In order to increase the sensitivity for detecting defects and reduce the voltage gain displacement by the lift-off variation, we demonstrated the improvement of voltage gain from the exciting coil to the detector coil using resonant circuits based on the equivalent circuit analysis. We used three (exciting coil, detector coil and metallic specimen) equivalent circuits that are coupled with each other through mutual induction. The two relationships (exciting coil and specimen, detector coil and specimen) were analyzed using a multi-layer model that consists of the coil, the air and the metal. The mutual inductance between the exciting and detector coils was taken from the result of preliminary experiment without resonance. According to the analysis, the voltage gain increased by up to 6.8 times and the voltage gain displacement reduced using resonance. In the experimental demonstration under the same conditions as the analysis, the increase of voltage gain was consistent with the analysis result within a prediction error of 4 % on average. From this demonstration, we confirmed that the resonant circuits were effective to increase the voltage gain and reduce the voltage gain displacement for improving the characteristics of eddy current testing.
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