Steering with phased arrays

A directional SHM transducer that does not require either phase-array delays or connection switching to achieve steering is described in Xu et al. [63]. This directional sensor achieves tuning into preferential direction at certain discrete frequencies, which are the solution of a firequency-wavenumber equation. The transducer consists of a skew array of piezo-wafer active sensors placed at pitch values d, d.2 and angles a, 5 about the 1 and 2 axes, respectively (Figure 16.29(a)). The transducer achieves directivity at certain fi equencies Figure 16.29(b) illustrates the directionality measured with SLDV on an isotropic aluminum 45° at 105 kHz, 120° at 150 kHz, — 17° at 200 kHz, 88° at 280 kHz. Thus, the transducer directionality is controlled by the excitation firequency in discrete steps. 

As the process of steering and focusing is repeated for a sequence of look angles, a sector-shaped image data set is acquired. The line density available from phased-array scanners is not as restricted as with curvilinear arrays, but some limitations exist in certain systems, due to the relatively large size of delay increments available in tapped delay lines. Heterodyned systems and digital beam-formers have less limitations in this area. 

The Phase-array transducer (Nguyen and Tsai, 1977) that consists of several IDT segments with one period displacement (phase difference) between adjacent segments, using frequency control SAW beam steering (Fig. 3.16). 

Optical feed a single feed, usually a monopulse horn, is used to illuminate the array with a spherical phase front (illustrated in Fig. 13.64). Power collected by the rear elements of the array is transmitted through the phase shifters to produce a planar front and steer the array. The energy can then be radiated from the other side of the array, or reflected and reradiated through the collecting elements. In the latter case, the array acts as a steerable reflector. 

---