Electrostatic Effect on AC-Based ECT and Its Elimination

The ac-based electrical capacitance tomography (ECT) system has been successfully used for visualizing gas-solid flow in various industrial processes, such as pneumatic conveyor and fluidized bed. However, the solid particles in gas-solid systems are charged due to the collisions between particles and pipe wall. The particle charging decreases the image reconstruction accuracy of the ac-based ECT system and even leads to the reconstruction image distortion. In this paper, the electrostatic effect on ECT by particle charging is first investigated theoretically, and then experiments on a belt conveyor rig are performed to verify the electrostatic effect. Experimental results showed that the particle charging causes the additional output of the ac-based C/V conversion circuit. When the additional output makes the output of the circuit in its saturation region, the reconstruction image from the ac-based ECT system is completely distorted. However, the particle charging has no effect on the ac-based ECT system when the output signal of the C/V conversion circuit is within its allowable output range. By choosing a suitable amplifier and appropriate feedback resistance and capacitance values (R-f and C-f), the saturation of the output signal of the C/V conversion circuit is avoided, and so, the electrostatic signal superimposed on the output capacitance signal of the C/V circuit can be completely removed by a bandpass filter due to their distinct frequency characteristics. As a consequence, the electrostatic effect on the ac-based ECT system can be eliminated by optimizing ac-based C/V conversion circuit.