DEVELOPMENT AND OPTIMIZATION OF A ROTATING
PHASED ARRAY INSPECTION SYSTEM
H. Rieder1, M. Spies1, R. Licht1 and P.Kreier2
1
Fraunhofer-Institute for Nondestructive Testing (IZFP)
University of Saarland, Bldg. 37, 66123 Saarbriicken, Germany
2
Innotest AG, Rosenstr. 13B, 8360 Eschlikon, Switzerland
ABSTRACT. This contribution describes the approach pursued in the development
of a miniaturized phased array system. Aiming at the detection of corrosion damage at
the inner and outer surface, an array transducer has first been optimized for immersion
inspection of pipes with high radial and axial resolution. In a second step, system components have been designed and developed on the basis of highly integrated electronic
components.
INTRODUCTION
Operating ultrasonic transducers as phased arrays, where each of the array elements can be pulsed with time delays, allows to control the beam shape and the sound
beam direction on a large scale. Appropriate array systems therefore offer excellent
capabilities for a variety of practical field applications. This contribution describes the
approach pursued in the development of a miniaturized phased array system. Aiming
at the detection of corrosion damage at the inner and outer surface, an array transducer
has first been optimized for immersion inspection of pipes with high radial and axial
resolution. On the basis of appropriate models the generated beam fields have been simulated and optimized for several radii of curvature of the components to be inspected.
In a second step, system components have been designed and developed on the basis of
highly integrated electronic components. Here, benefit has been drawn from the limited
and clear cut inspection parameters to be addressed, which are the frequency (up to 5
MHz), the sampling rate (20 ns) and the realtime capabilities. The inspection system,
which is splitted into a front-end and a back-end unit, is based on a DSP multiprocessor concept for fast signal processing, data evaluation and communication with a PC.
Additional performance is gained by intensively using FPGA technology for fast beam
steering and data preprocessing. To meet the requirements of real time applications a
bidirectional high speed optical link has been employed. The application-directed system development is exemplified and illustrated by simulation as well as experimental
results.
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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THE PHASED ARRAY TRANSDUCER
Transducer Modeling and Optimization
Targeting on the detection of corrosion damage at the outer surface of pipes a
transducer configuration has been elaborated previously [1]. The transducer has been
designed to meet the following specifications: focal spot of approximately 2 x 2 mm2
at the pipe backwall, main application to pipes of 300 mm diameter (wall thickness
10 mm), possible application to pipes of varying diameter (280 to 420 mm) without
dramatic loss in performance. The array with an aperture size of 25 x 20 mm2 is
operated at 2 MHz center frequency and consists of 32 elements (element width 0.8
wavelengths); its shape is convex in radial direction and concave in axial direction (ydirection), the radii of curvature being 75 mm, respectively. This probe has been further
modified in view of a simplification of the data processing algorithms and the electronics
of the inspection system. The transducer configurations have been elaborated by beam
field simulation using a point source superposition technique, which is briefly described
elsewhere in these proceedings [2], a detailed description can be found in [3]. The
beam field generated in the component using immersion technique is calculated threedimensionally, the evaluation is usually performed in the x-z- and the x-y-plane; also,
the particle displacement distribution at the component's outer surface is evaluated.
The optimization has been performed for three different pipe diameters: 280 mm, 300
mm and 420 mm. The following aspects have been examined in detail:
• increase of the diameter of the transducer carrying disk (and thus a decrease of
the transducer radius of curvature in axial direction);
• adaptation of the transmit-receive delay time discretization to reduce the requirements for the electronics (discretization in steps of 12.5 ns or 25.0 ns) and respective beam field evaluation;
• efficiency of a planar (in radial direction) aperture in view of a further improvement of the signal-to-noise ratio.
Results
For a pipe diameter of 300 mm, the immersion distance has been varied from 75
mm to 65 mm and 60 mm, where the axial radius of curvature has at the same time
been adapted. For the same water paths the calculations have also been performed for
280 mm and 420 mm pipe diameter. In Fig. 1, the results for the finally selected configuration are shown, which principally resembles the previous configuration [1]. However,
differences have been introduced in the shape (planar-convex instead of concave-convex)
and in the radius of the probe carrying disk (85 mm instead of 75 mm). Figure 1 shows
the beam fields generated in the components for the various diameters, as well as the
displacement distributions at the backwall (10 mm wall thickness). For the 280 mm
and 300 mm diameters, the focal spots (- 6 dB) are smaller than 2x2 mm 2 , while for 420
mm diameter the focusing in radial direction is at the first glance insufficient. However,
since the inspection unit progresses in axial direction, the performance of the transducer
is still sufficient for the detection of corrosion damage.
