Phased Array Principles
August 25, 2006 on 10:04 pm | In Probes, Technology, Phased Array |This article is excerpted from the paper, “The promise of ultrasonic phased arrays and the role of modeling in specifying systems” being presented by authors Guillaume Neau, Ph.D and Deborah Hopkins, Ph.D. at the ASNT Fall Conference & Quality Testing Show being 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 Figure 1, 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.
Figure 1: Principle of phased-arrays; delay laws calculated to focus at a given depth and angle.
Figure 2: 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.
Figure 3: Example of electronic scanning. A subset of the elements in the array are used to generate a focused beam at normal incidence; this beam is then translated across the test specimen by firing subsequent groups of elements without moving the probe.
Before and after the targeted focal spot, wavefronts are spherically converging and diverging, respectively. A few examples of delay-law computation are displayed in Figure 2. When no delay laws are applied (Figure 2a), 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 Figure 2b 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 2c and 2d are the same configurations as illustrated in 2a and 2b, respectively, except that the delay laws have been modified to focus the acoustic energy at a specified depth. In both images (2c and 2d), 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.
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Phased Array Principles
August 25, 2006 on 10:04 pm | In Probes, Technology, Phased Array |This article is excerpted from the paper, “The promise of ultrasonic phased arrays and the role of modeling in specifying systems” being presented by authors Guillaume Neau, Ph.D and Deborah Hopkins, Ph.D. at the ASNT Fall Conference & Quality Testing Show being 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 Figure 1, 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.
Figure 1: Principle of phased-arrays; delay laws calculated to focus at a given depth and angle.
Figure 2: 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.
Figure 3: Example of electronic scanning. A subset of the elements in the array are used to generate a focused beam at normal incidence; this beam is then translated across the test specimen by firing subsequent groups of elements without moving the probe.
Before and after the targeted focal spot, wavefronts are spherically converging and diverging, respectively. A few examples of delay-law computation are displayed in Figure 2. When no delay laws are applied (Figure 2a), 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 Figure 2b 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 2c and 2d are the same configurations as illustrated in 2a and 2b, respectively, except that the delay laws have been modified to focus the acoustic energy at a specified depth. In both images (2c and 2d), 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.
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[…] The advantages of phased-array systems include the ability to perform electronic scanning of the ultrasonic beam, which can reduce inspection times by eliminating or reducing the need move the probe. As illustrated in Figure 3, electronic scanning is accomplished by firing successive groups of elements in the array. A complete C-scan image can be obtained with a matrix phased array with the probe in a fixed position. The reliability of inspections can also be improved by reducing the need to move the probe. As is well known, good coupling between ultrasonic probes and the part undergoing inspection is crucial for good acoustic measurements. Each time the probe is moved, there is a risk of losing or degrading coupling. Thus, minimizing the number of times the probe is moved helps to maintain uniform conditions for multiple measurements. […]
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[…] The advantages of phased-array systems include the ability to perform electronic scanning of the ultrasonic beam, which can reduce inspection times by eliminating or reducing the need move the probe. As illustrated in Figure 3, electronic scanning is accomplished by firing successive groups of elements in the array. A complete C-scan image can be obtained with a matrix phased array with the probe in a fixed position. The reliability of inspections can also be improved by reducing the need to move the probe. As is well known, good coupling between ultrasonic probes and the part undergoing inspection is crucial for good acoustic measurements. Each time the probe is moved, there is a risk of losing or degrading coupling. Thus, minimizing the number of times the probe is moved helps to maintain uniform conditions for multiple measurements. […]
Pingback by Ultrasonic Phased Array Solutions » Advantages of Phased Arrays — August 28, 2006 #