An absolute high performance, self calibrating optical rotary positioning system

The next generation high performance Absolute Rotary Position System with self-calibration capability, incorporating a fast Digital Signal Processing (DSP) algorithm with digital servo control capability, is presented. Critical encoder calibration parameters and their effects on accuracy are discussed with respect to a novel multi-tier Sine/Cosine (S/C) read-out approach. This approach consists of all S/C tracks in place of digital tracks or a combination of coarse digital tracks and a fine S/C track set. The series of analog S/C signal sets are digitized simultaneously by Analog-to-Digital Converters (ADCs). Self-calibration is accomplished for each S/C tier (set of tracks) through the use of DSP algorithms. The DSP algorithms correct for amplitude, DC-offset, and phase errors for each tier of S/C signals. The DSP then computes position and merges the tiers resulting in an unambiguous absolute position word of desired resolution and accuracy. Independent processing of the data from multiple readstations provides digital cancellation of most of the installation and bearing errors. Combination of the self-calibrating encoder and digital servo employing the same time-shared DSP together with the signal processing techniques, described in the paper, have been successfully utilized in digital servo control applications. The resulting benefits in cost, reliability, servo optimization, flexible interface protocol, and the ability to recalibrate the encoder in situ are discussed. The resulting high-resolution position control system can be adapted to numerous applications in aerospace, defense, industrial and commercial markets in a cost-effective manner.