Research On Digital Demodulation Method In The Electromagnetic Detection And Tomography

Electromagnetic Tomography (EMT) is a process imaging technique based on magnetic induction principle. It reconstructs cross-sectional conductivity and permeability distribution in the object field. Phase sensitive demodulation extracts amplitude and phase information from induced voltage signals, which is a key part of an EMT system. Orthogonal sequential digital demodulation is an efficient method of phase sensitive digital demodulation.The method of orthogonal sequential digital demodulation is used in this paper. My research is electromagnetic induction measuring system based on half-period sampling and multiple-frequency excitation signal. Some main works and conclusions are presented as followings:1) The principle and method of half-period sampling demodulation are presented for the first time, and applies to Electromagnetic Tomography (EMT) technology. Electromagnetic induction measuring system based on half-period sampling demodulation is deeply studied. Firstly, analyze the orthogonal sequential demodulation in theory, and explain the feasibility of half-period sampling demodulation in the frequency domain. Secondly, prove conclusion of the theory through the MATLAB simulation. Finally, further verify the practical application values of half-period sampling demodulation, by electromagnetic tomography hardware experimental platform, through the contrast of half and the whole period sampling demodulation, and the study of the half-period sampling demodulation imaging results based on the different algorithms.2) The principle of multiple-frequency signal demodulation is studied and the data of the experiment are analyzed, proving that the multiple-frequency signal also can be used in digital EMT system in theory. Design a new type of experimental platform with 8-coil EMT sensor, and the EMT simulation system is used based on the multi-function data collection card, conduct the experiment of multiple-frequency signal demodulation, and analyze the results. The measurement experiment includes two parts: The first part, the 8-coil system is excited by the multiple-frequency signal, measuring the changing curves of amplitude with induction coil signal in different position. The second part, only two adjacent coils are used, to whether meet the orthogonal sequential digital demodulation standards, using PC program by the data acquisition card generated more than two kinds of frequency modulation signal, analyzes relative error of the amplitude with the modulation signals of different frequencies, multiple-frequency signal are studied without noise through the simulation in MATLAB.