Patent ID: 8173951

Claim:
An optical encoder including a scale having an incremental track that has a reflective phase grating formed along length measurement direction, and a detection portion that is displaced relatively to the main scale, comprising: a reference mark secured on the scale and composed of a reflection slit formed on at least one point of the incremental track; a pair of reference signal light receiving portions having a light irradiation portion that irradiates light to the scale, a main light receiving portion that is disposed at the surrounding centering around the light irradiation portion, receives interference fringes formed by the light irradiated from the light irradiation portion being diffracted by the main scale, has four light receiving windows, which are related to each other so as to be disposed along the length measurement direction and to have a phase difference of 90 degrees, and so as to be disposed along the direction perpendicular to the length measurement direction and to have a phase difference of 180 degrees, and outputs incremental signals four phases of which are different from each other, and the pair of reference signal light receiving portions being disposed in the direction perpendicular to the length measurement direction using the light irradiation portion as the center of point symmetry and output a reference signal which are secured at the detection portion; and a signal processing portion having a three-phase signal generation portion for generating three-phase signals having a phase difference of 90 degrees from the four incremental signals, quadrature sine wave signals generation portion for generating quadrature sine wave signals having a phase difference of 90 degrees by synthesizing the three-phase signals in terms of vectors, a direction judgment circuit for detecting edges by judging the direction of relative displacement of the detection portion to the main scale, an internal period counter for counting the periods of relative displacement, and a reference signal processing portion for generating a reference signal from the output of the reference signal light receiving portion.