Motor vehicles – Having four wheels driven – With differential means for driving two wheel sets at...
Reexamination Certificate
1998-10-19
2001-02-20
Johnson, Brian L. (Department: 3618)
Motor vehicles
Having four wheels driven
With differential means for driving two wheel sets at...
C180S249000, C701S088000, C701S089000
Reexamination Certificate
active
06189643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a differential limiting control apparatus for a four wheel drive vehicle. More specifically, the invention relates to a technology capable of controlling differential limiting force of a center the differential according to road and vehicle running conditions.
2. Description of Prior Arts
In recent years, many vehicles including four wheel drive vehicles are equipped with an anti-lock brake control system (hereinafter, referred to as ABS) to prevent sustained wheel-locking by controlling braking force on braking.
Generally, the ABS is constituted so as to control braking force by detecting wheel slippage on braking. In applying this system to four wheel drive vehicles, sophisticated control techniques are needed due to the relationship of driving force distributed between front and rear wheels.
For example, Unexamined Japanese Patent Application No. Toku-Kai-Shou 62-43355 discloses a technique in which the differential limiting of a center differential is released when depressing a brake pedal to operate the ABS.
According to the aforesaid technique, however, since the engagement of driving force between front and rear wheels is released even in case of depressing the brake pedal lightly, especially when the brake pedal is depressed repeatedly, frequent engagement and disengagement between front and rear wheels are performed and as a result a driver feels awkwardness due to the frequent changes of driving performance of the vehicle.
That is to say, the braking force applied equally to four wheels is suddenly applied more to front or rear wheels and as a result the under-steer characteristic of the vehicle becomes too strong or inversely it becomes too weak.
To solve such a problem, Examined Japanese Patent Application No. Toku-Kou-Hei 6-88504 proposes a technique wherein the differential limiting is released when a rotational deceleration of wheels exceeds a specified value on braking and after that the ABS is operated if the ABS operational condition is satisfied, so as to start the ABS control only when the wheels come closer to a locking state.
This technique still has a problem that when the rotational deceleration of wheels becomes large, the abrupt release of the differential limiting causes a sudden slip on the front or rear wheels and as a result the driver may feel a large change in the under-steer characteristic of the vehicle for an instant before the ABS control starts.
Further, for example, when an emergency braking is applied to the vehicle on a pavement having a high road friction coefficient, the wheel rotation is largely decelerated without causing slip. In this moment, since the engagement between front and rear wheels is abruptly released while a strong braking force is retained, a shock or an impact noise may be generated from the drive train of the vehicle in the moment the engagement is released.
SUMMARY OF THE INVENTION
In view of the aforesaid disadvantages of the prior arts, it is an object of the present invention to provide a differential limiting control apparatus capable of preventing an excess under-steer or over-steer characteristic due to engine brake exerted only to the front or rear wheels when the vehicle travels on a slippery road surface, like a snowy road, with the accelerator off. It is another object of the present invention to provide a differential limiting control apparatus capable of preventing a shock in the drive train of the vehicle due a sudden release of the engagement between front and rear wheels when an emergency braking is applied on a road surface with a high friction coefficient.
In order to achieve those objects, the differential limiting apparatus according to a first aspect of the present invention comprises: differential limiting force establishing means for establishing a differential limiting force of a center differential to be a specified value when the brake pedal is not depressed with the accelerator pedal released, or when the brake pedal is depressed with the accelerator pedal released, and when the braking force is smaller than a reference braking force; differential limiting force releasing means for releasing the differential limiting force of the center differential when the braking force is larger than the predetermined reference braking force; and differential limiting force correcting means for correcting the differential limiting force established in the differential limiting force establishing means to a first value close to zero when an anti-lock brake control apparatus starts to operate.
The differential limiting control apparatus according to a second aspect of the present invention comprises: differential limiting force calculating means, for calculating such a second value of differential limiting force of said center differential as distributing an engine braking force between front and rear wheels, at a distribution ratio corresponding to a front-to-rear wheel weight distribution ratio, when the brake pedal is not depressed with the accelerator pedal released, or when the brake pedal is depressed with the accelerator pedal released, and when braking force is smaller than a reference braking force; differential limiting force establishing means for establishing the second value as a differential limiting force of the center differential after the second value is calculated, so as to distribute the engine braking force between front and rear wheels at the distribution ratio corresponding to the front-to-rear wheel weight distribution ratio; differential limiting force releasing means for releasing the differential limiting force of the center differential when the braking force is larger than the predetermined reference braking force; and differential limiting force correcting means for correcting the differential limiting force established in the differential limiting force establishing means to a first value close to zero when the anti-lock brake control apparatus starts to operate.
The differential limiting control apparatus according to a third aspect of the present invention comprises: differential limiting force calculating means for calculating such a second value of differential limiting force of the center differential as distributing the engine braking force between front and rear wheels at the distribution ratio corresponding to the front-to-rear wheel weight distribution ratio when the brake pedal is not depressed with the accelerator pedal released, or when the brake pedal is depressed with the accelerator pedal released, and when braking force is smaller than a reference braking force; differential limiting force establishing means for establishing the second value as a differential limiting force of the center differential after the second value is calculated, when wheel rotation deceleration is smaller than a predetermined reference wheel rotation deceleration, so as to distribute the engine braking force between front and rear wheels at the distribution ratio corresponding to the front-to-rear wheel weight distribution ratio, and for correcting the differential limiting force calculated in the differential limiting force calculating means of the center differential to a third value smaller than the second value after the second value is calculated, when wheel rotational deceleration is larger than the predetermined reference wheel rotational deceleration so as to release the differential limiting force of the center differential even when braking force is small; differential limiting force releasing means for releasing the differential limiting force of the center differential when braking force is larger than the predetermined reference braking force; and differential limiting force correcting means for correcting the differential limiting force established in the differential limiting force establishing means to a first value close to zero when the anti-lock brake control apparatus starts to operate.
REFERENCES:
patent: 4770266 (1988-09-01), Yamaguchi et al.
patent: 5172959 (1992-12-01), Ei
Hiwatashi Yutaka
Takahashi Akira
Fuji Jukogyo Kabushiki Kaisha
Johnson Brian L.
Luby Matthew
Smith , Gambrell & Russell, LLP
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