Method for minimizing the amplitude of pressure modulation in an ABS control cycle

A brake system is described which calculates the duration of the pressure reduction signal. In this case, the wheel acceleration (or the wheel reacceleration) and the slip are determined in the unstable region of the wheel and related to prescribed magnitudes, from which the calculation of the reduction time of the new control cycle is performed in such a way that the wheel reacceleration following the pressure reduction corresponds to a precalculated magnitude. This signal duration is corrected in order to keep the amplitude of the pressure modulation small.

BACKGROUND OF THE INVENTION 
The invention relates to a method for controlling brake pressure in a motor 
vehicle wherein wheel speed signals are used to generate slip signals and 
wheel acceleration signals, which in turn are used to generate brake 
pressure control signals. 
The quality of the braking retardation of an ABS-controlled vehicle and the 
pedal reactions depend on the rapidity with which the required optimum 
pressure level is reached, the amplitude of the pressure modulation having 
to be very small. The amplitude of the pressure modulation is 
substantially dependent in this case on the magnitude of the preceding 
pressure reduction or on how long the period of activation of the 
discharge valve is. 
As an example, a method is known for dimensioning the discharge valve 
activation time which uses a constant magnitude to correct a value 
determined from the "prehistory" of the control, and can vary this time as 
a function of the current wheel behavior. 
Also known is a method which uses just such a time value from the 
prehistory, but calculates it with the aid of a correction value dependent 
on the wheel acceleration of the previous control cycle, the correction 
value being inversely proportional to the wheel acceleration. 
The disadvantage of these known solutions is that owing to the ever present 
system delay times the pressure reduction does not match the physical 
wheel behavior, or that because of roadway defects which are present the 
ABS control reacts with control fluctuations, for example in the case of 
small wheel reaccelerations, so that overall the aim of a small pressure 
modulation is not achieved. 
SUMMARY OF THE INVENTION 
These disadvantages are removed by the proposed brake system by determining 
the wheel acceleration (or the wheel reacceleration) and the slip in the 
unstable region of the wheel and relating them to prescribed magnitudes, 
from which the calculation of the reduction time of the new control cycle 
is performed in such a way that the wheel reacceleration following the 
pressure reduction corresponds to a precalculated magnitude. 
In this case, the invention proceeds from a brake system, in general an 
antilock system, for the wheels of a vehicle, consisting of sensors for 
determining the behavior of the Wheel motion, an evaluation circuit for 
processing the sensor signals and for generating brake pressure control 
signals, and a brake pressure control unit which is fed the brake pressure 
control signals for the purpose of pressure variation. It is possible in 
this way to form slip signals and acceleration signals of at least one 
wheel or a wheel group. The invention is distinguished in that the brake 
pressure control signals are determined in such a way that the 
acceleration of at least one wheel is limited to values which depend on 
the maximum slip value within a prescribable time interval. The maximum 
slip value can be determined for this purpose within the prescribable time 
interval from the slip signals. The brake pressure control signals are 
then determined in such a way that the acceleration of at least one wheel 
is limited to values which depend on the maximum slip value. 
It is particularly advantageous that brake pressure control signals whose 
duration is determined as a function of the maximum slip value determined 
are generated for the purpose of pressure variation. 
Furthermore, it can be provided in an advantageous embodiment of the 
invention that a basic duration and a correcting quantity for correcting 
the basic duration are determined in order to determine the duration of 
the brake pressure control signals, the correcting quantity being 
determined as a function of the maximum slip value determined. 
The prescribed time interval can correspond to a control cycle. The 
abovementioned correcting quantity is then used to correct the basic 
duration of the pressure variation in the following control cycle. 
Furthermore, it is advantageous that the maximum acceleration value is 
determined within the prescribable time interval from the acceleration 
signals of at least one wheel or a wheel group. A corrected desired value 
for the acceleration of the wheel or the wheels is then determined from 
the maximum slip value determined, whereupon this desired value is 
compared with the maximum acceleration value determined. The correcting 
quantity can then be determined as a function of the difference between 
the maximum acceleration value determined and the corrected desired value. 
Further advantageous embodiments of the invention are to be gathered from 
the exemplary embodiments described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Wheel speed sensors 1 and 2 at the wheels of an axle generate signals which 
are evaluated in an evaluation circuit 3. Brake pressure control signals 
for 3/3 brake pressure control valves 4 and 5 assigned to the wheels are 
generated in the evaluation circuit 3 as a function of the behavior 
determined of the wheel motion. The evaluation circuit calculates inter 
alia the activation period TAVI for the individual valves 4 and 5. 
The following description applies to one of the wheels. Precisely the same 
device is also provided for the other wheel. The correction circuit 
described in the following paragraphs is actually part of the evaluation 
circuit 3. 
