Design of a security screening system with a capacitance sensor matrix operating in single-electrode mode

Abstract

A planar capacitance sensor matrix may be used to detect dangerous objects for security screening because different materials under test (MUT) have different permittivities. Usually, each capacitance sensing element consists of two electrodes, one for excitation and the other for detection. A weakness of such a two-electrode structure is that its sensing depth is limited. To increase the sensing depth, a planar capacitance sensor matrix operating in single-electrode mode has been designed for a security screening system. In this paper, the design of the planar single-electrode capacitance sensor matrix is described and its performance is analysed. A model for the single-electrode capacitance sensor and its equivalent circuit is discussed. The sensor response to the properties of MUT is studied by simulation to determine the design parameters. A single-electrode capacitance measuring circuit is designed to measure capacitance from the planar single-electrode capacitance sensor matrix. Preliminary experimental results show that a steel knife can be imaged by the screening system with the planar single-electrode capacitance sensor matrix.