Abstract:
A method of operating a park lock mechanism in a vehicle includes determining a force acting on a vehicle park lock mechanism when the vehicle is in park, and providing an offsetting force to counteract the force on the park lock mechanism before the vehicle is shifted out of park. In doing so, harshness and noise that may be associated with shifting a vehicle out of park, especially when the vehicle is parked on an incline, can be reduced or eliminated.

Description:
FIELD 
       [0001]    The present disclosure relates generally to a vehicle parking brake or lock system and more particularly to a method of operating a park lock mechanism. 
       BACKGROUND 
       [0002]    Vehicles transmissions have a park setting or gear in which the transmission may be locked and the vehicle turned off. When a vehicle is parked on a hill or other grade, gravitational forces on the vehicle provide a load on the vehicle park lock system. With such a load on the park lock system, disengagement of the park lock system to permit the transmission to be shifted out of park may be difficult or noisy or have a harsh movement. 
       SUMMARY 
       [0003]    A method of operating a park lock mechanism in a vehicle includes determining a force acting on a vehicle park lock mechanism when the vehicle is in park, and providing an offsetting force to counteract the force on the park lock mechanism before the vehicle is shifted out of park. In doing so, harshness and noise that may be associated with shifting a vehicle out of park, especially when the vehicle is parked on an incline, can be reduced or eliminated. 
         [0004]    In at least one implementation, a method of providing an offsetting force to a park lock system in a vehicle includes providing an offsetting force on a transmission output shaft in a direction opposite to the force on the transmission output shaft due to the force of gravity on the vehicle. The offsetting force, in at least one example, can be applied without first determining the magnitude of the force needed, and the offsetting force can be applied until the net force on the park lock mechanism, or on the output shaft, is below a threshold. 
         [0005]    Further areas of applicability of the present disclosure will become apparent from the detailed description, claims and drawings provided hereinafter. It should be understood that the summary and detailed description, including the disclosed embodiments and drawings, are merely exemplary in nature intended for purposes of illustration only and are not intended to limit the scope of the invention, its application or use. Thus, variations that do not depart from the gist of the disclosure are intended to be within the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a side view of one example of a park lock system in its disengaged or unlocked position where a park lock mechanism is not engaged with a park gear of a vehicle transmission; 
           [0007]      FIG. 2  is a side view like  FIG. 1  showing the park lock system in its engaged or locked position; and 
           [0008]      FIG. 3  is a flow chart showing steps of a representative method for reducing the force on the park lock mechanism before the vehicle is shifted out of park. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring in more detail to the drawings,  FIGS. 1 and 2  illustrate a park lock system  10  for a vehicle. The park lock system  10  includes a lock mechanism  12  that engages a park gear  14  of the vehicle transmission to hold or lock the vehicle transmission in park. The lock mechanism  12  is removed or disengaged from the park gear  14  to permit the vehicle transmission to be shifted out of park and into another gear (e.g. reverse, neutral or a forward drive gear). 
         [0010]    In more detail, as shown in  FIGS. 1 and 2 , the park gear  14  is coupled to a vehicle transmission output shaft  16  for rotation with the output shaft  16  about an axis  18  of the shaft when the vehicle is moving. The gear  14  has a plurality of circumferentially spaced apart teeth  20  extending outwardly around its periphery. Gaps  22  are defined between adjacent teeth  20  in an alternating pattern of teeth and gaps. 
         [0011]    The park lock system  10  includes the lock mechanism  12  and an actuator  24  for selectively engaging the lock mechanism  12  with the park gear  14 . In the example shown, the lock mechanism  12  includes a pawl  26  pivoted at a first end about a pin  28  to move a second end including a lock tab  30  toward and away from the park gear  14 . This permits the pawl  12  to be moved between disengaged ( FIG. 1 ) and engaged ( FIG. 2 ) positions to selectively engage the pawl  26  with the park gear  14 . The lock tab  30  is adapted to engage a tooth  20  of the park gear  14  when the park lock mechanism  10  is actuated to lock the vehicle transmission in park. The pawl  26  may be yieldably biased, such as by a spring, toward its disengaged position where the lock tab  30  is free and clear of the park gear  14  to permit rotation of the park gear  14 . In this way, absent a force moving and holding the pawl  26  into its engaged position, the pawl  26  will normally be in its disengaged position. 
