Circuit configuration for brake systems with anti-lock control and/or traction slip control

A circuit configuration for brake systems with anti-lock control and/or traction slip control is equipped with circuits for cornering identification and for generating signals characteristic of cornering. The differential speed between the wheels of one axle is measured. The speed difference signal is standardized. An error signal which is system-inherent and occurs also when driving straight ahead is generated by means of a low pass filter having a long time constant. The difference between the standardized speed difference signal and the error signal represents the cornering signal. Further, a quantity depending upon the speed of a wheel is compared with the cornering signal and checked for reasonableness. A second reasonableness check is based on the comparison and the evaluation of the cornering signals which were generated by the front axle and the rear axle irrespective of one another.

BACKGROUND OF THE INVENTION 
The present invention relates to a circuit configuration for a vehicular 
brake system with anti-lock control and/or reaction slip control which is 
equipped with wheel sensors for generating electric signals representative 
of the wheel rotational behavoir. Electronic circuits are provided for 
conditioning, logically combining and processing the sensor signals and 
for generating braking-pressure and/or starting-torque control signals. 
Circuits are provided for cornering indentification and for generating 
signals characcteristic of cornering and for comparing these signals with 
the vehicle speed. 
A circuit configuration for automotive vehicles with traction control of 
this type is disclosed in German patent specification P 31 27 302. By 
means of the cornering-identification device described in this patent 
specification, a signal characteristic of cornering is obtained alone by 
evaluating the signals supplied by sensors which are arranged at the two 
wheels of the non-driven axle. In order to augment the driving stability, 
this cornering signal serves to reduce the starting torque of the vehicle 
enging during cornering if only one of the drive wheels tends to spin and, 
simultaneously, a threshold value of the vehicle speed is exceeded. The 
cornering identification is based solely on the measurement of the 
difference of the numbers of revolution between the two front wheels. 
Therefore, a difference between the numbers of revolution which is due to 
different tread radii of the two wheels, as caused by different tire 
profiles or tread wear, will result in an error signal, or the threshold 
of response of the cornering identification will be set to be so high that 
relatively narrow bends can only be detected which must be cornered with 
low speed for physical reasons. 
Further, a circuit configuration for a slip-controlled brake system is 
known with a cornering-identification circuit which in each case sums up 
the slip of the two wheels of one vehicle side and compares it with the 
sum of slip of the wheels on the other vehicle side (U.S. Pat. No. 
4,657,313 issued Apr. 14, 1987 and corresponding to German published 
patent application P 34 13 738). As soon as the difference of the slip 
value sums of both vehicle sides exceeds a limit value, the selection 
criteria will be temporarily changed, for instance from select-low to 
select-high, and thus the braking pressure variation. This way, the 
braking pressure variation is conformed to differing conditions when 
running straight and cornering, to the end that in each situation driving 
stability and steerability of the vehicle is maintained to the extent 
possible. Differences between the rotational speeds which are caused by 
different tread radii also impair the exactness of cornering 
identification and the evaluation of these signals in this known circuit. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to overcome the 
described shortcomings of known circuit configurations and to avoid the 
occurrence of error signals as well as faulty reactions in the cornering 
identification which are caused in particular by the different rolling 
circumferences of the individual wheels. 
It has been found that this object can be achieved in a relatively simple 
and technically progressive fashion by a circuit configuration of the type 
initially referred to whose special features resides in that the circuits 
for generating the cornering signals (CRV) are equipped with circuit 
branches for ascertaining a speed difference signal which represents the 
differential speed between the wheels of a vehicle axle, and with circuit 
branches for determining a system-inherent error signal (E.sub.sys) or 
correction signal which is independent of the instantaneous cornering 
action and which can be taken into account when evaluating the speed 
difference signals and when ascertaining the cornering signals (CRV). 
Hence the instant invention recognizes that reliable identification of the 
cornering of a vehicle becomes possible under critical conditions, in 
particular at high speed which allows only a large bend radius, if the 
system-inherent error is found by so-called "long-time observation" and 
its influence on cornering is eliminated. This is because such 
system-inherent errors which, for example, emanate from differences of the 
rolling circumference and, when comparing the rotational speeds of the two 
wheels of an axle, result in a difference and fake cornering, are 
definitely within the order of magnitude of the useful signals. 
