| Electromagnetic Tomography(EMT)is a relatively new member of the electrical tomography family,which works based on the eddy current in conductive objects.Among these tomography techniques,EMT is considered the most promising technique for non-destructive testing because of its contactless with the object.EMT is based on the principle of electromagnetic induction law.It detects the distribution of electromagnetic properties(in particular conductivity)in the target area through sensor arrays,and then uses the induced voltage and the reconstruction algorithm to show the distribution of internal conductivity or permeability.It can be used in many industrial and biomedical areas,such as industrial progress monitoring,mining,and medical tomography.The settings of the sensor array in EMT determine the number of induced voltages,which have an important influence on reconstructed image quality.This degree thesis was divided into three different parts:(1)To study the effect of the settings of coils on the quality of reconstructed images,the number and the position of the sensor array in EMT are discussed for optimization.In this part,the models with different numbers of coils and different contributions are established by using FEM software;the different algorithms are used to image reconstruction and the quality of the constructed images was compared by the correlation coefficient.The simulation shows that the image quality improved by increasing the number of sensors,but when increased to a certain number,the image quality will become worse if the number of the coils continue to increase.In order to increase the measurement data instead of increasing the number of the sensor array,the sensor array is rotated.By comparing the image quality before and after rotation,the result shows that the quality of the reconstructed image is obviously improved after the combination.The simulation results can be a guide for the settings of the sensor array in the design of the EMT system in future work.(2)A novel volume imaging based on the principle of electromagnetic tomography using a single layer sensor array,namely,3D-EMT,has been developed in this part.In the forward problem,three kinds of sensor arrays are established with different coil numbers,—8,12 and 16,to conduct simulations.Four typical conductivity distributions are used to verify the 3D EMT imaging.The sensitivity maps calculation is presented in3 D views and different layers of sensitivity maps are analyzed.The conjugate gradientiterative method has been used in image reconstruction.The reconstructed images are presented to compare the differences between 2D and 3D images.The results have shown the simulations of the 3D images with a single layer sensor array are better than2D's images in position,shape,and size.Three-dimensional imaging using a two-layer sensor array was studied in this part.The models with two layers of 16 coils,two layers of 20 coils,and two layers of 24 coils was established,and three-dimensional reconstructed imaging is performed.The simulation results show that the 3D reconstructed image can display more information on the z-axis than the two-dimensional plane reconstructed image.The three-dimensional reconstruction can clearly show the material distribution in the three-dimensional space,confirming the use of the multi-layer sensor array in EMT Feasibility of 3D reconstruction..(3)A single-layer three-dimensional experimental system was built,and the results confirmed the feasibility of using a single-layer sensor array for 3D imaging,indicating the possibility of three-dimensional imaging without saving hardware resources.A three-dimensional electromagnetic tomography system with a two-layer sensor is built,including a signal amplification circuit,a three-dimensional sensor array,a signal acquisition and amplification circuit,and a channel switching circuit.The practical design of the experimental system collects data and performs single-layer three-dimensional imaging experiments and two-layer sensor three-dimensional imaging.After analyzing the experimental results,it is found that three-dimensional imaging can show the size and shape information of objects in space.This paper can provide guidance for exploring the potential applications of electromagnetic imaging technology for single-layer 3D and multi-layer 3D imaging. |
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