Further Development in Differential Electrical Capacitance Tomography

Electrical capacitance tomography (ECT) technique has been developed due to its inherent simplicity and low cost. The tomographic images are obtained based on the permittivity differences between the investigated materials and background material (air, in most cases). The dynamic range of capacitance measurements in ECT could be 40 dB or higher. Simultaneously, the changes in the measured capacitances take up only 5% of the full measurement range. Therefore, the capacitance measuring circuit should be of high dynamic range as well as high precision, which constitutes one of the greatest challenges in ECT system design. The standing capacitances in ECT measurements may be canceled by differential measurement. In this paper, a recently developed differential ECT scheme has been presented. An active screen is employed as the differential electrode in the sensor, which is placed between the measuring electrode array and outer screen. Accordingly, a transmitter channel is implemented to accommodate the active screen, which enables the active screen to deliver multi-level AC signal. Simulation and experimental results show that the differential ECT system has a signal-to-noise ratio of 16 dB higher than the previous design, i.e. the non-differential system.