A Visual-Based Angle Measurement Method Using the Rotating Spot Images: Mathematic Modeling and Experiment Validation

An angle measurement method based on the rotating spot images and machine vision technology is proposed. This method abandons the traditional coding disk but uses image sensor to capture the images of a spot rotating synchronously with the measured shaft. Assuming the center of the rotation shaft is fixed, the mathematic model of the trajectory of the spot images is an ellipse. N spot images are used to fit the ellipse equation, where the eccentric angle of the ellipse is the measured rotation angle. The basic algorithms and a compact lensless transmission angle measurement structure are also proposed. The simulation and experiment results are verified the feasibility of the measurement method, where the 5.4” accuracy can be obtained and a random collection of N ≥ 36 spot images is recommended. Since the shape of the ellipse trajectory does not affect the measurement accuracy, this method allows the spot disk to be non-concentric with the rotating shaft, and allows the spot disk to be non-parallel to the image sensor. Therefore, the proposed method can achieve precision measurement under non-precision installation.

I. Introduction

Rotary optical encoders are the most widely used sensors for precision angular positioning in modern production engineering [1]. More than 90% of rotary encoders used in precision machine tools are estimated to be absolute type. The current rapid development of emerging areas, such as small intelligent robots, intelligent manufacturing, 3D printing, and drones, poses considerable challenges for high precision, miniaturized, and inexpensive absolute encoders [2], [3].