Coil Electromagnetic Acoustic Transducer Research Articles

Analytical modelling and analysis of the meander-line coil EMATs

The meander-line coil electromagnetic acoustic transducer (EMAT) is widely used in the field of ultrasonic nondestructive testing due to its convenience to generate specific mode of guided waves. Some design methods of the meander-coil EMATs are developed in the frequency-wavenumber domain while others in the time–space domain. In this paper, a unified theoretical framework is developed by proposing an analytical model from the system perspective. Signal transfers between different physical fields in EMAT excitation, wave propagation and EMAT reception are represented as linear time–space-invariant systems. Taking the Rayleigh wave EMAT detection as an example, the analytical model for the transfer functions of these systems is established. The analytical model is experimentally verified by different Rayleigh wave detection techniques: the conventional EMAT, the spatial pulse compression (SPC) EMAT, temporal-spatial pulse compression (TSPC) EMAT and detection cases employing the same receiving EMAT. From the system perspective, the received signal of EMAT is interpreted as the response of the filter system to the input signal. It is found that the meander-coil EMAT can be regarded as the frequency domain expression during the detection. And the frequency domain expression plays different roles in different techniques.

Defect detection and imaging using electromagnetic acoustic transducer with butterfly coil.

Electromagnetic ultrasonic detection technology utilizes the electromagnetic coupling method to generate and receive ultrasonic waves without a couplant, which is suitable for rapid detection. However, the detection can be affected by the spatial distribution of the acoustic field and the polarization direction of the shear wave, which can result in suboptimal detection performance. The acoustic field directivity of the shear wave generated by the butterfly coil electromagnetic acoustic transducer was measured using the transmission method. The data indicate that the acoustic pressure amplitude of the shear wave is maximized along the axis of the acoustic field, thereby meeting the requirements of synthetic aperture focusing technique imaging. We used the reflection method to detect the through-hole defects and investigated the effect of shear wave polarization direction. By comparing the experimental data and imaging results, it can be concluded that higher echo amplitudes are obtained when the polarization direction of the shear wave is perpendicular to the axis of the through-hole defects. Based on the explosive reflection model, the frequency domain phase shift migration (PSM) method converts the time-domain signal to the frequency domain for processing and uses a phase-shift factor for layer-by-layer imaging. We used the PSM method to process the experimental data, which not only produced high-resolution images but also had a high computational speed.

Enhancing Shear Horizontal Wave Generation With a Multilayer Coil Electromagnetic Acoustic Transducer

Enhancing Shear Horizontal Wave Generation With a Multilayer Coil Electromagnetic Acoustic Transducer

Design and Analysis of Meander Coil Electromagnetic Acoustic Transducer (EMAT) for Multi-mode Lamb Waves in Thin Plate

Design and Analysis of Meander Coil Electromagnetic Acoustic Transducer (EMAT) for Multi-mode Lamb Waves in Thin Plate

Generation of ultrashort Rayleigh wave pulses using the combination of temporal and spatial pulse compression technique

Rayleigh waves generated by electromagnetic acoustic transducers (EMATs) have been widely used in nondestructive testing and evaluations. To improve the signal to noise ratio (SNR) and spatial resolution of Rayleigh waves simultaneously, the combination of temporal and spatial pulse compression (TSPC) technique is proposed. By analyzing the topology diagram of the typical pulse compression technique, a frequency modulated signal acts as the stretcher and a wavelength modulated meander line coil acts as the compressor. Experimental results indicate that the ultrasound fields of Rayleigh waves using the proposed TSPC technique show a spike shape in the spatial domain. Compared with the typical meander-line coil EMATs, the TSPC technique shows significant improvement of the generated Rayleigh waves. The SNR has increased by 12 dB while the main lobe width has decreased by about 98%.

