Beam Steering
As noted in previous pages, the essence of phased array testing is an ultrasonic beam whose direction (refracted angle) and focus can be steered electronically by varying the excitation delay of individual elements or groups of elements. This beam steering permits multiple angle and/or multiple point inspection from a single probe and a single probe position.
As previously explained, ultrasonic beam characteristics are defined by many factors. In addition to element dimension, frequency and damping that govern conventional single element performance, phased array transducers behavior is affected by how smaller individual elements are positioned, sized and grouped to create an effective aperture equivalent to its conventional counterpart.
For phased array transducers N elements are grouped together to form the effective aperture for which beam spread can be approximated by conventional transducer models.
For phased array transducers, the maximum steering angle (at -6 dB) in a given case is derived from the beam spread equation. It can be easily seen that small elements have more beam spreading and hence higher angular energy content, which can be combined to maximize steering. As element size decreases, more elements must be pulsed together to maintain sensitivity.
Recalling that the practical limit for phased array transducer manufacturing restricts the smallest individual element width to 0.2 mm, the active aperture for a 16 element probe with 0.2 mm elements would be 3.2 mm. Creating an aperture of 6.4 mm would require 32 elements. While these transducers would no doubt maximize steering, the small apertures would limit static coverage area, sensitivity, and focussing ability.
The steering range can be further modified by using an angled wedge to change the incident angle of the sound beam independently of electronic steering.
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Grating Lobes and Side Lobes >>