| Electrical capacitance tomography technology is one of the process topographicimaging technologies. With many advantages, such as non-contact, fast response,low-cost and safe operation, ECT are widely used in modern industrial production andbecome a very popular detection technology in the PT realm. Two phase flow process inmany industrial sectors is universal. For the control of measurement, the energyconservation, the environmental protection and reliable operation of two phase flowconveying system, two phase flow parameters testing technology is very important. Byusing of the sensor array installed around in the pipeline or container, Capacitancetomography imaging technology get the projection data (capacitance value) in differentangles of non conductive objects. With the appropriate image reconstruction algorithm,we reconstruct the medium distribution of the object under test field. This makes twophases flow parameters an accurate measure method. Through image reconstruction ofthe internal pipe, it can provide real-time two/three dimensional visual information of thestate in closed pipeline, containers and equipment. It is convenient for the measurementof two phase flow parameters. So it is meaningful to study the capacitance tomographyimaging system based on image reconstruction algorithm.This paper is focus on electrical capacitance tomography system, discussing the"soft" field effect in the process of image reconstruction. The following are the majorwork in our thesis:After reading a lot of reference literature, we summarized present situation ofcapacitance tomography and the main methods of image reconstruction. We give anoverview the work principle, basic composition and sensor structure of capacitancetomography imaging system. We introduce the basic theory of electromagnetic fields, thefinite element model of electrical capacitance tomography, computing capacitance value,sensitivity field based on electric potential distribution, and calculation methods of fastsensitivity field. Due to "soft field" effect, the precision of the reconstructed image is nothigh and relative error is very large. In order to improve precision of the reconstructedimage and reduce the relative error, we proposed new image reconstruction methods byusing a changeable sensitivity of field to replace the constant one for the imagereconstruction.We use COMSOL software getting sensitive fields under different flow pattern. By comparing them with traditional sensitive field, it is can be seen from graphics thesensitivity in changeable sensitivity field is different from the sensitivity in fixedsensitivity field. Within the medium in pipelines, the sensitivity field also changes in thecorresponding position place.In order to validate the proposed method, we do simulation of ECT system by usingthe LBP and Land Weber algorithm. Simulation results show that the precision ofreconstructed image in different flow model using changeable sensitivity field is betterthan using fixed sensitivity. This also demonstrates our method is effective. Therefore weprovide a new method for improving the quality of image reconstruction in electricalcapacitance tomography system. |
