providing an electromagnetic acoustic transducer (EMAT) assembly proximate to the thin metal wall to produce and cause a magnetic field to exist therein;
energizing an eddy current coil of the EMAT assembly with an RF toneburst signal of known amplitude and frequency to generate a Lorentz force in the thin metal wall and cause it to vibrate and launch ultrasonic compressional waves into the liquid contents;
allowing the compressional waves to travel through the liquid contents and reflect off an interface, the reflected ultrasonic compressional waves returning to the thin metal wall and causing it to vibrate in the presence of the magnetic field produced by the EMAT transducer assembly, the vibrations of the thin metal wall inducing a voltage in the eddy current coil of the EMAT transducer assembly; and
measuring an amplitude of the induced voltage in the eddy current coil to determine the degree to which the ultrasonic compressional waves were attenuated during their passage through the liquid contents, the degree of attenuation being an indication of the condition of the liquid contents, and comparing the measured attenuation against preestablished values of attenuation representative of known conditions of the liquid contents to determine the condition of the liquid contents.
Abstract: 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.
U.S. Class: (73/599) (73/52) (73/643) (73/290V)
IPC: (G01N 29/20)
US Patents Cited: 2277037 3357556 3553636 3802252 3832885 3913383 4208915 4384476 4399514 4821573 4848924 5167157 5372042
OG Section: Electrical
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