A new chevron electromagnetic acoustic transducer design for generating shear horizontal guided wave

Guided wave electromagnetic acoustic transducers (EMATs) have a significant role in non-destructive testing and structural health monitoring. It can generate horizontally polarized shear waves propagating axially or circumferentially to characterize the shape and orientation of pipeline defects. This work proposes a new Chevron pattern-like coil structure design with a periodic permanent magnet (PPM) EMAT configuration. This specific design of the EMAT coil can generate a bi-directional shear horizontal wave (SH-wave) and reduce the side lobes. An optimal Chevron angle of coil wires assists in generating orthogonal propagatingwaveforms. These phenomena lead to the constructive interference of propagating waves and develop a resulting wave along the horizontal direction. A 3D FEM modeling and simulation have been carried out and validated with experimental results. The proposed EMAT results are compared with the conventional racetrack PPM EMAT model, which shows a significant improvement over conventional EMATs. A prototype of this proposed EMAT has been developed. It can be used to inspect surface defects in applications such as fuel transportation pipelines.

Study on the steering capability of a meander-line coil EMAT

Electromagnetic acoustic transducers (EMATs) are well-established as a means of ultrasonic wave generation and reception without the use of a mechanical coupling. When comprising a bias magnetic field and a meander-line coil (MLC), these waves propagate at an angle normal to the emission surface. With the appropriate frequency, the propagation pathway of these ultrasonic waves can be steered to a particular angle. This paper presents the methodology used to find the steering limit of an MLC EMAT and the results from simulations and experimental validations on aluminium. The results show that the maximum shear wave amplitude occurred at around 30°, the steering limit was approximately 50° and the simulations were validated by the experimental set-up to a satisfactory degree.

Enhancing the Lift-Off Performance of EMATs by Applying an Fe3O4 Coating to a Test Specimen

Electromagnetic acoustic transducers (EMATs) are noncontact ultrasonic transducers. The transduction efficiency of a particular EMAT on a given specimen is dependent on the lift-off distance, which is the distance between the EMAT coil and the specimen surface. The transduction efficiency drops dramatically with increased lift-off distance, requiring EMATs to be in close proximity to the specimen, usually within a few millimeters. This article proposes a new EMAT method of applying an Fe3O4 coating to the test specimen and quantitatively studying the enhancement effect of Fe3O4 coating on lift-off distance. To eliminate the interference of the electrical and magnetic properties of the tested specimen, a nonmagnetic and nonconductive glass specimen is selected. The experimental results on a glass substrate coated with Fe3O4 demonstrate the feasibility of EMATs generating and receiving ultrasonic waves through the coating, by a magnetoelastic mechanism. The transduction efficiency of EMATs on an Fe3O4 coating does not increase linearly with the bias static magnetic field, and the maximum measured signal amplitude value occurs at a relatively low flux density of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 0.12$ </tex-math></inline-formula> T. More specifically, it has been shown that the Fe3O4 coating can significantly enhance the lift-off distance of EMATs operating at 4 MHz to 8 mm on coated stainless steel. The performance of the Fe3O4 coating can be optimized, showing considerable potential to expand the application range of EMATs.

Measurement of Plate Thickness Based on the Optimal Unidirectional Generation of Ultrasonic Waves Using Dual Electromagnetic Acoustic Transducer

Unidirectional generation of shear horizontal (SH) ultrasonic guided waves can be achieved using dual-coil electromagnetic acoustic transducers (EMATs), through the interference mechanism when appropriately phased signals are used to drive each EMAT coil. Optimal unidirectional generation requires precise knowledge of the intended wave mode dispersion curve, which in turn, depends on the plate’s thickness. In this paper, we exploit optimal unidirectional generation with a dual-EMAT in order to measure plate thickness. The proposed thickness measurement method uses a dual periodic permanent magnet (PPM) EMAT, acting as transmitter, and two conventional PPM EMAT receivers on opposite sides of the transmitter, in order to receive the forward (enhanced) and backward (weakened) generated SH ultrasonic guided waves. Then, two consecutive frequency sweeps are used to generate the SH0 and SH1 modes, in order to seek the maximum ratio of the forward travelling wave amplitude to the backward travelling wave amplitude. The first sweep determines the optimum operating frequency for the first wave mode, which is a necessary parameter for the next sweep. The second frequency sweep determines the plate thickness. The new method was initially evaluated with a frequency-wavenumber-domain analytical model. The proposed method showed a much higher frequency-specificity, which was assessed by a quality factor figure of merit defined by the ratio of the centre frequency to the -3 dB frequency bandwidth, when compared to that obtained using the conventional method that is based solely on the amplitude of the generated wave, and is therefore less susceptible to error in the thickness estimation. The method was experimentally evaluated with a dual-PPM EMAT on aluminium plates, showing good agreement with simulations and providing more than ten-fold higher specificity than the conventional method, and accurate thickness measurements, with an error lower than 0.8%.

