Control system for improving the steering stability of motor vehicles equipped with antiskid brake systems

An antiskid control system for the individual regulation of fluid brake pressure at the respective wheels of a vehicle. The control system is modified so that a steering wheel experiencing high wheel/road adhesion is co-controlled during a cycle of wheel skid control at the steering wheel experiencing a low wheel/road adhesion value, such co-control being delayed at the onset of brake release at the low adhesion wheel, while the high adhesion wheel brake pressure is maintained constant to improve stopping distance.

BACKGROUND OF THE INVENTION 
The present invention concerns an antiskid control circuit for individually 
regulating the braking pressure at the steering wheels of a motor vehicle. 
On roadways with different coefficients of friction (.mu.-values) for the 
left and right wheels of an axle (split coefficients of friction), 
different coefficient of friction/slip curves (.mu./.lambda.-curves) are 
obtained for the two sides of the vehicle. Under these conditions 
differential brake pressure arises during braking, which results in high 
yawing moments and steering forces, and this reduces the steering 
stability. 
So-called "select low" control is well known. In "select-low" control, the 
wheel running at the higher .mu.-value is co-controlled in accordance with 
the regulated braking by the control circuit of the wheel running at the 
lower .mu.-valve. This has the advantage of reducing the variable steering 
forces that arise with split coefficients of friction, but the braking 
distance is unacceptably long. 
In regard to antiskid brake systems in which the control circuits of the 
wheels of one axle are connected with each other in accordance with a 
"select-low" control system and in which provision is made for co-control 
of the still unregulated wheel brake pressure of an uncontrolled wheel by 
the control circuit of a controlled wheel at a given moment of time, it is 
already known to delay for a predetermined time the brake pressure 
regulation of the uncontrolled wheel. However, the brake pressure for the 
wheel running at the higher .mu.-value (high adhesion wheel) is reduced 
within the period that a deceleration control signal (-b signal) in the 
control circuit of the wheel running at the lower .mu.-value (low adhesion 
wheel) (FIG. 3c) is still present. In this way the braking force and thus 
the braking moment are still reduced too soon, i.e., while the high 
adhesion wheel is in the stable range. At the low adhesion wheel, however, 
the braking moment is maintained longer, despite the brake pressure 
reduction, and is first removed when the low wheel has returned from the 
unstable range to the stable range through the critical .mu.-value of the 
associated .mu./.lambda. curve (FIG. 3d). In this way, despite the 
advantages that the known system has over comparable systems, there is the 
danger that yawing moments and appreciable steering forces will arise 
because the left and right wheels on the same axle can have different 
brake forces at the same time. 
SUMMARY OF THE INVENTION 
Therefore, the purpose of the present invention is to develop a circuit, in 
which an uncontrolled wheel brake pressure is held constant at least 
during the pressure release period of the controlled wheel to maximize 
vehicle stopping distance without developing critical steering moments. 
The solution of the problem thus involves a modified individual control of 
the wheels associated with a common axle. If a -b deceleration signal 
appears in the control circuit of the low adhesion wheel, e.g., after 
introduction of braking, and if the high adhesion wheel is not controlled, 
the brake pressure for the high adhesion wheel is maintained constant for 
a certain length of time, namely, until the low wheel, which has run into 
the unstable slip range, has started again and the maximum .mu.-value 
(also called the cricitical .mu.-value) on the associated .mu./.lambda. 
curve has been reached again. This maximum .mu.-value can be determined by 
well-known measuring techniques and can be realized by various well-known 
automatic control techniques (see FIG. 3a-c). From this time on, the brake 
pressure at the co-controlled wheel is reduced. 
By means of these measures the vehicle stopping distance is improved and 
the differential brake force between the two wheels is reduced to a 
minimum (see FIG. 3d-e). Due to the small differential force, the yawing 
moment and steering forces that still arise are negligibly small and do 
not affect the driving stability, especially since the changes in brake 
force on the vehicle wheels have the same sign.

