Design Of Grating High-precision Displacement Measurement System Based On FPGA

With the blockade and suppression of technology by developed countries,China has accelerated the pace of localization in the field of high-speed and high-precision advanced manufacturing equipment.The breakthrough of high-precision positioning and high-speed measurement methods and algorithms will not only guide the R＆D and production of precision equipment in the future,but also have far-reaching significance for China to move towards independent R＆D.Compared with expensive precision displacement measurement instruments such as laser interferometer and coding disk,grating ruler measurement sensor has the advantages of high resolution,compact structure,simple maintenance and low price while ensuring high accuracy.It has been widely used in various fields.The incremental grating displacement measurement instruments counts the moire fringe based on the physical scribed line,and then obtains the displacement value by decoding the count value.In order to improve the measurement accuracy of grating,moire fringes can be subdivided in different ways,or the distance of moire fringes can be reduced in the processing and manufacturing process.However,with the complex subdivision of moire fringes,the measurement process is very vulnerable to external noise interference,thus reducing the accuracy.Reducing the width of moire fringes will make the requirements of manufacturing process and manufacturing environment extremely complex and strict,and it is difficult to meet the product use requirements,Aiming at the mutual restriction and conflict between measurement accuracy and motion speed in the incremental grating measurement system,a grating real-time high-precision displacement measurement system based on FPGA is developed in this thesis.The research contents are as follows:Firstly,using the parallel processing advantage of FPGA,the idea of analyzing the grating displacement image information and outputting the displacement value through FPGA is proposed,and a set of grating real-time high-precision displacement measurement system based on FPGA is designed to replace the traditional grating ranging method.In this thesis,a high-frequency and high-resolution linear array camera is used to collect the moving grating image,and the high line frequency characteristics of the linear array camera are used to maximize the image quality and displacement information of the grating in the state of highspeed motion;At the same time,in order to improve the measurement accuracy of the system,this thesis designs a rotating structure to form an optimal angle between the photosensitive pixel of the linear array camera and the grating pattern of the grating ruler,so as to improve the image acquisition resolution,and then obtain a more subdivided grating image.Secondly,in the component selection scheme of the experimental platform,the optical image acquisition and control platform is composed of hardware such as high-frequency linear array camera,adapter,objective lens,linear array light source and light source controller.The image data is collected and processed at high speed by using Matrox image acquisition card and FPGA data acceleration board.In order to reduce the error caused by mechanical vibration in the process of high-speed movement of the moving platform on the measurement accuracy of the subsequent system,according to the structure and size of the motion platform,the threedimensional adjustment support of the camera is designed.Then,in the software algorithm processing scheme,the design of displacement algorithm accelerator is completed by Verilog HDL hardware programming language.The algorithm accelerator mainly includes the compilation of codes such as raster image acquisition,image preprocessing,motion direction recognition algorithm,automatic threshold algorithm and macro micro composite displacement algorithm.Aiming at the problem of huge storage of image data,a cross bank method is proposed for storage and processing;The development of human-computer interaction interface is completed by C# programming language,which is convenient for real-time observation and debugging of the real-time displacement value and motion direction output by the system in the process of experiment.Finally,the hardware parameters are set and the experimental platform is built.Under the same experimental conditions,the comparative experiment of displacement measurement is carried out with laser interferometer and system.The delay error of system processing time and the error of displacement measurement are analyzed and summarized.The experimental results show that the maximum moving speed measured by the system is 0.8m/s and the maximum resolution is 0.06 μm.The minimum processing time of the system is 148 ms and the displacement accuracy is ± 2.5μm.From the perspective of image processing,this thesis designs a real-time high-precision displacement measurement method based on FPGA grating,which provides a new idea and direction for the framework design of grating measurement system.