Abstract:
In response to driver selection of PARK, a vehicle transmission is commanded to momentarily select and hold NEUTRAL before PARK, while a vehicle wheel brake is applied. Such an arrangement avoids trapping torque in the driveline, and thus can obviate pawl ‘thump’ when PARK is next disengaged.

Description:
This invention relates to vehicle transmissions having a park lock function, and more particularly but not exclusively to automatic epicyclic transmissions. Aspects of the invention relate to a method, to a system, to a transmission and to a vehicle. 
     BACKGROUND 
     A conventional automatic transmission of a vehicle comprises a plurality of clutches and/or brakes, operable in conjunction with one or more epicyclic gear trains to provide a range of speed ratios. Typically a ‘park’ function is provided whereby an output shaft of the transmission is engageable with the gearbox casing to lock the transmission output shaft against relative rotation. The driveline downstream of the transmission is uninterrupted, so that engagement of the park function locks the vehicle wheels against movement. 
     A conventional driver-operated manual selector for such a transmission typically has the positions PRNDL in sequence, so that the park condition is at one end of those positions. The vehicle ignition switch is usually inhibited unless the transmission is in PARK or NEUTRAL, so that there is no danger of being able to start the vehicle engine with the transmission in gear. 
     A common means of providing the park lock function comprises a toothed wheel of the output shaft and a pivotable pawl mounted on the gearbox casing and resiliently urged into engagement between adjacent teeth of the toothed wheel. The pawl is disengaged by movement of the manual selector away from the PARK position. 
     It must be assured that the pawl will engage and disengage from the toothed wheel under all conditions of use. In particular disengagement must be reliable even if the output shaft has torque applied to it from the driven vehicle wheels. 
     Such a circumstance may occur if the vehicle is stopped on an incline, and the park pawl is engaged to hold the vehicle against movement without application of the handbrake. The park pawl stops the vehicle rolling away, and thus reacts to torque in the driveline. 
     In another circumstance PARK may be selected before the vehicle has ceased moving, and the park pawl may engage the toothed wheel as the vehicle handbrake is applied. In this case the pawl may trap torque within the driveline. 
     If torque is trapped or reacted to by the park pawl, it must nevertheless not prevent disengagement of the pawl at a reasonably low disengagement force. A known phenomenon includes rapid ejection of the pawl as a disengagement force is applied, which may be a consequence of the trapped torque. Such ejection is typically accompanied by an audible noise (e.g., “pawl thump”). 
     Internal measures may be applied to the transmission to solve the problem of such pawl thump, but the phenomenon may not be predictable or repeatable. It may for example be affected by a variable quality of transmission lubrication, slight variation in manufacturing tolerances, or wear and tear. 
     It would be desirable to find a solution for eliminating pawl thump, which does not require measures to be applied internal to the transmission casing. 
     SUMMARY 
     An illustrative example method of engaging a park pawl of a vehicle transmission includes commanding engagement of the park pawl,
         braking wheels of the vehicle against movement,   automatically engaging a neutral condition of the transmission, and   engaging the park pawl after a pre-determined delay.       

     Another illustrative example method of engaging a park pawl of a vehicle transmission includes
         commanding engagement of the park pawl,   braking wheels of the vehicle against movement; and   momentarily engaging a neutral condition of the transmission prior to engagement of the park pawl.       

