Non-contact surface resistivity measurement using cylindrical surface potential detector with a corona charger

A non-contact surface resistivity tester was developed by improving the charge decay measurement. The tester has a corona charger and surface potential probe in a cylindrical grounded body. The corona charger provides a surface charge on the test material and the surface potential detector measures the traveling motion of the surface charge. The traveling speed of the surface charge is dependent on the surface resistivity of the test material; therefore, the surface resistivity can be determined without contact to the material. An equivalent circuit was predicted with a one dimensional ladder circuit including the surface resistance of the material, the volume resistance of air, and the capacitance between the cylinder and the test material. From analysis of the equivalent circuit, the surface resistivity for an insulative surface was found to be a function of ρ_s/(ρ_vδ), where ρ_s is the surface resistivity of the test material, ρ_v is the volume resistivity of air, and δ is the gap between the test surface and the cylinder. For conductive and dissipative surfaces, the surface resistivity is dependent on ε₀ρ_s/δ, where ε₀ is the permittivity of air. The measurement concepts were experimentally verified using a needle type corona charger and a surface potential meter. The surface resistivity determined by surface potential measurement was in agreement with that determined using a contact type surface resistivity tester.