         [0012]    The actuator  24  may include a motor  32  and a drive member  34  driven by the motor  32  to move the pawl  26  from its disengaged position to its engaged position. The motor  32  may be any suitable reversible electric motor  32 . The motor  32  is coupled to a threaded spindle  36  to rotate the spindle in forward and reverse directions and thereby move the drive member  34 . The drive member  34  travels along the spindle  36  in a direction dictated by the direction of rotation of the spindle  36 . In the implementation shown, the drive member  34  moves toward the motor  32  when the spindle  36  is rotated in the forward direction and the drive member  34  moves away from the motor  32  when the spindle  36  is rotated in the reverse direction. 
         [0013]    In use, to engage the park lock system  10  the motor  32  is driven in its forward direction. This moves the drive member  34  toward the motor  32  along the spindle  36  until the drive member  34  engages the park pawl  26 . If the lock tab  30  of the pawl  26  is aligned with a gap  22  between adjacent teeth  20  in the park gear  14 , the pawl  26  is moved to its engaged position with the lock tab  30  received between adjacent teeth  20  as shown in  FIG. 2 . To provide a force holding the park pawl  26  in its engaged position, the drive member  34  may remain engaged with the park pawl  26 . Gravity or another force (e.g. engine or transmission force), will generally cause some movement of the vehicle and corresponding rotation of the park gear  14  to firmly engage the lock tab  30  with one of the teeth  20  on the park gear  14 . This provides a positive stop to movement of the park gear  14 . 
         [0014]    To shift the vehicle out of park, the park pawl  26  must be moved to its disengaged position. To do this, the motor  32  is driven in its reverse direction and the drive member  34  moves away from the motor  32  and the pawl  26 . As the drive member  34  disengages from the pawl  26 , the return spring biasing the pawl  26  moves the pawl  26  toward its disengaged position until the lock tab  30  is free and clear of the park gear  14  and the park gear can rotate without interference from the park pawl. 
         [0015]    At least when the vehicle is parked on an incline, the force of gravity on the vehicle will tend to rotate the park gear  14  against and firmly into engagement with the lock tab  30  of the park pawl  26 . This can provide a significant force on the park pawl  26  such that when the park pawl  26  is moved to its disengaged position, the park pawl  26  may abruptly move out of its engaged position and a loud noise and abrupt (but usually slight) movement of the vehicle may occur. Noise may also be attributed to the sudden movement of the park gear  14  upon release of the park pawl  26 . The noise and vehicle movement can be startling or unsettling to some people. 
         [0016]    To reduce or eliminate the noise and/or sudden movement of the vehicle upon disengagement of the park pawl  26  from the park gear  14 , a counterforce is provided to lessen (potentially reduce to zero) the force the park gear  14  places on the lock tab  30  at least when the vehicle is parked on a grade or incline above a threshold magnitude. The counterforce may be provided by an electric motor coupled to the output shaft  16 . The electric motor may provide a torque that counters the force tending to rotate the shaft  16  such that the force the park gear  14  provides on the lock tab  30  is reduced. In this way, the park lock system  10  is readily adaptable to an electric vehicle application where such an electric motor coupled to the output shaft  16  is already available. Of course, the counterforce could be provided in other vehicles including those using a combustion engine where the offsetting torque could be provided by the engine itself, or by another prime mover including an electric motor, if desired. 
         [0017]    Accordingly, the park lock system  10  in operation may utilize any method of applying a counterforce or offsetting torque to lessen the force acting on the park lock mechanism  12  before the park lock mechanism  12  is disengaged from the park gear  14 . In operation, one method of applying a counterforce is shown in the flow chart of  FIG. 3 . In general, that method for applying a counterforce or offsetting torque involves determining the amount of offsetting torque needed or desired at  40  and applying the determined torque to the shaft at  42 . Of course, other methods may be used, for example, applying an offsetting torque to the shaft  16  until only a predetermined force remains on the park pawl  26  (that is, reducing the force on the park pawl  26  until the force is below a threshold). In another embodiment, the park pawl  26  and/or the drive member  34  can be configured so engaging surfaces cooperate to facilitate a smooth, uninterrupted engagement/disengagement of the contacting parts ( 26  and  34 ) and engagement/disengagement of the lock tab  30  with the park gear  14 . For example, one or more contacting parts that engage/disengage may include a chamfered surface, curved surface profile, or otherwise complementary surface profile. 
         [0018]    In more detail, the method of  FIG. 3  includes a step  40  of determining the force acting on the park pawl  26 . This may be accomplished in many ways. One way is to sense or determine a longitudinal acceleration of the vehicle (generally, the attitude or relative height of a front of the vehicle compared to the back end of the vehicle). The longitudinal acceleration may be measured using an inertial sensor such as an accelerometer or tilt sensor, by way of a couple examples, without any limitation intended. From the longitudinal acceleration, the grade or incline on which the vehicle is parked may be calculated at  44  and a suitable counteracting or offsetting torque can be calculated based on the grade where the force applied to the park pawl  26  at the calculated or determined grade is known or can be determined as a function of the vehicle weight. 
