Fluid-pressure and analogous brake systems – Speed-controlled – Pressure reapply control
Reexamination Certificate
2000-08-14
2002-06-04
Schwartz, Christopher P. (Department: 3613)
Fluid-pressure and analogous brake systems
Speed-controlled
Pressure reapply control
Reexamination Certificate
active
06398321
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to anti-lock brake systems and in particular an algorithm for scheduling extra reapply pulses in a rear wheel antilock brake system.
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. Pat. 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
. Following the above described wheel departure and recovery cycles, it is known to open the isolation valve by supplying a number of very short reapply pulses, which are labeled
62
b
in
FIG. 2
, to the valve solenoid coil. The reapply pluses are intended to compensate for the progressive decrease in the brake lining friction coefficient due to heating and the increase in road friction that often occurs as the vehicle speed decreases. The reapply pulses also compensate for progressive thermal expansion of the rear brake drums which would cause the rear brake pressure to gradually decrease unless more brake fluid is added to the rear wheel brake system.
SUMMARY OF THE INVENTION
This invention relates to a rear wheel anti-lock brake system having an algorithm which schedules extra reapply pulses.
Referring again to the drawings, there is illustrated in
FIG. 3
, a graph of vehicle velocity vs. time which illustrates a known method for the time spacing of the reapply pulses. For simplicity, only the end portion of anti-lock brake cycle curve is shown in FIG.
3
. It is desirable to have the reapply pulses spaced further apart in time on a low mu road surface than on a high mu road surface. Accordingly, the RWAL control algorithm usually includes a binary decision subroutine for determining whether the vehicle is on a low or a high mu road surface. The control algorithm then implements a difference in the time spacing of the reapply pulses that is in a ratio of five to three with the shorter time period being applied for a high mu road surface.
For the examples shown, the horizontal axis is divided into 100 millisecond increments. The upper vehicle velocity curve, which is labeled
70
, is an example of the response of the control algorithm on a low mu road surface. For simplicity, a straight line is used for the upper vehicle velocity curve
70
. A reapply pulse is generated every 500 milliseconds, as indicated by vertical marks on the upper curve
70
.
The lower vehicle velocity curve in
FIG. 3
, which is labeled
75
is an example of the response of the algorithm on a high mu road surface. For simplicity, a straight line is used for the lower vehicle velocity curve
75
. For illustrative purposes, it is assumed that the initial vehicle speed, V
i
, is the same for both examples. A reapply pulse is generated every 300 milliseconds, as indicated by vertical marks on the lower curve
75
.
For the examples shown in
FIG. 3
, the RWAL control algorithm results in seven reapply pulses being generated when the vehicle is on a low mu road surface while only four reapply pluses are generated on a high mu road surface. It would be desirable to increase the number of reapply pulses on the high mu road surface to further enhance the braking of the vehicle.
The present invention contemplates scheduli
Kelsey-Hayes Company
MacMillan Sobanski & Todd LLC
Schwartz Christopher P.
Siconolfi Robert A.
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