Abstract:
A vehicle includes both a transmission park mechanism and an electronic parking brake. As a failure management strategy, a controller monitors vehicle movement to verify that the park mechanism is successfully restraining the vehicle against movement. Specifically, when the driver releases the brake pedal with the transmission park mechanism commanded to restrain the vehicle, the controller waits for a predetermined amount of time. If the vehicle moves during this time, the controller commands application of the electronic parking brake. The controller may also command application of the electronic parking brake if the vehicle does not come to a stop after the transmission park mechanism is commanded to restrain the vehicle.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of controlling vehicles. More particularly, the disclosure pertains to an apparatus and method to ensure that a vehicle is secured against movement after driver engagement of Park. 
       BACKGROUND 
       [0002]    Drivers of passenger cars equipped with an automatic transmission typically control the operation of transmission via a control device. The transmission control device is used to select several ranges, such as Park, wherein the transmission is locked to prevent the vehicle from moving, Neutral, wherein the transmission allows the vehicle to be moved freely, such as when being towed, Reverse, wherein the transmission allows the vehicle to move backwards, and one or more Drive ranges that enable forward motion of the vehicle. Usually, the transmission control device is in the form of a lever connected mechanically, such as via a cable, to the transmission. Typically, the lever is also mechanically connected to an indicator. The mechanical connection between the control device and the transmission is not impacted by whether or not the engine is running or whether electronic components in the vehicle are powered. 
         [0003]    In a shift-by-wire transmission arrangement, the mechanical connection between the transmission control device and the transmission is eliminated. Instead, the transmission control device is configured to transmit an electrical signal to an electronic controller. The controller electronically directs actuators to provide the transmission behavior associated with the selected range. The actuators may be motors that rotate when provided with electrical current, solenoids that regulate a hydraulic pressure in response to changes in an electrical current, or other devices that respond to electrical current or voltage. The gear shift module is not necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device may be an electro-mechanical interface (e.g., a series of buttons, lever or knob) that is used to instruct the transmission to switch between transmission ranges. 
         [0004]    When the Park range is selected, the transmission output shaft is held against rotation to prevent vehicle movement. The Park range may remain selected for long periods of time during which the operator typically leaves the vehicle unattended. During these periods, the engine and most electrical components are off. Therefore, the Park function of preventing vehicle movement must be maintained independent of the state of the engine or the vehicle electrical system. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    A method of operating a vehicle includes electronically commanding engagement of a transmission park mechanism in response to a park triggering condition and then, while the park triggering condition continues, responding to vehicle movement by automatically engaging an electronic park brake to restrain the vehicle. The park triggering mechanism may be, for example, a driver moving a shift lever to a park position to select a park mode. In some cases, the park triggering mechanism may occur while the vehicle is moving. In such cases, the method may include waiting until the vehicle speed decreases below a park mechanism ratcheting threshold speed before commanding engagement of the electronic park brake. After the vehicle comes to a stop and the driver releases a brake pedal, the method may include monitoring vehicle position for a predetermined time. If the driver turns the ignition key off during the predetermined time, the method may include postponing a shutdown process for the controller until the end of the predetermined period. 
         [0006]    A vehicle includes a transmission park mechanism, an electronic parking brake acting independently of the park mechanism, and a controller. The controller is programmed to respond to vehicle movement after release of a brake pedal while the park mechanism is commanded to provide park by applying the parking brake. The transmission park mechanism may be commanded to provide park by a driver moving a shift lever to a park position. The controller may monitor vehicle speed for a predetermined amount of time following release of a brake pedal with the vehicle stopped. Even if the driver turns an ignition key to an off position during the predetermined amount of time, the controller may delay a shutdown process until after the predetermined amount of time. 
