Ultrasonic wave-based structural health monitoring embedded instrument G. Aranguren, P. M. Monje, Valerijan Cokonaj, Eduardo Barrera, and Mariano Ruiz Citation: Review of Scientific Instruments 84, 125106 (2013); doi: 10.1063/1.4834175 View online: http://dx.doi.org/10.1063/1.4834175 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Lamb wave structural health monitoring using frequency-wavenumber analysis AIP Conf. Proc. 1511, 302 (2013); 10.1063/1.4789062 Investigation of contact acoustic nonlinearities on metal and composite airframe structures via intensity based health monitoring J. Acoust. Soc. Am. 133, 186 (2013); 10.1121/1.4770237 Benchmark problems for predictive fem simulation of 1-D and 2-D guided waves for structural health monitoring with piezoelectric wafer active sensors AIP Conf. Proc. 1430, 1835 (2012); 10.1063/1.4716434 DEVELOPMENT OF ULTRASONIC SURFACE WAVE SENSORS FOR STRUCTURAL HEALTH MONITORING OF COMPOSITE WIND TURBINE BLADES AIP Conf. Proc. 1335, 1639 (2011); 10.1063/1.3592125 Temperature effects in ultrasonic Lamb wave structural health monitoring systems J. Acoust. Soc. Am. 124, 161 (2008); 10.1121/1.2932071
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 125106 (2013)
Ultrasonic wave-based structural health monitoring embedded instrument G. Aranguren,1 P. M. Monje,1,a) Valerijan Cokonaj,2 Eduardo Barrera,3 and Mariano Ruiz3 1
Electronic Design Group, Faculty of Engineering of Bilbao, University of the Basque Country, Bilbao, Spain AERnnova Engineering Solutions Ibérica S.A., Madrid, Spain 3 Instrumentation and Applied Acoustic Research Group of the Technical University of Madrid, Madrid, Spain 2
(Received 24 July 2013; accepted 12 November 2013; published online 17 December 2013) Piezoelectric sensors and actuators are the bridge between electronic and mechanical systems in structures. This type of sensor is a key element in the integrity monitoring of aeronautic structures, bridges, pressure vessels, wind turbine blades, and gas pipelines. In this paper, an all-in-one system for Structural Health Monitoring (SHM) based on ultrasonic waves is presented, called Phased Array Monitoring for Enhanced Life Assessment. This integrated instrument is able to generate excitation signals that are sent through piezoelectric actuators, acquire the received signals in the piezoelectric sensors, and carry out signal processing to check the health of structures. To accomplish this task, the instrument uses a piezoelectric phased-array transducer that performs the actuation and sensing of the signals. The flexibility and strength of the instrument allow the user to develop and implement a substantial part of the SHM technique using Lamb waves. The entire system is controlled using configuration software and has been validated through functional, electrical loading, mechanical loading, and thermal loading resistance tests. © 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4834175] I. INTRODUCTION
The growth of the electronic systems is contributing to the appearance of many diverse applications for these devices in transport environments, industry, and homes. Even mechanical structures, which are seemingly far removed from electronic technologies, have led to the creation of a field of study known as structural health monitoring (SHM), which collects all of the technologies and techniques developed to monitor aeronautical structures, bridges, gas pipelines, wind turbine blades, and other large structures. Many advantages are observed using this technology compared to more conventional but less effective visual inspection techniques that are often used. When SHM is used, operation costs and inspection periods are reduced and measurement precision is increased considerably. The maintenance costs of an aeronautic firm represent up to 11% of the total company costs,1 and a meaningful portion of this percentage is the result of necessary structural maintenance. Maintenance is typically carried out at pre-arranged technical shops, where most of the panels on the structures are removed to gain access to the unreachable areas for visual inspection without specialized instrumentation. New SHM techniques can improve the security of airline travelers, detect failures before they become threatening problems, and greatly reduce structural maintenance costs.2 Staszewski et al.3 presented several non-destructive testing (NDT) techniques to ensure the structural integrity of aircraft. Apart from the above mentioned (and very extended) periodic visual inspections, many techniques have been described as promising solutions, such as eddy current foil sensors4 based on currents induced in conducting materials a) Electronic mail: [email protected]
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by time-varying magnetic fields, comparative vacuum monitoring (CVM),5 microwave antennas,6 fiber Bragg gratings (FBGs)7 based on optical fibers that reflect certain light wavelengths, magnetic particle inspection, acoustic-emission inspection, acoustic-ultrasonic inspection, and electromechanical impedance8 or radiographic inspections. Ultrasonic inspection methods are based on the waves described by the English mathematician Horace Lamb. Lamb waves are ultrasonic elastic waves that propagate along the surface of solid and thick plates and can be generated and acquired using piezoelectric transducers via the following steps: 1. Piezoelectric transducers are excited using a voltage from a signal generator to produce ultrasonic waves. 2. The ultrasonic waves propagate throughout the structure, interact with obstacles/flaws/borders in the structure, and reflect back to the transducer. 3. The piezoelectric transducers acquire the reflected signals containing information about the dimensions of the structure and the obstacles found that modify the trajectory of the waves. 4. Using appropriate post-processing, a comparison between the signals obtained before and after the appearance of any defect can determine its position and severity. This technique has many advantages compared to other SHM techniques. The propagation scope of Lamb waves is on the order of many dozens of meters, making it possible to study a large structure using a series of piezoelectric transducers placed in one of its corners. The other techniques require the installation of several sensors distributed through the entire structure, not on only one of its sides. Moreover, most of the previous techniques are only capable of detecting damage
