Optical encoder using modulated pitch grating (2nd report)

The light intensity diffracted from the superimposed dual transmission grating is sensitive to their relative lateral displacement. However, the light intensity as a function of lateral displacement generally deviates from the sinusoidal variation (signal distortion) and is affected by the air gap between them. In order to reduce the signal distortion, a new optical encoder using modulated pitch grating was proposed at 1st report. This paper presents the theory of the proposed encoder. The index scale has a modulated pitch grating, in which the distances between the grating slits vary a little. A photo-detector is assumed to be fixed on the rear surface of the index scale. The theoretical equation for the displacement signals and the basic design method of the modulated pitch grating are presented. The calculated results agree with experimental results.