Circuit for an electromagnetic rotation sensor

Abstract: The circuit includes a threshold comparator (7) which compares the signal (V i ) provided by the sensor (1-4) with that provided by a threshold-­ signal generator (8). The threshold-signal generator circuit (B) comprises a peak-detector circuit (10) connected to the sensor (1-4), a capacitor (11) connected between the output of the peak-detector circuit (10) and the threshold signal input of the comparator (7), and a reset circuit (12,14) controlled by the output of the threshold comparator (7) so as to cause the capacitor (11) to discharge every time the signal provided by the sensor (1-4) exceeds the threshold signal.

Description: The present invention relates to a circuit for processing a signal generated by a variable-reluctance electromagnetic rotation sensor of the kind defined in the preamble of Claim 1. A variable-reluctance rotation sensor typically comprises a rotor which has a series of teeth or notches in suitable positions, and with which an inductive pick-up is associated. When the rotor is rotated, each time one of its teeth passes in front of the pick-up, the latter outputs a signal whose amplitude is first of all positive and then passes through zero and becomes negative. Sensors of this type have various applications, in particular in the automotive field, for example for detecting the rate of rotation of a drive shaft or for detecting the instant at which a piston of the engine passes through top or bottom dead centre. In these applications, it is important to be able to determine correctly the instant at which the signal provided by the sensor passes through zero. A circuit of the above-specified type is described in EP-A-0 117 789. This prior signal processing circuit comprises a first comparator for detecting the zero-crossings in the input signal incoming from the sensor, and an inhibiting circuit including a second comparator which compares the amplitude of the incoming signal with a predetermined fraction of the peak value of said signal, previously detected and memorized in a storage capacitor. When the amplitude of an incoming signal does not reach the threshold thus defined, the inhibiting circuit disables the first comparator. The latter is thus prevented from providing an output signal at the zero-crossing of a possible low-amplitude parasitic signal superimposed onto the true sensor signal. The object of the present invention is to produce an improved circuit for processing the signal generated by a sensor of the aforesaid type. According to the invention, this object is achieved by means of a circuit having the features defined in Claim 1. US-A-4258324 discloses a signal conditioning circuit for the output of a magnetic position sensor. The known circuit uses a single comparator having hysteresis and discriminating between the noise component and the signal component by comparing the total signal to a variable threshold only after a positive going zero cross. The variable threshold is generated as a function of the peak amplitude of the total signal. In one embodiment of the invention the storage means comprise a capacitor and the reset circuit means include a switch connected to the capacitor and controlled by a monostable circuit which is controlled by the output of the threshold-comparator circuit. Further characteristics and advantages of the circuit according to the 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: Figure 1 shows a circuit according to the invention, and Figure 2 is a series of graphs showing the changes in the signal provided by a variable-reluctance rotation sensor and the corresponding signals generated in the circuit according to the invention as functions of time given on the abscissa. In Figure 1, a variable-reluctance rotation sensor is generally indicated 1 and comprises a rotor 2 having a plurality of teeth 3 in suitably selected angular positions. An inductive pick-up, indicated 4, is associated with the rotor 2. When the rotor 2 is rotated, each time one of its teeth 3 passes in front of the pick-up 4, the latter outputs a voltage signal with an amplitude which is initially positive and subsequently becomes negative, after passing through zero. In Figure 2, V i indicates an example of the changes in the signal output by the inductive pick-up 4, with reference to the passage of three teeth of the rotor 2 in front of the pick-up, as a function of time t. The output of the inductive pick-up 4 is connected through a resistor 5 to the input of a clamp circuit 6, formed in known manner, which "cuts off" the negative portions of the signal V i and compresses the dynamic range of the signal. The output of the clamp circuit is connected to a first input of a threshold comparator 7 with hysteresis and to the input of a threshold-signal generator circuit, generally indicated 8. The output of the latter circuit is connected to the second input of the threshold comparator 7. The threshold-signal generator circuit includes a sampling and storage circuit generally indicated 9, including a peak-detector 10 whose input is connected to the output of the clamp circuit 6. A storage capacitor 11 is situated between the output of the peak detector 10 and earth and a switching device 12 is connected in parallel therewith. A voltage divider, indicated 13, is interposed between the output of the sampling and storage circuit 9 and the threshold comparator 7. The output of the comparator is connected to the control input of a monostable cicuit 14 whose output controls the switching device 12 arranged in parallel with the storage capacitor 11. When the clamping circuit 6 receives the input signal V i of Figure 2, it outputs, correspondingly, a signal V e which has a peak in correspondence with each of the positive peaks of the signal V i . In Figure 2, the amplitudes of the three illustrated peaks of the signal V e are indicated V ep1 , V ep2 and V ep3 . When the signal V e applied to the peak detector 10 has a peak of amplitude V ep1 , the voltage across the capacitor 11 is brought to, and remains (as long as the switch 12 is kept in the non-conductive condition) at the level V ep1 , as indicated by the wave form V c in Figure 2. The voltage V c is applied to the input of the voltage divider which reduces it, for example by dividing it by 2. The voltage divider therefore applies a threshold voltage V s to the comparator 7 the trace of which is indicated in chain line in Figure 2. Thus, as long as V c remains at the level V ep1 , the threshold signal V s assumes the value indicated V s1 in Figure 2. As soon as the signal V e applied to the comparator 7 exceeds the threshold signal V s , as indicated at the time t₁ in Figure 2, the signal V₀ output by the comparator 7 changes level, for example from a high level to a low level. This signal activates the monostable circuit 14 which switches the switch 12 to the conductive condition for a predetermined period of time T, for example 5-10 µs. The capacitor 11 consequently discharges extremely rapidly through the switch 12 during this time interval, as shown in Figure 2. When the characteristic time period T of the monostable 14 has elapsed, the switch 12 returns to non-conductive condition and the capacitor 11 is charged to the new peak voltage V ep2 of the signal V e . The threshold signal V s is brought correspondingly rapidly to the value indicated V s2 in Figure 2. As soon as the signal V i output by the sensor passes through zero, at the instant indicated t₂ in Figure 2, the output of the comparator 7 returns to the high level again, as shown in the Figure. When the signal V e again exceeds the value of the threshold signal V s2 (at the time indicated t₃ in Figure 2) the output of the comparator 7 returns to the low level, again activating the monostable circuit 14 as indicated by the wave form V m in Figure 2 and switching the switch 12 to the closed condition for the time period T characteristic of the monostable circuit. The capacitor 11 is again discharged almost instantaneously and then starts to charge again as soon as the signal of the monostable 14 has disappeared. The voltage in the capacitor then returns to the new peak value V ep3 of the signal V e and the threshold signal is brought correspondingly to the level V s3 . The output of the comparator 7 returns to the high level at the moment t₄ at which the signal V i passes through zero again. The operation of the circuit then continues in the manner described above.

