Circuit configuration for dectecting wheel sensor malfunctions

A circuit configuration for detecting wheel sensor malfunctions includes circuits which process and analyze the sensor signals (s.sub.1 to s.sub.4), which ascertain the speed (v.sub.Rmax, v.sub.Rmin), deceleration and acceleration (a.sub.R) of the individual wheels and which compare these values with one another and compare them with predetermined threshold values (a.sub.0, v.sub.0, v.sub.1, -a.sub.1). Upon the detection of signals or combinations of signals typical of a sensor malfunction, the control will be disconnected after a predetermined period of time (T, T1+T2). When the measured acceleration values (a.sub.R) are below an overspeed threshold (a.sub.0) and the speed at any one of the remaining wheels is below a bottom speed threshold (v.sub.0), the control will be disconnected as soon as the speed of a wheel (v.sub.Rmax) exceeds a top speed threshold (v.sub.1). A time monitoring function is started in the presence of a measured acceleration value (a.sub.R) which is above the overspeed threshold (a.sub.0) and in the presence of a measured speed value (v.sub.Rmin) which is below the bottom speed threshold (v.sub.0) as soon as a measured speed value (v.sub.Rmax) exceeds a top speed threshold (v.sub.1). Anti-lock and traction slip control will be disconnected upon lapse of the predetermined time period.

BACKGROUND OF THE INVENTION 
The present invention relates to a circuit configuration, or system, for 
detecting wheel sensor malfunctions intended for automotive vehicle brake 
systems with electronic anti-lock control and/or traction slip control. 
Such a circuit configuration includes circuits which process and assess 
the sensor signals, which ascertain speed, deceleration and acceleration 
of the individual wheels and compare these values with one another as well 
as with predefined limit values. In addition, such a circuit configuration 
includes circuits which disconnect the control at least temporarily once 
signals or combinations of signals typical of a sensor malfunction are 
detected. 
Monitoring the individual component parts and the operability of an 
electronically controlled brake system is of great significance. This is 
because once malfunctions are detected, the conventional functioning of 
the brakes can be safeguarded by disconnecting the control. Since a great 
variety of errors and types of errors can occur, the control systems are 
monitored by different measures in practice. 
For instance, one monitoring method involves regularly generating test 
pulses and observing the reaction of the component parts to these pulses. 
Another monitoring method involves performing the signal processing in the 
electronic control unit of the system in parallel in separate circuits and 
monitoring the output signals generated this way for coincidence. A large 
number of errors can be detected by the so-called plausibility criteria 
method. This method of error detection is based on that specific measured 
values or combinations of signals are not physically possible when the 
system is intact. Thus, if such values or combinations of signals occur, 
an error is indicated. However, difficulties are involved in many cases to 
distinguish between signals caused by malfunctions and correct control 
signals. 
Some types of errors or problems cannot be detected at all, or not reliably 
or not fast enough by all known monitoring methods. The monitoring of the 
wheel sensors is among these problems. Erroneous signals occur in a 
variety of circumstances, e.g. in the presence of too large of an air gap, 
when the toothed disc loosens or is mounted incorrectly, in the event of 
pulse generator teeth missing partly or wholly, etc. In these 
circumstances, it is difficult to determine whether the absence of sensor 
signals on starting the vehicle is due to a standstill of the wheel or due 
to the sensor wheel missing. Problems of this kind are known among 
specialists under the phrase "detection on starting". 
An anti-lock system with a safety function is already known from European 
patent specification EP 0 075 932 B1, wherein specific pairs of signals 
are detected and an error is identified once a combination of signals 
occurs which is not possible during fail-safe operation. Specific sensor 
errors are unidentifiable this way because the signals caused by the 
malfunction can occur in certain control situations as well during a 
malfunction. 
SUMMARY OF THE INVENTION 
The circuit configuration of the present invention detects the sensor 
errors of different types occurring in practice very quickly and reliably 
and safeguards the conventional brake function in these cases by 
disconnecting the control. 
The circuit configuration of the present invention disconnects or disables 
the control unit in the presence of a measured acceleration value at one, 
two or three wheels which is below a so-termed overspeed threshold and in 
the presence of a measured speed value at the remaining wheel or at least 
one of the remaining wheels which is below a bottom speed threshold, as 
soon as a measured wheel speed value exceeds a top speed threshold. 
