High Accuracy Image-Based Rotary Encoder With Polar Transformation of DIG

Rotary encoders are commonly used in industry for high precision motor control. This article proposes a polar transformation of the digital image grating (DIG) method to realize a simple, accurate, and low-cost rotary encoder. It measures angles with a rather small camera view to get high accuracy angle results and uses DIG strategy to realize full range measurement. The DIG method estimates the rough angle position by code mark recognition and obtains the accurate angle position by precise image matching. First, a circle image pattern is designed and is put on the top of the rotator. Second, images of the circle image pattern are grabbed and transformed to polar coordinate system with polar transformation method. Last, the angle positions are calculated from the transformed images based on the DIG method. Simulations show that the proposed method is capable of measuring rotary angles with MSER below 1 arcsec. In experiments, the angle result precisions are about 1.6 arcsecs (MSER) with a rotary diameter of 60 mm, and the accuracy is ten times higher than the existing monocular methods.