Comparison study of high-sensitivity area-changed capacitive displacement transducers with low-impedance and high-impedance readout circuits

Abstract

Area-changed capacitive displacement transducers (CDTs) are widely used in the high-precision displacement measurement due to their high accuracy and large dynamic range. The preamplifier circuit is used to convert the capacitance variation signal into voltage, which requires low noise and is significant for the high-sensitivity area-changed CDTs. Current CDT preamplifiers are mainly categorized as the low-impedance preamplifier and the high-impedance preamplifier; however, their characteristics and application scopes have not been systematically compared. This paper builds comprehensive models of the low-impedance and the high-impedance preamplifiers. Then, three-electrode configurations with different electrode separations and gaps are designed to carry out displacement variation experiments with low-impedance and high-impedance readout circuits, respectively. The results show that the sensitivity decrease caused by the gap change with the high-impedance preamplifier is 70%, while the counterpart of the low-impedance preamplifier is 85%. When the gap is 0.1 mm and the width-to-separation ratio varies from 1:1 to 5:1, the sensitivity of the CDT based on the low-impedance preamplifier is increased by 64%, while the counterpart with the high-impedance preamplifier is increased by 22%. Hence, this paper gives the universal guiding rules of preamplification circuit selections for different CDT electrode configurations and application requirements. For a capacitive sensor design with large and unavoidable parasitic capacitance, the low-impedance preamplifier and a CDT with a large electrode width-to-separation ratio match best. For a capacitive sensor design requiring both a large sensitivity and good robustness to out-of-plane interferences, the high-impedance preamplifier and a CDT with a small electrode width-to-separation ratio match best.