一种新型窄磁铁聚焦式电磁超声表面波换能器

A New Electromagnetic Acoustic Transducer Design With Narrow Magnet for Generating Focused Surface Wave

窄磁铁电磁超声换能器(electromagnetic acoustic transducer,EMAT)利用较强的边缘磁场能够在铝板中激发出比常规EMAT幅值更大的超声波信号,但是窄磁铁磁场会导致激发的波形发生畸变,从而影响超声检测的可靠性。该文设计出一种新型窄磁铁聚焦式表面波EMAT,不仅修正窄磁铁磁场引起的波形畸变,而且使得激发的表面波幅值得到有效提升。首先,考虑磁场、相位、洛伦兹力贡献度等因素,推导出窄磁铁EMAT激励超声的合成公式,并通过并联分裂导线排布实现对波形畸变和频率偏移的修正。其次,研究磁铁与线圈尺寸参数对换能器激励效率的影响,并通过三维仿真对超声传播特性和聚焦效果进行分析。最后,制备窄磁铁聚焦式EMAT并进行表面波激励实验,结果表明:该文设计的表面波EMAT不仅实现对窄磁铁磁场引起的波形畸变的修正,而且同等磁通密度下激发的表面波信号时域幅值比常规EMAT提高55.92%,频域基频幅值提升69.7%,这为电磁超声表面波换能效率的提升及缺陷检测信号的有效提取提供新方法。

Due to the high magnetic flux density at the edge of the magnet,the amplitude of the ultrasonic wave generated by the narrow-magnet electromagnetic acoustic transducer(EMAT)on the surface of an aluminum plate is larger than that of the wave generated by the conventional EMAT.However,the magnetic field of the narrow magnet may cause distortion of the ultrasonic signal,which affects the reliability of ultrasonic detection.In this paper,a new narrow-magnet EMAT for generating focused surface waves is designed,which not only corrects the waveform distortion,but also enhances the amplitude of the surface waves effectively.First,a synthesis equation of the ultrasonic signals generated by the narrow-magnet EMAT is proposed,including parameters such as the magnitude and phase of the magnetic flux density,and the contribution of the Lorentz force.The distortion of ultrasonic signals and frequency offset are corrected by the parallel split wire arrangement.Then,the influence of the sizes of the magnet and the coil on the excitation efficiency of the transducer is studied,and the focusing and propagation characteristics of the ultrasonic waves are analyzed in a 3D finite element simulation model.Finally,a new narrow-magnet focused EMAT is developed and the experiments for generating surface waves using the EMAT are carried out.The experimental results verify that the EMAT designed in this paper not only corrects the signal distortion,but also enlarges the amplitude of the surface wave in time domain by 55.92%,and that in fundamental frequency by 69.7%,compared with conventional EMAT under the same magnetic flux density.This research can provide a new method for the improvement of the excitation efficiency of the surface wave EMAT and the effective extraction of defect-detection signals.