Electricity: measuring and testing – Magnetic – Displacement
Patent
1992-05-11
1995-07-04
Snow, Walter E.
Electricity: measuring and testing
Magnetic
Displacement
32420725, 327 79, G01R 19175, H03K 5153, G01B 714
Patent
active
054303702
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a signal conditioning circuit intended for use with an inductive or a variable reluctance sensor as used to determine ignition timing in motor vehicles.
1. Background of the Invention
In electronic ignition systems, a transducer is required to provide an indication of the current position of the engine crankshaft. For this purpose, it is known to use a sensor comprising a wheel of soft iron which has a number of regularly spaced teeth along its periphery and forms part of a magnetic circuit. The wheel is mounted for rotation with the crankshaft and as it rotates voltages are induced in coils of the sensor on account of the changing magnetic field caused by the varying reluctance in the magnetic circuit of the sensor. This generates a generally sinusoidal voltage at the output of the sensor. one of the teeth is omitted to enable the top dead centre position of the crankshaft to be determined.
The timing information is normally required by a digital control circuit and the output signal of the sensor must be processed to generate pulses occurring at top dead centre and at regular crankshaft angles. The processing is effected by a circuit for detecting zero crossings but problems are created for this circuit by the significant dynamic range of the sensor output signal. By the nature of an inductive sensor, the amplitude of the induced signal is speed dependent and varies from less than 1 volt when the engine is being cranked to more than 30 volts when the engine is running at its maximum speed.
A particular problem occurs at low cranking speeds in that in addition to the signal being weak and the signal to noise ratio being poor, the processing circuit has to contend with the fact that the engine speed tends to fluctuate rapidly making it impossible to carry out any form of time filtering or cycle to cycle correlation. The combined effect of all these difficulties can sometimes prevent synchronisation of the sparks and inhibit starting of the engine.
A zero crossing detector normally has hysteresis built into it in order to prevent undesired dithering on switching over of its output signal. This hysteresis is created by setting a higher threshold for positive going transitions than for negative going transitions. The reliability of detection of zero crossings is increased by widening the hysteresis band but this can only be done within the limits permitted by the overall signal amplitude. Thus at maximum engine speed, one may wish to set a hysteresis band greater than the peak to peak amplitude of the output signal of the sensor at cranking speeds.
2. Description of Prior Art
To mitigate this problem, it has already been proposed in U.S. Pat. No. 4,866,298 and in GB 2,116,325 to set a reference signal in the zero crossing detector which is linked to the maximum amplitude of the sensor output signal. However, this does not suffice, of itself, to achieve reliable synchronisation under all conditions.
Systems are described in GB 1,549,204 and U.S. Pat. No. 4,184,203 which use an inductive sensor and regulate the sensor output to produce a signal of substantially constant amplitude, however, these systems are not intended nor capable of identifying a missing tooth in a toothed wheel.
It has been proposed in GB 2,116,325 to generate a reference voltage which itself varies as a function of engine speed however this results in a system of considerable complexity.
OBJECT OF THE INVENTION
The invention seeks to provide a signal conditioning circuit for detecting zero crossings in the output of an inductive sensor which is or simple and reliable construction yet makes allowance for the dynamic range caused by the speed dependence of the output signal of the inductive sensor.
SUMMARY OF THE INVENTION
According to the present invention there is provided a signal conditioning circuit for detecting zero crossings in the output of an inductive or variable reluctance sensor, which comprises means for converting the output signal of the sensor into a signal having
REFERENCES:
patent: 4466022 (1984-08-01), Hill
Abolins Peter
Ford Motor Company
May Roger L.
Snow Walter E.
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