Electrical Capacitance Tomography With Differential Sensor

In electrical capacitance tomography, capacitance changes are used to determine the permittivity distribution in the imaging area. However, the changes are small compared with the standing capacitances, e.g., usually in the order of 10%-30%. For a single channel capacitance sensor, a differential configuration having a redundant pair of electrodes can be used to cancel the standing capacitance. However, there has been no report so far introducing such differential configuration into electrical capacitance tomography (ECT) sensors. This is mainly due to the fact that an ECT sensor is composed of an array of electrodes, e.g., 8, 12, and 16 and the capacitance measurements in ECT are required to interrogate capacitances of all electrode pair combinations, which introduce significant difficulty for a differential configuration. In this paper, a novel differential ECT sensor is proposed and designed, which consists of concentrically arranged dual array electrodes. It can also be thought as an additional array of electrodes being inserted between the measuring electrodes and the outer screen of a conventional ECT sensor. The new sensor design has been validated by the numerical simulation. A prototype sensor has been developed and evaluated with an field programmable gate arrays (FPGA)-based ECT measurement system, showing that with the proposed differential sensor, the dynamic range of the measured capacitance is reduced by $\sim 80$ % and an average improvement of 10.8 dB in signal-to-noise ratio is achieved.