DESCRIPTION AND OPERATION 
In the circuit shown in FIG. 1, it is assumed that supply valves and 
exhaust valves 2 and 4 are associated with a controlled wheel of an axle 
and that supply and exhaust valves 6 and 8 are associated with a still 
uncontrolled wheel. This means that the wheel associated with valves 2 and 
4 runs at the lower coefficient of friction (hereafter referred to as low 
wheel) and that the wheel associated with valves 6 and 8 runs at the 
higher coefficient of friction (hereafter referred to as high wheel). A 
deceleration control signal -b, an acceleration control signal +b, and a 
slip control signal .lambda. are generated in a well known manner when the 
wheel deceleration and acceleration rate threshold are exceeded, and when 
the wheel velocity falls below a predetermined percentage of the vehicle 
velocity or a vehicle reference velocity. 
The circuit has a timing circuit 10 that comprises a conventional forward 
and backward operating counter. The timing circuit 10 has an input 12 for 
counting up, two inputs 14 and 16 for counting back, and an output 18. 
Alternatively, input 16 may be a reset input for setting the counter to 
zero immediately. The exahust valve control signal of the low wheel is 
placed at the forward-counting input 12. The output of an AND gate 20 is 
placed at the backward-counting input 14. The AND gate connects the +b 
control signal and the .lambda. signal of the low wheel with each other, 
whereby the input for the .lambda. signal is negated. The output signal of 
AND gate 20 is also connected with the output signal of the forward and 
backward counter 10 in an AND gate 22. The output signal of AND gate 22 is 
combined with the exhaust valve control signal of the high wheel in an OR 
gate 24, by which the exhaust valve of the high wheel is controllable. The 
exhaust valve control signal of the high wheel is also placed at the 
second backward-counting input 16 of the timing circuit 10. The intake 
valve signals of the low and high wheel are combined in an OR gate 26. The 
output of OR gate 26 is connected with the intake valve of the high wheel. 
Another OR gate 28 combines the output signal of counter 10 and the intake 
valve control signal of the low wheel. The output of OR gate 28 is 
connected to the intake valve of the high wheel via OR gate 26, so that 
the intake valve of the high wheel is additionally controllable by the 
output signal of counter 10. 
The mode of operation of the circuit in FIG. 1 is as follows, whereby 
reference will be made to FIGS. 2a-f and 3a-e for purposes of explanation. 
In FIG. 3a-e the results achievable with a well-known select-low control 
system are also shown for purposes of comparison. 
After introduction of braking, the brake pressure on the wheels increases 
(see FIGS. 2b and 3c), and the velocity of the sensed low and high wheels 
(v.sub.low wheel and v.sub.high wheel) and the velocity of the vehicle 
(v.sub.vehicle) decreases (see FIGS. 2a and 3a). Upon appearance of the -b 
signal for the low wheel (see FIGS. 2c, 3b) valves 2 and 4 of the low 
wheel are controlled (see FIG. 2c, lower diagram), and the brake pressure 
is reduced (see FIGS. 2b, 3c), whereby the nature and manner of the 
pressure reduction as a function of the -b signal and the .lambda. signal 
will not be discussed here because this is not essential for the 
explanation of the invention. At the same time the intake valve 6 of the 
high wheel is controlled by the intake valve signal of the low wheel by 
line 30, so that the brake pressure that has been reached on the high 
wheel is maintained constant (see FIGS. 2d, 2b, 3c). 
At the same time the input 12 of counter 10 is controlled by the exhaust 
valve control signal of the low wheel and begins to count forward at a 
predetermined counting rate (see FIG. 2e) over the entire brake pressure 
reduction phase t.sub.A. One or more short pressure holding times can also 
occur in this control phase pressure reduction (see FIG. 2b, 3c). During 
these pressure holding times occurring in the pressure reduction phase, 
the counter stops counting and does not start to count again until an 
exhaust valve control signal is supplied again by the control circuit of 
the low wheel (see FIGS. 2e, c, 2b). 