         [0007]    A controller includes communications channels and control logic. The communications channels are configured to receive a signal indicating vehicle movement, such as a signal from a speed sensor. The communications channels is also configured to send a signal commanding engagement of an electronic parking brake. The communications channels may also be configured to send a signal controlling a parking pawl position and to receive a signal indicating the parking pawl position. The communications channels may also be configured to receive signals from a brake pedal and an ignition switch. The control logic is configured to respond to movement of the vehicle while a parking pawl is commanded to be in a park position by commanding engagement of the electronic parking brake. The control logic may be further configured to monitor vehicle speed for a predetermined amount of time following release of a brake pedal. Even if the driver turns the ignition switch to an off position during the predetermined amount of time, the control logic may delay a controller shutdown until the end of the predetermined amount of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is schematic diagram of a shift-by-wire vehicle powertrain and a braking system. 
           [0009]      FIG. 2  is a flow chart for a method of engaging a shift-by-wire park mechanism while a vehicle is in motion. 
           [0010]      FIG. 3  is a flow chart for a method of verifying that the park mechanism is engaged before shutting down a controller including countermeasures for park mechanism failure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    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. 
         [0012]      FIG. 1  schematically illustrates a vehicle powertrain. Mechanical connections are illustrated by single solid lines. Hydraulic connections are illustrated by double solid lines. Dashed lines represent the control signals. Engine  10  converts chemical energy in fuel to mechanical power at crankshaft  12 . Transmission  14  transmits the power from crankshaft  12  to transmission output shaft  16 . Transmission output shaft  16  rotates at a speed proportional to vehicle speed which may differ significantly from the speed at which the engine can efficiently produce the power. Transmission  14  accommodates these requirements. At low vehicle speed, transmission  14  reduces the speed and multiplies the torque for improved performance. At high vehicle speed, transmission  14  permits the engine to operate at a lower speed for quite, fuel efficient cruising. Transmission  14  includes a gearbox  18  which establishes a variety of forward speed ratios and at least one reverse speed ratio. Transmission  14  also includes a launch device such as torque converter  20  capable of transmitting torque when the driveshaft is stationary in order to start the vehicle moving. Differential  22  divides the power from transmission output shaft  16  between a left axle shaft  24  driving a left wheel  26  and a right axle shaft  28  driving a right wheel. In a rear wheel drive configuration, the differential is typically connected to transmission output shaft by a driveshaft. In a front wheel drive vehicle, the differential is typically integrated with the transmission. 
         [0013]    The driver controls transmission  14  by manipulating a transmission control device such as shift lever  32  to select the transmission range. When the driver selects either the Drive or Reverse range, controller  34  commands engagement of clutches in gearbox  18  to establish a power flow path with an appropriate sped ratio. When the driver commands either the Neutral or Park ranges, controller  34  either commands no clutches to engage or fewer clutches than required to establish a power flow path. Clutches that are not commanded to engage passively release such that no power flow path is established in Neutral and Park. 
         [0014]    Park is implemented by a park gear  36  fixedly coupled to the transmission output shaft  16  and a park pawl  38 . Controller  34  commands park pawl to move into one of two positions. When the shift level is in any position other than Park, controller  34  commands park pawl  38  to a non-Park position in which it does not engage with park gear  36  and output shaft  16  is free to rotate. In a Park position, park pawl  38  engages park gear  36  holding output shaft  16  against rotation. As long as the downstream components of the powertrain are intact and the wheels have traction, holding the transmission output shaft stationary holds the vehicle stationary. If output shaft  16  is rotating above a ratchet speed when park pawl  38  is moved to the Park position, park pawl  38  does not immediately engage park gear  36  but instead bounces off the park gear, called ratcheting, until the vehicle slows enough that the park pawl drops into engagement. This feature prevents sudden stopping of the output shaft at high vehicle speed if a driver accidentally moves the shift lever to Park. Position sensor  40  indicates to the controller which of the two positions park pawl  38  is presently in, permitting controller  34  to verify that the pawl has moved as commanded. Park pawl  38  is designed to remain in its present position in the absence of commands from controller  34 . 
