Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Indication or control of braking – acceleration – or deceleration
Reexamination Certificate
2001-04-03
2002-04-23
Camby, Richard M. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Indication or control of braking, acceleration, or deceleration
C701S078000, C303S150000
Reexamination Certificate
active
06377885
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a braking force control device, and in particular, to a braking force control device which controls the braking force applied to a wheel on the basis of the road surface &mgr;slope or a physical value equivalent thereto.
2. Description of the Related Art
Conventionally, antilock brake (ABS) control devices as follows have been proposed. Time series data of wheel speed is detected each time a predetermined sampling cycle occurs. On the basis of the detected time series data of the wheel speed, the history of changes of the wheel speed and the history of changes of the wheel acceleration are computed. On the basis of these computed values, a braking force slope (the slope of the braking force with respect to the slip speed) is estimated. The braking force applied to the wheel is controlled on the basis of the estimated braking force slope.
Braking force of a tire is generated by the slip between the tire and the road surface. In other words, the braking force of the tire is generated by the difference between the speed at which the tire advances (the advancing speed of the vehicle body) and the circumferential speed of the tire. Usually, an ABS control device computes the wheel slip and the wheel deceleration on the basis of a wheel speed signal, controls the increase, maintenance and decrease of the brake fluid pressure in accordance therewith, and prevents locking of the wheel.
As shown in
FIG. 1
, the characteristic of the frictional force between a tire and the road surface (the so-called &mgr;-S characteristic) varies in the direction of arrow X and the direction of arrow Y along the &mgr;-S characteristic at the time of pressure increase of ABS control. At the time of pressure reduction, the &mgr;-S characteristic cycles so as to fall in the &mgr; direction (the direction of arrow Z).
In order to effectively carry out ABS control by utilizing the &mgr;-S characteristic of the tire, at the time of pressure god increase, when there is slippage which moves away from the &mgr; peak, pressure is increased immediately (the direction of arrow X), and in a vicinity of the &mgr; peak, the pressure increase amount is suppressed to a slight amount (the direction or arrow Y) or is maintained, such that the time of staying in the vicinity of the &mgr; peak is as long as possible. On the other hand, at the time of pressure reduction, it is necessary to restore the slip immediately (the direction of arrow Z).
In a conventional ABS control device, the threshold value for carrying out pressure increase or pressure reduction is set so as to match the characteristic of a general tire. Accordingly, a problem arises in that this threshold value is not always the optimal value of a given tire for a given road surface.
In order to overcome this problem, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-165053 discloses a technique in which a friction force characteristic between the tire and the road surface is estimated, such that the ABS control ability is improved. In this conventional art, by utilizing the fact that the wheel acceleration generates a difference between the braking torque and the depressing force reaction force (the braking force applied to the vehicle), a slip ratio is determined such that the difference between the wheel acceleration and the vehicle deceleration becomes a predetermined value, and a target slip ratio is determined in consideration of the offset.
However, a wheel speed signal includes noise, and further, the vehicle acceleration is estimated from the wheel speed which includes the wheel slip. Thus, it is difficult to accurately compute the vehicle acceleration or the wheel acceleration. As a result, a problem arises in that the tire frictional force characteristic with respect to tire road surface cannot be accurately known. Further, in conventional methods, it cannot be judged what type of conditions there are at the &mgr;-S characteristic at the time of ABS control. As a result, it is extremely difficult to judge whether pressure should be increased immediately or whether pressure should be increased slowly.
Further, in Japanese Patent Application Laid-Open (JP-A) No. 2000-118375, the applicant of the present application has proposed an antilock brake control device which realizes a maximization of the braking force by carrying out follow-up control such that an estimated braking force slope becomes a specific target value in a vicinity of zero.
In this conventional art, the braking force can be made to become substantially a maximum by effecting follow-up control such that the estimated braking force slope becomes a target value near zero at the time of braking while the vehicle is advancing god straight forward. However, in a case in which a slip angle arises, i.e., in a case in which lateral slip arises at the wheel due to braking while turning, when follow-up control is effected such that the braking force slope becomes a target value in a vicinity of zero, a problem arises in that there is the possibility that the limit of the tire generation force characteristic will be exceeded.
This problem will be described with reference to
FIGS. 2 and 3
. As shown in
FIG. 2
, for example, when a slip ratio &kgr;
x
in the longitudinal direction (the vehicle longitudinal direction) and a slip ratio &kgr;
y
in the lateral direction are greater than or equal to about 0.15, the tire generation force exceeds 100%. No further tire generation force can be obtained, and a total slip region is entered. Note that point A in
FIGS. 2 and 3
is a point at which the braking &mgr; slope (the braking force slope) &agr;
x
becomes 3 when the lateral direction slip ratio &kgr;
y
is 0.13.
Further,
FIG. 3A
illustrates the relationship between the longitudinal direction slip ratio &kgr;
x
and the braking &mgr; (longitudinal direction braking force) at the time when the lateral direction slip ratio &kgr;
y
is 0.13.
FIG. 3B
shows the relationship between the longitudinal direction slip ratio &kgr;
x
and lateral &mgr; (lateral force).
FIG. 3C
shows the relationship between the longitudinal direction slip ratio &kgr;
x
and composite &mgr; (composite force). Further,
3
A also shows the braking &mgr; slope, and shows point A where the braking &mgr; slope a, &agr;
x
=3.
In accordance with
FIGS. 2 and 3
, as the longitudinal direction slip ratio &kgr;
x
increases, the braking &mgr; increases and the lateral &mgr; decreases. The composite &mgr; which is the synthesis thereof is saturated before point A is reached.
Namely, when the target value of the braking force slope is fixed to a specific value at the time when a lateral slip exists, regardless of the fact that the braking &mgr; slope is in a state before the peak, the composite &mgr; may reach the peak. In such a region, the lateral force may be reduced unnecessarily and the total slip region may be entered, which is not preferable from the standpoint of control stability of the vehicle.
SUMMARY OF THE INVENTION
The present invention is proposed in order to overcome the above-described drawbacks, and an object of the present invention is to provide a braking force control device which correctly grasps the road surface state and can carry out optimal control which is appropriate for the road surface state. Further, an object of the present invention is to provide a braking force control device which can improve control stability at the time of limit braking during turning.
In order to overcome the above-described problems, a first aspect of the present invention is a braking force control device comprising: wheel speed detecting means for detecting a wheel speed of each wheel of a vehicle; road surface &mgr; slope estimating means for, on the basis of the detected wheel speed, estimating, for each wheel a slope of a coefficient of friction &mgr; between the wheel and a road surface as a road surface &mgr; slope; and control means for, on the basis of the road surface &mgr; slope estimated for each wheel by the ro
Asano Kenji
Ishiguro Michio
Ito Shoji
Matsunaga Tsugiharu
Muragishi Yuji
Aisin Seiki Kabushiki Kaisha
Burns Doane , Swecker, Mathis LLP
Camby Richard M.
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