How to choose the right Phased Array probes for Ultrasound Phased Array

A transducer converts one form of energy into another, meaning a piezoelectric crystal converts electric energy into ultrasound energy, and reversely. Defining the right probe is the most important task to prepare for an application.
In fact, Phased Array probe should be determined even before selecting an instrument. Within Non Destructive Testing, and especially Phased Array ultrasonic testing, there are several types of probes each one dedicated to a specific area of application.
Phased Array probes can be standard or customized, and in front of a wide choice of shapes, sizes, frequencies and number of elements, it can appear to be a challenge choosing the good one. Hereafter is an introduction of some:

1D Linear Array probe:

The 1D linear is a Phased Array transducer with a vast majority of applications in its portfolio, and several configurations are now defined as standards. Like conventional UT, a 1D array probe can generate and receive an ultrasonic beam. Basic skills are beam steering, focusing, and electronic scanning in a single plane defined perpendicularly to the line of elements in the array.

The pitch is the distance center to center between each element. When defined as ʎ/2, it allows steering the beam without producing any grating lobe in the inspected material, it also defines today the current standard for designing the Phased Array probes. For single electronic scanning, a larger pitch can be defined to provide an increased coverage for better productivity.

Based on the application, required productivity and coverage, it is more or less a long transducer following the number of elements. Most common applications in regards to automated inspection system will meet the requirements for plate inspection, bars & billets inspection, composite inspection, tubes & pipes inspection, rail inspection, turbine discs, including reliable detection of surface, subsurface and wall thickness measurements. 

Linear Phased Array probe

2D Matrix probe:

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This type of Phased Array probes have the capability to deflect beams in several planes thanks to its elements spread on a 2D aperture, giving the ability to perform a complete volumetric inspection. When requiring to deflect beam within 3D space, the formula ʎ/2 need to be applied in both directions, potentially leading to a high number of elements.

Another advantage of such Phased Array transducer is to combine focusing and beam steering. In one direction we will use traditional ʎ/2 formula with the right pitch to reach the desired angle. On second direction we would use a larger pitch to adapt the focal depth, leading to an increase of energy of the beam, allowing thus better sensitivity and resolution.

For each application that requires the use of a 2D Ultrasound Phased Array probe, one objective is to maximize the reduction of elements to limit the cost, while keeping good acoustical results.

Matrix probes can be used to detect cracks in complex parts as for nozzle where the 3D geometry requires 3D beam steering skills.



TRL Probes:

TRL probes, based on both conventional and Phased Array probes, are dedicated to near surface inspections and enhanced focusing in complex material. Such Phased Array transducer combines two linear or matrix probes acting as separate transmitter and receiver, both settled on a common wedge to produce a focal area at the crossing of the two beams.

With the use of Ultrasound Phased Array TRL probes, also called dual array probes, it combines the advantages of acoustics to the versatility of Phased Array to provide efficient inspection close to the surface in rough structures. An example of application where such probe is performing well is weld inspection on complex materials.

TRL probes

The way of performing an inspection will influent on how to choose the right Phased Array transducer


Number of elements:

As the above information present most common types of Phased Array probes, this article also highlights one important criterion which is the number of elements. The number of elements is a key parameter as it will define the capabilities for Ultrasound Phased Array techniques, but also the specification for the multichannel system to drive it, which has economical consequence.

For each application, both services or systems in production or maintenance, the number of elements will be defined following the technical specifications.

The main parameter is the central frequency of the ultrasound wave; it will define the accuracy of the measurement (in time or depth, as for lateral resolution through the beam width) for the inspected material. The number of elements is then determined from the frequency value and from the Ultrasound Phased Array application.

Beam Steering:

Sectorial scanning consists in steering the beam electronically using the same Phased Array transducer position and aperture. It is necessary here to apply a fine pitch on the Phased Array probe based on the formula ʎ/2, where ʎ is the wavelength inside the inspected material considering the desired wave polarity (transverse or longitudinal). The number of elements depends on the inspected depth in the material: the size of the aperture must be large enough to supply energy and to focus the beam at the desired depth. Thus, the number of elements may increase quickly with frequency and depth as the pitch is small. 

Electronic scanning:

When one application does not require to deflect the beam with Phased Array probes, we can increase the pitch for a wider area to be covered! For a given number of elements, a larger pitch mechanically increases the size of the Phased Array transducer and it produces a larger coverage and a better productivity.

The size of the pitch will depend on the beam width in the region of interest in order to guaranty the overlapping during the electronic scanning. Thus, the pitch depends on the frequency and on the scanning step; It can reach several wavelengths.

Electronic scanning can be applied for thin or thick materials and higher the frequency is, the smallest the pitch will be. An Electronic scanning combined with sectorial scanning must apply the formula ʎ/2, and it would not benefit from an extended coverage.

Socomate International provides a wide range of Ultrasound Phased Array instrument for automated inspection system in many industries. To find the right probe and equipment for your application, contact us today!

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