Absolute rotary encoder system based on optical sensor for angular measurement

Abstract

A goniometer is a device used to measure the angular range of motion of various human joints and muscle groups, such as the elbow, knee or waist. However, the majority of mechanical goniometers are heavy, provide inaccurate measurements and are affected by dust or stains. Electronic and optical fiber-based goniometers demonstrate better capability than their mechanical equivalents; however, due to their higher costs and complicated setup requirements, their usage is not viable. Optical-based encoder patterns were GC-ARE previously proposed and discussed in some studies. Such optical-based encoder patterns achieve the same angular resolution as traditional Gray code encoder patterns with many shortcomings, such as a high error rate and low tolerance, which arise as a result of the encoding method. This paper presents a design for a reliable and non-intrusive phase shift code absolute rotary encoder (PSC-ARE), which is based on an optical sensor, providing a low-cost, high-precision absolute rotary encoder system. An inexpensive optical mouse sensor is used to capture the code image and the phase shift code transforming time-domain data into frequency domain, which is used to decode the corresponding absolute angle. The PSC-ARE proposed in this study shows a performance improvement over conventional designs owing to the reduced interference from dust and other environmental particles and provides lower sensitivity to calibration errors. Furthermore, the proposed design is not only more compact and portable than the traditional goniometer, and it can also provide a longer lifespan as no stepper motor is included. Finally, the proposed design demonstrates high tolerance to “unintentional defocus” effects and noise effects.