Patent ID: 7102759

Claim:
An optical displacement sensor comprising: a coherent light source which emits a light beam; a scale which is movable with respect to the coherent light source along a relative movement direction, and which includes a grating pattern formed on a pattern-formed surface thereof to have a predetermined period in the relative movement direction, wherein the light beam is emitted by the coherent light source to the pattern-formed surface along a path which is substantially perpendicular to the pattern-formed surface; and a photodetector having a light receiving surface which detects movement of a diffraction image of the light beam reflected and diffracted by the grating pattern, said light receiving surface including a plurality of sets of light receiving areas, wherein the light receiving areas in the sets are arranged at intervals of p 2 in the relative movement direction, and the plurality of sets of light receiving, areas are deviated with respect to each other in the relative movement direction by an odd multiple of ¼×p 2 ; wherein p 2= np 1( z 1+ z 2)/ z 1 where: n is a natural number; p 1 is a spatial period of the grating pattern on the scale; z 1 is a distance along a principal axis of the light beam from a beam emitting surface of the coherent light source to the pattern-formed surface of the scale; and z 2 is a distance from the light-receiving surface of the photodetector to the pattern-formed surface of the scale; wherein the coherent light source and the photodetector are arranged on a same side of the scale, and the light receiving surface of the photodetector is substantially parallel to the pattern-formed surface of the scale; and wherein relative movement of the scale with respect to the coherent light source is detected based on an output of the photodetector.