Extending the Incidence Angle of Shear Vertical Wave Electromagnetic Acoustic Transducer with Horizontal Magnetization.

Angled shear vertical (SV) waves have been successfully employed in the non-destructive testing of welds, pipes, and railways. Non-contact meander-line coil electromagnetic acoustic transducers (EMAT) have many benefits in generating angled SV waves. The most important benefit is that the incidence angle of an SV wave can be controlled by the excitation frequency. However, the incidence angle of a traditional SV-wave EMAT is reported to be under 45 degrees in many cases. In this work, such cases are tested, and the problems of the received signal at large incidence angles are found to be due to wave interference and small signal amplitudes. An equivalent finite element (FE) model is established to analyze the problem, and the main reason is found to be the head wave. An alternative configuration of angled SV-wave EMAT with horizontal magnetization is proposed to reduce the influence of the head wave. Finally, the results from simulations and experiments show that the proposed EMAT has a larger signal amplitude and significantly reduced interference in large-incidence angle scenarios. Moreover, an incidence angle of an SV wave of up to 60 degrees can be achieved, which will help improve the performance and capability of nondestructive testing.

A new pulse electromagnet structure to model a transverse magnetic field to pick up the out-of-plane for laser-EMAT integrated system

In order for an electromagnetic acoustic transducer (EMAT) to be able to pick up the out-of-plane vibrations of ultrasonic waves generated by the laser and to facilitate the movement of the system, a new pulse electromagnet EMAT is proposed, which can be combined with a laser to form a new laser–pulse electromagnet EMAT integrated system. Because of the unique structure of the pulse electromagnet EMAT, it can not only generate a uniform transverse magnetic field in the shallow surface of the specimen just below the EMAT receiving coil but also leave enough space above the magnetic field to ensure the smooth radiation of the laser source on the specimen. The optimal parameters of the probe are determined by combining the finite element simulation model and the orthogonal analysis method. The simulation and experimental results show that the pulsed electromagnet can generate at least 0.22 T transverse magnetic field in the specimen when each coil of C-electromagnet, respectively, injects 20 A. The pulsed electromagnet combined with the EMAT coil is used to receive various modes of ultrasonic vibration velocity with the out-of-plane component. It lays a foundation for the research on the simultaneous detection of railhead surface and internal defects by using the new laser–pulse electromagnet EMAT integrated system through one scan.

Numerical Study and Optimal Design of the Butterfly Coil EMAT for Signal Amplitude Enhancement.

The low energy conversion efficiency of electromagnetic acoustic transducers (EMATs) is a critical issue in nondestructive testing applications. To overcome this shortcoming, a butterfly coil EMAT was developed and optimized by numerical simulation based on a 2−D finite element model. First, the effect of the structural parameters of the butterfly coil EMAT was investigated by orthogonal test theory. Then, a modified butterfly coil EMAT was designed that consists of three−square permanent magnets with opposite polarity (TSPM−OP) to enhance the signal amplitude. Finally, the signal amplitude obtained from the three types of EMATs, that is, the traditional EMAT, the EMAT optimized by orthogonal test theory, and the modified EMAT with TSPM−OP, were analyzed and compared. The results show that the signal amplitude achieved by the modified butterfly coil EMAT with TSPM−OP can be increased by 4.97 times compared to the traditional butterfly coil EMAT.

