Research Of The Electromagnetic Tomography Technology

Electromagnetic tomography (EMT) has been developed since later1990s, which is one of process tomography technologies based on the principle of electromagnetic induction. It is used to detect and reproduce the distribution of electrical conductivity or magnetic permeability in object field space. It has characteristic of non-contact, non-invasive, low cost and real-time. Combining the simulation system with the lab hardware system, and combining theory with practice are research methods in this paper. From the simulation system, the physical model of the EMT system and the sensitivity model are established, image reconstruction algorithms are designed and realized, and the appropriate hardware parameters are selected. In terms of hardware system, a reasonable laboratory EMT system is built. The reconstruction algorithms used in simulation system are applied to lab EMT system, and the quality of reconstruction image is evaluated. The main jobs of the paper are as follows:(1)The existing EMT systems are summarized. According to the system characteristics, the EMT systems are classified.(2)The physical model of the EMT system is derived. The mathematical model of the EMT system according to the Maxwell equations is derived. Linear model is built by finite element method.(3)The effect of reconstruction image caused by the position shift and angel deflection of the detection coils is analyzed. It is concluded that when the detection coils are offset, the reconstruction image is tended to the edge of the object field.(4)The sensitivity model is built. Threshold sensitivity matrix calculation method based on electrical field centre lines (EFCL) is applied to speed up generation of a sensitivity matrix, and improve reconstructed images quality. The reconstructed images with the threshold sensitivity method have superior image quality than those with normal method, and time spent on calculating sensitivity is no more than50percent of normal method.(5)The new sensitivity normalization model is established based on the analytical formula. The formula of the new sensitivity normalization is given. The normalization effect of new model is compared with the original. The new sensitivity normalization is applied to the image reconstruction, and evaluated the image quality. It is conclusion that the new sensitivity normalization model is more close to the original trend, and got higher reconstruction image quality.(6)Based on conjugate gradient algorithm, modified conjugate gradient algorithm for electromagnetic tomography (EMT) is proposed, which improves quality of reconstructed image and convergence speed efficiently. Results of image reconstruction are given using Landweber iteration algorithm, Tikhonov regularization algorithm, conjugate gradient algorithm and modified conjugate gradient algorithm.The results are evaluated in terms of image error, correlation coefficient, and convergence for image reconstruction. The conclusions are that the modified conjugate gradient algorithm is superior to the others.(7)A set of8coils which arranged on the same layer EMT system is developed. In this system, excitation coils and detection coils are used alternately, and the frequency is10MHz. Image reconstruction methods and structure parameters used in the simulation system are applied to this laboratory EMT system. From the image evaluation including the image error and relevance, this lab system has good image reconstruction ability.