This circuit configuration serves to very quickly detect a sensor 
malfunction when the vehicle starts to drive and the acceleration is below 
the overspeed threshold. 
Once a considerable deceleration occurs, i.e. when a wheel deceleration is 
below a predetermined deceleration limit value, a time monitoring function 
with a predetermined shortened duration will be started. The time 
monitoring function can be in the range between 20 and 60 seconds or 
roughly can amount to 30 seconds according to a preferred embodiment of a 
circuit configuration of the present invention. 
To detect sensor malfunctions, for example when overspeeding occurs when 
the vehicle is starting to drive, a circuit configuration according to the 
present invention is devised such that under certain conditions, a time 
monitoring function will operate. These conditions include a measured 
acceleration value at up to three wheels which is above the overspeed 
threshold at any time from the starting of the vehicle, and a measured 
speed value on at least one of the remaining wheels which is below a 
bottom speed threshold, and a measured speed value which exceeds a top 
speed threshold. After the period of time predetermined by the time 
monitoring function has lapsed, the control unit is disconnected. 
According to another preferred embodiment of the present invention, the 
time monitoring function accumulates the time periods in which a measured 
speed value exceeds the top speed threshold, while the time monitoring 
function is reset to the initial position once the measured speed value 
falls short of the bottom speed threshold. That is to say, the time 
element of the time monitoring function is stopped in those phases in 
which the measured speed value, after the top speed threshold is exceeded, 
drops below this top threshold again. Consequently, the elapsed time 
during these phases is not taken into account. The predetermined duration 
of the time monitoring function expediently ranges between 1 and 3 
minutes, and preferably amounts to roughly 2 minutes. 
According to an embodiment of the present invention, the time monitoring 
function is initiated in the presence of a measured acceleration value on 
at least one wheel which is above the overspeed threshold at any time from 
the starting of the vehicle and in the presence of a measured speed value 
which is below the bottom speed threshold, as soon as a measured speed 
value exceeds the top speed threshold. In another embodiment of the 
invention, when the above conditions are met and when the fastest wheel 
has a constant or approximately constant rotational behavior, the duration 
of the time monitoring function is shortened by a predetermined value in 
dependence on the acceleration of this wheel. Once a variation of the 
speed of the fastest wheel in the predetermined time unit is measured to 
be below 0.1 to 0.2 g roughly, the duration of the time monitoring 
function will be shortened by a value in the range between 10 and 50 
seconds, e.g. by 20 seconds. Once an acceleration of this wheel occurs 
after the top speed threshold has been exceeded and this acceleration is 
in excess of a predetermined value (e.g. .+-.1 g), the commencement of the 
time span for detecting the constant wheel rotational behavior will be 
shifted to the point of time when this acceleration occurs. 
Finally, according to a preferred embodiment of the present invention, 
after the top speed threshold has been exceeded, the control unit is 
switched over for the duration of the time monitoring function to a 
control pattern where the control is dependent on the deceleration and 
acceleration of the individual wheels, yet is not dependent on the 
measured slip values. Thus, in this phase, the control remains unaffected 
by an incorrect slip measurement which is due to a sensor malfunction, for 
instance.

DETAILED DESCRIPTION OF THE DRAWINGS 
The curve illustrated in FIG. 1 refers to a starting-to-drive operation 
"without overspeed" . The speed variation of the fastest wheel v.sub.Rmax, 
the speed of the slowest wheel v.sub.Rmin, the acceleration of the fastest 
wheel a.sub.R, as well as speed and acceleration thresholds are 
illustrated. The term "a.sub.0 " designates a so-called overspeed 
threshold, the term "v.sub.0 " designates a bottom speed threshold and the 
term "v.sub.1 " designates a top speed threshold. A bottom speed threshold 
of v.sub.0 =5 km/h and a top speed threshold of v.sub.1 =20 km/h are 
chosen in an embodiment of the present invention. A typical value for the 
overspeed threshold is a.sub.0 =0.3 g, with "g" referring to the constant 
of acceleration due to gravity. 
