Patent ID: 6577401
Filing Date: 2003-06-10
Classification: G01D

Abstract:
A displacement measuring apparatus for detecting a displacement of a measured object by using a scale that moves with the movement of the measured object, the displacement measuring apparatus comprising:a coherent light source; a diffraction grating that is marked with lines in a direction perpendicular to a movement direction of the scale and that splits a light beam emitted from the light source into two light beams of a non-diffracted light and a diffracted light; a liquid crystal device that is disposed on an optical path of one of the two light beams, and that modulates the phase of the light beam between areas on a cross-sectional plane perpendicular to a proceeding direction of the light beam, thereby to obtain a desired phase difference in an interference signal between this light beam and the other light beam on the other optical path; reflection mirrors that make the two light beams incident again into an incident point of the diffraction grating; and a photoelectric conversion element that converts an interference light between the two light beams recombined by the diffraction grating into a plurality of electric signals corresponding to phase modulation areas of the liquid crystal device, wherein the light source and the photoelectric conversion element are provided on the same one side of two sides sandwiching the diffraction grating and the reflection mirrors and the liquid crystal device are provided on the same other side sandwiching the diffraction grating, that is opposite to the side of the light source and the photoelectric conversion element, and the displacement of the diffraction grating is measured based on a plurality of output signals from the photoelectric conversion element; said apparatus further comprising: a branching circuit that splits the plurality of output signals from the photoelectric conversion element; a phase difference detecting circuit that detects a phase difference between the split output signals; a corrected-signal generating circuit that calculates a corrected signal to make the phase difference between the output signals to &pgr;/2 (rad), and outputs this corrected signal; and a liquid crystal device driving circuit that converts the corrected signal into a voltage signal for correcting a phase of the liquid crystal device, wherein the phase difference between the output signals is always maintained accurately at &pgr;/2 (rad).