Phased-array probes are composed of several piezoelectric
crystals that can transmit/receive independently at different
times. To focus the ultrasonic beam, time delays are applied to
the elements to create constructive interference of the wave
fronts, allowing the energy to be focused at any depth in the
test specimen undergoing inspection.
This principle is illustrated in the right picture, where delay laws
have been computed to focus the acoustic beam at a specified depth
and angle. As shown in the figure, each element radiates a spherical
wave at a specified time. The superposition of these wavelets
results in an almost planar wave front at the specified location.
Before and after the targeted focal spot, wave fronts are
spherically converging and diverging, respectively. A few
examples of delay-law computation are displayed in the figure
hereunder. When no delay laws are applied, the resulting
ultrasonic beam is unfocused and is equivalent to the beam
generated by a conventional flat transducer. The natural “pseudo
focalization” evident in the image corresponds to the near-field
distance of the probe. The configuration illustrated in b
results in the same ultrasonic beam that would be generated by a
conventional flat transducer used in conjunction with a wedge.
Examples of delay-laws and visualization of the radiated
acoustic beam (displacement field). Calculations made using CIVA
simulation software: (a) no delay-laws applied, (b) steering
only, (c) depth focusing and (d) combined steering and depth
In this case, there is no focusing of the ultrasonic energy; the
applied delay laws result in steering of the ultrasonic beam.
Figures c and d are the same configurations as illustrated in a
and b, respectively, except that the delay laws have been
modified to focus the acoustic energy at a specified depth. In
both images (c and d), it is evident that the focal spot is
narrower and more localized. To obtain the same results with a
conventional probe would require using a specially designed
crystal shaped to obtain the desired focal point.
Principle of phased-arrays; delay laws calculated to focus at a
given depth and angle
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