Motor vehicles – Having four wheels driven – With differential means for driving two wheel sets at...
Reexamination Certificate
1998-02-02
2002-05-14
Vanaman, Frank (Department: 3611)
Motor vehicles
Having four wheels driven
With differential means for driving two wheel sets at...
C180S197000
Reexamination Certificate
active
06386308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an apparatus for controlling an actual drive force to be applied to drive wheels of a motor vehicle, and more particularly to techniques for applying a braking torque to at least one drive wheel to control the actual drive forces of the drive wheels.
2. Discussion of the Related Art
Generally, such a wheel drive force control apparatus is provided for a motor vehicle which includes (a) a drive power source such as an engine and/or an electric motor, (b) at least one pair of drive wheels driven by the drive power source, (c) a differential gear unit disposed between the drive power source and each pair of drive wheels, for connecting the right and left drive wheels of the pair so as to permit the drive wheels to be rotated at different speeds and to equally distribute the driving torque of the drive power source to the drive wheels, so that a drive force is produced between each drive wheel and a road surface, and (d) a pair of brakes for braking the respective drive wheels, independently of each other.
Since the driving torque values distributed to the two drive wheels of each pair from the drive power source through the differential gear unit are always equal to each other, the sum of the actual drive forces of the two drive wheels is two times the actual drive force of the wheel whose critical drive force with respect to the road surface is smaller than that of the other drive wheel. The critical drive force is the maximum drive force that can be produced between the drive wheel and the road surface without slipping of the drive wheel on the road surface, while the differential action of the drive wheels by the differential gear unit is not limited. The critical drive force of each drive wheel depends upon a friction coefficient &mgr; of the road surface and a vertical load acting on the drive wheel. The critical drive force of one of the two drive wheels of the pair may become smaller than that of the other drive wheel, due to a lower value of the friction coefficient &mgr; of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. In this case, the above-indicated one drive wheel slips on the road surface, and the actual drive force of that drive wheel decreases, so that the driving torque distributed to the other drive wheel by the differential gear unit is reduced. As a result, the actual drive force of the above-indicated other drive wheel decreases, whereby the sum of the actual drive forces of the two drive wheels decreases, making it difficult to achieve smooth starting or acceleration of the vehicle. Namely, in the presence of a difference between the critical drive forces of the two drive wheels, the driving torque to be distributed by the differential gear unit to the drive wheel whose critical drive force is larger cannot be made large enough to assure the actual drive force of that drive wheel that is sufficiently close to its critical drive force, even while the critical drive force of that drive wheel is considerably larger than the actual drive force.
In the light of the above drawback where there exists a difference between the critical drive forces of the right and left drive wheels, the differential gear unit is conventionally provided with an exclusively designed limited slip differential (LSD) mechanism which is adapted to limit the differential action of the differential gear unit, for increasing the actual drive of the drive wheel whose critical drive force is larger, to a value close to the critical value.
However, the provision of such a LSD mechanism in the differential gear unit for each pair of drive wheels inevitably results in a considerable increase in the cost of manufacture and the weight of the differential gear unit.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus for controlling actual drive forces to be applied to drive wheels of a motor vehicle, so as to maximize the actual drive forces, by utilizing a brake provided for each drive wheel and without using an exclusively designed limited slip differential mechanism.
The above object may be achieved according to a first aspect of the present invention, which provides an apparatus for controlling an actual value of a drive force produced between a road surface and each of a pair of drive wheels of a motor vehicle including (a) a drive power source for driving the pair of drive wheels, (b) a differential disposed between the drive power source and the pair of drive wheels and connecting the pair of drive wheels so as to permit a difference between rotating speeds of the drive wheels and to evenly distribute a driving torque of the drive power source to the pair of drive wheels, for thereby producing the drive force between the road surface and each drive wheel, and (c) a pair of brakes for braking the pair of drive wheels, respectively, independently of each other, the apparatus being characterized by comprising: a pair of wheel speed sensors for detecting the rotating speeds of the pair of drive wheels; and a brake control device responsive to output signals of the pair of wheel speed sensors, for activating one of the pair of brakes which corresponds to one of the drive wheels which has a smaller critical value of the drive force with respect to the road surface than the other of the drive wheels, to thereby apply a braking torque to the above-indicated one drive wheel for increasing an apparent value of the drive force of that one drive wheel, in order to increase the actual value of the other drive wheel which has a larger critical value of the drive force.
The critical drive force of one of the two drive wheels may become smaller than that of the other drive wheels, due to a lower value of the friction coefficient &mgr; of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. The vertical load may be reduced upon vertical movement or jumping of the drive wheel away from the road surface. If the brake for the above-indicated one drive wheel having the smaller critical drive force value is activated in the above condition, a braking torque is applied to that one drive wheel in the same direction as the actual drive torque based on an actual friction force between the road surface and the drive wheel in question. Namely, the braking torque and the actual drive torque are simultaneously applied to that drive wheel in the same direction. As a result, the apparent value of the drive force of the drive wheel having the smaller critical drive force, as seen from the other drive wheel having the larger critical drive force, is increased, and the slipping of the drive wheel having the smaller drive force is restricted, whereby the speed difference between the two drive wheels is reduced. Accordingly, the drive torque to be applied to the drive wheel having the larger critical drive force is increased by an amount corresponding to the braking torque applied to the drive wheel having the smaller critical drive force, so that the actual drive force of the drive wheel having the larger critical drive force is increased toward the critical value.
The wheel drive force control apparatus according to the first aspect of this invention was developed based on the above finding. In the present apparatus, the brake control device is adapted to activate the appropriate brake for braking the drive wheel having the smaller critical drive force value, when the vehicle is running with the two drive wheels having the different critical drive force values with respect to the road surface. The application of a braking torque to the drive wheel having the smaller critical drive force results in an increase in the apparent value of the drive force of that drive
Nishikata Hiroto
Takahira Yousuke
Toyota Jidosha & Kabushiki Kaisha
Vanaman Frank
LandOfFree
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