Design of a Three-Axis Surface Encoder with a Blue-Ray Laser Diode

A compact optical three-axis surface encoder, which can detect displacements in the XYZ-directions simultaneously by employing a scale grating with a short pitch (0.57 μm) and a single laser beam with a short wavelength (405 nm), is described in this paper. This surface encoder mainly consists of a blue-ray laser diode, a pair of two-dimensional diffractive gratings serving as scale grating and reference grating, respectively, and two quarter photodiodes (QPD) utilized for detecting output optical density. The displacements of the scale grating along X-, Y- and Z-directions are measured by analyzing interference signals caused by the phase shifts and light path difference of diffracted beams from the scale grating and reference grating. Basic measurement principle is illuminated, a compact surface encoder is designed and evaluation experiments are carried out.