Developments in Ultrasonic Phased Array Inspection I Understanding of Key Number in Phased Array D. Braconnier, Phased Array Consultant for KJTD, Japan B.-S. Yoon, H.-J. Lee, Nuclear Power Laboratory, Korea ABSTRACT Phased array technology for NDE started on a commercial basis in the 80’s~90’s in Germany and France. From the second part of the 90’s up until today, the number of phased array equipment manufacturers has increased from three companies to more than a dozen. However, this technology can give rise to many difficulties, misunderstandings of its limitations, and a lack of quality and easy to understand educational material. This paper presents the main specification parameters for phased array equipment and studies its influence on actual applications. It also includes experimental results to accompany adequate conclusions which should assist the reader in understanding what really matters in terms of technical effectiveness. For example, some of the important phased array concepts are the number of elements, bandwidth, and delay range and resolution. INTRODUCTION Ultrasonic testing (UT) phased array techniques consist of using UT probes with a number of small elements called arrays. Groups of elements are enabled, making an aperture, and the phase (delay) between elements is managed by the instrumentation in order to create constructive interferences on a focal point. In some cases, the groups of elements are scanned to virtually move the aperture along the probe by enabling and disabling elements. The advantages of phased arrays are mainly the ability to change the focal depth, the capacity to alter the concept of depth of field by updating the lens whilst receiving echoes (DDF), faster inspections by using electronic scanning, and, in some cases, less complex robot systems and beam deflection, which provides the user access to hard to reach inspection zones. This paper presents phased array technology from the user’s point of view, with resources which can be found on the market, but does not cover advanced or specific techniques only available in academia or research laboratories. This document will analyze an optimal set of parameters, which, if misunderstood, would lead to meaningless conclusions when making purchasing decisions based on brochures from equipment manufacturers. HISTORICAL BACKGROUND Originally, phased arrays applied with ultrasonic technology started long before the 1970’s with underwater sonar. NDT applications were in fact first experimented on mainly in Universities or Research Institutes. Around the middle of the 1970s, medical applications boomed, and medical UT with phased arrays never stopped improving. From the 1980s, the German institutes IZFP and BAM began offering equipment commercially for NDT field applications based on non-optical CCD analog delay lines. The process was limited to low frequencies, but was enough for train bogie inspections and some nuclear applications where focusing and deflection were key points. In the early 1990s, NDTSystems in France was offering a system that was able to focus and scan electronically based on variable analog delay lines and multiplexers. Much of phased array’s acceptance in the NDT field came from the success in the nuclear field with applications like inspecting fast neutron steam generators for Superphenix or crack detections in anti-rotation keys for shrink-on turbine discs. During the second part of the 1990s, RDTech provided the first commercial based semi-digital phased array that mainly addressed the Nuclear and Power Generation field. Also, in the USA, Infometrix had an annular phased array solution available for inspecting titanium on aircrafts. Since 2000, the number of phased array suppliers has increased tremendously, to the point where we can now find over 30 instrument and probe makers worldwide throughout Europe, North America and Asia. This recent boom has given rise to a numbers race for purely marketing reasons; such as the number of elements, bandwidth, delay range and delay resolution. In the 1990s, the maximum number of elements inside an aperture was from 16 to 32, it increased to 64 in 2005. In 2009, a third of makers began claiming to be able to handle between 128 to 256 elements, bandwidths ranging from 10 to 30MHz and delay ranges varying from 5µs to more than 200µs with steps from 20ns to as low as 1ns. But is such a race to improve figures really necessary? SOME BASIC TECHNICAL EXPLANATIONS CONSIDERING THE NUMBER OF ELEMENTS A major advantage of phased arrays is the ability to electronically focus an ultrasonic beam. Keep in mind that focusing is not a question of geometry. Even if all of the elements are directed to the target, it doesn’t necessarily mean that the focus point will be on the target. Focusing is a matter of phase. This means that the signals from all of the elements must reach the target with the same phase. This is important in cases of large bandwidth signals such as echoes. An element with a width in the range of the wavelength will have a wider directivity diagram. So, if all elements have enough