Every piezoelectric element in a phased array probe behaves exactly like a piezoelectric crystal in a standard UT probe. Furthermore, if all the elements in an array are excited at the same time, the ultrasonic beam produced is closely similar to that produced by a single crystal of the same dimension. The difference between the beam produced by a phased array probe and that produced by an equivalent standard single crystal probe does not lie in the physical shape or size of the piezoelectric elements, but in the electronic management of the excitation of the elements in the array.
By introducing a unique time delay in the excitation of every single element in the array to two nanosecond precision (2x10 -9 sec), the ultrasonic wavefront experienced by the specimen being tested is now angled. For all inspection purposes, the phased array probe is now operating as a standard single crystal angled probe of the same dimensions as the array. This manipulation is generally known as 'beam steering'.
Another very important feature attainable by electronically manipulating the excitation of the elements in the array is the focusing of the beam. By carefully calculating the time delays of the single elements in the array on the basis of Snell's refraction law, it is possible to obtain constructive interference at a desired locus following Huygen's principle. The beam has now been electronically focused. The focal spot size is generally equal to, if not smaller than, that of a conventional focused probe.
A combination of beam steering and focusing is also easily attainable. In this way, a phased array system can closely simulate a single probe inspection which would otherwise require various standard probes at different beam angles and focusing depths.
It has to be noted that, although the beam produced by a phased array is almost identical to that of conventional probes, low amplitude diffraction grating lobes are created in addition to normal probe side lobes due to the constructive interference principle. If incorrectly specified, phased array generated ultrasonic beams can have very poor signal-to-noise performance.
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