Circuit for an electromagnetic rotation sensor

Abstract: The circuit comprises a first threshold comparator (2,3) with hysteresis which is arranged to provide a reference signal (V c ) indicative of the zero-crossing of the sensor signal (V p ). Also connected to the sensor (P) is an integrator circuit (4) whose output is connected to the input of a second threshold comparator (7). The latter provides an inabling output signal (V e ) only when the integrated signal (V i ) exceeds a reference signal (V₂) whose amplitude varies in a predetermined manner in dependance on the amplitude of the integrated signal (V i ). An inabling circuit (14) is connected to the two threshold comparators (2,3;7) and outputs the signal (V c ) generated by the first comparator (2,3) only when the second comparator (7) emits an enabling output signal (V e ).

Description: The present invention relates to a circuit for processing the signal generated by a variable-reluctance electromagnetic rotation sensor of the kind defined in the first part of Claim 1. Variable-reluctance electromagnetic rotation sensors of the so-called phonic wheel type have various applications, particularly in the automotive field, for example, for the detection of the rate of rotation of the drive shaft or for detecting the moments at which pistons of the engine pass through top or bottom dead centre. These sensors typically each comprise a toothed rotor which is rotated by a rotary member, and an associated inductive pick-up. The detection of the passage of the rotary member with which the rotor is associated through predetermined angular positions is detected by the comparison of the signal of the pick-up with the reference signals and/or the detection of the zero-crossing of the pick-up signal. The signal typically provided by an electromagnetic rotation sensor of the type described above has an amplitude which varies greatly in dependence on the rate of rotation of the toothed rotor and in general is rather "dirty" in that its amplitude can be altered significantly as a result of pulsed interference picked up. The information obtainable by the comparison of the amplitude of this signal with reference levels can consequently be erroneous and is therefore in general not very reliable. A circuit according to the first part of Claim 1 is disclosed in EP-A-0 166 699. That prior circuit includes a threshold comparator which compares the amplitude of the integrated signal with a constant-level threshold signal; only when the integrated signal exceeds the constant threshold signal this comparator provides an enabling signal to a logic gate which, in this case, allows the signal indicative of the zero-crossing of the signal generated by the sensor to proceed. In a similar prior circuit disclosed in US-A-3 944 936 a threshold detector compares the amplitude of the integrated signal with a constant threshold signal generated by a bias control internal to said detector, and only when the integrated signal exceeds said constant threshold the said detector provides an enabling signal to a logic circuit which allows the signal indicative of a zero-crossing of the signal generated by the sensor to be validated. A circuit for processing the signal provided by a differential electromagnetic rotation sensor is disclosed in EP-A-0 117 789. This processing circuit includes a comparator for detecting the zero-crossing of the signal provided by the sensor, and an inhibiting circuit which inhibits said comparator when the amplitude of a half-wave of the sensor signal does not reach a threshold corresponding to a predetermined fraction of the peak amplitude of the preceding half-wave. The inhibiting circuit comprises a threshold signal generating circuit including a peak detector, storage means coupled to the output thereof and a control circuit for resetting the storage means each time the sensor signal exceeds said threshold provided the output of the comparator is in a predetermined state. Tests and measurements carried out by the inventors, however, have ascertained that, for an identical angular position of the shaft with which the sensor is associated, the integral of the sensor signal has an amplitude which varies as the rate of revolution of the shaft varies in a unit of time. This amplitude is therefore not a characteristic which is invariable with variations in the rate of rotation of the rotor of the sensor. Starting from the experimental finding, the object of the present invention is to provide a circuit of the above-mentioned type which has even more reliable and precise operation than those of the prior art. This object is achieved, according to the invention, by means of a circuit of the type specified above having the features defined in Claim 1. Embodiments of the invention are defined in dependent Claims 2 and 3. Further characteristics and advantages of the present invention will become clear from the detailed description which follows with reference to the appended drawings, provided by way of non-limiting example, in which the block diagram of a circuit according to the invention is shown. In the appended drawing, a variable-reluctance electromagnetic rotation sensor of the phonic wheel type is generally indicated 1. In general, this comprises a toothed rotor W with which an inductive pick-up, indicated P, is associated. The inductive pick-up P is connected to the input of a first threshold comparator with hysteresis, indicated 2. This comparator is arranged so that its output changes state (for example, changing from high level to low level) when the signal V, provided by the pick-up exceeds the level of a threshold signal V₁ provided by a generator 3. The level of the threshold signal V₁ is conveniently a function of the amplitude of the signal V p . The comparator 2 changes state again when the signal V p falls below zero. In this case, each leading edge of the signal V c provided at the output of the comparator 2 is indicative of a zero-crossing of the signal V p . The pick-up P is also connected to the input of an integrator circuit, generally indicated 4 and produced, for example, with the use of an operational amplifier 5 and a feedback capacitor 6. The signal V i provided at the output of the circuit 4 corresponds to the integral of the signal V p provided by the pick-up, and is applied to the input of the second threshold comparator 7, as well as to a sampling and storage circuit, generally indicated 8. The latter circuit includes a peak-detector 9 to whose input the signal V i is applied, and a storage capacitor 10, connected to the output of the peak detector. A controlled switch, indicated 11, is arranged in parallel with the capacitor 10 and is constituted, for example, by a transistor. The switch is controlled by the output of a monostable circuit 12 whose input is connected to the output of the threshold comparator 7. In the embodiment illustrated, the capacitor 10 and the switch 11 are connected between the output of the peak detector and earth. However, the circuit 8 may be produced in an equivalent form, in which the capacitor and the associated switch are connected between the output of the peak detector and the non-earthed pole of the supply voltage. The output of the sampling and storage circuit 8 is connected to a divider circuit 13 which is arranged to divide the voltage of the signal applied to its input by a predetermined factor, for example by a factor of 2. The output of the voltage divider 13 is connected to a threshold comparator circuit 7. The outputs of the threshold comparators 2 and 7 are connected to two inputs of a logic circuit indicated 14. In operation, when a tooth of the rotor W passes in front of the pick-up P, the latter emits a signal whose amplitude is initially positive and then suddenly changes its sign, passing through zero. The signal is integrated by the circuit 4 and the resulting signal V i is applied to the input of the peak detector 9. In this situation, the switch 11 is in its non-conducting condition or state, so the capacitor 10 is charged at the peak voltage reached by the integrated signal V i . The threshold signal V₂ supplied to the comparator 7 by the voltage divider 13 therefore has an amplitude which is proportional to the peak value reached by the integral of the signal provided by the pick-up P. As soon as the signal V i exceeds the level of the signal V₂, the comparator 7 provides an output signal which enables the logic circuit 14 to output the signal V c . The signal output by the threshold comparator 7 also activates the monostable circuit 12 which causes the switch 11 to switch to the conduction condition or state for a predetermined period of time, causing the rapid discharge of the capacitor 10. When this very short period of time has elapsed, the switch 11 returns to the non-conducting condition or state so that the capacitor 10 is ready to store the new peak value reached by the integrated signal V i . It is clear from the above explanation that the amplitude of the threshold V₂ is variable in dependence on the amplitude of the signal V i emitted by the integrator circuit 4. The operation of the circuit described as a whole can thus be made practically insensitive to variations in the amplitude of the integrated signal (V i ) which in practice may not be negligible.

