Patent ID: 7282699

Claim:
An optical encoder comprising: a scale having a sector or disk-like shape on which an optical grating including a discontinuous portion is formed, the scale being adapted to rotate about a rotation center; a light source for illuminating the scale with light; a first detection head and a second detection head, which are movable relative to said scale, the detection heads having a plurality of light receiving sensors arranged in correlation with the pitch of said optical grating to receive light diffracted by said optical grating and output encoder signals, the sensors being composed of a plurality of groups each having a same number of sensors; a circuit board on which the first and second detection heads are mounted at different positions such that the first and second detection heads form an angle of approximately 180 degrees with respect to the rotation center of said scale and the direction of the displacement of the detection heads caused by the expansion and contraction of the circuit board is tangential to the sector or disk-like shape; and a control portion that calculates relative positional relationship between said scale and said first and second detection heads based on the encoder signals, wherein said control portion determines an origin position based on a change in the encoder signals as the discontinuous portion of said scale passes one of said first and second detection heads and determines position information by effecting calculative processing on encoder signals output from said first and second detection heads with reference to the origin position.