Differences in the rolling circumference of 5% may easily be caused by 
different wear of tires. On the other hand, a speed difference of 5%, with 
the rolling circumference being the same, is an evident signal for 
cornering. 
Other causes for differences between the signals derived from the left and 
the right wheel of an axle when driving straight onward are tolerances of 
the mechanic and electronic component parts, etc. System-inherent errors 
change due to the changing of tires, due to displacement or aging of the 
component parts, for which reasons gauging actions would have to be 
required frequently to eliminate these errors. Yet this "gauging" is 
constantly performed by way of the circuit in accordance with the present 
invention. As will be explained hereinbelow, after the changing of a tire, 
the invention circuit configuration would be prepared for a new 
system-inherent error after a very short time. The circuit configuration 
according to the present invention requires but a few seconds to detect 
the new error or the new situation. 
An exact cornering identification is of major advantage both for anti-lock 
control and for traction slip control. This applies for instance for 
measures for the weakening of yawing torques or for the pressure increase 
or the pressure control in a skidding vehicle. To control the traction 
slip when starting or accelerating in a bend, precise cornering 
identification, possibly even a bend radius identification, is likewise 
necessary. 
In a preferred embodiment of the inventive circuit configuration, the 
discrepancy in percent of the speed of one wheel from the speed of the 
second wheel of the same axle is ascertained for generating the speed 
difference signal. The difference in speeds between the wheels of an axle 
is suitably standardized by division by the speed of the instantaneously 
slower wheel, if necessary, also by division by the speed of the faster 
wheel or by an averaged speed. This is of benefit for the further signal 
processing. 
It is furthermore provided according to an embodiment of this invention for 
the determination of the system-inherent error signal or correction signal 
to supply the speed difference signal or the standardized differential 
speed to a low pass circuit having a large time constant in comparison 
with the duration of conventional cornering, e.g., a first-order low-pass 
filter with a time constant of at least 40 seconds. The time constant of 
this filter will then so-to-speak define the period of time which is 
necessary for "acquiring" the system-inherent error after the changing of 
a tire or after the ignition has been switched on. 
Low-pass filters with switchable time constants can also be used which 
during travel straight ahead are dimensioned, e.g. to a time constant of 
roughly 30 to 120 seconds and which, on identification of cornering, can 
be switched over to a time constant of approximately 150 to 300 seconds. 
Under certain conditions, "freezing-in" of an acquired system-inherent 
error for a specific time span is expedient. For this purpose, the 
inventive circuit configuration is provided with a circuit branch which 
can be supplied with the standardized speed difference signal and with a 
constant comparative value and which, when a maximum value of the speed 
difference signal is exceeded, precludes that this value is taken into 
account when determining or updating the error signal. It will thereby be 
prevented that a short-time disturbance which manifests itself in a too 
large speed different signal, or that a large useful signal (speed 
difference signal) which cannot be due to a system-inherent error of the 
type described and therefore must not be `acquired`, will have effects on 
the error signal. The differnce between the speed difference signal and 
the system-inherent error signal can be evaluated as cornering signal 
according to the instant invention. 
Further, according to another embodiment of this invention, there are 
circuits which derive a cornering limit value from the instantaneous wheel 
speed, in particular from the speed of the slower wheel, and compare it 
with the instantaneous cornering signal and issue a spurious signal when 
the cornering signal exceeds the cornering limit value. 
Cornering identification may further be improved by comparison and 
evaluation of the signals of the above-mentioned type derived from two (or 
more) axles of a vehicle. To this end, according to another embodiment of 
this invention, the output signals of the complete circuits allocated to 
the individual axles are delivered to an evaulation circuitry and checked 
for reasonableness. In doing so, the evaluating circuits are supplied by 
the individual complete circuits with information which imply the 
identification of cornering, the bend radius, the travel straight ahead, 
left-hand bend or right-hand bend, differential speed, etc., and all or 
some of these data. 
