Optimal design of spiral coil electromagnetic acoustic transducers considering lift-off sensitivity operating on non-ferromagnetic media

The generation efficiency of electromagnetic acoustic transducers (EMATs) dramatically reduces with increasing lift-off, thus restricting their applications. This paper aims to provide methods to decrease lift-off sensitivity. The process of generating a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using the finite element method. An equivalent excitation circuit, including an impedance-matching network and a limited power source, is presented for calculating the excitation current. Such an equivalent circuit has not been given adequate attention in previous publications in this field, but it has proven to be of great importance in determining the efficiency of ultrasonic generation. The predictions of static magnetic field, eddy current density and excitation current are validated by experimental results. The conversion efficiency and lift-off sensitivity of the EMAT are analysed with respect to impedance matching parameters, magnet dimensions, the presence of a copper backplate between the magnet and the coil, and backplate-to-coil distance. The results indicate that impedance matching parameters, magnet diameter and plate-to-coil distance have obvious effects on lift-off sensitivity. The backplate can be used to decrease lift-off sensitivity, and it can rearrange the source current density across the cross-sectional area of the coil conductor. Furthermore, compared with the original EMAT, the lift-off sensitivity of the optimised EMAT can be decreased by 7.9 dB at 2 mm lift-off.