Proximity sensing system and inductance measuring technique

A method for monitoring the proximity of a target object to a sensor unit wherein the sensor unit includes a sensor inductor whose inductance L.sub.S varies in a known relationship with the proximity of the target object, the sensor unit also having a temperature-dependent or otherwise variable equivalent resistance in series with said inductance L.sub.S and a temperature-dependent or otherwise variable equivalent capacitance in shunt with said inductance L.sub.S, comprising the steps of:

• periodically establishing, during each of successive system periods T.sub.S, a current i.sub.S through the sensor unit, and thereby through the sensor inductor, said sensor current having a time-variable waveform;
• and, for each such system period T.sub.S,
• thereafter constraining the voltage v.sub.S across the sensor unit to be constant when said sensor voltage reaches a reference voltage V.sub.L, whereby the effect of the equivalent capacitance is cancelled;
• while so constraining said sensor voltage to be constant, determining the rate of change of said sensor current with respect to time when said sensor current is zero, whereby the effect of the equivalent resistance is cancelled;
• establishing an indication of the inductance L.sub.S when said sensor current is zero in accordance with the relationship ##EQU9## and, providing a target proximity indication representative of the proximity of the target object to the sensor unit in accordance with a known relationship between the proximity of the target object and the inductance L.sub.S.

An inductance measuring apparatus for determining the inductance L of a network, wherein the network includes an equivalent resistance in series with the inductance L and an equivalent capacitance in shunt with the inductance L, said apparatus comprising:

• current means for establishing a current i through the network, said network current having a time-variable waveform;
• control loop means for thereafter constraining the voltage across said network to be constant when said network voltage reaches a reference voltage V.sub.L whereby the effect of the equivalent capacitance is cancelled;
• retrieval means for determining, while said network voltage is so constrained to be constant, the rate of change of said network current with respect to time when said network current is zero, whereby the effect of the equivalent resistance is cancelled;
• and,
• measuring means for determining the inductance L in accordance with the relationship ##EQU11##

A proximity sensing system for monitoring the proximity of a target object to a sensor unit wherein the sensor unit includes a sensor inductor whose inductance L.sub.S varies in a known relationship with the proximity of the target object, the sensor unit also having a temperature-dependent or otherwise variable equivalent resistance in series with said inductance L.sub.S and a temperature-dependent or otherwise variable and equivalent capacitance in shunt with said inductance L.sub.S, the proximity sensing system comprising:

• current means for periodically establishing, during each of successive system periods T.sub.S, a current i.sub.S through the sensor unit and, thereby, through the sensor inductor, said sensor current having a time-variable waveform;
• control loop means for thereafter constraining the voltage v.sub.S across said sensor unit to be constant when said sensor voltage reaches a reference voltage V.sub.L, whereby the effect of the equivalent capacitance is cancelled;
• retrieval means for determining while said sensor voltage is so constrained to be constant, the rate of change of said sensor current with respect to time when said sensor current is zero (whereby the effect of the equivalent resistance is cancelled) so as to establish an indication of the inductance L.sub.S in accordance with the relationship ##EQU13## and, proximity determination means responsive to said inductance indication to provide a target proximity signal representative of the proximity of the target object to the sensor unit in accordance with the known relationship between the proximity of the target object and the inductance L.sub.S.