Finally, in another embodiment of this invention, the presence of a 
characteristic difference between the signals of different vehicle axles 
which, for instance, are attributable to there being applied a spare wheel 
with a widely differing (e.g. 20%) tread radius, change-over of the logic 
and compensation of this discrepancy will be effected.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The drawing figures serve to illustrate the circuits in accordance with the 
present invention which generate the cornering signals and which provide 
an electronic circuit configuration for the control of vehicle brake 
systems with anti-lock control and/or traction slip control. In FIGS. 1 to 
3, the signals at the inputs of the individual circuit blocks or circuit 
branches are referred to by A and B, while the output signal is assigned a 
Y. 
According to FIG. 1, the circuits are supplied with electric signals 
(V.sub.R, V.sub.L) by way of the inputs E1 and E2 of the complete circuit, 
which signals have been obtained by means of sensors arranged at the front 
wheels in the illustrated embodiment and which in each case represent the 
rotational behavior of one of the two front wheels. The wheel sensors and 
the normally necessary circuits for conditioning the signals are not shown 
in FIG. 1 as they are no part of this invention. In a difference former 1, 
a speed difference signal v is formed whose sign permits recognition of 
whether a right-hand bend or a left-hand bend is present. During travel 
straight ahead and under ideal conditions, that means equal tread radii 
etc., the signal at the output Y of stage 1 becomes zero. In a subsequent 
divider stage 2, the differential speed v will be standardized by dividing 
the difference signal v by the lesser of the two wheel speeds V.sub.min. 
To this end, the lesser speed V.sub.min was found by a selection stage 3 
and delivered to the input B of stage 2. The standardized differential 
speed v.sub.n =v/v.sub.min at the output of the divider 2 is supplied by 
way of a transfer switch 4 to a low pass 5, herein a first-order low-pass 
filter with switchable time constant. The output signal of this low pass 5 
represents the error signal or correction signal E.sub.sys which is a 
measure for the system-inherent errors which will cause a speed difference 
signal also when the vehicle drives straight onward. Finally, the 
difference between the standardized differential speed V.sub.n and the 
error signal or correction signal E.sub.sys is formed in another 
difference former 6. This signal CRV=V.sub.n -E.sub.sys serves for 
cornering identification, and if the resolution of this signal so allows, 
is even a measure of the bend radius. It can be recognized from the sign 
of the cornering signal CRV whether a right-hand or a left-hand bend is at 
issue. 
The low-pass filter 5 is switchable. In the initial position which it 
assumes as long as the signal at the control input I is zero, the time 
constant T.sub.1 of the low-pass filter 5 is set to a value between 40 and 
100 seconds. If, however, "cornering" was detected in the preceding 
calculating cycle (loop), this is signalled to the low-pass filter 5 by 
way of the control input I. A signal at the input I results in change-over 
of the stage 5 or increase of the time constant to T.sub.2 =200 to 300 
seconds. Change-over of the time constants can simply be realized by 
increase of the ohmic resistance of an RC-member. Consequently, during 
cornering, the change of the error signal E.sub.sys at the output of stage 
5 as a consequence of the value of the standardized differential speed 
V.sub.n being increased due to cornering is delayed considerably. During 
travel straight ahead, however, system-inherent errors are "acquired" with 
the speed of the time constant T.sub.1. 
Disturbances which cause a too high standardized differential speed V.sub.n 
that is in excess of a predetermined threshold value, as well as useful 
signals which are larger than the signals which can be caused by 
system-inherent errors, are suppressed by an additional circuit branch 
containing another difference former 7. This threshold value is symbolized 
by the constant k.sub.2 at the input B of the difference former 7. 
Whenever the standardized differential speed V.sub.n exceeds a predefined 
value, a signal will result at the output of stage 7 which causes 
change-over of the switch 4 by way of the control input I. Thereby, the 
error signal E.sub.sys is returned by way of the input B of the switch 4 
to the input of the low pass 5, in consequence whereof the acquired error 
is "frozen" or the error signal E.sub.sys is kept constant for the 
duration of the disturbance. 