In the starting operation to which FIG. 1 relates, a specific wheel 
acceleration of the fastest wheel a.sub.R is detected and remains below 
the acceleration threshold a.sub.0. The speed characteristic curve 
v.sub.Rmin of a wheel does not reach the bottom speed threshold v.sub.0. 
In this case, the occurrence of a sensor malfunction is detected at time 
t.sub.1, when the speed of the fastest wheel v.sub.Rmax reaches the top 
speed threshold v.sub.1. Upon detection of the malfunction, the anti-lock 
and traction slip control unit is disconnected or a control is prevented 
in any other way. This is because such a differing wheel rotational 
behavior is not possible in an intact system or a system with intact wheel 
sensors. 
In addition to the thresholds a.sub.0, v.sub.0, v.sub.1 described above, a 
bottom acceleration threshold "-a.sub.1 " is also depicted in FIG. 2. Upon 
starting of a vehicle, e.g. when upshifting the manually or automatically 
operated gearbox, it is typical for a "remarkable" wheel deceleration to 
occur, which involves a wheel deceleration falling short of the threshold 
-a.sub.1 at the point of time t.sub.2. This is interpreted as an 
"overspeeding in the standing position" and initiates a shortened time 
monitoring function. FIG. 2 shows the typical wheel variation v.sub.Rmax, 
v.sub.Rmin and a.sub.R in a similar situation which could be caused by 
switching over, by driving over a patch of ice or the like. 
FIG. 3 refers to a starting operation "with overspeed". The acceleration 
a.sub.R exceeds the overspeed threshold a.sub.0 at time t.sub.3 at one, 
two or three wheels. Simultaneously, the speed v.sub.Rmin of at least one 
wheel remains below the bottom speed threshold V.sub.o which herein was 
defined to be 5 km/h. A similar signal variation could be due to 
1. a starting action with up to three defective (i.e. not issuing a signal) 
wheel speed sensors, or 
2. overspeed when standing on a road surface with a low coefficient of 
friction. 
Therefore, a time monitoring function is initiated at time t.sub.4 when the 
fastest wheel whose speed is referred to as v.sub.Rmax exceeds the top 
speed threshold v.sub.1. The control will be disconnected upon the 
expiration of a predetermined period of time, which could amount to two 
minutes. This predetermined period of time can be shortened in dependence 
on specific criteria capable of identifying the existence of a wheel 
sensor malfunction. 
In the starting situation according to FIG. 4, the time scale was changed 
in comparison to FIGS. 1 to 3 to explain the time monitoring function. As 
in the example of FIG. 3, the wheel acceleration a.sub.R exceeds the 
overspeed threshold a.sub.o at the point of time t.sub.5, and the speed 
v.sub.Rmax of the fastest wheel exceeds the top speed threshold v.sub.1 at 
the point of time t.sub.6. Consequently, the time monitoring function is 
started to operate at the point of time t.sub.6. The contents I of a 
corresponding accumulator or time counter begin to accumulate at time 
t.sub.6, as illustrated in FIG. 4. The counter will be stopped and the 
counter contents will remain constant if, like in the example according to 
FIG. 4, the measured speed value v.sub.Rmax at time t.sub.7 temporarily 
drops below the top speed threshold v.sub.1 and exceeds this threshold 
again at the point of time t.sub.8. This "time interruption" between 
t.sub.7 and t.sub.8 consequently prolongs the duration until disconnection 
of the control unit after the first error detection at time t.sub.6. 
At time t.sub.9, the predetermined duration T ends. The predetermined 
duration could range between one minute and two minutes. In the example 
shown in FIG. 4, the control unit is disconnected after the predetermined 
duration plus the time difference between t.sub.7 and t.sub.8 from time 
t.sub.6 expires. The duration T is composed of the time spans T1+T2 in 
this case. 
The diagrams according to FIGS. 5 and 6 also relate to situations in which 
the speed variation v.sub.Rmax of the fastest wheel indicates overspeeding 
of this wheel. In both cases, the acceleration a.sub.R exceeds the 
acceleration threshold a.sub.0 at some time from the starting of the 
vehicle. The monitoring time T is initially set to a value between one and 
two minutes. In order to be able in such a case to considerably reduce the 
monitoring time T commencing at the time when the top speed threshold 
v.sub.1 is exceeded, the variation of the speed .DELTA.v/.DELTA.t is 
measured with a relatively coarse screen pattern, after the speed 
threshold v.sub.1 is exceeded. 
