Novel Common-Differential Inductance Coils With Dual Signal Conditionings for Separation of Lift-Off and Defects

Lift-off has a significant impact on defect discrimination and qualification. Estimating lift-off and evaluating defects is a critical issue for eddy current testing (ECT). In this article, a novel sensor that separates lift-off and defects is proposed. This uses common- and differential-mode coils with a dual signal conditioning circuit. Specifically, it comprises two hollow rectangular excitation coils located on the first and third layers of the printed circuit board (PCB), serving as the two arms of an ac bridge circuit. The output from the bridge signal is used for lift-off estimation since the coaxial configuration and small coil fill ratio make it specifically sensitive to lift-off variation. The other pair of rectangular receiver coils are situated on the second and third layers and placed in the middle of the hollow transmitter. This forms a transformer with excitation coils and is only sensitive to defects. The signals from these two complementary outputs allow lift-off to be used as auxiliary information for defect discrimination. Simulations and experiments were carried out to validate the method. This proves that a combination of coil configurations with different signal conditioning techniques can be employed for separating multiple parameters.