Via a line 6, the evaluation circuit transmits the period TAVI of the 
pressure reduction signal to a subtractor 7 which corrects this period and 
feeds the corrected signal back to the evaluation circuit for processing. 
The evaluation circuit 3 supplies the wheel slip SRI determined there and 
the wheel reacceleration ARI for the purpose of determining the 
correction. The wheel slip SRI can, for example, be determined in a known 
way in accordance with SRI=(Vf-Vr)/Vf from the measured wheel speed Vr and 
the vehicle longitudinal speed Vf. The wheel reacceleration or the wheel 
acceleration ARI can be obtained by differentiating the rotational wheel 
speed or the wheel speed Vr with respect to time. 
The maximum value SRIM and ARIM of these variables is determined and stored 
in a control cycle in blocks 8 and 9, respectively. A desired slip value 
SRS of, for example, 7% is prescribed at a terminal 14. A subtractor 10 
forms the difference DSR=(SRIM-SRS)*C.sub.1 +C.sub.2. C.sub.1 and C.sub.2 
are system constants which can be, for example, 0.7 and 0, respectively. A 
desired value ARS of the wheel acceleration is determined from the 
difference DSR in a transfer element 11 by means of a linear function 
ARS=f(DSR); this desired value is compared in a subtractor 12 with the 
maximum value ARIM determined. The difference DAR=(ARIM-ARS)*C.sub.3 is 
fed to a transfer element 13 which forms therefrom via a linear function 
TAVK=f(DAR) the correction value TAVK for the duration of the pressure 
reduction signal, which is fed to the subtractor 7. DAR can be, for 
example, 1 g, from which it follows that TAVK=3 ms; C.sub.3 can be, for 
example, 3 ms/g. 
A correction of somewhat different type is described in the following 
lines. A correction ARK for the desired value of the wheel reacceleration 
is calculated from the slip deviation DSR=SRIM-SRS, SRS being prescribed 
as, for example, 5% in relation to the reference speed ARK=DSR* C.sub.1 '. 
The desired value of the acceleration is corrected using the correction 
value of the wheel acceleration. The constant has a magnitude of, for 
example, C1'=0.7%/g. 
ARS=ARK+C2' 
Consequently, the desired value ARS of the wheel acceleration is available; 
the constant can then be, for example, C2'=3.5 g. 
DAR=ARIM-ARS 
DAR as a result of the actual/desired comparison is converted by means of 
the constant C3 into a correction time for the following reduction. 
TAVK=DAR * C3 
A suitable magnitude is, for example, C3=4 ms/g. 
If, for example, the extreme values of a wheel deflection are 
SRIM=9% and 
ARIM=8 g, 
the reduction time correction by which the following reduction is computed 
from the old value is calculated as follows: 
______________________________________ 
DRS = 9% - 5% = 4%, 
ARK = 4% .multidot. 0.7 g/% 
= 2.8 g, 
ARS = 3.5 g + 2.8 g 
= 6.3 g, 
DAR = 8 g - 6.3 g 
= 1.7 g 
TAVK = 1.7 g .multidot. 4 ms/g 
= 6.8 ms 
______________________________________ 
Since the preceding reduction was too large, the wheel had, at 8 g, too 
large a reacceleration; consequently, the new reduction is initialized 
predictively smaller by approximately 7 ms than the old value. 
EQU TAV=TAVI-7 ms 
It may be said in summary that the antilock control system according to the 
invention consists of sensors for determining the behavior of the wheel 
motion, an evaluation circuit for processing the sensor signals and for 
generating brake pressure control signals, and a brake pressure control 
unit which is fed the brake pressure control signals for the purpose of 
pressure variation. In this case, slip signals and acceleration and 
deceleration signals of the wheels or wheel groups are formed and pressure 
reduction signals which have been determined and whose duration TAVI is 
determined in the evaluation circuit are gene-rated. The core of the 
invention is that the maximum slip value SRIM and the maximum wheel 
reacceleration value ARIM are respectively determined within a control 
cycle, and that a corrected desired value ARS of the wheel acceleration is 
obtained from the maximum slip value SRIM, and that this desired value ARS 
of the wheel acceleration is compared with the maximum wheel 
reacceleration value ARIM and the difference DAR is used to form a 
correcting quantity TAVK =f (DAR) for the duration TAVI of the pressure 
reduction pulse in the following control cycle, by means of which the 
duration TAVI of the pressure reduction is corrected (TAV=(TAVI-TAVK)). 
A brake system is described which calculates the duration of the pressure 
reduction signal. In this case, the wheel acceleration (or the wheel 
reacceleration) and the slip are determined in the unstable region of the 
wheel and related to prescribed magnitudes, from which the calculation of 
the reduction time of the new control cycle is performed in such a way 
that the wheel reacceleration following the pressure reduction corresponds 
to a precalculated magnitude. This signal duration is corrected in order 
to keep the amplitude of the pressure modulation small.