Vehicle wheel anti-skid or anti brake-lock system

To reduce disturbance and interference signals, and particularly recurring disturbance signals due to vibration or out-of-round conditions of rotating elements coupled to a speed transducer, a low-pass filter (11) is connected to the speed transducer and, in addition, a high-pass filter (12) which, however, is disconnected upon sensing that the speed of the wheel or of the vehicle is below a predetermined limit, or that the anti brake-lock system unit (13) has responded. Thus, disturbance signals, and particularly periodic disturbance signal which might cause oscillatory conditions to arise in the filter and transducer circuitry, are effectively suppressed. To further increase the reliability of response, the output from the filter circuits is connected to threshold circuits with a variable threshold level in which comparators compare a peak signal with a weighted, then existing signal, so that the comparison level threshold is continuously shifted based on previously occurring peaks, so that disturbance signals are reliably excluded from affecting signal processing within the anti brake-lock system unit (13).

REFERENCE TO RELATED PUBLICATIONS 
German Patent Disclosure Document DE-OS 17 80 062, WEHDE et al.; German 
Patent Disclosure Document DE-OS 19 16 518, ATKINS (corresponding to U.S. 
patent application Ser. No. 716,709, filed Sept. 23, 1968). 
The present invention relates to vehicle wheel anti-skid or anti brake-lock 
systems, and more particularly to such systems used in automotive vehicles 
in which wheel speed sensors provide output signals representative of 
wheel speed and rotation in order to provide derived signals, such as 
wheel acceleration or deceleration, vehicle speed, and the like. 
BACKGROUND 
Anti brake-lock or vehicle anti-skid systems utilize sensors coupled to the 
wheels. Such sensors are subject to mechanical disturbances, particularly 
vibrations, and may also respond by spurious signals to uncontrollable 
conditions in the transducer elements, such as out-of-round conditions of 
transducer components, and the like. Consequently, malfunction or 
interference with proper, controlled operation of the anti-skid system may 
occur. Vibrations and out-of-round conditions, particularly, may cause 
interference or disturbance signals which are within the frequency and/or 
amplitude range of the actual signal which is intended to be derived. If 
such disturbance signals are, erroneously, evaluated as actual wheel 
signals, malfunction of the anti-skid system may occur. 
It has previously been proposed--see German Patent Disclosure Document 
DE-OS 17 80 062--to utilize filter circuits coupled to a speed transducer, 
in which the time constants of the filters are suitably selected to 
exclude, as far as possible, disturbance signals. Later, it was 
proposed--see German Patent Disclosure Document DE-OS 19 16 518 (based on 
U.S. application Ser. No. 716,709, filed Sept. 23, 1968, ATKINS, assigned 
Kelsey-Hayes Co.)--to construct filters which have low-pass 
characteristics utilizing an R/C series circuit arrangement. 
It has been found that filters may, in connection with the remaining 
circuits in which they are used, form oscillatory systems if output 
signals from wheel rotation transducers are subjected to vibration or 
other recurring disturbances. If the circuit becomes oscillatory, 
disturbance and noise signals will be enhanced, which, in spite of other 
precautions taken to exclude disturbance signals, may lead to erroneous 
response, and hence malfunction of the anti-skid system. 
THE INVENTION 
It is an object to provide an anti-skid or anti brake-lock system in which 
disturbance effects, particularly recurring disturbance effects, are 
essentially eliminated. 
In accordance with the invention, the filtering network includes two 
filters, in which the second one has a band-pass characteristic different 
from the first, preferably formed as a high-pass filter, whereas the first 
one is a low-pass filter; and controlled switching means are provided to 
selectively connect or disconnect the second filter in dependence on 
predetermined operating parameters, as represented by signals, which occur 
within or are available within the anti-skid system. Typically, the 
control signals which may selectively connect the second filter may be 
based on wheel speed, vehicle speed, or response of the wheel anti-lock 
system. 
In accordance with a feature of the invention, the output from the system 
is conducted to a threshold sensing circuit in which the threshold level 
dynamically changes with level of the signal, so that the response of the 
threshold circuit will change with signal amplitude. If the signal 
amplitude increases, the response level of the threshold circuit, 
likewise, is increased, so that the threshold circuit will have a variable 
response level, following changes in amplitude of the signal to which it 
is to respond, so that the threshold level will remain a predetermined 
fraction of a signal level regardless of the amplitude of signal being 
applied thereto. 
The system has the advantage of substantially improved noise signal 
rejection. Inherent resonance effects due to circuitry between the sensor 
or transducer element and the filter, and which may include the filter 
component, can be readily suppressed. 
In accordance with a feature of the invention, the second, selectively 
connectable filter preferably is an R/C circuit element in which the 
resistance portion is selectively connectable, based on operating 
parameters such as vehicle or wheel speed, for example. Shifting the 
threshold level of a threshold detector in accordance with amplitude of 
the signal being applied thereto has the additional advantage that large 
output signals will exceed the threshold level by a predetermined 
percentage, rather than a predetermined fixed level, thus resulting in 
extremely good noise rejection.

