High-precision Absolute Photoelectric Shaft Encoder Quality Optical Signal Extraction Method

The absolute rotary optical encoder has its own characteristics compared with the incremental encoder with regard to the methods of extracting high quality grating photoelectric signals. However, it is usually not convenient to apply the grating signal spatial filtering methods used in the incremental encoders to the absolute rotary encoders. It is thus necessary to carry out a dedicated study on the methods of extracting high quality grating photoelectric signals for the absolute encoders. In this study, a novel comprehensive method of extracting high-quality grating photoelectric signals in the absolute rotary encoders is developed. It uses combined stop index gratings for spatial filtering of grating signals to enhance their sine property. It uses phase vector correction of grating signals to improve their orthogonality. It also utilizes some known research results on improving the quality of grating photoelectric signals, such as multi-line averaging, signal subtraction processing, multi-reading head addition and averaging, etc. According to Fourier's optics theory of correlation function and power spectrum, and based on the discussion on the correlation function of the transmittance function of two the line and space gratings, the power spectrum of Moir'e fringes and the frequency spectrum of the output photoelectric signals of the transmission-type double-grating optical reading system are analysed with the conclusion that the frequency spectrum of the photoelectric signal is equivalent to the product of the frequency spectra of two gratings (i.e. principal and indicating gratings) and the frequency spectrum of the receiving stops. Based on the power spectra analysis of transmittance function and correlation function of grating, a new spatial filtering method is proposed, which has a novel design of combined stop index grating for carrying out spatial filtering for fine grating signals. A mathematical model for this novel combined stop index grating is established. This new method of spatial filtering for fine code grating signals has been implemented on absolute rotary encoders first time in China. A combined cosine stop index grating and an innovative combined "convolution" stop index grating have been developed and demonstrated in real engineering applications. GBJ420 high-precision absolute rotary optical encoder used in AS-1 beam directional control system in national high-technology Project 806-2 that adopts this novel fine processing technology of grating photoelectric signals has produced high quality photoelectric signals. The primary four technical specifications of the signals are measured as follows: the error of sine ≤1.5%, orthogonality ( error of phase difference ) ≤1.5°, constant amplitude property ≤1.5% , and residual d.c level ≤2.5%. The angle-measuring accuracy of the encoder is (), and its error of electronic subdivision is ()。...