Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Indication or control of braking – acceleration – or deceleration
Reexamination Certificate
2001-10-12
2004-05-11
Black, Thomas C. (Department: 3663)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Indication or control of braking, acceleration, or deceleration
C701S072000, C701S078000, C303S155000, C303S189000
Reexamination Certificate
active
06735510
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to control systems for automotive vehicles. The present invention specifically relates to a control of a brake system of an automotive vehicle for improving vehicle transient and steady state performance in a combined braking and steering maneuver by the vehicle, and an integrated control of a brake and steering system of an automotive vehicle for improving vehicle transient and steady state performance in a combined braking and steering maneuver by the vehicle.
2. Description of the Related Art
Recently many vehicles have been produced with brake systems which can independently control brake forces (i.e., torques) of individual wheels. Many automakers and automotive suppliers are also developing brake by wire systems (e.g., electric or electro-hydraulic) which will give designers more freedom than ever before in controlling braking forces of individual wheels in response to instantaneous conditions of motion as well as access to additional measured signals. At the same time, some vehicles are offered with active rear wheel steer systems, and an intense development efforts continue in the area of augmented front steer or steer by wire systems.
Most efforts in the area of brake control algorithms are focused on improving brake control in an anti-lock braking system (ABS) mode of operation and a vehicle stability enhancement (VSE) mode of operation. These modes of operations are active only when a vehicle is at or very close to the limit of adhesion. During base braking, the brake force distribution typically used is symmetric left to right. Thus, it is not affected by vehicle cornering and is optimized for straight line braking. Therefore, transient response of many vehicles in combined steering and braking maneuvers is less than ideal with a tendency of the vehicle to oversteer and to prematurely enter into ABS mode of operation due to reduced normal loads on the pair of inside tires.
More specifically,
FIGS. 1A-1C
illustrate the fundamental physical principles of a vehicle
10
in a combined braking and right hand cornering maneuver. As shown in
FIG. 1A
, vehicle
10
is subjected to a longitudinal force F
LO
equaling m*a
x
and a lateral inertial force FLA equaling m*a
y
, where m is a mass of vehicle
10
, a
x
is a longitudinal acceleration of vehicle
10
, and a
y
is a lateral acceleration of vehicle
10
. A pitch and roll moment of vehicle
10
during the maneuver is due to the longitudinal force F
LO
and the lateral inertial force F
LA
in combination with various pitch forces P
LF
, P
RF
, P
LR
, and P
RR
, and various roll forces R
LF
, R
RF
, R
LR
, and R
RR
, applied to a left front tire
11
a
, a right front tire
11
b
, a left rear tire
11
c
, and a right rear tire
11
d,
respectively. The pitch and roll moment of vehicle
10
is balanced by various normal forces N
LF
, N
RF
, N
LR
, and N
RR
being applied to left front tire
11
a
, right front tire
11
b
, left rear tire
11
c
, and right rear tire
11
d
, respectively. As a result, a normal load distribution among tires
11
a
-
11
d
is shifted from rear tires
11
c
and
11
d
to front tires
11
a
and
11
b
due to braking, and from inside tires
11
b
and
11
d
to outside tires
11
a
and
11
c
due to cornering. Consequently, as shown in
FIG. 1B
, left front tire
11
a
carries the largest normal load and right rear tire
11
d
carries the smallest normal load.
The vectors of forces V
LF
, V
RF
, V
LR
, and V
RR
in the yaw (horizontal) plane of vehicle
10
developed by each tire
11
a
-
11
d
, respectively, must remain within a corresponding friction circle
12
a
-
12
d
, respectively, whose radii are equal to products of a surface coefficient of adhesion &mgr; and the corresponding normal force N
LF
-N
RR
. If tires
11
a
-
11
d
are on a relatively uniform surface, the maximum available tire forces in the yaw plane are approximately proportional to normal forces N
LF
-N
RR
. With the brake proportioning techniques known in the art, the brake forces on both sides of vehicle
10
are approximately the same. Thus, during braking, the friction potential of outside tires
11
a
and
11
c
is underutilized while inside tires
11
b
and
11
d
enter ABS too early. In a 3-channel system, an ABS mode is entered on both rear wheels
11
c
and
11
d
simultaneously whereby a further reduction in longitudinal forces is generated.
Another undesirable consequence of traditional brake force distribution during a braking and cornering maneuver is that vehicle
10
exhibits a tendency to oversteer, especially under light to moderate braking.
FIG. 1C
illustrates a simplified bicycle model of vehicle
10
for explaining the aforementioned oversteer condition of vehicle
10
. Prior to braking during a steady state cornering, a lateral force F
yfa
applied to a front axle (not shown) of vehicle
10
and a lateral force F
yra
applied to a rear axle (not shown) of vehicle
10
balance each other whereby a yaw moment M
Z
about a center of mass
13
of vehicle
10
is approximately zero in accordance with the following equation [1]:
M
z
=F
yfa
*a−F
yra
*b=
0 [1]
where a is a longitudinal distance between the front axle and center of mass
13
, and b is a longitudinal distance between the rear axle and center of mass
13
. Lateral force F
yfa
and lateral force F
yra
correspond to side slip angles of front tire
11
a
and rear tire
11
c
, respectively, with the side slip angle of front tire
11
a
being larger than the side slip angle of rear tire
11
c.
When brakes are applied to front tire
11
a
and rear tire
11
c
, normal force N
LF
is increased on front tire
11
a
and normal force N
FR
is reduced on rear tire
11
c
. Thus, if the side slip angles of front tire
11
a
and rear tire
11
c
were to be maintained, an increase in lateral force F
yfa
on the front axle that is nearly proportional to normal force N
LF
would occur while a decrease in lateral force F
yra
on the rear axle that is nearly proportional to normal force N
LR
would occur. This imbalance between lateral force F
yfa
and lateral force F
yra
increases yaw moment M
Z
in accordance with the following equation [2]:
M
z
=F
yfa
*a−F
yra
*b>
0 [2]
Consequently, the yaw rate of vehicle
10
increases until a new steady state is reached. Another effect of braking is a reduction of lateral force F
yfa
and lateral force F
yra
due to development of longitudinal forces (not show). This effect produces an opposite result than illustrated in
FIG. 1C
, but the effect is significantly small for light and moderate braking, and therefore the first effect dominates. In this new steady state, the side slip angle of rear tire
11
c
is larger than prior to braking and the slide slip angle of front tire
11
a
is lower than prior to braking. This is essentially one of the definitions of vehicle oversteer.
There is therefore a need for a brake control method for overcoming the aforementioned shortcomings described herein. The present invention addresses this need.
SUMMARY OF THE INVENTION
The present invention provides a novel and unique method and system for improving vehicle transient and steady state performance in a combined braking and steering maneuver by using side to side proportioning of braking forces during braking in a turn. Accordingly, the present invention applies to any brake system that provides means of controlling brake forces among wheels in various proportions (e.g., a hydraulic brake system, an electric brake by wire system, and a hybrid of a hydraulic brake system and an electric brake by wire system). While the present invention is not limited to any particular implementation scenario, the intended area for implementing the present invention is mainly in the range of a performance envelope below the activation of prior art brake control algorithms related to ABS and VSE.
One form of the present invention is a method of dynamically controlli
Black Thomas C.
Delphi Technologies Inc.
Donnelly Arthur D.
Smith Michael D.
LandOfFree
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