If a +b signal is present at the low wheel without a .lambda. signal being 
simultaneously present, or if the .lambda. signal has already disappeared, 
the +b signal controls (through AND gate 20) one of the backward counting 
inputs 14 of the counter 10, which then starts to count backward at a 
predetermined rate. 
During the entire forward and backward counting phase of the counter 10, 
i.e., when the counter is set at a value&gt;0, an output signal is present at 
its output 18 (see FIG. 2f, e). 
As soon as the backward counting process begins, i.e., in the circuit shown 
in FIG. 1, when a +b signal is present at the low wheel, in the absence of 
a .lambda. signal, the AND gate 22 connects through and controls the 
exhaust valve 8 of the high wheel via OR gate 24, and pressure reduction 
is initiated on the high wheel (see FIGS. 2d, e, f, b, c, and 3b, c). 
The pressure reduction occurs over a time interval t.sub.2, which can 
depend, in a predeterminable way, on the time interval t.sub.A of the low 
wheel corresponding to the brake pressure reduction control phase, for 
example, t.sub.2 =t.sub.A /a or t.sub.2 =t.sub.A -b or t.sub.2 =(t.sub.A 
.+-.c)/d etc. The quantities a, b, c and d can in turn depend, for 
example, on the length of the time interval t.sub.A, the velocity 
corresponding to a vehicle velocity (reference velocity), the number of 
control cycles that have occurred, the magnitude of the control pressure 
etc. After termination of time interval t.sub.2, i.e., after termination 
of the pressure reduction phase at the high wheel, the high wheel is 
co-controlled by the control circuit of the low wheel (see FIGS. 2b, c, d, 
and 3c). 
The co-control phase can begin with a pressure holding phase or with a 
pressure control phase, depending on whether the time interval t.sub.2 
ends before or after the beginning of the pressure buildup phase on the 
low wheel. 
The control signal of the exhaust valve 8 of the high wheel also 
disappears, according to the circuit in FIG. 1, when the counter 10 has 
counted back to zero, or when the +b signal of the low wheel has 
disappeared because the AND gate 22 is then switched to the blocking 
state. 
The control action described above repeats itself for each control cycle as 
long as no skid control signal of the co-controlled high wheel itself 
appears to control the exhaust valve 8. For example, as soon as a -b 
signal appears in the control circuit of the co-controlled high wheel, the 
second backward counting input 16 of counter 10 is controlled. Depending 
on the nature of input 16, the counter is either reset or the counter 
begins to count backward, preferably at a considerably higher rate than is 
the case with the "normal" backward counting process described above. This 
has the effect of interrupting brake pressure regulation of the high wheel 
by the low wheel, in the case of a roadway having different coefficient of 
friction values, to prevent a mutual influence on the control circuits of 
the two wheels. 
It is also possible to set the counter back either immediately or after a 
predeterminable time delay, when the brake pressure control of the 
previously co-controlled high wheel's own control circuit is initiated. 
By controlling the intake valve 6 of the high wheel with the output signal 
of counter 10 over the additional OR gate 28, the control of the intake 
valve of the high wheel can still be maintained under influence of the low 
wheel, and thus the high wheel brake pressure can be maintained, even 
when, the .lambda. signal at the low wheel disappears. 
The circuit in FIG. 1 can also be realized alternatively by replacing the 
forward and backward counter 10 with a chargeable and dischargeable 
capacitor, as, for example, in an R-C coupling network. 
Let us now consider FIGS. 3d and 3e, which show a graphic comparison of the 
achievable braking forces and the differential braking forces that develop 
for the "modified individual control system" described here and for the 
"select-low" control system mentioned in the introduction. 
It is clearly seen that the braking force F for the high wheel in the 
"select-low" control system still differs so much from the braking force 
on the low wheel (see FIG. 3d), that considerable differential forces 
.DELTA.F are still found between the two wheels (see lower curve in FIG. 
3e). The differences between the braking forces are significantly smaller 
with the modified individual control system of the invention, so that 
significantly smaller differential forces .DELTA.F occur (see upper curve 
in FIG. 3e). As a result, the steering stability of vehicles on roadways 
with different coefficients of friction for the left and right side of the 
vehicle is considerably improved. 