A check of the cornering signal CRV according to reasonableness criteria is 
achieved by means of an additional circuit branch (8, 9). For this 
purpose, first the speed V.sub.min of the slower wheel of the axle 
considered is compared with table values in stage 8. The output signal 
CRV.sub.grenz of stage 8 is directly or indirectly a measure for the bend 
radius below which the vehicle cannot drop at the measured vehicle speed 
V.sub.min without jeopardizing the driving stability, and thus it is a 
measure for the maximum value or limit value of the cornering signal CRV. 
As is known, a narrow bend can be cornered by a vehicle at low speed only. 
It may be recognized from a comparison of the limit value CRV.sub.grenz 
with the cornering signal CRV, whether a like bend radius is possible at 
all at the measured speed, the said limit value being derived from the 
instantaneous speed V.sub.min and being available at the output of stage 
8. The comparison is performed with the aid of the difference former 9. If 
the reasonableness criterion is not fulfilled, stage 9 will signal a 
malfunction. 
Two further circuit branches 10 and 11 serve to evaluate the sign of the 
cornering signal CRV and hence to determine whether the vehicle is 
cornering to the right or to the left. Finally, "travel straight ahead" is 
still signalled by way of the output of another branch 12 in the absence 
of either a right-hand bend signal or a left-hand bend signal. 
FIG. 2 shows an evaluation matrix of evaluation circuit 13 which is 
supplied with the signals of the front axle and rear axle generated with 
the complete circuit according to FIG. 1. By means of individual complete 
circuits (1 to 12) of the type shown in FIG. 1, the front-axle and 
rear-axle signals are generated irrespective of one another. 
Checking the front axle and rear-axle signals for reasonableness by virtue 
of the evaluating circuit 13 permits detection of errors or disturbances 
and improves the signal analysis. Apart from special cases, the 
corresponding signals derived from the front axle and the rear axle must 
be coincident. The results of the evaluations made by the circuit 13 are 
combined in an integrator 14 inserted after the evaluating circuit 13. 
After a comparison with limit values which are symbolized by the constants 
k.sub.7 and k.sub.8 of the subsequent stages 15 and 16 in FIG. 2, the 
signal W.sub.I at the output of the integrator 14 allows a reliable 
indication of cornering and the direction of the bend, that means 
"right-hand bend" or "left-hand bend". A cornering-identification signal 
is generated by way of an OR-gate 17. 
When comparing the input information at the inputs I.sub.1 to I.sub.6 with 
the corresponding information at the inputs I.sub.7 to I.sub.12, 
contradicting input values will make the output signal W of the circuit 13 
minimal, while in the event of input values that make sense it will assume 
a maximum amount. The integrator 14 inserted after will then supply a 
signal W.sub.i which sums up the results of the consecutive evaluations. 
FIG. 3 illustrates an additional circuit which serves to recognize that a 
spare-wheel (mini-spare wheel) is fitted. Such spare wheels have a rolling 
circumference which is roughly up to 20% less than that of the standard 
wheels. The mounting of a like spare wheel shows in that a relatively high 
differnetial speed and thus a seeming cornering signal CRV occurs at one 
axle only. 
That is to say, the comparison of the CRV-signals of front axle VA and rear 
axle HA by means of an evaluating circuit 13' results in a difference 
signal typical of the mounting of a mini-spare wheel. "Long-time 
observation" also will permit detection of this case, and to enable a 
correction of the erroneous wheel speed produced by the spare wheel. To 
this effect, the low-pass filter 18 is furnished with a time constant of, 
for example, 2 to 30 seconds. The output signal of this low-pass filter 18 
is compared in the subsequent comparator 19 with a typical threshold value 
symbolized by the constant k.sub.g. If the output signal of stage 18 
exceeds the threshold value k.sub.g for a long time, this leads to 
conclude that the mini-spare wheel is mounted. The control can be adapted 
to the special situation by reducing the rotational speed measured at the 
location of the spare wheel by an amount calculated from the lesser wheel 
diameter.