声束的形成
The response of any ultrasonic test system is a combination of factors: the transducer used, the type of instrument used and its settings, and the acoustic properties of the test material. The responses produced by phased array transducers, like those from any other ultrasonic transducers for NDT, will be related both to transducer design parameters like frequency, size, and mechanical damping, and to the parameters of the excitation pulse that is used to drive it.
Four important transducer parameters will have a number of interrelated effects on performance.
Frequency-- As noted in the previous section, the test frequency has a significant effect on near field length and beam spreading. In practice, higher frequencies can provide better signal to noise ratio than lower frequencies since they offer potentially sharper focusing and thus a tighter, more optimized focal spot. At the same time, penetration in any test material will decrease with frequency because of increasing material attenuation as frequency goes up. Applications
involving very long sound paths or test materials that are highly attenuating or scattering will require use of lower frequencies. Commonly, industrial phased array probes are offered with frequencies betwen1 MHz and 15 MHz.
Element size-- As the size of individual elements in an array decreases, its beam steering capability increases. The minimum practical element size in commercial probes is typically around 0.2 mm. However if the element size is less than one wavelength, strong unwanted side lobes will occur.
Number of elements-- As the number of elements in an array increases, so can the physical coverage area of the probe and its sensitivity, focusing capability, and steering capability. At the same time, use of large arrays must often be balanced against issues of system complexity and cost.
Pitch and aperture-- Pitch is the distance between individual elements, aperture is the effective size of a pulsing element that is usually comprised of a group of individual elements that are pulsed simultaneously (virtual aperture). To optimize steering range, pitch must be small. For optimum sensitivity, minimum unwanted beam spreading, and strong focusing, the aperture must be large. Today's phased array instruments most commonly support focal laws for up to 16 element
apertures. More advanced systems allow up to 32 or even 64 element apertures.
The key concepts for a general understanding phased array beam can be summarized as follows: A group of elements is fired with a programmed focal law. This builds the desired transducer aperture and beam characteristics.