Claims: A circuit (5-14) for processing the signal (V i ) generated by a variable-reluctance electromagnetic rotation sensor (1-4), the sign of the amplitude of said signal (V i ) changing from positive to negative and from negative to positive passing through zero; the circuit (5-14) comprising: a threshold signal generator circuit (8) including a peak detector circuit (10) which is intended to be connected to the sensor (1-4), and resettable storage means (11) connected to the output of the peak-detector circuit (10), for generating a variable threshold signal (V S ) whose level varies in accordance with the peak values stored in said storage means (11); threshold comparator means (7) connected to the output of the threshold signal generator circuit (8) and having a signal input for connection to the sensor (1-4), for providing an output signal (V₀) which changes from a first level to a second level when the signal (V i ) generated by the sensor (1-4) passes through zero changing from a positive amplitude to a negative amplitude; and reset circuit means (12, 14) controlled by the threshold comparator means (7), for resetting the storage means (11) almost instantaneously each time the signal (V i ) provided by the sensor (1-4) exceeds the threshold signal (V S ); characterised in that said threshold comparator means comprise a single threshold comparator (7) having a threshold input for receiving said variable threshold signal (V s ) and having a hysteresis which is variable in dependence on said threshold signal (V s ) whereby said single threshold comparator (7) is arranged to provide an output signal (V o ) which changes from said second level to said first level when the signal (V i ) generated by the sensor (1-4) exceeds said variable threshold signal (V s ); said storage means (11) being connected to the threshold input of said threshold comparator (7). A circuit according to Claim 1, characterised in that the storage means comprise a capacitor (11) and the reset circuit means include a switch (12) connected to the capacitor (11) and controlled by a monostable circuit (14) controlled by the output of the threshold comparator (7). A circuit according to Claim 1 or Claim 2, characterised in that it further includes a clamp circuit (6) to be interposed between the sensor (1-4) and the inputs of the peak detector (10) and the threshold comparator (7); the clamp circuit (6) being adapted to remove respectively the negative or positive portion of the signal (V i ) of the sensor (1-4) and to compress the dynamic range of the sensor signal (V i ).

Inventors: LASAGNA DIEGO, SEMERARO FRANCESCO, LASAGNA, DIEGO, SEMERARO, FRANCESCO
Applicants: MAGNETI MARELLI SPA, MAGNETI MARELLI S.p.A.
Application year: 1989
Main classification: G01R19
IPC codes: H03K5, G01D5, G01B7, G01R19