Fluid-pressure and analogous brake systems – Speed-controlled – Multiple control signal with multiple threshold
Reexamination Certificate
2000-06-30
2001-09-18
Butler, Douglas C. (Department: 3613)
Fluid-pressure and analogous brake systems
Speed-controlled
Multiple control signal with multiple threshold
C303S177000, C303S121000
Reexamination Certificate
active
06290312
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to algorithms for anti-lock brake systems and in particular to a control algorithm which provides improved control of wheel brake pressure on speed sensitive road surfaces.
Braking a vehicle in a controlled manner under adverse weather conditions, such as rain, snow or ice, generally requires precise application of the vehicle wheel brakes by the vehicle operator. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure which causes the vehicle wheels to lock-up such that excessive slippage between the wheels and the road surface takes place. Wheel lock-up conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout. Accordingly, 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 the vehicle wheel brakes to prevent lock-up of the associated vehicle wheels. One type of ABS controls only the rear vehicle wheel brakes. Such a system is referred to as a RWAL in the following description.
A typical prior art RWAL system
10
is illustrated in FIG.
1
. The RWAL system
10
includes a normally open solenoid valve
22
connected between the vehicle master cylinder
14
and the controlled rear wheel brakes
20
a
and
20
b.
When actuated, the normally open solenoid valve
22
closes to isolate the rear wheel brakes
20
a
and
20
b
from the master cylinder
14
. Accordingly, the normally open solenoid valve
22
is referred to below 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 RWAL system
10
also includes a normally closed solenoid valve
26
, which is referred to below as a dump valve. The dump valve
26
is selectively opened to reduce the pressure at the rear wheel brakes by bleeding brake fluid from the rear wheel brakes
20
a
and
20
b
to an accumulator
28
. The isolation and dump valves
22
and
26
are mounted within a control valve
21
.
The vehicle brake system master cylinder
14
provides a source of pressurized hydraulic brake fluid to the RWAL system
10
. Thus, a separate hydraulic source, such as a motor driven pump, which is usually included in a four wheel ABS, is not needed. This reduces the complexity and cost of manufacturing the RWAL system
10
, which is typically referred to as a passive system. The RWAL system
10
further includes an electronic control module
30
which is electrically connected to a wheel speed sensor
40
and to the isolation and dump valves
22
and
26
. The control module
30
can be mounted directly upon the control valve
21
or located remotely therefrom.
The control module
30
includes a microprocessor (not shown) which is programmed to control the RWAL system in accordance with a control algorithm and parameters permanently stored in a Read Only Memory (ROM). Typically, the control algorithm is trimmed for the particular vehicle in which the ABS is installed. The microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL system
10
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 ABS electronic control module
30
continuously receives speed signals from the wheel speed sensor
40
. During a vehicle braking cycle, the ABS microprocessor monitors the rear wheel speed and deceleration. The microprocessor calculates a theoretical speed ramp, which represents the speed the vehicle would travel if decelerated at a predetermined maximum rate, such as, for example, 1.0 g. The microprocessor compares the actual rear wheel speed to the theoretical ramp. If the rear wheel deceleration reaches a predetermined value, such as, for example, 1.3 g, the microprocessor determines that the rear wheel brakes
20
a
and
20
b
may be approaching a rear wheel lock-up condition. Accordingly, the ABS microprocessor closes the isolation valve
22
to isolate the rear wheel brakes
20
a
and
20
b
from the master cylinder
14
. If the rear wheel speed departs form the theoretical ramp in addition to, or in place of, the deceleration condition, the ABS microprocessor determines that the rear wheel brakes
20
a
and
20
b
are certainly approaching a lock-up condition and the microprocessor maintains the isolation valve
22
in the closed position. The ABS microprocessor then selectively opens the dump valve
26
to reduce the pressure applied to the rear wheel brakes
20
a
and
20
b
to 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 to initiate a second wheel speed departure to adjust the rear wheel brake pressure upward.
The operation of the RWAL system is illustrated by the graphs shown in FIG.
2
. The upper curve shows the rear wheel speed as a function of time while the lower curve shows the rear wheel brake pressure as a function of time. The middle curves illustrate the operation of the isolation and dump valves
22
and
26
as a function of time. The solid curve labeled
60
represents the velocity of the rear wheels while the dashed curve labeled
64
represents the vehicle velocity. 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 a constant level P
e
, as shown in the lower curve.
If the vehicle transitions from a low mu to a high mu road surface, a key feature included in the algorithm utilized by the RWAL system
10
is a corresponding increase in the braking effort exerted by the rear wheel brakes
20
a
and
20
b
to utilize the increased mu. An example of such a transition is shown at t
8
in FIG.
2
. The road surface transition can be detected when the microprocessor detects an increased vehicle deceleration due to the greater braking effect of the uncontrolled front wheel brakes
19
a
and
19
b
upon the higher mu road surface. Typically, if the vehicle deceleration increases by a predetermined amount, such as 0.25 g, from a vehicle deceleration value measured near the beginning of the stop, it is known to reopen the isolation valve
22
to generate an unlimited series of reapply pulses
62
b.
The resulting increased pressure to the rear wheel brakes
20
a
and
20
b
initiates a third wheel speed departure, which is labeled
60
c
in FIG.
2
. At time t
10
, a dump pulse
63
d
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
.
SUMMARY
This invention relates to an improved control algorithm for an anti-lock brake system which provides improved control of wheel brake pressure on speed sensitive road surfaces.
It has been determined that on speed sensitive surfaces such as wet ceramic tile or wet jennite, mu increases gradually as vehicle speed decreases, even though there is no change from one type of surface to another. This condition is often referred to speed sensitivity of the road surface. Eventually, near the end of a stop, the increased mu can cause the change in vehicle deceleration to exceed the threshold value for detecting a low-to-high mu transition and the microprocessor initiates an additional wheel speed departure. However, the associated pressure cycling activity occurring near the end of the stop does not significantly improve the deceleration of the vehicle. Accordingly, it would be desirable to avoid such pressure cycling activity near the end of the anti-lock braking cycle. Additionally, it would be desirable to increase the braking effort of the controlled rear wheel
Butler Douglas C.
Kelsey-Hayes Company
MacMillian Sobanski & Todd, LLC
LandOfFree
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