Conductivity measurement using a dual-coil eddy current sensor with constant lift-off difference

Eddy current (EC) testing enables efficient non-contact measurement of the electrical conductivity of metallic materials, but its accuracy is often affected by the lift-off effect. In this work, a dual-coil EC sensor with a constant lift-off difference is proposed, and a simple lift-off suppression feature for conductivity measurement is extracted from its single-frequency response. Based on the classical Dodd – Deeds model, a simplified analytical expression for the proposed dual-coil sensor is derived, showing that the phase tangent difference between the two coils is linearly related to the square root of conductivity and is theoretically independent of lift-off distance. To validate the effectiveness of the proposed method, experiments were conducted on metallic samples with different conductivities. The results show that the phase tangent difference maintains a stable linear relationship with the square root of conductivity and exhibits low sensitivity to lift-off variation, with the relative error of conductivity measurement remaining within about 5%. Therefore, the proposed method can effectively reduce the influence of lift-off variation and provides an efficient and feasible ECT scheme for rapid conductivity measurement of metallic materials owing to its single-frequency excitation, simple sensor structure, and clear conductivity-related linear relationship.