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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