For the time span Dt, a magnitude of 1 to 3 seconds, roughly 2 seconds, is 
chosen. Once a variation .DELTA.v/.DELTA.t is measured which is below a 
predetermined value (e.g. 0.1 to 0.2 g), this implies an approximately 
constant rotational behavior of this wheel. In this case to which FIG. 5 
is relating, the monitoring time T is shortened by a specific amount in 
the range between 10 and 50 seconds, for instance by roughly 20 seconds. 
This minor speed variation or deceleration of less than 0.1 or 0.2 g 
within the monitoring interval .DELTA.t is interpreted as detection of a 
"stationary or quasi stationary driving action". 
In addition to this measurement of the speed variation in the coarse 
time-slot pattern or within the monitoring time interval Dr, the 
acceleration dv/dt is still measured. To this end, the speed variation 
within a working clock which can range between 5 and 10 milliseconds for 
instance is determined. Once an abrupt change in acceleration occurs 
during this stationary or quasi stationary driving operation, which is 
shown at time t.sub.13 in the example according to FIG. 6, the 
commencement of the monitoring interval .DELTA.t is shifted from the point 
of time t.sub.12 in FIG. 6, at which the speed threshold v.sub.1 was 
exceeded, to the point of time t.sub.13. Such short-time major 
accelerations can be due for instance to road trouble, to a road surface 
covered by slick ice or to similar circumstances. 
FIG. 7 illustrates the connection of the most important component parts of 
a circuit configuration according to this invention. First, signals 
v.sub.R1 to v.sub.R4 representative of the wheel speeds are generated from 
the output signals representative of wheel rotational behavior of wheel 
sensors S.sub.1 to S.sub.4 in evaluating circuits 1 to 4. Thereafter, 
valve control signals are produced from these signals in a known fashion 
by way of an ABS/TSC logic circuit 5 and are applied to a valve block 6 
for the purpose of braking pressure modulation. Circuits 7 to 10 which 
serve to obtain the acceleration signals a.sub.1 to a.sub.4 by way of the 
differentiation of the speed signals are also shown in FIG. 5. 
These acceleration signals are required by the ABS/TSC control logic 
circuit 5 and also assist in detecting sensor malfunctions. Therefore, the 
acceleration signals a.sub.1 to a.sub.4 are supplied also to a time 
monitoring arrangement circuit 11 which presets the predetermined times in 
response to the respective situation until the control unit is 
disconnected in the case of malfunction. 
Further, the inventive circuit configuration according to FIG. 7 comprises 
circuits 12, 13 for determining the instantaneously lowest (12) and 
highest (13) wheel speeds, respectively v.sub.min and v.sub.max. In a 
comparator 14, the highest speed v.sub.Rmax is compared with the lowest 
speed v.sub.Rmin, and subsequently the duration T is predetermined in 
dependence on the output signal of the time monitoring circuit 11 in a 
circuit 15. The duration T will last from the time when the top speed 
threshold v.sub.1 is exceeded until the disconnection of the control. The 
output signal of the circuit 15 is therefore supplied via a time element 
16 to a main relay 17 which finally disconnects the control when a sensor 
malfunction is detected. 
The output signal of the circuit 13 which selects the instantaneously 
highest wheel speed v.sub.Rmax is further supplied to a low-pass filter 18 
having a predetermined time constant in the seconds range. Once the speed 
signal v.sub.Rmax, namely the output signal of the circuit 13, passes over 
into a stationary or quasi stationary range, a time-reducing circuit 19 is 
set by means of the low-pass filter 18 and, in turn, drives the time 
element 16 and presets the predetermined duration in this situation until 
the disconnection of the control. The "reset" of the time-reducing circuit 
19 is controlled by means of a second low-pass filter 20 which evaluates 
the output signal of the time-monitoring circuit 11. The time constant of 
the low-pass filter 20 is in the milliseconds range, e.g. between 5 and 10 
milliseconds. 
The output signal of the time monitoring circuit 11 which evaluates the 
wheel acceleration is compared in the circuit 15 with the output signal of 
the comparator 14 and hence is made use of also for driving the time 
element 16.