Non-contact proximity sensor using capacitive-inductive oscillator - has beat frequency circuit with regulating loop for compensation against humidity variations

摘要

A non-contact proximity sensor is based upon an oscillator dependent upon capacitive and inductive effects. A regulating loop is included to correct the oscillator for external disturbances due to changes in ambient conditions e.g. humidity. The sensing oscillator (1) output is mixed with a fixed frequency output from another oscillator (4) in a beat frequency detector stage (5). A band pass filter (7) is set to the required beat frequency range and a rectifier stage (8) generates a switch signal for an output stage (9). A regulating loop with reference frequency source (11), regulator (12) and capacitive diode (13) has a long time constant to provide compensation for any change in capacitance of the sensing oscillator stage due to humidity variations.