Periodic permanent magnet (PPM) array electromagnetic acoustic transducers (EMATs) can efficiently generate and receive shear horizontal (SH) ultrasonic waves. Conventional PPM EMATs typically generate waves which simultaneously propagate both forward and backward. This can complicate the received signals and make it difficult to locate the position of scatterers. Unidirectional generation of ultrasounds can be achieved if two ultrasonic sources are separated by a predefined distance and are excited with the proper delay. Relying on this principle, EMATs have been previously designed aiming to generate other modes of ultrasonic waves. The main challenge when extending this conception to an SH-wave EMAT is how to restrict each coil to its specific magnet array. We present the concept of a unidirectional SH EMAT consisting of two racetrack coils and two interlaced PPM arrays, that are slightly shifted sideways, in such a way that the generated wavefronts still properly interfere. The design was fabricated and experimentally evaluated in an aluminum plate generating the SH0 guided wave mode. The forward to backward generated wave ratio is above 20 dB and well agrees with finite element simulations.

中文翻译：

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带有侧移周期永磁体的单向剪切水平波产生电磁声换能器

周期性永磁体（PPM）阵列电磁声换能器（EMAT）可以有效地生成和接收水平剪切（SH）超声波。常规的PPM EMAT通常会产生同时向前和向后传播的波。这会使接收到​​的信号复杂化，并难以确定散射体的位置。如果两个超声源相隔预定距离并以适当的延迟激发，则可以实现超声的单向生成。依靠这一原理，以前已经设计了EMAT，旨在生成其他模式的超声波。将这一概念扩展到SH波EMAT时的主要挑战是如何将每个线圈限制为其特定的磁体阵列。我们提出了一种单向SH EMAT的概念，该EMAT由两个跑道线圈和两个交错的PPM阵列组成，它们稍微向侧面偏移，以使生成的波前仍然适当地干涉。该设计是在产生SH0导波模式的铝板上制造并进行实验评估的。前后产生的波比在20 dB以上，并且与有限元模拟非常吻合。