This article is excerpted from the paper, “The promise of ultrasonic phased arrays and the role of modeling in specifying systems” presented by authors Guillaume Neau, Ph.D. and Deborah Hopkins, Ph.D. at the ASNT Fall Conference & Quality Testing Show held in Houston on October 23 - 27th, 2006. You may download the paper here.
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 wavefronts, allowing the energy to be focused at any depth in the test specimen undergoing inspection.
This principle is illustrated in the figure below, 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 wavefront at the specified location.
Principle of phased-arrays; delay laws calculated to focus at a given depth and angle.
Before and after the targeted focal spot, wavefronts 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.
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.
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 focusing.