Patent ID: 7966158

Claim:
A measurement apparatus configured to measure a position of an object using a plurality of different phase signals obtained based on displacement of the object, the apparatus comprising: a distortion correcting unit configured to generate a plurality of correction signals by correcting the corresponding plurality of different phase signals using a corresponding distortion coefficient which is stored in the apparatus; a square-root of sum of squares arithmetic unit configured to calculate a square-root of sum of squares of the plurality of correction signals; a first normalizing unit configured to generate a plurality of first normalized signals by dividing the corresponding plurality of correction signals by the square-root of sum of squares; a phase arithmetic unit configured to calculate a phase corresponding to the position of the object based on the plurality of first normalized signals; an averaging arithmetic unit configured to average a plurality of the square-root of sum of squares over one cycle of the plurality of correction signals; a second normalizing unit configured to generate a plurality of second normalized signals by dividing the corresponding plurality of correction signals by the averaged square-root of sum of squares; and a distortion coefficient arithmetic unit configured to update the distortion coefficient with respect to the plurality of different phase signals based on the corresponding plurality of second normalized signals, wherein the distortion coefficient arithmetic unit is configured to update the distortion coefficient with respect to one of an amplitude error coefficient, a zero point error coefficient and a phase error coefficient of the plurality of different phase signals based on two signals in corresponding the plurality of second normalized signals that correspond to predetermined two phases calculated by the phase arithmetic unit.