Study and comparison of pulse compression techniques with three types of excitation for high-temperature EMAT detection

To address the low conversion efficiency and high ultrasonic attenuation in the detection of hot castings and forgings, the pulse compression technique (PCT) is an important and efficient method for improving the signal-to-noise ratio (SNR) and range resolution of ultrasonic echoes. A finite element model was established for the process of detection using a spiral coil electromagnetic acoustic transducer (EMAT) with three types of excitation, these being chirp signals, Barkercoded signals and combined Barker-coded signals, and the PCT processions were also provided. The SNR and range resolution of the PCTs with three types of excitation were compared using simulations and verified by experiments. The self-developed and water-cooled EMAT with three PCTs was used to measure a high-temperature forging and the pulse compression effect was also compared. The results indicate that the PCT with Barker-coded excitation has the highest SNR and the PCT with chirp excitation and combined Barker-coded excitation processes a higher range resolution. When the tested forging is at 710°C and compared with the traditional tone-burst excitation, the SNR from the PCT with combined Barker-coded excitation can be improved by 8.5 dB and the ultrasonic echo width can be reduced by 100%.

Noncontact reception of ultrasound from soft magnetic mild steel with zero applied bias field EMATs

Electromagnetic acoustic transducers (EMATs) without added magnets or bias currents receive ultrasound as bulk or Lamb waves from soft magnetic iron or mild steels. EMATs typically include bulky permanent or electromagnets for biasing magnetic fields in ultrasound reception. New compact-coils-only EMATs without additional magnets excite bias fields with pulsed strong coil currents, limiting detection time. Here, short premagnetization pulses in a simple EMAT coil allowed longer ultrasound reception from mild steel and virtually zero externally applied bias field. An unexpected long-lasting magnetic imprint inside the metal was clearly responsible and not normally expected with soft magnetic ferrous materials. Suggestions are made for mechanisms of electroacoustic coupling and permanent magnetization, even of transformer irons, after the termination of an exciting field. This contributes to novel applications of compact-coils-only EMATs without added magnets.

Design Method of Multiwavelength EMATs Based on Spatial Domain Harmonic Control.

Conventional electromagnetic acoustic transducers (EMATs) are generally only used to generate and detect guided waves with a single wavelength, which increases their sensitivity at that particular wavelength but limits their application scenarios and the accuracy of defect assessment. This article proposes a design method for multiwavelength EMATs based on spatial-domain harmonic control. First, the EMAT model is analyzed, where it is then outlined that the eddy-current density distribution of the specimen is equivalent to the spatial low-pass filtering of the coil-current density distribution. This shows that the multiwavelength guided waves can be achieved as long as the spatial distribution of the coil-current density contains those multiple harmonics that are desired. It is then proposed that the structure of the EMAT coil is equivalent to the spatial sampled pulse sequences of a spatial signal. The coil parameter design based on pulse modulation technology is proposed. Taking a dual-wavelength EMAT design for Lamb waves as an example, details of the coil parameter design are presented. The simulation and experiment with the dual-wavelength EMAT proved the correctness of the proposed method. Finally, an experiment with a three-wavelength EMAT demonstrated the feasibility of the proposed method in designing multiwavelength EMATs.

Optimal Design of Spiral Coil EMATs for Improving Their Pulse Compression Effect

When chirp pulse compression technology (PCT) is applied in Electromagnetic Acoustic Transducers (EMATs), the frequency response characteristic of the EMAT testing system may be inconsistent with the chirp signal bandwidth. Furthermore, it causes a loss in the frequency spectrum and distortion of the ultrasonic echo waveform, leading to a decrease in the signal-to-noise ratio (SNR) and range resolution of the pulse compressed signal. To solve this problem which has not been further researched in the previous publications, we proposed a new circuit-field coupling finite element model for ultrasonic wave generation of the spiral coil EMAT for enhancing the pulse compression effect, and the excitation circuit for the EMAT is also considered in this model. Furthermore, the effects of the diameter of the spiral coil wire, matching method of the impedance matching network, and parameter of the matching components on the peak and width of the main lobe after pulse compression were analysed through simulations and experiments. Subsequently, the optimal combination of the EMAT coil conductor diameter, impedance matching method, and matching component parameters were determined and verified by experiments. The results show that the SNR and range resolution of the pulse compressed ultrasonic signal can be largely enhanced when the diameter of the spiral coil wire, impedance matching method, and the parameter of the matching components are chosen carefully. When the aforementioned parameters are appropriately selected, the amplitude of the main lobe can be increased 3.16 times, and the width of the main lobe is reduced by 31.7%.