contribution on a target point somewhere in front of the probe, it’s possible to delay their signals so that all the waves from the probe reach the target at the same time. Focusing is most commonly applied on both emission and reception, but sometimes only one or the other. If the aperture is large or the focal point is near, then the lateral resolution (sharpness) is good, but the depth of field is short. Also, the distance between the point of maximum sensitivity and the focal point along the propagation axis is short. On the contrary, if the aperture is small or the focal point is far, then the lateral resolution is large, and the depth of field is long. In actual fact, the lateral resolution is proportional to the focal depth and directly proportional to the aperture versus the wavelength, where the aperture is seen from the target point. The depth of field varies according to the square of the ratio between the focal depth and the aperture. By varying the lateral position of the focal point, it’s possible to steer the beam in order to create a sector scan. Also, the sensitivity along the depth and the depth of field can be adjusted by varying the position of the focal point along the depth. If we consider Dynamic Distance Focusing (DDF), we can simply reconsider the depth of field notion entirely. DDF consists of changing the reception electronic lens (delay pattern) in real time, so that the electronic lens is applied at the time corresponding to the depth where the echo is located on its return path. In such a case, the depth of field definition corresponds to an overall depth of field versus the depth of field from each electronic lens. Note that DDF can only be applied in reception as it is impossible to have different emission lenses propagating at the same time. Phased arrays have “N” elements, but an aperture uses only “n” elements. The selected aperture of “n” elements can be shifted along the “N” elements of the phased array with “s” element step. This virtual scanning technique is called electronic scanning. It has the advantage that the switching time between 2 apertures is as fast as the electronics allow. In addition, it is possible to alter the scan step or even a random selection of the order of the apertures which are scanned. The advantage is higher scanning speeds, and even opportunities to avoid one extra axis on the mechanical scanner. There are several kinds of phased arrays, the most famous being: annular arrays, linear small phased arrays, long linear phased arrays, line focus phased arrays, curved phased arrays, TRL phased arrays, Matrix TRL phased arrays and Matrix phased arrays. WHY A LARGE NUMBER OF ELEMENTS IS IMPORTANT Meaningless Focusing Range for Linear Probes The focusing technique has meaning only within a certain range. This range is given by the near field distance. In order to summarize with a good enough approximation, an aperture can indeed have efficient focusing only in a depth before the near field distance. The near field distance corresponds to the distance where the aperture naturally diffracts (refer to Fourier and Fresnel theories). Beyond this distance, we can consider that focusing will not be effective. Let us consider the minimum distance from where the phased array can focus effectively. It will be given by the highest angle of the element located at the extreme position of the aperture versus the position of the target point. The reference is done with the element directivity diagram. If the element is small, its directivity diagram will allow it to be sensitive to a wide range of directions (angles). If the element is large, the sensitivity is higher, but the range is narrower. If we consider an aperture limited to “n” elements, we can clearly understand that if the elements are large, we can have efficient focusing in locations farther from the probe, but the focusing from the aperture will not be near it. On the contrary, we can understand that, if the elements are small, the aperture will be able to focus effectively near the probe, from a short depth. However, the depth to which the effectiveness will hold will decrease. This means that if we want to focus on a zone near and far from the probe we have to increase the number of elements “n” inside the aperture. When deflection is used with large angles, the apparent aperture from the extreme angle target location decreases according to the cosine of the angle. At the same time, the near field distance decreases according to the square of the cosine of the aperture. So, large angle deflection considerably reduces the depth range where the focusing can be effective. Note that this is the main reason why the focus point set in phased array instruments does not match what the operator usually sees in the display. Grating Lobes Grating lobes are an undesirable characteristic of phased arrays. Grating lobes should not be confused with side lobes, where side lobes are an effect of the diffraction of the aperture. Side lobes are usually very close to the main lobes and quite short and low in amplitude. Grating lobes are the