Highly accurate, absolute optical encoder using a hybrid-positioning method

An absolute optical encoder with nanometric positioning repeatability is introduced and investigated. The encoder consists of an improved scale grating and a compact two-probe reading head. The scale grating contains multiple unevenly spaced distance marks that are superimposed onto grating grooves. The two probes, which are located in the sensor head, read the marks and the grating grooves to determine the position and displacement simultaneously. For one of the two probes, a mask with the same code design with the marks on the scale grating generates a pulse signal to locate the position of the marks approximately-with an accuracy of less than about half a grating period (0.5 μm). For the other probe, light beams are configured for grating interferometry to measure displacement with nanometer-scale resolution. In this Letter, high-quality sinusoidal signals of grating interferometry are used to accurately locate the marks on the scale grating, with stable 100-subdivision phase information. The testing results show that the positioning repeatability can reach 10 nm for a motion range of several tens of millimeters.