Research On Precise Displacement Image Measurement Technology Based On Diffraction Grating

Precision displacement measurement technology is mainly used in aerospace field,military field and ultra precision machinery field.The displacement measurement system based on diffraction grating is based on grating spacing.It has the advantages of strong anti-interference and high precision.Now,it has been widely used.The classic grating measurement system mainly uses the detection of sine waves to demodulate the displacement.However,due to its split term optical path,it has a larger volume.There are also problems of large phase edge calculation error and limited subdivision resolution.Therefore,this paper proposes an image-based displacement measurement method based on diffraction grating.In this paper,an image-based displacement measurement optical system and image demodulation algorithm are designed,which can realize image sub-pixel-level positioning and nano-level displacement detection.This paper refers to a large number of domestic and foreign literature.The typical grating measurement system and demodulation method are analyzed.The advantages and disadvantages of each method are summarized.The grating diffraction principle,Doppler effect,optical polarization principle and displacement signal demodulation principle are studied.A precise displacement image demodulation method based on diffraction grating is proposed.The principle of non-local mean(NLM)denoising is studied.According to the characteristics of the Euclidean distance curve of the fringe image,an inverse kernel NLM denoising algorithm is designed.By using MATLAB software,it is compared with NLM denoising algorithm of typical kernel function.Then,the signal-to-noise ratio results are evaluated.The experimental results show that after denoising by the inverse kernel NLM algorithm,the signal-to-noise ratio of the image is 30.1370 d B,which is better than the traditional kernel NLM denoising algorithm.A sub-pixel positioning and demodulation algorithm for gray extreme values is designed.Experiments compare it with image correlation method and optical flow method.The results show that: in the range of [0,2?],the measurement accuracy of the gray scale extreme value method is ?=1.797 pixel.The result is better than the comparison algorithm.Furthermore,the continuous multi-frame displacement fringe image is simulated,and then the method in this paper is used for demodulation.The simulation experiment results show that the displacement measurement accuracy ?=1.093 pixel.The average error after removing the gross error is 0.0624 pixel.The physical size error of the pixel is 0.1374?m.Laid a theoretical foundation for high-precision actual displacement measurement and demodulation.Design and build a displacement measurement optical system based on diffracted light interference and superimposed fringes.2-3 images are collected,and then static positioning experiments are carried out to verify the algorithm.The results show that this algorithm can achieve sub-pixel image positioning.And nano scale positioning of displacement.Finally,the optical system is simulated with Fred software.The sources of experimental errors are analyzed,and the influence of each part of the errors on the experimental results is also analyzed.The method proposed in this paper is also suitable for image-based displacement signal demodulation of other displacement measurement systems,such as self-coherent Moiré fringe system and photoelectric shaft encoder.It provides a research foundation for the development of signal demodulation technology in the field of grating precision displacement measurement.