An improved second-generation electrical capacitance tomography system

A new electrical capacitance tomography (ECT) system has been developed at UMIST having a number of improvements over our previous design. The new system uses AC-based instead of charge-discharge capacitance transducers which allow an increase in resolution from 0.21 to 0.025 fF (peak value of noise level). Phase-sensitive demodulation is employed in order to be able to discriminate between the effects of the conductive and capacitive components of the unknown admittance. Each channel has its own demodulator thus allowing parallel measurement. The latter, coupled with high-frequency (500 kHz) operation, results in an acquisition rate of 100 frames per second. Additionally, each channel uses an AC programmable-gain amplifier (PGA) before demodulation, which, combined with another PGA after standing value compensation and an adjustable excitation signal result in a very wide dynamic range. Finally, excitation signals can be applied to several electrodes at the same time, and, thus, the system can be employed to investigate the possibility of using multi-electrode excitation patterns.