DETAILED DESCRIPTION 
A wheel speed sensor 10 is coupled to the wheel of a vehicle. Such sensors 
or transducers, typically, are magnetic transducers which provide sine 
wave output signals upon passage of ferromagnetic elements in front of a 
pick-up coil. Other types of transducers may be used. The output signal of 
the vehicle rotation transducer is used in the automatic anti brake-lock 
control unit (ABS) 13 to derive from the signals from the transducers 
other signals representative of slip, speed, acceleration or deceleration 
of the respective wheel. Further signals can be derived therefrom, for 
example by averaging, and modifying, in accordance with known criteria, 
wheel speed signals in order to derive a vehicle speed signal. The various 
signals are processed--as well known--in the control unit 13 to provide 
output signals representative of control action, and if the control unit 
13 should respond at all. 
Each one of the wheel transducers 10 is connected to a low-pass filter 11, 
as known. The customary inductive-type speed sensors provide output 
signals which increase with increasing wheel speed. The low-pass filter 
11, connected to the transducer 10, dampens frequencies at higher range, 
so that the signal obtained from the filter 11 is essentially linear. The 
low-pass filter 10, however, also suppresses disturbance signals so that 
threshold circuits, customarily included within the control unit 13, will 
not respond. 
Under certain operating conditions, and particularly under the influence of 
out-of-round conditions within the transducer system of which the 
transducer 10 is a part, vibration, and the like, the transducer 10 and 
the low-pass filter 11 may form a resonance system. This is particularly 
so if the sensor or transducer 10 is periodically mechanically disturbed, 
for example due to vibration or other similar periodically recurring 
conditions. Resonance systems, as well known, cause substantial signal 
level increases. Under such conditions, thus, disturbance signals may be 
unduly enhanced, and may cause erroneous response of a threshold circuit 
within the control unit system 13. 
In accordance with a feature of the invention, a high-pass filter 12 is 
connected to the low-pass filter 11, and selectively connectable in 
circuit with the low-pass filter 11 and the control unit 13. Of course, 
the control unit 13 will have similar signals applied thereto from the 
other wheels of the vehicle, as indicated by the broken connecting lines 
with the arrows leading to the control unit 13. 
Switch 14 is provided for selective connection or disconnection of the 
high-pass filter 12. Switch 14 is operated in dependence on the output 
from an OR-gate 15. The OR-gate 15 is controlled by two output lines 16, 
17 connected to the control unit 13. The output line 16 carries a signal 
derived from vehicle speed, and, if the speed drops below a predetermined 
minimum speed, OR-gate 15 is enabled to close switch 14. Line 17 carries a 
signal which is representative of response of the control unit 13, which 
may occur, for example, if one of the wheels of the vehicle is about to 
block, which may lead to skidding, as sensed by the control unit 13. If at 
least one of these two signals is present on lines 16, 17, OR-gate 15 is 
enabled and switch 14 will close. Under those conditions, then, the 
high-pass filter 12 is bridged, so that it will no longer influence signal 
processing from the transducer 10 and the filter 11 in the control unit 
13. Thus, the high-pass filter 12 will be excluded from influencing the 
signal if the speed of the respective wheel, or vehicle speed--in 
dependence on the nature of the signal on the line 16--is below a 
predetermined reference level; or if the ABS unit 13 has responded. The 
reason for bridging the high-pass filter 12 is this: At low vehicle or 
wheel speeds, the output signals from the transducer 10 are low, and no 
additional attenuation by further circuit components of the signal should 
result; further, at low wheel or vehicle speeds, disturbance signals with 
relevant amplitude at the relevant frequency are not expected. Further, 
such disturbance signals usually do not occur during the time that the 
control unit of the anti-skid or anti brake-lock system has responded; any 
disturbance signals which occur during response of the control unit 13, 
can be suppressed by signal processing within the control unit, as well 
known. 
FIG. 1 illustrates the simplest case in which the high-pass filter 12 is 
merely bridged or shunted by the switch 14. Other switching arrangements 
may be used and, of course, the switch 14 may be replaced by an electronic 
switch, such as a controlled semiconductor. Further, of course, the 
shunting circuit formed by switch 14 need not be of the ON/OFF type; 
rather, the effectiveness of the filter 12 can be decreased with decrease 
of vehicle or wheel speed, for example by attenuating the effect of the 
filter 12 by including in the parallel circuit a variable resistor which, 
in a limiting case, forms a continuous conductor, such as a transistor 
which provides a shunting path to the filter 12 of variable resistance, 
changing in dependence on the level of a control signal applied through an 
analog OR-gate 15 between a high or essentially blocked value, 
intermediate levels, to an essentially zero resistance or entirely 
conductive level. 
FIG. 2 illustrates an embodiment in which the speed transducer 10 is 
connected to a low-pass filter formed by the series circuit of a resistor 
20 and a capacitor 21. This low-pass filter is connected to a high-pass 
filter formed by a capacitor 22 and resistor 23. Switch 14 is provided to 
change the characteristics of the high-pass filter by, selectively, 
connecting a further resistor 24 in parallel to resistor 23 upon closing 
of switch 14. Of course, similar effects can be obtained by switching the 
capacitor 22. Switch 14 is shown only in symbolic representation and, of 
course, can be replaced by an electronic switch of the ON/OFF type, or of 
the gradually increasing resistance type, for example a transistor. 
