Research Of Velocity Measurement For Photoelectric Encoder Based On Moiré Fringe

Photoelectric encoder is a kind of angular displacement sensor by using grating moiré fringe. By utilizing the relative displacement of the grating, and by adopting optic and electronic subdivision, the encoder can translate displacement information into corresponding digital signal output. They can accomplish tasks like high accuracy measurement of rotation angle, speed, and acceleration. They have been widely used in fields such as industrial control, automatic detection and precision measurement, due to its advantages of high accuracy, small volume, anti-jamming,long life, etc.In high precision measurement control system, the speed feedback of the photoelectric encoder is extremely important, thus acquiring accurate speed information of the photoelectric encoder is of great significance. In order to get the real-time speed of photoelectric encoder more accurately, this paper analyzed and studied the measurement methods based on moiré fringe signal. By consulting a large number of papers on speed measurement home and abroad, a speed measurement method based on moiré fringe is proposed on the premise of understanding their merits and flaws. After the analysis of the principle and theory of the generating of moiré fringe photoelectric signal, and its error types, the impacts of moiré fringe photoelectric signal error to the accuracy of speed measurement isdeduced.A total differential speed measuring method based on moiré fringe signal of photoelectric encoder is proposed, and the impacts that influence the accuracy of speed measurement is analyzed. Starting from the perspective of the moiré fringe photoelectric signal, joined with the calibration algorithm, the speed of the photoelectric encoder is obtained by adopting total differential operation. In the process of speed calculation, the angle output is not essential, which has effectively reduced the accuracy loss of speed measurement caused by angle measurement error.The angle and speed information can be obtained simultaneously, so that the time delay of the system is reduced.Another speed measuring method of single moiré fringe based on Hilbert transform is proposed. The generation process of moiré fringe is studied, and the principle and merits of adopting Hilbert-Huang transform for the instantaneous frequency extraction are analyzed. The relation between the frequency and speed of the moiré fringe signal of photoelectric encoder is deduced.By Hilbert-Huang transform of the moiré fringe signal, the speed value of the photoelectric encoder is extracted. The method proposed can completely avoid speed measurement error caused by Orthogonal and continuous error and the impact of dc component of the moiré fringe signal, which has effectively improved the velocity accuracy.The two speed measurement methods are applied to some 21 bit photoelectric encoder produced by Chinese Academy of Sciences. By adopting total differential speed measuring method, under condition of lower speed delay, the root mean square error of the speed measurement is reduced from 0.0367 rad/s of M method to0.0216 rad/s; by adopting single moiré fringe speed measurement method based on Hilbert transform, under the condition of a slight increase of measurement delay, the root mean square error is reduced from 0.0367 rad/s to 0.0134 rad/s of M method.The two moiré fringe speed measurement methods proposed in this paper is capable of improving the accuracy and stability of speed measurement of photoelectricencoders.