| Electromagnetic tomography(EMT)is a process tomography technology based on the principle of electromagnetic induction.Compared with ECT and ERT,which are only sensitive to a single electrical parameter,EMT has a dual-mode characteristic that is sensitive to both conductivity and permeability parameters,and has a broad industrial application prospect.However,the existing EMT system has many problems,such as low measurement accuracy,less projection data and imperfect structure,which seriously restrict the application and promotion of EMT technology.In-depth study of the above issues and the realization of highquality visual inspection of the EMT system can fully take advantage of the EMT system’s visual monitoring of industrial processes.In this thesis,EMT technology is the research background,combined with COMSOL finite element analysis software,using the research methods of numerical simulation and physical experiment verification,the high-quality visualization method of EMT system is studied.Based on the analysis of theoretical model and simulation experiment of EMT,the sensitive field and imaging characteristics of EMT system are studied.Aiming at the problem of low imaging accuracy caused by weak sensor field and uneven distribution in EMT system,a new dual coil excitation mode is studied.In view of the problem that the EMT system has few projection data and affects the quality of reconstructed image,a method of sensor position reconstruction compensation is proposed,which can make the measured projection data double without increasing the number of sensors.On this basis,the image fusion algorithm is studied to improve the imaging accuracy of the reconstructed image.The main work done in this thesis is as follows:(1)The 3D EMT sensor system model is studied.Based on the multi physical field finite element simulation software COMSOL and MATLAB script file,the 3D EMT system simulation model is established.Based on the calculation of boundary measurement value and sensitivity coefficient matrix of the positive problem,the influence of medium distribution and excitation signal on the sensitive field is analyzed.(2)Aiming at the problem of poor imaging accuracy caused by the imperfect sensor structure of EMT system,the spatial resolution,imaging correlation coefficient and image reconstruction error are determined as the evaluation indexes.Based on the analysis of the influence of single structure parameter on sensitive field and each index,a multi-objective optimization design method based on response surface method is proposed to realize the comprehensive optimization design of sensor structure.The results show that the optimized sensor structure can improve the correlation coefficient of image reconstruction by more than 14%.(3)Aiming at the problems of large voltage dynamic range and poor imaging accuracy caused by weak signal and sensitive field intensity of EMT technology,a double coil adjacent excitation measurement mode is proposed.The simulation results show that the excitation mode can enhance the measurement signal of the single-layer 8-coil EMT system,reduce the dynamic voltage range of the system by 40%,reduce the design difficulty of the signal acquisition circuit,and effectively improve the reconstruction accuracy of the system for the shape of the object in the central area.(4)Aiming at the problem of less projection data and low reconstruction quality of image reconstruction algorithm in EMT system,a compensation algorithm based on sensor position reorganization is proposed,which can obtain more projection data reflecting the distribution of media in the field by rotating sensor coil.Based on this,the image reconstruction is completed by combining PCA(principal component analysis)data fusion method.The simulation results show that this method can improve the reconstruction accuracy of the system for multi conductor distribution.In the experiment of three conductor distribution,the imaging correlation coefficient increases nearly 20%.(5)Combined with 2D image reconstruction method of EMT technology,3D image reconstruction algorithm of EMT is studied.The 3D EMT sensor models of 2×6 coil and 2×6 cross coil structure are designed respectively.Boundary voltage and sensitivity of sensor system are analyzed.The image reconstruction of LBP(linear back projection)algorithm in the 3D EMT system is realized.(6)The experimental platform is built under the laboratory conditions,and the sensors made according to the optimization results are used for experimental verification through simulation analysis and physical experiments.Through the experiment,the structure optimization of the sensor and the effectiveness of the proposed method in EMT system are verified. |
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