DE-OS German 32 34 637 teaches a circuit configuration of the afore-described type, by which the sensor signals are edited for an electronic anti-locking system. The information on the wheel rotating pattern required for the control is obtained with the aid of sensor signals. For this purpose, a toothed disc rotates along with the wheel, with the disc cooperating with a stationary inductive transducer. The output signal of the inductive transducer is available in the form of an a.c. voltage which in frequency and amplitude is proportional to the wheel speed. With the aid of a trigger circuit, the sensor signals are edited, i.e. boosted and converted into a square wave signal or a pulse sequence, the frequency of which corresponds to the speed. The trigger circuit, in addition, contains filters to attenuate noise signals to the extent possible.
The heavy dependence of the amplitude on the speed results in that, at low wheel speeds, the output signals will become so weak that they can only be distinguished from the inevitable noise signals by taking additional steps. Moreover, because the voltage induced in the transducer is greatly dependent on the air gap between the transducer and the toothed disc and, hence, on the operating tolerances, eccentricities of the wheel or the positioning of the wheel, the output signal, for identical wheel speeds, can assume highly different amplitudes. In adverse circumstances, i.e. at a low speed and a large air gap, the output signal may become extremely weak. This also applies if, through costly correcting and adjusting efforts, the mechanical tolerances are kept low.
Another process and circuit configuration for editing the sensor output signals have been described in DE-OS German 35 43 058, which is concerned with an enhanced separation of noise signal and useful signal. For that purpose, two low-pass filters are provided between the sensor output and the trigger circuit, which generate a useful signal on the one hand, and a reference signal on the other hand. The two signals are compared. In response to the difference of the two signals, with the aid of a comparator, a pulse-shaped output signal, viz. the edited sensor signal, is generated. The reference signal, with the aid of a control signal obtained through a matching circuit, dynamically follows the useful signal.
The strong dependence of the amplitude of the output signal of the sensor on the speed and on the air gap between the transducer and the toothed disc does, however, still renders difficult the layout of a trigger circuit of this type. The response threshold, under consideration of the largest air gap permitted in the tolerance range, and under consideration of the lowest speed to which the controller responds, will have to be rather low, such as 100 mV, so that the trigger circuit in such an extreme case will reliably respond. In the event of an air gap which is small by accident or by the tolerance conditions, and a resultant relatively strong noise signal, there will, however, be danger for the trigger threshold to be reached also by the noise signal. The prior known grinding of the tires along the road causing so called "frictional vibrations", the dynamic changes in the air gap as a result of roadway shocks and numerous other causes create noise signal levels exceeding the response threshold designed for the lower limit range.