The output from the filter circuits 20, 21 and 22, 23 , with or without 
connection of resistor 24, provides the filtered utilization signal, which 
is applied to an evaluation circuit 25. The output of the evaluation 
circuit 25 is connected to two threshold circuits K.sub.1, K.sub.2 
connected as comparators. The output from evaluation circuit 25, thus, is 
connected to the direct input of an operational amplifier forming 
comparator K.sub.1 and the inverting input of a second, and preferably 
similar operational amplifier forming comparator K.sub.2. The output 
signal U.sub.S is, additionally, connected to a peak detector 26. The 
output from the peak detector 26 is connected to the inverting input of 
the first comparator K.sub.1 and, further, through an inverter 27 to the 
direct input of the other comparator K.sub.2. The outputs of the 
comparators K.sub.1, K.sub.2 are connected to the SET and RESET inputs of 
a flip-flop FF, respectively, as shown in FIG. 2. 
Operation, with reference to FIG. 3: The filtered sensor voltage U.sub.S, 
derived from the evaluation circuit 25, is shown in the top graph of FIG. 
3. The states of the comparators K.sub.1, K.sub.2 are shown in the next 
subsequent graphs, and the state of the flip-flop FF in the last line of 
the graphs of FIG. 3. 
The circuit including components 26, 27, K.sub.1, K.sub.2 and FF is used to 
provide output signals representative of a threshold which changes with 
increasing signal amplitude. In order to obtain such a changing threshold, 
the peak value of the signal U.sub.S at the output of the evaluation 
circuit 25, as determined by the peak signal circuit 26, is sensed and 
stored for one signal undulation, as clearly seen in FIG. 3, see top graph 
U.sub.S. The output signal of the peak value circuit 26 is shown at 31. 
This signal is connected to a weighting circuit, for example a voltage 
divider, which forms a weighted signal of somewhat smaller or lower value, 
to determine a threshold level which varies with the overall level or peak 
value of the signal being applied thereto, so that the threshold will 
change as a function of the peak value. The weighted signal is shown by 
broken line 32. The weighting of signal 31 to obtain signal 32 can be 
carried out directly within the peak value detector 26 or, separately, by 
suitable adjustment setting or biassing of the comparators K.sub.1, 
K.sub.2. 
The graphs of FIG. 3 show input signals 30 of increasing amplitude, and 
hence an increasing threshold level. Of course, as the signals decrease, 
the threshold level likewise will decrease. 
The comparators K.sub.1, K.sub.2 compare the weighted peak value of U.sub.S 
--see broken line 32--with the instantaneous peak value of the signals, 
see chain-dotted line 31. Referring to FIG. 3: After the first undulation 
or period of U.sub.S, the weighted value, line 32, is applied to 
comparator K.sub.1. At time T.sub.1, the second undulation reaches the 
value of line 32, causing the flip-flop FF to be SET. The flip-flop FF is 
RESET when the negative threshold in the second comparator K.sub.2 is 
reached. This negative threshold, illustrated by a broken-line curve, is 
the inverse of the curve 32. As can be clearly seen, the curve 32, at time 
T.sub.2, is at a greater difference level from zero or null than the curve 
31 was at time T.sub.1. Thus, the RESET time of the thresold T.sub.2 now 
has considered the increase in signal amplitude of the second undulation 
above the peak value 30 of the first undulation. The signal voltage, prior 
to reaching the negative portion of the output signal, has passed through 
a maximum so that, in the time after T.sub.1, the weighted value 32 has 
shifted, thus shifting the response value of the second comparator 
K.sub.2. Thus, with response of the comparator K.sub.2 at time T.sub.2, 
the flip-flop FF is RESET. 
The cycles will repeat between the times T.sub.3 /T.sub.4 and T.sub.5 
/T.sub.6, respectively. As is clearly apparent from the graphs, the 
switching threshold of the comparators K.sub.1, K.sub.2 follows the signal 
voltage 30, and increases with increasing signal voltage level. 
Any interference or disturbance or noise voltages, thus, are increasingly 
suppressed as the signal voltage increases, so that malfunction or 
erroneous response of the control unit 13 is thereby prevented. The output 
signal from the flip-flop FF is connected to the control unit 13, the 
circuit portion between the evaluation circuit 25 and the flip-flop FF 
being, for example, connected just in advance of the control unit 13. 
Various changes and modifications may be made, and features described in 
connection with one of the embodiments may be used with the other, within 
the scope of the inventive concept. 
The control unit 13 is well known in the literature and in industry, and 
could, for example, take the form described in U.S. Pat. No. 3,620,437. 
The evaluation circuit 25 comprises DC-blocking means and potentiometer 
means for adjusting the trigger level of comparators K.sub.1 and K.sub.2.