Theoretical and experimental analysis of lift-off effects on probe impedance in eddy-current testing

The impedance of a probe-coil above a conductive plate with finite thickness was derived and expressed in series expansion, and then the impedance characteristics of the coil due to lift-off variation were analyzed. Furthermore, three-dimension eddy-current problem for a coil above the plate with a defect was calculated by finite element method. Effects of lift-off on coil impedance change produced by a defect were investigated. The results show that amplitude of defect signal is adversely affected by lift-off variations, but phase difference between defect signal and lift-off loci is almost unchangeable. After analyzing the characteristics of the phase difference with the chang of frequency, a conclusion that defect signal is approximately vertical to lift-off noise at a well-selected frequency is obtained. The impedance calculation model and theoretical analysis are confirmed to be correct by experiments, which are valuable for removing lift-off disturbance by using phase rotation and signal enhancement technique in a practical eddy-current testing.