Advances in Phased Array
The advancements in probe and controller hardware are expected to enable a new generation of phased array applications in a broad spectrum of industries. Ultrasonic phased arrays have come into their own in the past 10 years and have replaced conventional ultrasonics in many industrial applications. Recent advancements in phased-array technology, including massively parallel systems, a broader frequency range, conformable arrays and a variety of matrix probes, are creating unprecedented possibilities for maximizing resolution and inspection speed. At the same time, rapidly improving computer performance and powerful modeling capabilities allow 3-D displays in real time that greatly ease data interpretation. These advancements represent a leap in inspection flexibility that is enabling new applications.
Phased Array Principles
In contrast to conventional ultrasonic probes that use a single active element, phased array probes are composed of several piezoelectric crystals that can transmit and receive independently at different times. To focus and steer the ultrasonic beam, time delays are applied to the elements to create constructive interference of the wavefronts, allowing the energy to be focused at any point in the test specimen undergoing inspection.
Advantages of Phased Arrays
Phased arrays have several advantages over conventional ultrasonic probes that derive from the ability to dynamically control the acoustic beam transmitted into the structure under examination. Whereas a conventional probe has one focal length and a fixed size, shape and orientation, a phased array probe allows the operator to change the focal point and shape of the ultrasonic beam to optimize each inspection. A single phased array probe can, therefore, replace an entire toolbox of conventional probes and eliminate the associated calibrations and setups.
Phased arrays can reduce inspection times by eliminating or reducing the need for mechanical scanning by taking advantage of the ability to perform electronic scanning, accomplished by firing successive groups of elements in the array. Eliminating or reducing mechanical scanning also increases the reliability of the measurements by eliminating changes in-or loss of-coupling, which is a risk each time the probe is moved.
In addition to focusing and beam steering, operators also can optimize scanning strategies. For example, sectorial scanning is a useful tool in many cases for improving defect sizing. Most phased array controllers also offer dynamic-depth focusing, which allows measurements to be made at several depths in the same amount of time as it takes to perform a single-depth measurement using a conventional probe, while also maintaining consistent resolution throughout the thickness of the part. Another useful feature in some systems is the ability to specify focal spots in a computer-aided design (CAD) drawing of the test specimen.
Advancements in signal processing and modeling are greatly expanding options available for data visualization and analysis, improving the reliability of inspections.
Latest Developments and New Applications
Advancements have been made in both phased array controllers and probes. Controllers are available in either parallel (all channels can be active at the same time) or multiplexed architectures (only a subset of the total number of channels can be active at the same time). For example, to drive a 128-element probe with all elements active simultaneously requires a controller with 128 parallel channels. In contrast, a multiplexed system with 128 channels that limits the number of active elements to 32 would be sufficient for an application that uses 32 elements of the probe at a time to focus the ultrasonic beam and then electronically scans the beam by translating the active elements along the length of the probe.
A higher number of active channels provides greater focusing flexibility and greater depth penetration. A higher number of total channels allows larger probes to be used, permitting electronic scanning over larger areas, thereby reducing inspection times. Systems are available today with up to 128 parallel channels, and with multiplexed configurations up to a total of 512 channels (with 128 active at one time), with much bigger systems under development.
Until recently, phased array controllers had a frequency operating range from about 1 to 25 megahertz (MHz). To meet the needs of the nuclear power industry, a low-frequency system-50 kilohertz (kHz) to 5 MHz-was developed by the French Commissariat a l’Energie Atomique (CEA) and a phased array manufacturer to allow inspection of 1.2-meter-thick concrete barriers. Because of the thickness of the material and the heterogeneity of the concrete, the optimal frequency lies between 50 and 300 kHz. Using conventional ultrasonics at these frequencies results in signals with poor signal-to-noise ratios. The objective of the joint project was to use phased array techniques to improve detection resolution and sizing accuracy, as well as to validate simulation tools for heterogeneous material.
The methodology was tested on a representative concrete block containing artificial defects. A 250-kHz probe was first used to perform a conventional inspection of the test block. Eight identical 250-kHz probes were then phased to allow beam-focusing techniques to be employed. In this case, the probes were controlled by a low-frequency phased array system that can drive up to 64 conventional ultrasonic probes at one time.
To achieve high-resolution images, “full-matrix capture” was used to acquire the data. This means that each probe is fired in sequence, with all other probes acting as receivers. The full matrix of data allows post-processing beam-reconstruction techniques to be used to provide an image that is equivalent to what would be obtained by computing separate delay laws to focus the ultrasonic beam at each measurement point in the sample. The phased-array techniques resulted in a substantial improvement in resolution.
A second example illustrates how “full-matrix capture” is used to provide high-resolution images. In this case, measurements were performed on a calibration block with four side-drilled holes, indicated by the circles superposed on the ultrasonic images. The measurements were performed using a 64-element phased array probe. Each element was fired in sequence with all other elements acting as receivers. The difference in the images in moving from left to right is the number of points used in the reconstructed image. The graininess of the images disappears and the resolution of the defects improves with more points. With respect to probes, advancements include new generations of matrix and conformable arrays. Matrix probes are available in a variety of geometries, from rectangular and annular arrays, to geometrically shaped probes. Two-dimensional matrix probes allow focusing, steering and scanning of the ultrasonic beam in 3-D.
The images display the results of perpendicular sectorial scans projected inside a 3-D computer-aided design (CAD) drawing of the test specimen for three positions of the probe. The ultrasonic echoes coming from the side-drilled holes and sawcuts present in the block are clearly visible in the images. Real-time visualization of the ultrasonic images inside the CAD drawing is useful in detecting, locating and sizing defects.
Conformable arrays are a good solution when the inspection surface has an irregular shape. The measured displacements are accounted for when calculating the time delays applied to the individual elements to ensure that the beam is correctly focused inside the test specimen.
A tremendous advantage of phased array technology is that it is highly customizable and well suited to automation. The size, shape and geometry of probes can be designed for flexibility so that the same probe can be used for a variety of inspections, or they can be optimized for a specific application. Similarly, the phased array controller can be sized for flexibility or maximum portability. Handheld systems are routinely used for field inspections, whereas very large phased-array units have been integrated with robotic systems for large-scale automated inspections, for example, in the aerospace industry for fuselage inspection. Miniaturization continues and will increase the number of channels available in handheld systems. At the same time, the ability to electronically scan in 3-D in conjunction with development of massively parallel systems will enable new large-scale applications in which inspection speed and reliability are the industry drivers.
The advent of lower-frequency systems opens the door to inspecting thicker samples and heterogeneous materials, which, in turn, enables new applications for civil structures. Increased computing power and modeling capability are allowing sophisticated post processing and image reconstruction to achieve easily interpreted high-resolution images. Advancements in signal processing will continue to improve resolution and sizing capability, and will enable automated go/no-go decision-making. The expanding hardware and software options can make it more difficult for the operator to specify equipment, but highly accurate simulation tools allow modeling even complex parts, making it possible to optimize system and probe requirements.
The advancements in probe and controller hardware, in conjunction with developments in signal post processing and simulation, are expected to enable a new generation of phased array applications in a broad spectrum of industries. Partnerships between suppliers, potential operators and research institutes have proven to be an effective mechanism for application development, and the suitability of phased array technology to customization should accelerate development.