result of constructive interference between 2 adjacent elements with different delays but the same phase. This means that there will be an angle where the signal will be in phase, but the delay will correspond to 1 cycle. As usual, the echoes handled by probes ring over several cycles. Grating lobes have unique characteristics, they are very wide when elements are large, and they are also very sensitive, so it’s not a good idea to ignore them. If beam deflection isn’t applied and an element periodicity of 1 wavelength or smaller is used, then we will never reach conditions where grating lobes will appear. However, in order to avoid grating lobes when deflecting the beam, the element size must be even smaller than half the wavelength. Let’s consider using a 5MHz probe and water for coupling. We would need elements no larger than 0.3 to 0.15 mm to ensure that grating lobes do not appear. These are very small element width values, so it will take a large number of elements in the aperture to focus far from the probe. For example, 32 elements of 0.3 mm at 5MHz with a water path of 50mm will only provide a Fresnel distance of approximately 15mm in steel, meaning that it will not be effective to focus further. For such reasons, having a large number of elements when making an aperture is important. 1.2mm pitch probe 0.5mm pitch probe Figure 1 - 5MHz comparison① 1.2mm pitch (left) and ② 0.5mm pitch (right) with Sector scans of the same aperture width. We can clearly see the grating lobe in ① and how they are not appearing in ②. Matrix probe (2D array) Matrix probes are quite interesting. As they are 2D arrays, the element sampling is done along both axes. The major and obvious advantages are the ability to use either electronic point focusing, deflection with skew or tilt angles, or possibly all at the same time. The drawback is, since the sampling is on a 2D array instead of a 1D array, the number of elements varies at the square of the Concluding Remarks on the Number of Elements There are of course several other examples, but this paper won’t list them all. We can conclude that several applications can be addressed with only 8, 16 or 32 elements, but there are also applications requiring 64, 128 or even more than 256 elements in the aperture. BANDWIDTH, DELAY Re81.91845(n)-0.20.7265(R)4.1-87(96-4.0024( )-610.576(e)-1.90024( )-0.1495860)-2L495 In Figure 3, curves for different pitches are plotted (0.5, 0.7 and 1 mm). One can observe that the error due to the quantization of the focal law increases as the pitch decreases. It makes sense, because the delay precision is more important when the elements are close together. But it can still be considered negligible for the most part of actual applications. Figure 3 - Beam profiles (at the focal plane) of phased array radiation with quantized focal law. Influence of the pitch. F0 = 5 MHz, c = 5890 m/s, 32 elements, focal distance = 40 mm, delay step = 20 ns, θ = 0°, pitch = 0.5, 0.7, 1 mm. Figure 4 shows the beam profiles according to three focal distances: 20, 40 and 60 mm. The error is almost null on the high part of the main lobe in all 3 cases. Then, down to -25dB, the error is greater for the focal distance of 60 mm. On the other hand, the focusing at 20 mm involves a small error all over the x plane. It seems that the longer the focal distance, the greater the error. This short study elicits that a delay quantization inferior to 20 ns in the case of 5MHz array does not affect the profile of the beam. The difference appears underneath -25dB at the bottom of the main lobe, and is still negligible. We can therefore affirm that a delay step of less than 20 ns, 1/10 of the period of the cycle of the probe, does not have any benefit. This being said, recent specifications of 1ns delay resolutions are unnecessary and are purely for marketing reasons. Figure 4- Beam profiles (at the focal plane) of phased array radiation with quantized focal law. Influence of the focal distance. F0 = 5 MHz, c = 5890 m/s, 32 elements, pitch = 0.5 mm, delay step = 20 ns, θ = 0°, focal distance = 20-40-60 mm. Delay range There are also technical inconsistencies for delay ranges of the electronic lens. Except for particular applications like laboratory experiments, if we take the case of a linear array of “n” elements in the aperture, there is no way that the electronic instrument will have to set a maximum delay of more than (n x T ) where T=1/F (the cycle of the probe frequency). The reason is quite simple: if the delay difference between 2 consecutive elements is equal to 1 cycle, more or less, (if we were to be more rigorous we would take into account the space between the elements), the signal integration within these elements will be null. To get the maximum delay over 1 aperture, in the worst case, we multiply the limit (1 cycle) between 2 elements by the number of elements inside the aperture. Let us now apply this with actual values: Table 1 Probe Frequency Cycle Element in the aperture 10MHz 5MHz 5MHz 2MHz 1MHz 100ns 200ns 200ns 500ns 1000ns 32 32 64 32 16 Max electronic lens delay needed 3.2µs 6.4µs 