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d = 300 mm
d = 420 mm
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FIGURE 1. Beam fields (top) and focal spots (bottom) at the backwall for the 300 mm and
the 420 mm pipe. The features for the 280 mm diameter pipe are comparable to the 300 mm
case.
In Fig. 2 the delay times calculated for backwall focusing using Fermat's principle are shown; in these plots, the delay times resulting from the rounding algorithm
used in the steering electronics (discretization in steps of 12.5 ns or 25.0 ns) are also
shown. The beam field calculations reveal that for the 'rough' delay time discretization
of 25 ns no appreciable changes result. Finally, Figure 3 displays the maximal beam
field amplitudes for the standard pipe geometry (300 mm) for the various shapes, i.e.
convex vs. planar, and immersion distances. It can be recognized that decreasing the
(axial) radius of curvature and using a (in radial direction) planar transducer geometry
leads to an increase of the maximum field amplitude of about 7 %.
The transducer has been build up using piezocomposite material, as described
in Ref. [4]. Finally, it has been incorporated into a carrying disc (Fig. 4), which in
turn is mounted onto the frontend unit of the elaborated rotating inspection system.
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d = 300 mm
FIGURE 2. Exact and approximated delay times for array elements No. 1 (center) to 16
(rightmost). The delay times for elements No. 17 to 32 are symmetrical.
THE PHASED ARRAY INSPECTION SYSTEM
System Structure
Within an industrial-financed project with Innotest AG (Switzerland), a highly
integrated, modular electronic unit called ADAPT-US is currently developed at IZFP.
This system is based on modern field programmable gate array (FPGA) and digital
signal processing (DSP) technology [5] and is realized as a distributed processor system
in a frontend/backend design (Fig. 5). The technical data characterizing the system
are given in Tables 1 and 2. The frontend unit is an autonomous FPGA and DSP
based system, which is linked to a backend system by different interfaces. The unit is
capable to work as a general ultrasonic inspection system or as a phased array system.
The frontend system is a special development of IZFP. The design is easy to adapt to
different applications by the use of VHDL (Hardware Description Language for Very
high-speed integrated circuits') programming techniques of the FPGA and the use of
ANSI-C software for the signal processors. The backend system uses commercially
available high end DSP based hardware and software components.
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Immersion Distance [mmj
FIGURE 3. Curved versus planar aperture (in axial direction) efficiency for 300 mm pipe
diameter. The maximum backwall signal amplitude is plotted versus the immersion distance.
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TABLE 1. Technical characteristics of the inspection system.
Technical Data
3.9 liters
54 Watts
up to 5 kHz
distributed system based on FPGA and DSP
high speed bidirectional by an fiber optic
interface with up to 320 Mbit/sec;
synchron serial interface (SSI) with 20 Mbit/sec
based on ANSI C and numerical assembler
Frontend Unit
compact size
low power consumption
repetition rate
topology
communication
open software system
Backend unit
PCI-lnterface
2 DSP and 2 mezzanine interfaces
high speed fiber optic interface; SSI interface
signal processing, data processeing, ndt
algorithms
software
For the given application, a rotating inspection system for pipes, the system
works as a focusing phased array system with 16 delay channels. The use of SMD(surface
mounted devices)-technology, field programmable gate arrays and digital signal processor technology allows for a compact assembly of the phased array device. The embedded
multiprocessor system with the calculation power of four signal processors renders additional benefit in view of the implementation of modern signal processing algorithms for
realtime applications, such as data filtering, data compressing, peak detection, various
spectral analysis methods and special NDT evaluation algorithms.
The basic analogue/digital phased array module consists of a four channel unit
with one gate array (Fig. 6) as a channel processor (CP), which comprises all components to control standard ultrasonic or phased array applications, including transmitters, decoupling network, receivers, programmable amplifiers and A/D-converter modules. Within the gate array, based on Virtex technology from Xilinx, the transmit- and
receive delay units, fast fifo memories and a general bus interface is realized. Additional
logic blocks are preserved in the gate array for future implementation of preprocessing
algorithms.
I
FIGURE 4. Phased array probe (left) and probe carrying disc (right) to be mounted on the
frontend of the inspection system.
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FIGURE 5. Link structure of frontend and backend unit.
FIGURE 6. Four channel inspection unit (CP).