         [0015]    The vehicle also includes brakes  42  and  44  to slow left axle  24  and right axle  28  respectively. These are hydraulically actuated by depressing brake pedal  46 . Finally, the vehicle includes an electronic park brake (EPB)  48 . The EPB is mechanically connected to both brake  42  and  44  and engages both wheel brakes in response to a signal from controller  34 , independently of the position of brake pedal  46 . Although electrical power is required to apply the EPB, the EPB is designed to remain in the applied position restraining the rear wheels from rotation without electrical power. The driver can command either application or release of EPB  48 . However, controller  34  is not configured to command release of the EPB. 
         [0016]    Normally, the controller is powered on when ignition key  50  is turned to an ON position and powered off when the ignition key is turned to an OFF position. However, the controller can delay the shutdown process is some circumstances. 
         [0017]      FIG. 2  is a flow chart for the process used by controller  34  to respond to movement of shift lever  32  into Park while the vehicle is moving. In initial state  60 , the vehicle is moving with the Shifter in Drive or Reverse. Parking pawl  38  is in the non-Park position. Control moves to  62  in response to a driver moving shifter  32  into the Park position. At  62 , the controller commands the park pawl  38  into the Park position. Then, an engagement timer is started at  64  and the controller waits at  66  until either the timer expires or position sensor  40  indicates that pawl  38  has moved into the Park position. If the timer expired, the controller commands application of EPB  48  as a countermeasure to potential failure modes that may prevent pawl  38  from responding to commands in a timely manner. In the usual case of confirmation that pawl  38  has moved into the Park position before expiration of the timer, control moves to  70 . If the vehicle is above the speed at which the parking mechanism is designed to ratchet, the controller waits at  70  until the speed decreases below the ratcheting speed. The controller may determine vehicle speed in any of a variety of ways, including a signal from an output shaft speed sensor or a signal from an anti-lock braking system. Once the vehicle speed is confirmed to be less than the ratcheting speed at  70 , the controller checks at  72  to determine if the transmission output shaft stopped. If the output shaft has not stopped, then the controller commands application of EPB  48  as a countermeasure to potential failure modes that may prevent pawl  38  from engaging with parking gear  36 . In the usual case, final state  74  is achieved in which the vehicle is stopped with pawl  38  engaged with park gear  36  to hold the vehicle stationary. 
         [0018]      FIG. 3  is a flow chart for the process used by controller  34  after the vehicle is stopped to ensure that the park mechanism is holding the vehicle stationary as opposed to only the braking system holding the vehicle stationary. The initial state  74  of this process is the final state of the process illustrated in  FIG. 2 . The vehicle is stopped with the Shifter in Park. Parking pawl  38  is in the Park position but controller  34  has not yet confirmed that the park pawl dropped in from the ratcheting mode to engage park gear  36 . Controller  34  is still active at  74 . Control moves to  76  in response to a driver releasing brake pedal  46 . Then, park verification timer is started at  76  and the controller waits at  78  until either the timer expires or vehicle motion is detected. If vehicle motion is detected, the controller commands application of EPB  48  at  80  as a countermeasure to potential failure modes that may prevent pawl  38  from engaging with and remaining engaged with park gear  36 . In the usual case, the timer expires and control moves to  78 . If ignition key  50  is still on, the controller waits at  82  until the ignition key is turned off. Once the ignition key is off, the controller commands a shutdown at  84  and final state  86  is achieved. 
         [0019]    Although Park is ordinarily engaged in response to driver movement of a shift lever as depicted in  FIGS. 2 and 3 , other events may trigger engagement of Park while the shift lever is not in the Park position. For example, the vehicle may include a sensor to detect the presence of a person in the driver&#39;s seat and automatically command Park if the driver&#39;s seat is unoccupied. As another example, the vehicle may automatically command Park if the driver&#39;s door is opened with the driver&#39;s seat belt not fastened. 
         [0020]    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. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. 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.