12.4µs 16µs 16µs These cases are quite extreme, but the reason why it is difficult to conceive larger apertures when the frequency is dropping is due to the fact that the element size will become too large for a good coupling to the part; or to put it more simply, to ensure a correct handling of the probe. In the case of matrix probes, despite the fact that there are many elements in the aperture, the total delay is reduced along the side or diagonal of the array, because the elements are distributed along two dimensions instead of just one. From this, we can see that instruments providing a delay range of more than 20 to 40µs are over-specified and are seemingly targeting submarine applications. BESIDES THE NUMBER OF ELEMENTS, THE DELAY RANGE AND RESOLUTION, WHAT ARE THE OTHER KEY PARAMETERS FOR PHASED ARRAY EQUIPMENT AND PROBES? Crosstalk Crosstalk, in electronics as well as in the probe, is a parameter that is usually not specified, but can be very important. The reason for such little attention is probably due to the difficulty in achieving a high quality criterion and a reliable method of measurement. Remember that focusing in a phased array instrument requires the beam to be constructed without parasite lobes, or decreases in sensitivity, or lateral resolution. Crosstalk not only influences this ability to focus well, but also increases the dead zones after high reflection interfaces like surface echoes. More generally, crosstalk can decrease axial resolution. In some cases, crosstalk isn’t so critical, but in others, it could ruin the inspection. Figure 5 - Comparing a 5MHz 0.5mm pitch with -12dB crosstalk ① (left) and with less than 60dB crosstalk ② (right) In Figure 5, for both configurations, the same aperture widths were set when focusing at a 30mm depth where the laws are close to each other. Imaging is not possible in ① but possible only in ②. This is to say that crosstalk can disturb the shape and the sensitivity of the beam creating problems for demanding applications. Dynamic Range Currently, phased array instruments are built around digital processes. A lot can be said about dynamic range, but beyond mentioning this topic, it isn’t within the scope of this paper. Let’s just keep in mind that there are a lot of parameters to take into account when specifying the focusing process and guarantying an acceptable linearity along a working dynamic range. From the Perspective of the Probe Radius mode Although a large majority of phased array probes are made with piezo composite materials, which by nature provide less radius mode (Transversal mode of vibration, Poisson effect, from the piston mode) than conventional monolithic ceramic based phased array probes, it is something that shouldn’t be ignored. Also, the radius mode has the same effect as crosstalk and is even more critical in some cases. Reliability The probe’s main characteristics and performance must last over time. Reproducibility Apart from laboratory experiments, inspections are generally not executed only once. Most users are interested in doing the same inspection over a long period of time and several times at once, as inspection procedures imply, so one probe is usually not enough. The probe maker must be able to guarantee the number of probes over the time when it is manufactured and delivered, that the probe complies with specifications and meets tolerances. This is quite obvious, but actually very difficult to achieve in the NDT field, which seems to all too easily require specific cases or characteristics. CONCLUSION Phased array technology gives way to easier, faster and more precise inspections. In some cases, it’s the only solution, such as when access to the scan area is difficult or when the scan area itself is limited. Phased array has boomed in the market the past decade and its success must be justified. However, as it is still a new technology with complex physical and mathematical descriptions, new parameters are apprehended with the previous techniques. It’s important to pay close attention to what the real target of the inspection is so that proper means are used. Although it’s easy to succumb to evaluating equipment and methods based only on direct number comparisons, thinking that “bigger is better”, it is essential to keep in mind that success arises from gaining know-how and a good understanding of new techniques. That being said, application entities, standards organizations, education centers, institutes and laboratories must not give in to the pressures from equipment manufacturers, who are too easily tempted to flood the market with easy and convincing magical offers which don’t actually fit the needs. Fads and trends can play a big role in any field, but NDE is based on Physics and complex technology. One needs to distinguish between promising methods and just mere attraction to the latest number trends. Perhaps equipment manufacturers are not necessarily in the best position to define the main characteristics of their own equipment in a field as complex as phased array NDE. 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