A Highly Sensitive Low-Cost Electrical Impedance Tomography System for Gas Bubble Detection in Process Engineering
Martin Gevers, Kai-André Mach, Thomas Musch
Cape Tpown, South Africa, ISIPT (Hg.) 2012 – 6th International Symposium on Process
Electrical impedance tomography (EIT) is a well suited measurement method for continuous monitoring of gas bubbles in conductive liquids. The conductivity distribution over the tube cross-section can be determined. This paper describes the design of a cost effective highly sensitive EIT system. It consists of an electrode ring, a digital system control unit, a floating current source and a parallel voltage measurement unit. The realized digital system control unit is based on a field programmable gate array (FPGA) which accounts for the critical timing of the measurement process. A stable and constant excitation current is an important issue for the conductivity reconstruction. For this reason a controlled current source has been developed, which consists of a direct digital synthesizer (DDS) and a controlled voltage-to-current converter. The control loop consists of a proportional and an integral part. The deviation of the output current has been measured for increasing loads and a constant input level. The output current variation of the controlled current source is less than 0.1 % for load resistance up to 20 k?. The frequency response has been measured with a load resistance of 10 k?. For frequencies up to 1 MHz the output current variation is less than 0.6 %. A parallel voltage measurement unit has been designed as an active electrode configuration for the measurement of the induced voltages. The relative measurement error is smaller than 0.1 % with respect to the input voltage. The functionality of the complete EIT system has been verified with conductivity reconstructions in different test scenarios.