Patent ID: 7281432

Claim:
A displacement sensor comprising: an excitation coil and a detection coil arranged in a predetermined positional relationship; an amplifier having an input end connected to an output end of the detection coil; a phase shifting circuit provided between an output end of the amplifier and an input end of the excitation coil and configured to provide phase shifting to nullify a phase difference produced between an input waveform inputted to the excitation coil and an output waveform outputted from the detection coil by changing a frequency when the phase difference is produced between the input waveform inputted to the excitation coil and the output waveform outputted from the detection coil; frequency measuring means for detecting a frequency deviation produced by the phase shifting circuit shifting the phase; and a rod-like probe of a magnetic body which is inserted into at least one of air core portions of the excitation coil and the detection coil and a sectional area of which is changed in a longitudinal axis direction for detecting a displacement amount of an object to be measured from the frequency deviation produced by displacing the rod-like probe in the longitudinal axis direction while maintaining a resonating state of a closed loop including a space between the excitation coil and the detection coil.