Orthogonal Optimal Design Method for Spiral Coil EMAT Considering Lift-off Effect: A Case Study

This paper provided a method to improve the conversion efficiency and lessen the lift-off effect of spiral coil electromagnetic acoustic transducer (EMAT) for some specific situations, for example, irregular sample surface, online inspection and varying coating. Moreover, a new evaluation method for the performance of EMAT is proposed. To study the effect of the structural parameters on the performance of EMAT, the orthogonal test method with five factors and four levels is employed. Consequently, the degree of five factors on the lift-off performance and the theoretical maximum value of amplitude are obtained. The results show almost all factors have an obvious impact on the theoretical maximum value of amplitude, while only the magnet-to-coil distance can affect the lift-off performance effectively. The experiment results well agree with the numerical results, in which the improvement of the lift-off performance and the theoretical maximum value of amplitude is 31.94% and 78.79% respectively. In addition, the conversion efficiency of EMAT is increased by 125% when the lift-off is 1 mm. These imply the performance of spiral coil electromagnetic acoustic transducer is improved dramatically and the detectable lift-off is increased as well.

Differential Coil EMAT for Simultaneous Detection of In-Plane and Out-of-Plane Components of Surface Acoustic Waves

Surface acoustic waves, such as Rayleigh or Lamb waves, can be used to characterise the geometry of surface-breaking defects. For accurate measurement of defect length and depth, the in-plane (IP) and out-of-plane (OP) components of the displacement or velocity can be measured and compared. Electromagnetic acoustic transducers (EMATs) offer a simple, non-contact method of doing this, as it is possible to design detection EMATs such that they are sensitive predominantly to either the IP or the OP velocity component. Here we present a new design of differential coil EMAT which measures both components simultaneously, and through simple processing gives the IP and OP velocities. This is compared to previous designs, showing that these components are obtained accurately. The system is used to characterise three artificial surface-breaking defects in aluminium and one thermal fatigue crack in 10 mm thick steel plate under thin coating.

Modes Control of Lamb Wave in Plates Using Meander-Line Electromagnetic Acoustic Transducers

The multimode and dispersion characteristics of Lamb waves make them difficult to apply to nondestructive evaluation. This paper presents a paired configuration of a meander-line coil electromagnetic acoustic transducer (EMAT) to generate a single-mode symmetric and antisymmetric Lamb wave in aluminum plates. In the paired structure, the bias magnetic field of the EMAT that generates symmetric mode Lamb waves is perpendicular to the plate surface, while the bias magnetic field of the EMAT that generates antisymmetric Lamb waves is parallel to the plate surface. The symmetric and antisymmetric exciting forces generated by these two EMATs are consistent with the dispersion equations of single symmetric and antisymmetric Lamb wave modes, respectively. The numerical simulations and experiments verified that the presented paired configurations of meander-line coil EMATs can effectively control the generation of single-mode Lamb waves at low frequencies.

Multiple Wavemode Scanning for Near and Far-Side Defect Characterisation

The combination of ultrasonic inspections using different wavemodes can give more information than is available with single mode inspection. In this work, the response of shear and Rayleigh waves to surface-breaking defects propagating on the near-side and far-side of a sample is investigated. The directivity of shear waves generated by a racetrack coil electromagnetic acoustic transducer (EMAT) is identified and used to set an ideal separation for a pair of transmit-receive EMATs. Defects are indicated by a reduction in the transmitted Rayleigh wave amplitude, and by blocking of the shear wave. Used together, these can identify features in the bulk wave behaviour which are due to near-face surface-breaking defects, and give a full picture of both surfaces. By using a combination of the two wavemodes, the angle of propagation and length of any near-side defects can additionally be identified. A scanning method for samples is proposed.