Digital Optoelectrical Pulse Method for Vernier-Type Rotary Encoders

Abstract

This paper proposes the digital optoelectrical pulse (DOP) method for vernier-type rotary encoders in high-resolution applications. A 25-b encoder using the DOP method is demonstrated. In rotary encoders, the low-bit data of an angle measurement are easily generated using a periodic signal. The high-bit data of an absolute angle measurement can be obtained using various methods such as binary code, gray code, pseudorandom code, $M$ code, or the vernier method. The vernier method has a significant advantage over the above methods for small-sized rotary encoders because it requires fewer grating tracks than do other methods. The vernier method uses grating tracks to generate periodic signals rather than digitally coded signals. In the vernier method, periodic signals with slightly different frequencies are used. The high-bit data of an absolute angle measurement can be obtained using the difference of two periodic signals. Complete absolute angle data are obtained by concatenating the high-bit data and the low-bit data. However, the high-bit data and the low-bit data are, in general, not synchronized. As a result, significant intermediate-bit errors occur at the concatenation point. The DOP method is developed in this paper to mitigate these intermediate-bit error problems. A system on a chip (SoC) that uses the proposed method is designed and fabricated using a 0.18- $\mu \text{m}$ CMOS image sensor process. A 25-b absolute encoder is constructed using an SoC that provides absolute angle data without intermediate-bit errors.