A two-probe linear encoder by using an arrayed scale grating stitched by multiple separate short gratings

Abstract

A line encoder composed of a two-probe reading head an arrayed scale grating stitched by multiple separate gratings was proposed, constructed and evaluated in this research for expanding the measurement range. The scale grating is stitched by multiple separate short gratings fabricated by a holographic lithography that is able to provide sub-micron order grating period. Differing from the conventional stitched grating, in which the gap of the adjacent two gratings should be strictly controlled so that a continuous phase can be ensured, the gap in this research is not necessary to be controlled, neither the gap width nor the phases, by using a two-probe reading head. In contrast to the reading head in conventional linear encoder, the two-probe reading head was designed in such a manner that a collimated laser beam with a diameter of 1mm was divided into two parallels probes (probe A/B) by a specially designed prism lens. These two probes are projected perpendicularly onto the transmission type arrayed scale grating. Taking probe A as an example, after passing the grating, the positive and negative first-order diffraction beams follow a typical grating interferometry principle, including propagating direction changing, co-path by a beam splitter and a polarize for a phase delay module and finally interfere with each other, from which the displacement can be obtained. The two probes propagate along similar optical path. The two probes can ensure as least one can be projected onto the grating. By stitching the two probe outputs suitably, the continuous output of the scale grating can be measured accurately. The jig for stitching the separate gratings was investigated and tested. The two-probe reading head prototype was constructed and evaluated.