to torque relationship is now swapped with a decreasing relationship that allows the control device to decrease the clutch pressure in a controlled manner. In the event of an under-filled clutch, if the controlled pressure drops below a calibrated level, the control pressure gets clipped to that of the calibrated value. 
         [0021]    The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.