افزایش دامنه سیگنال موج سطحی EMATها براساس تحلیل شبیه‌سازی 3 بعدی و روش آزمون متعامد

The amplitude of an ultrasonic signal generated by electromagnetic acoustic transducers (EMATs) is typically low when compared to those generated by contacting transducers, which restricts the application of EMATs in the fields of nondestructive testing and nondestructive evaluation. The transmission process of a surface wave EMAT is studied, based on a previously established 3-D model, with the aim of enhancing the amplitude of ultrasonic waves generated by the EMAT. The effect of changing various EMAT parameters on the surface wave is investigated, by utilizing the orthogonal test method. Results indicate that after optimization, the signal amplitude of the EMAT has increased by 25.2%.

Electromagnetic acoustic transducers (EMATs) are a type of non-contact, ultrasonic transducer, capable of generating or detecting ultrasonic waves. They can be particularly attractive for some applications in nondestructive testing (NDT) and nondestructive evaluation (NDE), requiring no liquid couplant, having the flexibility to generate and detect multiple wave modes, with the capability of operating on hot or moving components [1] and [2]. The ultrasonic wave amplitude excited by EMATs is usually very weak compared with that of the piezoelectric transducers, as the generation mechanisms are relatively inefficient. In practice, one attempts to compensate for the poor generation efficiency by maximizing transmitting power, often using narrowband and ultra-low noise receivers, and by introducing various signal processing methods [3], which inevitably increases the complexity and the cost of EMAT-inspection systems. Researchers have simulated and analyzed the working processes of EMATs, based on various physical models and have performed extensive investigations of the EMAT mechanisms [3], [4], [5], [6], [7], [8], [9], [10] and [11], in an attempt to reduce the issues associated with their low efficiencies. This previous work has extended the understanding of EMATs' operation principles, laying the foundation for the improvement of EMAT performance. The excitation of ultrasound generated by an EMAT is a multi-physics process, which involves the interaction of eddy currents, static and dynamic magnetic fields and the sample. To facilitate analysis, previous work has typically established simplified EMAT models, based on several preconditions, including using 2-D models to represent 3-D situations, assuming the static magnetic field is uniform, neglecting some of the influence of dynamic magnetic fields, or building models only for the intermediate working process of EMATs [3], [4], [5], [6], [7], [8], [9] and [10]. Based on these simplified models, researchers have previously studied the influence of a few EMAT parameters (such as lift-off distance, magnet-to-coil width ratio) on the amplitude of the ultrasonic signal and have enhanced the ultrasonic amplitude merely by improving the intermediate physical fields, including the eddy current, magnetic fields or Lorentz force fields, of the transmission processes of EMATs [3], [4], [5], [6], [7], [8], [9] and [10]. To date, however, there has been little work studying the method of enhancement of ultrasonic amplitude, by investigating and comparing the influence of various EMAT parameters on the amplitude, with a holistic approach considering the EMAT's transmission process as a whole. A novel 3-D modeling method has recently been proposed, for the meander-line-coil surface wave EMATs operating on aluminum plates [11]. The accuracy and integrity of the model have been significantly improved, modeling a more complete transmitting process of the surface wave EMATs, with consideration of the influence of the dynamic magnetic fields on the generation process. This paper presents an improved and extended study of the influence of various EMAT parameters on the amplitude of generated ultrasonic waves, and proposes an approach for surface wave EMATs to enhance their signal amplitude. This paper is organized as follows: firstly, the transmission process of a surface wave EMAT is studied, based on the simulation of a 3-D model; secondly, using the orthogonal test method, the influence of various parameters on EMAT generated ultrasonic wave amplitude is obtained, and these parameters are optimized to maximize the wave amplitude; and finally, experiments are conducted to validate the results of the proposed method.

Aiming at enhancing the signal amplitude of the surface wave EMATs for the inspection of aluminum plates, an optimal design method is proposed by using the orthogonal test method based on the 3-D model we previously established. The influence of various parameters on the amplitude is investigated and compared. Experiments indicate that the signal amplitude of the optimized transducer has increased by 25.2%, which verifies the validity of the proposed method. Within the given ranges of variation of EMAT parameters, conclusions are drawn as follows: (1) Although lift-off distance and excitation current play the most significant role in the enhancement of signal amplitude of surface waves, which are excited by the Lorentz forces due to static magnetic field, proper choice of EMAT parameters, like the thickness of magnet, the width of coil conductors, and the size relationship between magnet and coil, can significantly increase the signal amplitude. (2) For the given levels within the ranges of variation, increasing the excitation current, the thickness of magnet and the length of coil conductors, and decreasing the lift-off distance, and the width of coil conductors are advantageous to the enhancement of the signal amplitude. Moreover, the size relationship between the magnet and the coil should be that the magnet is 1.22 times wider than the coil and meanwhile shares the same length as that of the coil conductors. (3) Lift-off distance, excitation current and the width of coil conductors have similar magnitude effects, and have a much stronger influence on the signal amplitude of surface waves excited by the Lorentz forces due to the dynamic magnetic field than the static magnetic field. This makes it possible to adjust the properties of the excited surface waves according to the requirements of real applications.