Research And Design Of A New Electrical Capacitance Tomography Detector System

Electrical capacitance tomography (ECT) based on capacitance sensingarray is a kind of process tomography (PT). It has the advantage of rapidresponsive, non-invasion, easy mounting and low in cost. It can acquire visible2D or3D distribution information of closed vessel or pipeline. The visibletechnique is of a great developing potential process parameter measurementrecently.But there are some problems and difficulties in ECT designs and ECTapplications: Firstly, the measured capacitance and its change induced by themeasured materical concentration variation are very small, but the straycapacitances of co-axial screening cable and CMOS switches are relative verylarge, so the capacitances deteciting is easily affected by the stray capacitances.Secondly, the electric fields in the sensor’s detecting region is affected by thepermittivity distribution, non-uniformity of the space responsive sensitivity isvery high, and the image reconstruction is undetermined. Thirdly, the ElectricalCapacitance Tomography system widely adopts analogue device, which is limitedin the speed and precision of the tomography due to the performance of theanalogue device.This dissertation goes into research of ECT key technique in its designing. Anew ECT detector system has been developed.Firstly,aiming at the non-uniformity problem of sensitivity distributioncaused by the "soft field" effect of the static electric field in the measurementregion of capacitance sensor, on the basis of deeply analyzing the capacitancemeasuring principle in electrical capacitance tomography system, an TripleElectrodes excitation and Double Electrodes examine method is proposed. Byusing the new method, a more uniform electric field can be got, and theindependent measurement data is also increased, which improves the ill-posed ofthe problems, enhances the uniformity of capacitance sensitivity. Secondly,the detection of very small capacitance is one of the most difficultproblems in developing ECT system because of the very small capacitance, so theAC-based capacitance detecting method is adopted, all the electrodes use thesame excitation source and C/V converting circuit.Thirdly,a new ECT is designed in this paper,which of data acquisitionsystem of core is FPGA. Voltage from C/V switching circuit is taken sample byA/D converter in the new ECT. It adopted FPGA as the key control module. Thefunction of FPGA as follows: control the emergence of alternatingstimulus,adjust the state of electrode and take charge of state switchover amongpassageway. Beyond that, the data switched by A/D of demodulation andfiltration is controled by FPGA,then the data is transmitted by USB to PC. A newECT reduced the stable time of the system and improved the speed and theprecision of collection.