Expanding-Aperture Annular Array1

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Abstract

A dynamically-focused annular array system for contact B-scanning has been developed. The design is based on a constant F-number approach, whereby, at short focal lengths, the aperture is increased in proportion to the focal length. This approach allows the use of larger area array elements, thus increasing the sensitivity of the system. Other major advantages include a substantial reduction in the time delays and refocusing rates required for the lens synthesis with a corresponding reduction in the electronic complexity of the system. The initial design employs an array operating at 2.25 MHz, with four annuli active at the near focal length of 1.5 cm. As the focal length increases, the array expands to a maximum of twelve rings, with 4.0 cm outer diameter, for focal lengths greater than 12 cm. A single, tapped delay line with 1 μs total duration provides the time delays for focusing on receive. A variable point or line focus is provided on transmit. Experimental measurements of the focusing properties of the system include beam profiles showing mainlobe width and sidelobe levels and B-scans of a standard test object.

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