Development of a novel optical encoder based on two commercial DVD pick-up heads

Abstract

This study develops a novel optical encoder based on two commercially available DVD optical pick-up heads and a single reflective grating with a sinusoidal surface profile. In the proposed configuration, the laser beams output from the two pick-up heads are passed through a beam splitter and an objective lens and are focused on the surface of the grating. By adjusting the distance of the two DVD pick-up heads from the grating and carefully controlling their angles of rotation in the X-Y plane, the distance between the two focus spots on the grating surface is regulated such that it equals one quarter of the grating pitch. When the grating is displaced, the laser spots trace the surface of the grating, causing a shift in the position of the reflected light beams on the four-quadrant photodiodes within the DVD pick-up heads. The resulting quadrature sinusoidal signals output by the pick-up heads are processed to generate two square-wave signals with a 90° phase difference and a triangular waveform, respectively. By counting the zero crossings of the square waves and interpolating the triangular waveform, both the direction and the value of the grating displacement can be derived.