Optical Encoder Having Pitch-Modulated Photodiode Array

In this paper, a novel optical encoder having pitch-modulated photodiode array as an index scale has been proposed for integrating the gratings with the detectors. The pitch-modulated photodiode array, which obtains four phase-shifted signals simultaneously, is specially designed to reduce higher harmonics distortions of displacement signal and then fabricated by using integrated circuit process technology. In a preliminary study, the effect of the sensitivity distribution of the photodiode on the measurement has been evaluated. In the encoder experiment, it is shown that the 3rd and 5th order distortions for the pitch-modulated photodiode array are reduced by approximately a factor of more than 10, and are almost independent of the air gap between the main scale and the index scale. The proposed encoder is effective for obtaining precise positioning.