Electromagnetic acoustic transducer (EMAT) for ultrasonic inspection of liquids in containers

摘要

An ultrasonic inspection technique using a specially designed electromagnetic acoustic transducer (EMAT) launches and receives longitudinal ultrasonic waves into a thin metal wall or thin metal foil seal of a container, causing it to vibrate and launch ultrasonic compressional waves into liquid contained therein. The contents of plastic containers having a metal foil seal forming one wall are easily inspected. The EMAT establishes a magnetic field in the surface of the metal parallel to the surface. Radio frequency (RF) eddy currents are also induced by the EMAT in the surface of the metal. A Lorentz force is generated in the metal surface according the vector product of J, the current density, and H, the magnetic field, and the force generated by the interaction of the perpendicular components of the magnetic field H and the eddy currents J is directed normally to the surface of the metal. This normal force oscillates with the frequency of the induced eddy currents creating ultrasonic compressional waves which propagate normal to the surface of the metal. In such thin-walled metal containers or thin metal foil seals, where the thickness of the metal is much shorter than the ultrasonic wavelength in the metal, the generation and reception process is analogous to the operation of a loudspeaker in air. In this application, the thin metal wall or foil acts as a membrane, with the Lorentz forces generated in the wall or foil causing the metal membrane to vibrate, generating ultrasonic waves in the liquid. Because the thin wall or foil is much more compliant than a thick piece of metal, much larger displacements are generated at the metal-liquid interface than for the thick-wall case, resulting in much larger signal amplitudes than in the thick-wall case.