Electrostatic Effect on AC-Based ECT and Its Elimination

Abstract

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.