Four CP modules have to be linked together for a 16 channel phased array unit
or a general 16 channel ultrasonic inspection unit. An additional electronic print for
data preprocessing (DP) controles the CP modules. This module, which is based on an
integer DSP and a gate array, supervises the complete inspection, collects the raw data
from the CP modules and performs user defined preprocessing algorithms, such as peak
detection, averaging and numerical filtering. The DP additionally supports a scanner
control interface for passive evaluation of predefined drive patterns, such as comb or
meander patterns.
A floating point DSP processor (SHARC/Analog Devices) is the main processor (MP) at the frontend unit for controlling, data acquisition, signal processing and
communication with the backend unit. The master is linked to the PC by a high
speed fiber optic or a synchron serial interface for bidirectional data transfer. The data
streams via the interface are processed by two floating point digital signal processors
(SHARC/Analog Devices) at a PCI-interface for further signal processing. For the
given application of pipe testing, the complete frontend electronics are put to a casing,
which rotates in the water.
The design of ADAPT-US typically refers to a distributed system and allows for
the modification of the number of channels as well as the modification of data evaluation
by C-written software or by VHDL.
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FIGURE 7. Top: rf signal acquired during focusing on a spherical object (2 mm diameter),
the reflections from the sphere (far left) as well as the reflections from the bottom of the water
tank can be recognized; bottom: result of the peak detection process.
First Evaluation Results
After completion of the development of the ultrasonic phased array frontend
unit, the validation process of the implemented ultrasonic data preprocessing algorithms
has been started. The ultrasonic data acquisition and the various data preprocessing
algorithms, such as peak detection, gating technique or numerical filtering, are simultaneously processed by the gate array. As a first result, Fig. 7 shows the ultrasonic rf
data acquired during focusing on a small sphere in water. The amplitude result of the
real-time peak detection algorithm is applied to the modulus of the half-waves of the rf
signal and the according time-of-flight values. A few microseconds after the inspection
record has been finished, the results of different processing steps are available to the
master processor for further processing.
CONCLUSION
A wide variety of applications and benefits of ultrasonic array technology exists, especially when components with non-planar surfaces and materials with complex microstructures have to be inspected. However, ultrasonic array systems depend
on various sophisticated components, which are the piezo-composites used as transducer material, the electronic units to build up efficient driving devices and finally the
application-specific software to account for the proper calculation of the element time-
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TABLE 2. Characteristics of the analogue/digital components refering to the ultrasonic specifications.
Frontend Unit
analogue bandwith
channels
element number
converter
amplifier range
transmitter /receiver delay
pulser
pulse repetition rate
modes
adder
beam steering switch time for delay tuning
complete tuning cycle (gain, delay and record)
basic print size
volume
power consumption
methods for data acquisition
Technical Data
300 kHz to 8 MHz (-3dB-drop)
16 parallel
modulo 4
12 Bit/40MS/s (20,10,5 MS/s selectable)
40 dB per channel in steps of 0.1 dB
< 32 us in steps of 25 ns
Spike of 300 Volt
max. 5 kHz
phased array and parallel US
fixed to 1 6 Bit, all logic integrated in FPGA
4 us
40 us
1 5 0 x 9 0 x 10 mm
3.9 liters
54 Watts
RF signal, peak values, numerical filter, time
gates
delays. IZFP is therefore pursuing a multifold strategy aiming at the improvement
of each of these components. One outcome of this strategy is the rotating inspection
system described in this contribution. Further work on system evaluation and the
acquisition of in-field data is underway.
REFERENCES
1. Spies M., 'Current Activities in Ultrasonic NDE Simulations - A German
Perspective', in Review of Progress in QNDE, Vol. 18, eds D.O. Thompson and
D.E. Chimenti (Plenum Press, New York, 1999), 647-655
2. Spies M., 'Prediction of Transient Flaw Signals of the Ultrasonic Benchmark
Problem', in these proceedings
3. Spies, M., J. Acoust Soc. Am. 110, 68-79 (2001)
4. Spies M., Gebhardt W. and Rieder H., 'Boosting the Application of Ultrasonic
Arrays', in Review of Progress in QNDE, Vol. 21, eds D.O. Thompson and D.E.
Chimenti, Melville, New York, American Institute of Physics (AIP Conference
Proceedings 615), 847-854 (2002)
5. Rieder, H.; Bolwien, K.H. and Lattner, W., 'A Multi-Channel Ultrasonic Inspection
System for Aluminum Can Splice Testing', in Proceedings of the Annual Meeting
of the German Society for NDT (DGZfP) 1997 (DGZfP Publishing, Berlin, 1997),
671-678 (in German)
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