         [0019]    Then, if there is a request  46  to shift the vehicle out of park and to another gear, the motor may be energized to provide the desired counteracting torque to the shaft  16 . The counteracting torque applied can be sensed at  48 , and when the desired counteracting torque, or a greater torque, has been applied to the shaft  16 , the counteracting torque is no longer applied and the vehicle may then be shifted out of park at  50  and into another gear. A suitable controller, which may be a stand alone unit or part of an existing vehicle controller or control system, may be used to determine the offsetting force needed to be applied to the output shaft  16 , and to control and monitor the application of the offsetting force to the output shaft  16 . 
         [0020]    Another way to determine the force on the park pawl  26  is to actually sense the force acting on the park pawl  26 , such as with a strain gauge or other sensor. Such a force sensor  60  is shown diagrammatically in  FIG. 1  on the lock tab and may be engaged by teeth  20  of the park gear  14  in use. After or while the offsetting force is provided, this force sensor  60  could also be used to determine when the force on the park pawl  26  has been reduced sufficiently to permit disengagement of the park pawl  26 . Further, an initial determination of the magnitude of the load on the park pawl  26  is not needed. Instead, the offsetting force could be provided until the net force on the park pawl  26  is below a desired threshold. In other words, the magnitude of the offsetting force can be increased until the force on the park pawl  26  is decreased to a desired magnitude. In this example, an accelerometer or inclinometer could be used to determine the direction of the force to be applied to the output shaft  16  to offset the force on the park pawl  26  (where the direction will be different if the vehicle is parked with its front end facing uphill than if the vehicle is parked with its front end facing downhill). 
         [0021]    Other steps may be included or substituted for steps identified in the representative flow chart and described method. For example, before beginning the method a determination can be made at  52  as to whether the vehicle is stationary and in park. If not, then no counteracting torque should be applied so the method should not be performed. Another step  54  may be to determine if the vehicle is parked on an incline that is greater than a threshold incline. The threshold incline can be chosen so that on inclines less than the threshold, the force on the pawl  26  will be within an acceptable range without any offsetting force applied to the output shaft  16  such that disengagement of the park pawl  26  will not cause unacceptable noise or vehicle movement. In this way, if the vehicle is not parked on a greater than threshold incline, then no offsetting force need be applied to the output shaft  16  and the method can be stopped. 
         [0022]    Also, a system check may be provided at  56  before the counteracting torque is applied. The system check may include one or more steps designed to prevent application of the counteracting torque in certain conditions. For example, the system check may verify one or more of the following: 1) that the park lock is not already disengaged; 2) the vehicle currently is on a grade; 3) there is an operator request to disengage park; 4) the vehicle brake pedal has been/is applied (perhaps by sensing brake pedal switch(es) or receiving an indication from the system or vehicle ECU); 5) a signal indicative of the vehicle grade that was stored from the previous ignition cycle (e.g. when the vehicle engine or motor was shutoff) is within a threshold value of the current signal regarding the grade the vehicle is currently on; 6) the integrity of the indicated/signaled grade is verified (perhaps through a Controller Area network (CAN), Cyclic Redundancy Check (CRC) and/or Rolling Counter (RC) check; 7) a traction motor angular displacement is less than a threshold; and 8) that the counteracting torque applied (achieved torque) is within a threshold of the desired or calculated torque intended to be applied. In at least some implementations, the counteracting torque would not be applied if any of the above conditions were not satisfied. The system check could also verify that sufficient brake system pressure exists to hold the vehicle stationary if the park lock is disengaged, and that a hand brake has been applied before disengaging the park lock, if desired. 
         [0023]    Of course, other method steps may be added or substituted for the steps shown in  FIG. 3 , and not all steps are needed in any desired method. Further, the steps could be done in a different sequence or in a different combination of steps to provide an offsetting torque to the output shaft or park gear. 
         [0024]    Further, while a particular park lock mechanism  12  and system  10  are described, the method of providing an offsetting force to the transmission output shaft  16  could be used with any park lock mechanism  12  and system  10 . That is, the electric motor driven system, including the drive member  34 , pawl  26 , etc, are not needed. The force on any type of vehicle park lock system  10  could be reduced by providing a counteracting or offsetting force or torque to the transmission output shaft  16  or park gear  14 .