Nonlinearity compensation and accuracy improvement method for an optical rotary encoder

This paper presents a method for the nonlinearity compensation of an optical rotary encoder. The proposed method is based on the application of 1) a special 4-bit mixed analog-digital circuit used for the generation of a quasi-linear signal, and 2) a two-stage nonlinear ADC which performs linearization and digital conversion of the quasi-linear signal at the same time. The quasi-linear signal is obtained by combining fragments of phase-shifted sinusoidal signals, where each fragment is presented with a 4-bit digital code. In the continuation, the quasi-linear signal is linearized with the two-stage nonlinear ADC of a compact design based on the application of a single flash ADC in both conversion stages. Additionally, the design of the flash ADC is modified so that the number of employed comparators is equal to the resolution of the flash ADC. For instance, by linearizing an optical rotary encoder using the 4-bit mixed analog-digital circuit and the 20-bit two-stage nonlinear ADC containing 10 comparators, the maximal value of the absolute measurement error can be reduced to 3.23·10-5°.