Improvement of unidirectional focusing periodic permanent magnet shear-horizontal wave electromagnetic acoustic transducer by oblique bias magnetic field

摘要

We propose a new unidirectional point focusing shear horizontal (SH) guided wave electromagnetic ultrasonic transducer (EMAT) with an angled periodic permanent magnet (PPM) in this work. The angled PPM developed here provides an angled bias magnetic field to achieve the EMAT's unidirectional focusing capability. The characteristics of the magnetic field distribution are analyzed by numerical and theoretical calculations. The simulation and experimental results are proven to be in good agreement when studying the bidirectional normalized amplitudes of the displacement of the single-coil SH-guided EMAT with oblique permanent magnets. Both the proposed angled transducer structure and the traditional paralleled transducer structure are performed and simulated using the three-dimensional Finite Element Method (FEM) to compare their unidirectional focusing capabilities. The results show that the SH guided wave EMAT of the focusing coils with an oblique permanent magnet enhances the signal on the focusing side and weakens the signal on the other side effectively. This performance can suppress the influence of the reflected signal from the unfocused side and further improve the ultrasonic signal's resolution. Moreover, it is shown in the study that increasing the oblique angle of the PPM makes it difficult to increase the signal strength at the focal point when the angle reaches a certain value, but it is still effective at weakening the signal on the unfocused side. (C) 2019 Elsevier B.V. All rights reserved.