Fluid-pressure and analogous brake systems – Speed-controlled – Split coefficient of friction
Reexamination Certificate
1999-12-23
2002-03-19
Schwartz, Christopher P. (Department: 3613)
Fluid-pressure and analogous brake systems
Speed-controlled
Split coefficient of friction
C303S113100, C303S175000
Reexamination Certificate
active
06357840
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to anti-lock brake systems and in particular an algorithm for testing the road surface with a small pressure release.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. One type of ABS controls only the vehicle rear wheel brakes. Such a system is referred to as a RWAL in the following.
A typical prior art RWAL is illustrated at
10
in FIG.
1
. As shown in
FIG. 1
, the RWAL
10
is installed on a vehicle having a hydraulic braking system consisting of a brake pedal
12
coupled to operate a dual reservoir master cylinder
14
. When the vehicle operator depresses the brake pedal
12
, the master cylinder
14
supplies hydraulic fluid under pressure from a front reservoir
14
a
through a hydraulic line
16
a
and from a rear reservoir
14
b
through a hydraulic line
16
b
to a conventional combination or proportioning valve
18
. The combination valve
18
includes a first output line
18
a
adapted to supply hydraulic fluid at a first predetermined pressure to actuate a pair of vehicle front wheel brakes
19
a
and
19
b.
The combination valve
18
also includes a second output line
18
b
which supplies hydraulic fluid at a second predetermined pressure to actuate a pair of vehicle rear wheel brakes
20
a
and
20
b.
The RWAL
10
shown in
FIG. 1
utilizes a control valve
21
to selectively control the application of pressure to the rear wheel brakes
20
a
and
20
b
when the system is in an anti-lock braking mode. The control valve
21
includes a normally open solenoid valve
22
connected between the line
18
b
and a line
24
which supplies pressurized brake fluid to the controlled rear wheel brakes
20
a
and
20
b.
During an anti-lock braking cycle, the normally open valve
22
isolates the rear wheel brakes
20
a
and
20
b
from the master cylinder
14
and is commonly referred to as an isolation valve. The isolation valve
22
also can be selectively opened to increase the pressure at the rear wheel brakes
20
a
and
20
b.
The control valve
21
also includes a normally closed solenoid valve
26
, which is connected between the line
24
and a fluid accumulator
28
. The normally closed valve
26
is commonly referred to as a dump valve. The dump valve
26
is selectively opened to reduce the pressure at the rear wheel brakes
20
a
and
20
b
by bleeding brake fluid from the rear wheel brakes to the accumulator
28
. In the RWAL
10
, the master cylinder
14
provides a source of pressurized hydraulic brake fluid during an anti-lock braking cycle, thus eliminating the need for a separate source of pressurized hydraulic fluid, such as a motor driven pump, which is usually included in a four wheel ABS.
The RWAL
10
further includes a computer control module
30
which is electrically connected to a wheel speed sensor
40
. The control module
30
can be mounted directly upon the control valve
21
or located remotely therefrom. The control module
30
includes a RWAL microprocessor (not shown) which is programmed to control the RWAL
10
in accordance with a RWAL control algorithm and parameters permanently stored in a Read Only Memory (ROM). The RWAL microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL illustrated in
FIG. 1
is included in U.S. Patent Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the RWAL control module
30
continuously receives speed signals from the wheel speed sensor
40
. The RWAL microprocessor monitors the speed signals for potential rear wheel lock-up conditions. When the vehicle brakes are applied and the RWAL microprocessor senses a first rear wheel speed departure, which is indicative of an impending wheel lock-up condition, the RWAL microprocessor is responsive thereto to close the isolation valve
22
to isolate the rear wheel brakes
20
a
and
20
b
from the master cylinder
14
. The RWAL microprocessor then selectively opens the dump valve
26
to reduce the pressure applied to the rear wheel brakes
20
a
and
20
b
and thereby correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve
22
to initiate a second wheel speed departure.
The operation of the RWAL
10
is illustrated by the graphs shown in FIG.
2
. The upper solid curve labeled
60
represents the velocity of the rear wheels while the dashed curve labeled
61
represents the vehicle velocity. The operation of the isolation valve
22
and the dump valve
26
is illustrated by the curves labeled
62
and
63
, respectively. The lower curve, which is labeled
64
, shows the pressure applied to the controlled rear wheel brakes.
During an anti-lock braking cycle, the first and second wheel speed departures are labeled
60
a
and
60
b,
respectively. Following correction of the second wheel speed departure, which occurs at time t
7
, the rear wheel brake pressure is maintained at a constant level P
e
. If the vehicle transitions from a low mu to a high mu surface, the braking effort exerted by the rear wheels can be increased. Such a transition is detected when the RWAL microprocessor senses an increased deceleration of the vehicle caused by the uncontrolled front wheel brakes
19
a
and
19
b.
Accordingly, it is known to generate a series of reapply pulses
62
b
which reopen the isolation valve
22
. The increased pressure initiates a third wheel speed departure, which is labeled
60
c
in FIG.
2
. At time t
10
, a dump pulse is generated to open the dump valve
26
to reduce the rear wheel brake pressure to a level P
g
to correct the third rear wheel departure. Thereafter, the rear wheel brake pressure is held at the level P
g
, which is greater than the previously held level P
e
. It is further known to generate a fourth wheel speed departure, which is labeled
60
d
in
FIG. 2
, to assure that optimum rear wheel braking is provided. The fourth wheel speed departure results in the rear wheel brake pressure being further increased to P
h
which is greater than P
g
.
SUMMARY
This invention relates to an algorithm for testing the road surface with a small pressure release.
As described above, it is known to cause a pair of rear wheel speed departures in a RWAL, during which the actual rear wheel speed drops below the actual vehicle speed, when a transition of the vehicle from a low to a high road surface mu is detected. A pair of such generated wheel speed departures are shown in FIG.
3
A. During anti-lock braking cycles on very low mu surfaces, such as surfaces having a mu which is less than 0.3, it has been observed that the rear vehicle wheels can experience a gradual wheel speed departure while the rear wheel brake pressure is held constant. These wheel speed departures are commonly referred to as “wheel speed sneakdown” and are well under the 1.3 g deceleration threshold used to trigger an anti-lock braking cycle. A wheel speed sneakdown condition is illustrated at
70
in FIG.
3
B. Because the slowing of the actual wheel speed below the actual vehicle speed under wheel speed sneakdown conditions is similar to the reaction of the rear wheels when the vehicle transitions from a low to high mu road surface, the wheel speed sneak down can be misinterpreted by the RWAL microprocessor as a transition from a low mu road surface to a high mu road surface. Accordingly, the RWAL microprocessor may react to the wheel speed sneak down condition by initiating forced wheel speed departures, as also illustrated in FIG.
3
B. However, since the vehicle has not transitioned to a higher mu surface, the forced wheel speed departures would deplete the limited amount of pressurized brake fluid available from the master cylinder without a corresponding increase in braking effort. Accordingly, it would be desi
Kelsey-Hayes Company
MacMillan Sobanski & Todd LLC
Schwartz Christopher P.
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