Coupling mechanism of electromagnetic acoustical transducers for ultrasonic generation

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. (c) 2006 Acoustical Society of America.