Patent ID: 7501811

Claim:
A displacement measuring apparatus of a rotating member comprising: an encoder supported by a portion of a rotating member concentrically, said encoder changing a characteristic of a detected face thereof alternately with respect to circumferential direction; a sensor supported by a portion which is not rotated in a state where a detecting portion thereof is opposed to the detected face, said sensor changing an output signal thereof with respect to the characteristic of the detected face; a filter circuit performing a filtering process on the output signal of the sensor or a processing signal obtained based on the output signal, said filter circuit erasing an error component based on an error with regard to the change in the characteristic of the detected face over a circumferential direction in a variation of the output signal or the processing signal; and a computing unit calculating a displacement amount of the rotating member based on the output signal or the processing signal on which the filtering process is performed by the filter circuit, said computing unit being provided with a function of calculating the relative displacement amount based on a pattern of changing the output signal or the processing signal passing through the filter circuit.