Phased Pulsing and Its Effects
Whenever waves originating from two or more sources interact with each other, there will be phasing effects leading to an increase or decrease in wave energy at the point of combination. When elastic waves of the same frequency meet in such a way that their displacements are precisely synchronized (in phase, or 0 degree phase angle), the wave energies will add together to create a larger amplitude wave. If they meet in such a way that their displacements are exactly opposite (180 degrees out of phase), then the wave energies will cancel each other. At phase angles between 0 degrees and 180 degrees, there will be a range of intermediate stages between full addition and full cancellation. By varying the timing of the waves from a large number of sources, it is possible to use these effects to both steer and focus the resulting combined wave front. This is an essential principle behind phased array testing.
In conventional transducers, constructive and destructive interference effects create the near field and far field zones and the various pressure gradients therein. Additionally, a conventional angle beam transducer uses a single element to launch a wave in a wedge. Points on this wave front experience different delay intervals due to the shape of the wedge. These are mechanical delays, as opposed to the electronic delays employed in phased array testing. When the wave front hits the bottom
surface it can be visualized through Huygen's Principle as a series of point sources. The theoretically spherical waves from each of these points interact to form a single wave from at an angle determined by Snell's Law.
In phased array testing, the predictable reinforcement and cancellation effects caused by phasing are used to shape and steer the ultrasonic beam. Pulsing individual elements or groups of elements with different delays creates a series of point source waves that will combine into a single wave front that will travel at a selected angle. This electronic effect is similar to the mechanical delay generated by a conventional wedge, but it can be further steered by changing the pattern of delays. Through constructive interference, the amplitude of this combined wave can be considerably greater than the amplitude of any one of the individual waves that produce it. Similarly, variable delays are applied to the echoes received by each element of the array to sum the responses in such a way as to represent a single angular and/or focal component of the total beam. In addition to altering the direction of the primary wave front, this combination of individual beam components allows beam focusing at any point in the near field.
Elements are usually pulsed in groups of 4 to 32 in order to improve effective sensitivity by increasing aperture, which reduces unwanted beam spreading and enables sharper focusing.
The returning echoes are received by the various elements or groups of elements and time-shifted as necessary to compensate for varying wedge delays and then summed. Unlike a conventional single element transducer, which will effectively merge the effects of all beam components that strike its area, a phased array transducer can spatially sort the returning wavefront according to the arrival time and amplitude at each element. When processed by instrument software, each returned focal law
represents the reflection from a particular angular component of the beam, a particular point along a linear path, and/or a reflection from a particular focal depth. The echo information can then be displayed in any of several standard formats.
