Patent ID: 7007554

Claim:
A displacement sensor comprising: a light emitting element configured to make light incident on an object having a displacement to be measured; a light receiving element configured to detect a reflected wave from the object having a displacement to be measured; an amplifier having an input connected to an output of the light receiving element; a phase shifting circuit provided between an output of the amplifier and an input of the light emitting element, the light emitting element, the light receiving element, the amplifier, and the phase shifting circuit together forming a closed-loop resonance circuit, the phase shifting circuit being configured to nullify a phase difference by deviating a frequency being output by the phase shifting circuit when the phase difference is produced between an input waveform input to the light emitting element and an output waveform output from the light receiving element in responsive to displacement of the object; and frequency measuring means for detecting the frequency deviation produced the phase shifting circuit as a result of the nullification of the phase difference.