Liu Zenghua, Deng Liming, Zhang Yongchen, Li Aili, Wu Bin, He Cunfu
College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China.
Ultrasonics. 2020 Apr;103:106094. doi: 10.1016/j.ultras.2020.106094. Epub 2020 Jan 30.
In the traditional electromagnetic acoustic transducer (EMAT) based on Lorentz force mechanism, to meet the principle of constructive interference, the coil center distance is generally set to be half of the wavelength of the specified mode. The fixed center-to-center coil produces a Lorentz force under the action of a uniform static magnetic field provided by the magnet, thereby producing a specified mode signal that satisfies the constructive interference. In the above principle, the center distance of the coil is fixed, and applied with a uniform static magnetic field, which the coils with different center distances are combined with the dispersion curve to control the mode of the generated signal; that is, tuning the signal mode by changing the center distance of the coil. Another way to tune the signal mode is by changing the configuration of the magnet. Adopting appropriate waves for the identification of individual types of defects facilitates faster and more accurate detection. When using EMAT, some specifications of EMAT need to be changed, which can be inefficient and costly. To solve the problem, a mode-tuning magnetic-concentrator-type electromagnetic acoustic transducer (MT-MC-EMAT) is proposed in this study. This type of EMAT controls the mode of the generated signal by controlling the center distance of the static magnetic field provided by the magnet; that is, designing a new type of double-layer variable-pitch meander coil and different magnetic concentrators to select each coil. This method can tune the mode of the excitation signal by replacing the magnetic concentrator without changing a series of parameters, such as the coil, magnet, and excitation frequency. Different types of magnetic concentrators were added to a traditional EMAT to guide and concentrate the magnetic field of the permanent magnet, thereby changing the distribution of the magnetic flux density. These magnetic concentrators corresponded to meander coils with different pitches to satisfy constructive interference and achieve signal mode tuning. Both finite element simulation and experiment proved that the mode generated by this transducer was tunable after adding the different types of magnetic concentrators. Furthermore, experiments were conducted to examine the transducer characteristics. Finally, the configuration of the MT-MC-EMAT was optimized through orthogonal experiments. The influence of each parameter on the transducer efficiency of the proposed MT-MC-EMAT was studied, and the optimal parameter combination was confirmed.
在基于洛伦兹力机制的传统电磁声换能器（EMAT）中，为满足相长干涉原理，线圈中心距通常设定为特定模式波长的一半。固定的线圈中心距在磁体提供的均匀静磁场作用下产生洛伦兹力，从而产生满足相长干涉的特定模式信号。在上述原理中，线圈的中心距是固定的，并施加均匀静磁场，将不同中心距的线圈与色散曲线相结合来控制所产生信号的模式；也就是说，通过改变线圈的中心距来调谐信号模式。另一种调谐信号模式的方法是改变磁体的配置。采用合适的波来识别单个类型的缺陷有助于更快、更准确地检测。使用EMAT时，需要改变EMAT的一些规格，这可能效率低下且成本高昂。为解决该问题，本研究提出了一种模式调谐磁集中器型电磁声换能器（MT-MC-EMAT）。这种类型的EMAT通过控制磁体提供的静磁场的中心距来控制所产生信号的模式；也就是说，设计一种新型的双层变节距曲折线圈和不同的磁集中器来选择每个线圈。该方法可以通过更换磁集中器来调谐激励信号的模式，而无需改变一系列参数，如线圈、磁体和激励频率。将不同类型的磁集中器添加到传统EMAT中，以引导和集中永磁体的磁场，从而改变磁通密度的分布。这些磁集中器对应不同节距的曲折线圈，以满足相长干涉并实现信号模式调谐。有限元模拟和实验均证明，添加不同类型的磁集中器后，该换能器产生的模式是可调谐的。此外，还进行了实验来检测换能器特性。最后，通过正交实验对MT-MC-EMAT的配置进行了优化。研究了各参数对所提出的MT-MC-EMAT换能器效率的影响，并确定了最佳参数组合。
