Evaluation of Integrated Electrodes for Electrical Capacitance Tomography

Philip Williams and Trevor York

Industrial Process Tomography, Dept of Electrical Eng and Electronics, UMIST, PO Box 88, Manchester, M60 1QD. (wpm@fs2.ee.umist.ac.uk, t.york@umist.ac.uk)

The authors are members of the Virtual Centre for Industrial Process Tomography

Abstract - Noise immunity in the charge-discharge circuit, which has proved very effective for electrical capacitance tomography, is improved by increasing the clock frequency which, as a result, increases the gain of the circuit. This frequency is limited by the stray capacitance which is largely due to the connecting wires. The present paper describes a system which eliminates the coaxial wires by mounting the circuitry directly onto the electrodes. This is achieved by implementing as much of the circuitry as possible on a custom integrated circuit.

Twelve "integrated electrodes" have been fabricated. These have been tested on a 10 cm diameter plastic pipe. Using 1 metre of coaxial cable the sensors exhibit a worst case signal to noise ratio of 48dB. When the circuitry is mounted directly onto the electrodes this is increased to 58dB. This is the first implementation of an electrical capacitance tomogram, using the charge-discharge circuit, that is based entirely on CMOS technology.

Keywords: electrical capacitance tomography, custom IC, stray capacitance