Coupling mechanism of electromagnetic acoustical transducers for ultrasonic generation

Abstract

Electromagnetic acoustic transducers (EMATs) generate ultrasonic waves in metals through an electromagnetic coupling mechanism. A concept for EMAT generation, using a coil alone without a permanent magnet, but with a pulse generator and a sample, is introduced. A simplified equivalent coil circuit is given and has been validated by experimental measurements. Such an equivalent circuit can be used for variations in excitation current calculations, which have often been neglected in previous publications in this area but have proved to be of great importance in considering the efficiency and frequency characteristics of ultrasonic generation. The equivalent coil inductance is dependent on the distance between the coil and the sample, due to coil interactions with eddy currents and the Ampere current of the magnetization mechanism. Analytical solution for an annular coil above metal samples is given, and the influences of the lift-off, coil radius, material magnetic permeability and electrical conductivity on the equivalent coil inductance are discussed. Experimental measurements agree well with predictions.

Electromagnetic acoustic transducers for in- and out-of plane ultrasonic wave detection

“…An equivalent current J eq in air that generates an identical magnetic field in the coil of the receiving EMAT to what is generated by the current sheet, J ed, at depth z 0 , is assumed and is given by, Considering the magnetic source of size of w 0 wide and l 0 long with its centre at the origin of the coordinate system, the static magnetic field B 0 is mainly limited to this area. The magnetic field, B, at point (x, y, z) produced by the current sheet element due to the particle vibration within the area between -w 0 /2 to +w 0 /2 in x-axis and -l 0 /2 to +l 0 /2 in y-axis, where B 0 has significant amplitude, is given by 10) where r is the distance between the eddy current element and the field point .…”

“…Only particle vibrations within the skin depth of the electromagnetic wave can produce an effective contribution to the induced voltage signal [3,[9][10] . An equivalent current J eq in air that generates an identical magnetic field in the coil of the receiving EMAT to what is generated by the current sheet, J ed, at depth z 0 , is assumed and is given by, Considering the magnetic source of size of w 0 wide and l 0 long with its centre at the origin of the coordinate system, the static magnetic field B 0 is mainly limited to this area.…”

“…They operate via the Lorentz force or magnetostriction mechanisms or the use of both simultaneously [9][10][11][12] . EMATs are non-contact devices, and therefore ultrasonic waves can be measured with minimal disturbance to the ultrasonic wave itself or to the material.…”

“…The ultrasonic field generated by a circular coil EMAT can be modeled to analyze the influence of the various parameters of the EMAT design upon the ultrasonic generation mechanism [9][10] .…”

Eddy current generation enhancement using ferrite for electromagnetic acoustic transduction

“…Generating an eddy current in the core of a coil would lead to a reduction in the inductance of the coil and a reduction in the eddy current that could be generated in the target sample derived from a fixed voltage source at a particular frequency of current discharge. 2 In the case of electromagnetic acoustic transduction an eddy current is generated in the electromagnetic skin depth of a sample. This eddy current interacts with the coil in the current itself to produce a repulsive force normal to the sample surface, generating an ultrasonic wave.…”