Electromagnetic Transducers for Generation and Detection of Ultrasonic Waves

Abstract

Electromagnetic Acoustic Transducers (EMATs) provide a noncontact method of generating ultrasound in metals. In its simplest form, an EMAT is a coil of wire and a magnet. An RF signal applied to the coil induces surface currents in the metal. These surface currents are acted upon by a Lorentz force due to the presence of the static magnetic field. This disturbance is transferred to the lattice of the solid by collisions resulting in body forces which drive the elastic wave. EMATs can be used to produce all of the wave modes produced by conventional piezoelectric and magnetostrictive transducers.

There are many advantages to EMATs which include operation without couplant, flexibility of wave modes, repeatability in sensor fabrication, and high temperature applications. The most significant disadvantage to EMATs is the low conversion efficiency; the loss is at least 50 dB, one way. However, achieving even this level of conversion efficiency requires special instrumentation, including carefully designed high-power RF drivers and very-lownoise preamplifiers. This means that EMAT applications should be chosen carefully. Inspite of this, EMAT technology is successfully making the transition from the laboratory to a versatile field inspection method.