Patent Application
20120191311
The present invention relates to a method and apparatus aiding a driver to park a vehicle having a transmission with a parking pawl.
Vehicles with automatic transmissions are typically equipped with a parking pawl. The parking pawl engages a slot on a toothed wheel on the vehicle's transmission output shaft when the transmission mode selector is put into its parking mode for locking the driven wheels. Depending on the number of teeth on the toothed wheel, the output shaft may make up to one-eighth of a rotation before the parking pawl engages with the toothed wheel. Consequently, a vehicle parked on a slope may roll six inches or more before the parking pawl locks the transmission. The rolling vehicle may stop with a sudden jerk when the parking pawl engages the slot. Not only may this abrupt stop irritate the vehicle's occupants, the significant force exerted on the parking pawl may damage the toothed wheel or the parking pawl itself.
It is the objective of the present invention to reduce the sudden force exerted on the parking pawl when a vehicle is parked on a slope.
It is a further objective of the present invention to diminish the jerk perceived by the vehicle occupants when the parking pawl engages the slot on the toothed wheel when the vehicle is parked on a slope.
These objectives are achieved by holding the vehicle brakes engaged after the driver releases the brake actuator and slowly reducing the brake force until the vehicle starts moving. Such a gradual release of the brakes allows the vehicle to move until the parking pawl engages, but it prevents the vehicle from freely accelerating pursuant to the downgrade force of the slope. Accordingly, the vehicle will not obtain a high speed that would result in a sudden jerk. Due to the lower speed, the force acting on the parking pawl is also reduced compared to a vehicle allowed to roll freely until the parking pawl engages.
According to the invention, the system can estimate a holding brake force sufficient to keep the vehicle immobilized on the slope. When the vehicle has stopped on the slope, the system can reduce the brake force to the holding brake force when the vehicle driver releases the brake pedal. Because the holding brake force will hold the vehicle on the slope, the slow reduction of brake force can start at the level of the holding brake force.
If the vehicle has an active suspension, it can first be allowed to settle, e.g. to lower the chassis or to vent air springs, before the brake force is reduced. That way, there will not be any other interfering operations while the method is performed.
While the brake force may be reduced continuously, using incremental steps may make the control easier to handle: The brake force remains at a certain level to observe whether the vehicle starts to move before the force is further reduced.
Once the system determines that the vehicle has reached its final position, it terminates the method by reducing the brake force to zero.
In the drawings,
While a vehicle is in a driving mode in which the vehicle may move, the parking pawl 7 is held in a position where the tooth 5 is removed from the slot 6 so that the transmission output shaft 2 can move freely.
When the vehicle is parked, the vehicle's gear selector (not shown) is moved to “P,” the parking mode, and the parking pawl 7 is released. The tooth 5 may not immediately match up with a slot and may come to rest on a tooth instead of a slot of the toothed wheel 1.
When the vehicle is parked on a level surface, the tooth 5 will remain on the toothed wheel 1 where it came to rest. But when the vehicle is parked on an uphill or downhill slope, it can accelerate in the slope's downward direction until the tooth 5 engages the next slot 6. Depending on the number of teeth on the toothed wheel 1 and on the initial resting position of the tooth 5, the distance the vehicle rolls can be six inches or more. On a steep slope, the vehicle can reach a speed that results in a noticeable jerk when the tooth 5 latches into the next slot 6 and abruptly stops the movement. This sudden stop can have a startling effect on the vehicle occupants and might over time cause damage to the tooth 5 or the teeth of the toothed wheel 1.
Vehicle 9 rests on a slope 8 with an inclination angle α. Gravity acts vertically downward on the vehicle with the gravitational force FG. The gravitational force can be broken down into two vector components: the normal force FN perpendicular to the surface or the slope and the downward force FD parallel to the surface of the slope. The downward force FD is in a linear correlation with the vehicle mass and with the sine of the inclination angle α—if friction forces are neglected.
In order to keep the vehicle from rolling downhill before the parking pawl engages, a holding brake force FH countering the downward force must be applied. The magnitude of the brake force must be equal to the downward force.
The type of brake system used determines which physical quantity is manipulated to vary the brake force. In a hydraulic brake system, it is the pressure acting on a brake piston. But in an electrical brake, the quantity is the electrical current generating a magnetic field.
The following example of
Assuming this has occurred by time t1, the brake force is lowered in one step to a holding force FH, which is reached at time t2. This holding brake force FH corresponds to the holding brake force FH of
Now, assuming that the holding force FH was properly chosen and the vehicle 9 has not started to move, the brake force FB is lowered in small increments until the vehicle 9 starts rolling and the vehicle speed v has a value other than zero at a time t3. If the vehicle 9, after starting to move, slows down to a stop again, the brake force FB can be incrementally lowered further. This is done to make sure that the vehicle 9 has not just stopped because of an uneven road surface or some other unrelated impediment. The process of lowering the brake force until the parking pawl 7 has engaged the toothed wheel 1 can take several seconds, depending on the distance the vehicle 9 travels until the tooth 5 catches in slot 6.
But if the vehicle 9, despite lowered brake force, does not start moving again, the system assumes that the parking pawl has engaged with the toothed wheel. This determination can be based on several factors, alternatively or concurrently. Such factors include but need not be limited to:
Brake force difference or ratio between the last movement and now;
Lapse of time;
Absolute present brake force FB; and
Distance traveled by vehicle 9 since the gear selector was put in the parking mode.
Once the electronic controller 10 determines that the parking pawl 7 is engaged, it can direct the vehicle's brake system to release the brake force. The vehicle 9 may have an automatic parking brake that can take over before the brake force is released.
In a vehicle 9 with a non-hydraulic brake system, a parking brake function may be performed by the operating brake. In such vehicles, the brake force might not be reduced any further at all after the parking pawl engages the toothed wheel.
The above represents only one example of implementing the method to avoid a sudden stop when parking a vehicle on a slope. The method relies on a gradual brake force reduction, which may occur in steps as shown or on a continuous curve. The slope of the continuous curve, just as the height and duration of each incremental step can be optimized for time (steeper curve, greater increments and shorter time intervals) or for comfort (less steep curve, smaller increments, longer time intervals). The parameters for optimizing the method can be empirically determined.
Also, initially lowering the brake force to an estimated holding force (or holding pressure) will accelerate the completion of the process.
The broad teachings of the disclosure can be implemented in many ways not specifically pointed out. Accordingly, the true scope of the disclosure is not limited to the particular examples discussed in detail. Further modifications become apparent by studying the drawings, the specification, and the following claims.
