Abstract
The light intensity diffracted from superimposed dual transmission gratings having uniform pitch is sensitive to their relative lateral displacement. In the case that change of light intensity is used for measurement of lateral displacement, the intensity change has some distortion (deviation from sinusoidal variation) and is affected by changes in the air gap between two gratings. This paper shows that an index grating with modulated pitch can be used to reduce signal distortion. The index grating does not have uniform pitch lines but has periodical patterns of UN-uniform pitch lines. Experiment shows that the signal distortion is reduced and is almost independent of the gap. This method can reduce interpolation error under ±0.125% of grating pitch, and is effective for an encoder in precision machining.
