Volume 2, Number 2 April 2003
TNT FYI:
Ultrasonic Phased Array
Advances in electronics miniaturization, computer processing capabilities and fabrication methods for ultrasonic transducers have reduced the cost of implementing ultrasonic phased array inspection systems. Though still expensive relative to conventional UT equipment, both manual and automated inspections using phased array are finding greater application in industry.
Advantages of Dividing Large into Small
Large conventional ultrasonic probes give good flat coverage but have a small beam angle. Small elements have a much larger beam divergence angle. It is this large angle that makes possible the most useful features of the phased array technique - beam steering and dynamic focusing. In its simplest form, an array is a single large transducer (piezoelectric material) cut into very small segments or elements. It takes less energy to excite the smaller mass of these tiny elements. As a result, they transfer and receive energy with much greater efficiency. Typically, element sizes range from 0.02 - 0.1 in. (0.5 - 2.54 mm) although arrays can have custom element sizes and can be arranged in custom configurations with the particular geometry developed to meet an ultrasonic application need. Linear, annular (circular elements divided into doughnut-shaped elements) and matrix arrays are basic configurations.
Virtual Transducer
The basic premise for all phased array transducers is that the small, individual elements are independently driven. Computer software defines which elements are activated. They may be pulsed in groups to simulate conventional transducer excitation or individually. The elements selected at any point in a sequence are a small fraction of the total number within an array. Selection of the elements to be activated, the delay sequence for firing, element amplitudes and delays in reception are all programmed into operating software. In effect, the computer creates a virtual transducer. Electronically controlling the time at which each individual element in the virtual transducer is activated varies the sound field characteristics resulting in a range of different focal points and beam refraction angles.Beam Steering and Dynamic Focusing. Beam steering is the ability to dynamically synthesize an ultrasonic beam of any angle within the overall beam spread of an individual element by sequentially firing each element in an array to create a wave front following the desired angle. Selecting the array firing order and pulse delays can also be used to dynamically focus an ultrasonic beam. Both beam steering and dynamic focusing can be changed on a pulse by pulse basis to effectively sweep through test material. They may also be combined to give a resultant beam both focused and angled. Additionally, excitation and receiver gain settings for each of the sets of elements can be controlled allowing amplitude density profiles in the integrated sound beam to be adjusted. This electromechanical process is call beam forming.
Electronic Positioning. Since practically all aspects of the sound beam are being controlled electronically, many iterations, or sequences of iterations, can be run in nearly real-time. This allows a single array to inspect a component with variable inspection angles and focusing depths almost simultaneously. For instance, depending on the array design and the component thickness, a one-dimensional linear array, with the major axis oriented normal to a pipe weld, may interrogate close to an entire planar cross-section of the weld by sweeping through a series of inspection angles without having to mechanically index the transducer. Theoretically, an entire pipe weld can then be examined with a single circumferential scan motion. By electronically moving the virtual transducer across the entire physical array, the sequential electronic positioning of the virtual transducer creates the same physical scanning motion that would be achieved by mechanically positioning a traditional transducer over the same physical distance equal to the total length of the array. The electronic scanning motion of automated inspection with phased array replaces mechanical motion, reducing wear and operating costs and increasing system reliability.
Conclusion. Initial costs for the electronics and software development required to operate phased array systems are more expensive than conventional ultrasonic counterparts. Also, array probes are much more expensive than standard ultrasonic transducers and each is generally designed for only a few applications at best. As more applications become commonplace, it is expected that demand for phased array systems and certain array probes will rise and manufacturers will produce these as off-the-shelf items. Use of phased array systems is expected to increase, simply to minimize inspection times or to address complex geometries and material conditions not being adequately inspected with conventional methods.
Adapted from texts contributed by Michael T. Anderson of Pacific Northwest National Labs and Terry Banach of Agfa NDT, Krautkramer Testing Machines.
Editor's note: Interested technicians are encouraged to browse the Internet for more information. Many phased array equipment manufacturers have Web sites with informative text, animated drawings and tutorials regarding phased array.
[ The NDT Technician ]
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