A multi-objective structural optimization of an omnidirectional electromagnetic acoustic transducer

Abstract

In this paper an axisymmetric model of an omnidirectional electromagnetic acoustic transducer (EMAT) used to generate Lamb waves in conductive plates is introduced. Based on the EMAT model, the structural parameters of the permanent magnet were used as the design variables while other parameters were fixed. The goal of the optimization was to strengthen the generation of the A0 mode and suppress the generation of the S0 mode. The amplitudes of the displacement components at the observation point of the plate were used for calculation of the objective functions. Three approaches to obtain the amplitudes were discussed. The first approach was solving the peak values of the envelopes of the time waveforms from the time domain simulations. The second approach also involved calculation of the peaks, but the waveforms were from frequency domain model combined with the forward and inverse Fourier transforms. The third approach involved a single frequency in the frequency domain model. Single and multi-objective optimizations were attempted, implemented with the genetic algorithms. In the single objective optimizations, the goal was decreasing the ratio of the amplitudes of the S0 and A0 modes, while in the multi-objective optimizations, an extra goal was strengthening the A0 mode directly. The Pareto front from the multi-objective optimizations was compared with the estimation from the data on the discrete grid of the design variables. From the analysis of the results, it could be concluded that for a linearized steel plate with a thickness of 10mm and testing frequency of 50kHz, the point with minimum S0/A0 could be selected, thus the multi-objective optimization effectively degenerated to the single objective optimization. While for an aluminum plate with a thickness of 3mm and frequency of 150kHz, without further information it would be difficult to select one particular solution from the Pareto front.

Debonding Detection in Aluminum/Rigid Polyurethane Foam Composite Plates Using A0 Mode LAMB Wave EMATs

“…A modified EMAT configuration that can increase the horizontal magnetic flux density in the eddy current area [ 19 ] is shown in Figure 4 . According to Equation (1), for this EMAT configuration, the Lorentz force in the z-axis direction increases, which benefits the generation of A 0 mode Lamb waves [ 28 , 29 ].…”

“…A modified EMAT configuration that can increase the horizontal magnetic flux density in the eddy current area [19] is shown in Figure 4. According to Equation (1), for this EMAT configuration, the Lorentz force in the z-axis direction increases, which benefits the generation of A 0 mode Lamb waves [28,29]. The center frequency of the meander-line coil is determined by the spacing interval of adjacent wires and its relationship can be described by [30]…”

Characterizing excitability of Lamb waves generated by electromagnetic acoustic transducers with coupled frequency domain models

“…The first approach is implemented with the time-domain models, in which the bias magnetic field (for the practical EMATs) comes from the magnetostatic simulation while the eddy current distribution, and the generation and propaga- 1 These approaches were already discussed extensively in [23], where optimization of an omni-directional EMAT was conducted. tion of the Lamb waves are from time-dependent simulations.…”

Accelerated optimizations of an electromagnetic acoustic transducer with artificial neural networks as metamodels

“…Real coding is employed. As described in Wang et al (2017), the internal status of the MOGA program is tracked carefully to avoid unnecessary evaluations of the objective functions, so as to decrease the number of evaluations and the total time needed to accomplish the optimization. This is possible because the program is stochastic in nature, so when the value of an individual is not changed by the genetic operation, the corresponding objective function should not be computed.…”