     These methods may comprise successive steps. 
     These methods include a time delay to separate the engagement of the park pawl from commanding such engagement. During the delay the wheels of the vehicle are braked to prevent movement of the vehicle, and neutral is engaged to permit the driveline to settle so that no residual torque is retained. Typically wheels associated with the handbrake are braked, for example using wheel brakes or a transmission brake in the driveline downstream of the transmission. After a sufficient settling period, the park pawl is engaged. The pre-determined delay is typically less than one second, and in one embodiment is around 330 MS. 
     The methods may include the step of braking the vehicle wheels automatically, for example by applying an electric park brake (EPB) of the vehicle by command from a suitable detector of a transmission selector. 
     After engagement of the park pawl, the vehicle wheel brake may be automatically released. While this arrangement prevents the park pawl from trapping torque through early engagement, it does not prevent subsequent torque loading of the pawl due to an incline. Accordingly in one embodiment the wheel brake does not automatically release upon engagement of the park pawl. 
     The methods of the invention may be initiated as the selector leaves the position next to ‘PARK’ (e.g., when the selector moves from the REVERSE position) or may be initiated as the PARK position is reached. It will be understood that a suitable detector can be provided at any location of the selector which is associated with movement to the PARK position. 
     An illustrative example control system is for a vehicle automatic transmission having a park pawl engageable with an output shaft. The control system includes a selector for commanding a condition of the transmission and a transmission actuator responsive to the selector to engage the commanded condition. The control system comprises a selector detector to detect selection of a park condition, a brake controller to command application of a vehicle wheel brake, and an actuator controller to command the transmission actuator to momentarily engage and hold a neutral condition of the transmission prior to engagement of the park pawl. 
     Another illustrative example control system for a vehicle automatic transmission having a park pawl engageable with an output shaft includes a selector for commanding a condition of the transmission, a transmission actuator responsive to the selector to engage the commanded condition, a selector detector to detect selection of a park condition, and a brake controller to command application of a vehicle wheel brake, and an actuator controller to command the transmission actuator. Upon selection of a park condition, the control system is arranged to brake the wheels of the vehicle against movement, engage a NEUTRAL condition of the transmission, and engage the park pawl after a pre-determined delay. 
     Typically engagement of the park pawl occurs based on a single continuous movement of the selector that is too fast to momentarily engage REVERSE when moving from NEUTRAL toward PARK. Such movement is equivalent to a conventional arrangement having direct movement of the transmission selector. 
     A conventional automatic transmission typically has a mechanical condition selector in the form of a selector shaft passing through the transmission casing, for causing the transmission to adopt one of its operating conditions (e.g. PRNDL). The input shaft may be rotary or linear, and is typically coupled directly to a drivers manual selector via a mechanical linkage. A time delay may be provided by a dash pot or a similar device in order to delay engagement of the park pawl according to an embodiment of the invention. 
     In an embodiment of the invention PARK is selected remotely. Thus. for example, a conventional manual selector/linkage may be replaced by an actuator responsive to a command signal. In one embodiment an electronic signal commands an electric actuator, for example a stepper motor, which in turn is coupled to the mechanical selector shaft. 
     Remote selection of park permits control of the transmission from the dashboard or steering wheel by, for example, electric switches. Such an arrangement also obviates the usual mechanical linkage and thus avoids constraints associated with providing a path for a linkage. Greater design freedom is the desirable consequence. 
     A particular advantage of remote operation is that a delay may be provided in the signal transmission path with relative ease, such as by a microprocessor electronically delaying operation of an actuator of the transmission upon receipt of a command to engage PARK. 
     Remote operation is typically by a slave actuator mounted on the transmission casing and operable to directly move the selector input shaft. 
     In one embodiment of the invention the delay may be adjustable or selectable according to vehicle specification or condition of use. Thus, if necessary, the delay may be set according to an equipment specification of a base vehicle, or may be set in use, for example according to vehicle loading. In the latter case a lightly loaded vehicle may be treated differently than a vehicle at maximum load. 
     The driver control for remote operation of the transmission may comprise a rotary selector which retracts to an inoperable condition on switching off of the vehicle engine. According to an aspect of the invention, such a selector retracts upon engagement of a park pawl rather than upon selection of ‘PARK’. Any suitable means of indicating such engagement is possible, for example an electronic detector of the position of the park pawl relative to the transmission casing. 
     Within the scope of this document the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims, or in the following description and drawings may be taken independently or in any combination. For example, features described in connection with one embodiment are applicable to all embodiments except where there is incompatibility of such features with a particular embodiment. 
     One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a vehicle incorporating an embodiment of the present invention. 
         FIG. 2  schematically illustrates a control system designed according to an embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1  a vehicle  11  has an engine  12  and a front wheel drive automatic transmission  13 . The selector input shaft of the transmission  13  is coupled to an electric actuator  14  mounted on the transmission casing, and is controlled via an electric cable  15  extending to a rotary selector  16  mounted on a vehicle console  17  in the vehicle interior. 
     An illustrative example control system  300  is for the vehicle automatic transmission having a park pawl  310  that is engageable with an output shaft  312  of the transmission. The control system  300  includes the selector  16  for commanding a condition of the transmission and the transmission actuator  14  responsive to the selector  16  to engage the commanded condition. The control system comprises a selector detector  318  to detect selection of a park condition, a brake controller  320  to command application of a vehicle wheel brake  322 , and an actuator controller  324  to command the transmission actuator  14  to momentarily engage and hold a neutral condition of the transmission prior to engagement of the park pawl  310 . 
     As illustrated the vehicle  11  is on a slope, and accordingly it will be appreciated that torque is applied to the transmission park pawl  310  if the vehicle wheel brake  322 , which may be a parking brake, is not applied or is ineffective. Release of the pawl  310 , even if the vehicle brake  322  is in the meantime applied, is thus under torque loading, and may be rapid, which results in a typically audible noise as the pawl  310  thumps against an abutment in the released condition. As noted earlier, torque may also be trapped due to early engagement of the park pawl  310  and wind-up of the transmission; this leads to the same pawl thump release phenomenon. 
     In the invention selection of PARK via the rotary selector  16  does not immediately cause the park pawl  310  to engage. The brake controller  320  is commanded to engage an electronic park brake (e.g., the brake  322 ), and the actuator  14  is commanded to engage or hold the transmission  13  in NEUTRAL for a short period, typically less than 500 milliseconds before allowing the park pawl  310  to engage. This delay, while the vehicle park brake is engaged, allows any residual torque in the transmission to dissipate. The vehicle park brake  322  may be associated with a driven or non-driven axle. 
     If the selector  16  is in DRIVE, the actuator  14  is commanded to pause in neutral while the selector is manually moved from DRIVE to PARK. If the selector  16  is in REVERSE, the actuator  14  is commanded to shift into NEUTRAL and pause, while the selector  16  is manually moved from REVERSE to PARK. The final movement of the actuator  14  from NEUTRAL to PARK passes through the REVERSE position, but is sufficiently rapid to avoid engagement of the REVERSE speed ratio—such rapid motion is conventional. 
     It will be appreciated that in selecting PARK from REVERSE, the actuator  14  is commanded to first move away from the PARK condition towards NEUTRAL. This motion is momentary, and should be imperceptible to the vehicle driver. 
     Thus the invention can avoid trapping residual torque in the transmission driveline, and so long as the vehicle park brake  322  is effective and can eliminate park pawl thump as PARK is disengaged. 
     This invention is described with reference to an automatic transmission  13  having a conventional PRNDL selector  16 . However it will be understood that the principle of the invention is applicable to any transmission having a park lock function and a neutral condition. The invention is also applicable to rear wheel drive and all wheel drive vehicles, and to vehicles having an automated manual transmission with a park function. 
     Typically functionality of the invention is provided in an electronic control unit of the vehicle responsive to the selection of the transmission condition, and commanding operation of an appropriate actuator. Such a control unit is conventional, and may include logic elements to ensure appropriate command of the actuator upon selection of the park function.