Phased array transducers

FIGURE 6.7 Intracardiac ultrasound (A) shows the left atrium (LA) imaged from a phased array transducer in the right atrium. A contour of the LA is incorporated into an electroanatomical map (B) and also merged with a preacquired Computed tomographic angiogram. 

Another model-free method for active damage detection is based on directional transducers. For passive sensing. Section 16.9 has already discussed the use of directional strain rosettes (both piezo and optical) to effectively detect an acoustic source without the need for a structural model. For active sensing, directional transmission as well as reception of guided wave can be achieved with phased-array transducers. 

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). 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

An improvement over the individual transducers is thephased-array transducer systems that may have many elements (e.g., from 16 to over 128 elements) in a linear arrangement [23,27]. The phased-array electronic systems will pulse multiple small (e.g., 1 mm wide) elements in a timed sequence to create an ultrasonic beam with directional or focused properties based on the time phasing of the transducer pulses... 

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. 

Turnbull DH, Foster FS (1992) Simulation of B-scan images from two-dimensional transducer arrays Part H-com-parisons between linear and two-dimensional phased arrays. Ultrason Imaging 14 344-353... 

The use of annular array transducers provides high-resolution images over large areas. In fact, concentric annular ring elements driven by a phase-controlled pulse can shift the focal point along the transducer axis. 

The University developed a method of determination of the material residual strength, based on measurement of the change of phase velocity of ultrasonic waves, as well as an ultrasonic flaw detector-tomograph with multi-element transducers of the type of phased acoustic array. It enables control of the internal structure of materials and products of up to 300 mm thickness, with the resolution of up to 0.5 mm. In the same university, work on NDT is also carried out in the welding and electro-acoustic departments. 

Medical scanners have an array of transducers in each probe and are capable of providing thousands of measurements of the small changes in the velocity of sound as it passes through various phase changes within the body. This information is fed to computers that create a television picture of the internal tissues of the human body. 

Applications in organic liquids are another suitable field for coated resonators and are much more easily performed than in aqueous surroundings. In contrast to the complex effects in aqueous phases, the main interference to the mass effect in organic liquids occurs from viscosity, which can be compensated for using a dual array with an uncoated transducer and/or a non-imprinted coated device, as in the gas phase. The best compensation for non-specific effects and temperature fluctuations is achieved with a dual/ternary electrode geometry on one quartz plate. 

High frequency applications in which the wavelength is comparable to the scale of the composite macrostructure, show the full potential of composite structures. Impedance, bandwidth, and radiation pattern can be controlled in such systems in a sophisticated manner impossible in single-phase systems. By prepoling PZT fibers or ribbons before the assembly of the composite, it is possible to construct polar solids of new type for use in complex transducer arrays operating in scanning and focusing modes. 

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