Shift-by-wire vehicle and method to verify securement

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.

TECHNICAL FIELD

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

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.

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.

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

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.

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.

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.

DETAILED DESCRIPTION

FIG. 1schematically 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. Engine10converts chemical energy in fuel to mechanical power at crankshaft12. Transmission14transmits the power from crankshaft12to transmission output shaft16. Transmission output shaft16rotates at a speed proportional to vehicle speed which may differ significantly from the speed at which the engine can efficiently produce the power. Transmission14accommodates these requirements. At low vehicle speed, transmission14reduces the speed and multiplies the torque for improved performance. At high vehicle speed, transmission14permits the engine to operate at a lower speed for quite, fuel efficient cruising. Transmission14includes a gearbox18which establishes a variety of forward speed ratios and at least one reverse speed ratio. Transmission14also includes a launch device such as torque converter20capable of transmitting torque when the driveshaft is stationary in order to start the vehicle moving. Differential22divides the power from transmission output shaft16between a left axle shaft24driving a left wheel26and a right axle shaft28driving 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.

The driver controls transmission14by manipulating a transmission control device such as shift lever32to select the transmission range. When the driver selects either the Drive or Reverse range, controller34commands engagement of clutches in gearbox18to establish a power flow path with an appropriate sped ratio. When the driver commands either the Neutral or Park ranges, controller34either 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.

Park is implemented by a park gear36fixedly coupled to the transmission output shaft16and a park pawl38. Controller34commands park pawl to move into one of two positions. When the shift level is in any position other than Park, controller34commands park pawl38to a non-Park position in which it does not engage with park gear36and output shaft16is free to rotate. In a Park position, park pawl38engages park gear36holding output shaft16against 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 shaft16is rotating above a ratchet speed when park pawl38is moved to the Park position, park pawl38does not immediately engage park gear36but 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 sensor40indicates to the controller which of the two positions park pawl38is presently in, permitting controller34to verify that the pawl has moved as commanded. Park pawl38is designed to remain in its present position in the absence of commands from controller34.

The vehicle also includes brakes42and44to slow left axle24and right axle28respectively. These are hydraulically actuated by depressing brake pedal46. Finally, the vehicle includes an electronic park brake (EPB)48. The EPB is mechanically connected to both brake42and44and engages both wheel brakes in response to a signal from controller34, independently of the position of brake pedal46. 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 EPB48. However, controller34is not configured to command release of the EPB.

Normally, the controller is powered on when ignition key50is 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.

FIG. 2is a flow chart for the process used by controller34to respond to movement of shift lever32into Park while the vehicle is moving. In initial state60, the vehicle is moving with the Shifter in Drive or Reverse. Parking pawl38is in the non-Park position. Control moves to62in response to a driver moving shifter32into the Park position. At62, the controller commands the park pawl38into the Park position. Then, an engagement timer is started at64and the controller waits at66until either the timer expires or position sensor40indicates that pawl38has moved into the Park position. If the timer expired, the controller commands application of EPB48as a countermeasure to potential failure modes that may prevent pawl38from responding to commands in a timely manner. In the usual case of confirmation that pawl38has moved into the Park position before expiration of the timer, control moves to70. If the vehicle is above the speed at which the parking mechanism is designed to ratchet, the controller waits at70until 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 at70, the controller checks at72to determine if the transmission output shaft stopped. If the output shaft has not stopped, then the controller commands application of EPB48as a countermeasure to potential failure modes that may prevent pawl38from engaging with parking gear36. In the usual case, final state74is achieved in which the vehicle is stopped with pawl38engaged with park gear36to hold the vehicle stationary.

FIG. 3is a flow chart for the process used by controller34after 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 state74of this process is the final state of the process illustrated inFIG. 2. The vehicle is stopped with the Shifter in Park. Parking pawl38is in the Park position but controller34has not yet confirmed that the park pawl dropped in from the ratcheting mode to engage park gear36. Controller34is still active at74. Control moves to76in response to a driver releasing brake pedal46. Then, park verification timer is started at76and the controller waits at78until either the timer expires or vehicle motion is detected. If vehicle motion is detected, the controller commands application of EPB48at80as a countermeasure to potential failure modes that may prevent pawl38from engaging with and remaining engaged with park gear36. In the usual case, the timer expires and control moves to78. If ignition key50is still on, the controller waits at82until the ignition key is turned off. Once the ignition key is off, the controller commands a shutdown at84and final state86is achieved.

Although Park is ordinarily engaged in response to driver movement of a shift lever as depicted inFIGS. 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's seat and automatically command Park if the driver's seat is unoccupied. As another example, the vehicle may automatically command Park if the driver's door is opened with the driver's seat belt not fastened.