Design and testing of a linear encoder capable of measuring absolute distance

Abstract

A linear encoder that can be used for commercial absolute precision-positioning applications was designed and tested. This prototype consists of an improved single-track grating scale and a compact two-probe reading head. The specified grating scale was fabricated in such way that multiple reference codes can be superimposed physically on incremental grooves. The reading head is based on a two-probe design, where one probe is used to read the continuous incremental grooves by grating interferometry, while the other probe identifies the discrete reference codes by superimposing the codes with a stationary mask. In other words, the encoder enables the simultaneous measurement of both displacement and position of a precision stage. The so constructed linear encoder prototype was tested thoroughly. The period of the incremental grating structure was calibrated before the measurement range was set to 20 mm to include the two reference codes. The experimental results were verified, and an accuracy for the measured incremental displacement of at least 1 μm was achieved with comparison with a laser interferometer used as the reference. Especially the distance between two reference codes was calculated to evaluate the positioning accuracy of the grating system. The positioning accuracy exceeds 0.22 μm according to our statistical analysis of 12 groups of repeated measurements.