A novel eddy current testing for permeability measurement reducing lift-off and tilt angle effect

As a significant parameter, metal permeability is closely related to the heat treatment state, fatigue state and hardness, which can be used to evaluate structural information. Eddy current testing (ECT) is suitable to measure permeability of metallic materials due to its advantages of being non-destructive and high sensitivity. However, lift-off variations and coil tilting will introduce measurement error. In order to solve the problems, a novel eddy current testing method immune to lift-off and tilt angle variation is investigated for permeability measurement. First of all, the analytical solution is established and analysed for tilting coil configuration. According to the analytical solution, the inductance variation of coil is only determined by lift-off and tilt angle under high-frequency measurement (~MHz). Furthermore, two absolute coils parallel to each other are designed to decouple to the lift-off and tilt angle. Combining the estimated lift-off and tilt angle, the permeability of metallic plate can be measured by the calibrated self-inductance variation in low-frequency excitation. Numerous experiments are carried out to evaluate the performance of the proposed method, and the results indicate the relative measurement error is only 2.5% with 45° to 75° inclination and 3.5 mm lift-off distance fluctuation.