Claims: A circuit for processing the signal (V p ) generated by a variable-reluctance electromagnetic rotation sensor (1, W, P) comprising: first threshold comparator means (2, 3) for comparing the signal (V p ) provided by the sensor (1, W, P) with a first threshold signal (V₁) and providing an output reference signal (V c ) indicative of the zero-crossing of the signal (V p ) provided by the sensor (1, W, P), an integrator circuit (4) for integrating the signal (V p ) provided by the sensor (1, W, P), second comparator means (7) having one of its inputs connected to the output of the integrator circuit (4) and the other one to the output of a threshold signal generator circuit (8, 13), for providing on its output an enabling signal (V e ) when the signal (V i ) provided by the integrator circuit (4) exceeds a second threshold signal (V₂) provided by the threshold-signal generator circuit (8, 13), and a logic circuit (14) having first and second inputs connected to the outputs of the first and second comparator means (2, 3; 7) respectively and arranged to transmit the output reference signal (V c ) only when the second comparator means (7) provides the enabling signal (V e ), characterised in that said threshold-signal generator circuit (8, 13) is connected to the output of the integrator circuit (4) and is arranged for generating the second threshold signal (V₂) whose variable amplitude is proportional to the peak amplitude of the signal (V₁) emitted by the integrator circuit (4). A circuit according to Claim 1, characterised in that the threshold-signal generator circuit comprises: a peak-detector circuit (9) having its input connected to the output of the integrator circuit (4), storage means (10) connected to the output of the peak-detector circuit (9), and circuit means (11, 12) for resetting the storage means (10),piloted by the second comparator means (7) so as to reset the storage means (10) each time the signal (V i ) provided by the integrator circuit (4) exceeds the second threshold signal (V₂). A circuit according to Claim 2, characterised in that the storage means comprises a capacitor (10) and that the reset means (11, 12) comprises a switch (11) connected to the capacitor (10) and piloted by a monostable circuit (12) controlled by the output of the second comparator means (7).

Inventors: LASAGNA DIEGO, MANDARA FRANCESCO PAOLO, LASAGNA, DIEGO, MANDARA, FRANCESCO PAOLO
Applicants: MAGNETI MARELLI SPA, MAGNETI MARELLI S.p.A.
Application year: 1989
Main classification: G01R19
IPC codes: G01R19, H03K5, G01D5, G01P3