Counter 10 in FIG. 1 is designed in such a way that the backward counting 
process has precedence over the forward counting process, which means that 
even when an exhaust valve control signal of the low wheel is present at 
counter input 12, the counter counts backwards as soon as, for example, a 
signal is sent to input 16. This is especially important during a 
transition to homogeneous roadway conditions. It is important to prevent 
the development of too high a differential pressure between the wheels of 
a given axle, and in such a case it is important to ensure that the high 
wheel's own control takes over as quickly as possible. 
Let us now consider FIGS. 4, 5 and 6, which show the relationships for 
different exhaust criteria on the high wheel, oriented essentially on the 
control signals present at the maximum coefficient of friction of the 
.mu./.lambda. curve. 
When it is assumed that the .lambda. signal disappears at the maximum 
coefficient of friction, the braking pressure on the high wheel must be 
released upon decline of the .lambda. signal (see FIGS. 4b, c (.lambda. 
diagram), and d (valve control signal diagram), in preparation for a 
subsequent reapplication of brake pressure. In this case the counter 10 in 
the circuit in FIG. 1 counts forward for the duration of the -b signal and 
maintains the counter value that has been reached until the disappearance 
of the .lambda. signal. 
At small control amplitudes it often happens that no .lambda. signal is 
produced. In this case it is expedient to initiate the pressure release on 
the high wheel upon appearance of the +b control signal (see FIGS. 5b, c 
(b signal diagram), and d (valve control signal diagram)). The counter 10 
then counts forward for the duration of the -b signal and maintains the 
counter value that has been reached until the appearance of the +b signal. 
In accordance with FIG. 5, the braking distances are longer due to earlier 
pressure reduction. In addition, greater braking force differences develop 
between the two wheels. For this reason, it is advantageous to combine the 
+b and .lambda. signals in a suitable way in order to achieve better 
braking force utilization and better driving stability. If no .lambda. 
signal appears, brake pressure control on the high wheel is introduced 
upon appearance of the +b signal. If a .lambda. signal does appear, the 
brake pressure control on the high wheel is initiated upon disappearance 
of the .lambda. signal. 
The pressure release on the high wheel can also begin at the end of the 
pressure control phase on the low wheel or after the disappearance of the 
-b control signal on the low wheel. 
The behavior of the reference velocity relative to the velocity of the low 
wheel can be used as an additional criterion for the brake pressure 
reduction on the high wheel. At the point of intersection of the reference 
velocity and the velocity of the low wheel, the low wheel is already 
running at a sufficient speed again, so that it can be used as a criterion 
for the beginning of the brake pressure reduction on the high wheel (see 
FIGS. 6a, b, d). 
If OR gate 28 is not incorporated in the circuit in FIG. 1, the intake 
valve control lines of the low and high wheels can be combined in a single 
line and connected with a common intake valve for the two wheels. 
As has already been proposed, the brake pressure control phase preferably 
consists of two phases. In the initial phase after the pressure holding 
phase, brake pressure is rapidly introduced over a predeterminable 
interval of time, and in the second phase the brake pressure is introduced 
more slowly or in pulsed form. 
It should be noted that in order to achieve smaller braking distances with 
the different coefficients of friction in question here, it is 
advantageous if a differential pressure develops between the wheels of an 
axle, whereby, of course, care must be taken to ensure that the resulting 
differential braking forces do not cause any perceptible yawing moments in 
the vehicle. However, the differential pressure should never rise above an 
acceptable mean value because the pressure gradients run in phase 
opposition during the pressure supply and pressure exhaust at equal 
pressure levels. 
The circuit of the invention also operates in the event of disturbances 
that trigger control signals. However, the effect of such disturbances on 
the quality of the brake pressure control is small because such 
interference signals are almost always of very short duration, so that the 
counter 10 in the circuit of FIG. 1 counts forward only slightly during 
this period of time and counts back down at a much faster rate after the 
disappearance of the interference signal.