A lift-off measurement and compensation method based on a comprehensive EMAT-PEC probe

Abstract

In the field of industrial testing, lift-off fluctuations of the probe are inevitable, leading to unstable signals and inconsistent detection results, especially in electromagnetic acoustic transducer (EMAT). Besides, EMAT has a low energy utilization, since the induced pulsed eddy current (PEC) is not fully utilized during the transduction process. To address these issues, this paper proposes a novel composite detection method which is able to measure the lift-off based on PEC and compensate for the lift-off effect on EMAT using a designed comprehensive EMAT-PEC probe. A finite element (FE) model of the EMAT consisting of a permanent magnet and a type of butterfly coil is established, and the spatial distribution of the PEC density induced by the butterfly coil is analyzed. Based on this, comparison of the effectiveness among different EMAT-PEC probe configurations for lift-off detection is implemented, and an optimal EMAT-PEC probe with a compact structure is designed. This probe is capable of obtaining both EMAT signal and PEC signal synchronously through a single excitation, ensuring the accuracy and efficiency of the compensation process. The blind test is then performed to simulate the random lift-off fluctuation, and the experimental results demonstrate that the relative errors of the measured lift-off and the compensated peak amplitudes of EMAT signals are all less than 7%, verifying the effectiveness of the method. For industrial applications, based on the developed method, a probe array with different lift-off combinations is designed to simplify the calibration process, and an application in probe inclination detection is introduced, proving the method has good application foreground.