This invention relates to a parking lock mechanism which locks an output shaft of an automatic transmission of a vehicle when a parking range is selected by a driver.
In a vehicle provided with an automatic transmission, a so-called shift by wire parking lock mechanism is known which locks the output shaft of the automatic transmission by the action of an actuator such as a motor according to the movement of a shift lever to a parking range. If the driver unexpectedly moves the shift lever to the parking range while the vehicle is running, however, a shock occurs due to sudden locking of the output shaft of the transmission which may give a discomfort to driver or passenger of the vehicle. Further, sudden locking of the output shaft may give damage to the parking lock mechanism.
In this connection, Tokkai Hei 5-280637 published by the Japanese Patent Office in 1993 discloses a parking lock mechanism wherein, when a certain predetermined vehicle speed condition is satisfied while the vehicle is running, operation of the parking lock mechanism is permitted after emitting an alarm sound, and when the vehicle speed condition is not satisfied, the operation of the parking lock mechanism is ineffectual.
In this parking lock mechanism, as the transition to the parking lock state is notified in advance to the driver, the driver is not startled by an unintended shock, but there is no difference from the state where there is a transition to the parking lock state when the vehicle is running.
In other words, all the kinetic energy of the vehicle is stopped by the parking lock mechanism alone, so a large shock nevertheless occurs, and the parking lock mechanism must also be strong enough to stop the kinetic energy of the vehicle.
The output shaft is restricted by the engaging of a locking pawl with a parking gear which rotates together with the output shaft of the automatic transmission. When the vehicle speed exceeds a predetermined value, the locking pawl does not enter the teeth of the locking gear as the teeth of the rotating parking gear push aside the locking pawl. However, the driver or passengers hear an unpleasant noise when the parking gear pushes aside the locking pawl.
Further, when the shift lever is moved to the parking range before the driver stops the vehicle, the parking mechanism does not operate if the vehicle speed condition is not satisfied. If the intended parking lock operation is not performed, it may even occur that the driver mistakenly considers the vehicle has a fault, and hastily operates the shift lever repeatedly.
It is therefore an object of this invention to mitigate the shock due to the operation of a parking lock mechanism on a vehicle when the vehicle is running.
It is another object of this invention to mitigate the load on the parking lock mechanism due to the operation of the parking lock mechanism when the vehicle is running.
It is yet another object of this invention to prevent an unpleasant noise due to the operation of the parking lock mechanism when the vehicle is running.
It is yet another object of this invention to perform a parking lock operation intended by the driver while still achieving the above objects.
In order to achieve the above objects, this invention provides a parking lock device which locks an output shaft of an automatic transmission of a vehicle according to a change-over of a shift lever to a parking range. The parking lock device comprises a parking lock mechanism which locks a rotational movement of the output shaft, a sensor which detects a vehicle speed, a sensor which detects the change-over of the shift lever to the parking range, a braking device which brakes the vehicle, and a microprocessor. The microprocessor is programmed to determine whether or not the vehicle speed is less than a first predetermined vehicle speed, activate the braking device to decelerate the vehicle when the shift lever has changed over to the parking range at a vehicle speed not less than the first predetermined vehicle speed, and prevent the parking lock mechanism from locking the rotation of the output shaft until the vehicle speed becomes less than the first predetermined speed.