Research Of The Electrical Capacitance Tomography Based On Ac High Voltage Excitation

Electrical Capacitance Tomography (ECT) has been developed since later1980s, which is used to detect the2D/3D distribution informantion of different dielectric constant materials in measured object. Recently, the research of ECT system focuses on measuring low capacitance and image reconstruction algorithms. The paper introduces a high voltage excitation ac-based low capacitance measuring circuit for tomography and designs ECT systems based on the low capacitance measuring circuit. The main jobs of the paper are as follows:(1) The paper reviews the technology of low capacitance measurement for tomography, and then proposes a high voltage excitation ac-based low capacitance measuring circuit. The performance of new measurement circuit is tested with a structured reference capacitance whose resolution is about attofarad order of magnititude. The testing results show that the new measurement circuit has high resolution of several attofarads, high stability (no more than6aF during4hours) and high linearity (the correlation coefficient more than0.9992).(2) An8-electrode ECT sensor for high voltage excitation is optimized according to the condition number of sensitivity matrix, the uniform of sensitivity map and the range of changes in capacitance. A switching arrangement circuit is designed for high voltage excitation signal. An8-electrode ECT system is constructed with750Vpp ac excitation. Comparing experimental results show that the new ECT system has better discernment performance for little duty ratio object in the sensor than an ac-based ECT with20Vpp excitation.(3) The paper also discusses how to choose a pair of dielectic constants for calculating the sensitivity matrix under the known dielectric constant condition. Some typical distribution patterns with unknown dielectric constant is used to test the image reconstruction ability of high voltage ac excitation8-electrodes ECT system, and the experimental resultants show that the new ECT system has a good ability of reconstruction image.(4) A24-electrode ECT system based on improved ac high voltage excitation low capacitance measurement circuit is constructed in laboratory. Ten different distributions in sensor are used to test the24-electrod ECT system with the support of filtered-LBP, Landweber iterative and Tikhonov iterative algorithms. The experimental results show that all distributions can been reconstructed by the ECT system exactly but the core pattern in the centre of the sensor. The results also prove that the LBP algorithm can reconstruct a good quality image when the number of electrode of sensor array is up to24. That indicates that more electrodes benefits to the quality of reconstructed image.