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the moment it is being generated and therefore require continuous operation to look for changes over a preconfigured threshold. Lamb waves can detect damage in the structure long after they have appeared; the evolution of the damage can even be determined from its first appearance. Rose presented an improved understanding of the fundamental principles of ultrasonic Lamb waves and the main areas of focus of this technology.9 In conclusion, Lamb wave-based techniques require less equipment and thus less weight and lower power consumption. Weight and power consumption savings are two essential requirements that must always be kept in mind when designing new systems for deployment on an airplane for SHM. A wide variety of SHM instrumentation is available. CompactRIO from National Instruments is an extended PC-based platform for testing, measurements, and controls with many possibilities for laboratory tests.10 Scan Genie from Acellent is a FBG interrogation system used to detect Lamb waves and acoustic emission events in both aluminum and composite substrates.11 Another example is DiAMon Plus, from Inasco Hellas, a dielectric and electrical monitoring system for real-time sensing of composites in manufacturing.12 Other equipment that uses ultrasonic techniques are USPC from Dr. Hilger,13 which provides non-contact ultrasonic testing with air using a single-shot technique, and the SMART platform from Alenia,14 a hybrid system that combines piezoelectric transducers and optical fibers. Apart from these systems, there are numerous experimental solutions that have been built using commercial signal generators and oscilloscopes.15 However, most of this equipment cannot be automated or used in real time because it is necessary to disassemble parts of the aircraft to perform the structural inspections. With the objective of improving the operational safety of aerospace structures and reducing an aircraft’s structural maintenance costs, the authors started a research and development line termed the Phased Array Monitoring for Enhanced Life Assessment (PAMELA). The result of this project is the PAMELA SHMTM system, which integrates the necessary hardware (PAMELA HW), including the integrated phased-array (PhA) of piezoelectric transducers and software (PAMELA SW). This SHM system has been designed for laboratory use and on-aircraft experiments. Moreover, PAMELA SHMTM can also be used to test other types of structures, including ships, buildings, bridges, pressure vessels, oil tanks, and pipelines. In this paper, the design of PAMELA HW is presented in Sec. II, the design of the PhA transducer in Sec. III, the analysis techniques available for the system and PAMELA SW in Sec. IV, and the laboratory tests performed to validate the system in Sec. V.
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FIG. 1. General diagram of the PAMELA HW system.
nents (generation, acquisition, and processing) have been developed and integrated into the same electronic instrumentation. The PAMELA HW can generate any type of excitation signal for up to 12 piezoelectric transducers. Concurrently, it can acquire the response signals that propagate throughout the structure being tested and also perform the signal processing for damage detection in situ. The general diagram of the integrated electronic architecture inside PAMELA HW is presented in Figure 1. The Processor Module configures and controls the remaining modules in PAMELA HW and performs the signal processing required for each analysis algorithm. This module is based on an embedded PowerPC processor, which is inside a Virtex 5 field programmable gate array (FPGA) device from Xilinx Inc. The Excitation Module generates the excitation signals for each of the 12 output channels. The shape of the excitation signals is digitally synthesized for any desired signal inside the FPGA. An analog circuit converts the signals into analog voltages up to 40 V peak-to-peak. This module allows each channel to be excited independently of each other. The main characteristics of the excitation signals are shown in Table I. The Adaptation Module receives the excitation signals and adapts each of them to the corresponding piezoelectric transducer. It connects the Excitation Module and Acquisition Module with the piezoelectric transducers by providing
TABLE I. Configurable characteristics of the excitation signal. Characteristic
II. HARDWARE INSTRUMENTATION OVERVIEW
An ultrasonic wave-based SHM system consists of many piezoelectric transducers, one or more circuits to generate the actuator excitation signals, one or more circuits that acquire the ultrasonic signals from the transducers, and a processing system. In the system presented in this paper, these compo-
Number of channels Frequency range Voltage range Shape of signal Delay between signals
Value 12 independent or combined 30 kHz to 1 MHz 0–40 V peak-to-peak Sinusoidal, triangular, Hanning-windowed signal, white noise, sine sweep. . . 0–2500 ns (10 ns step)
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FIG. 3. Integrated PhA transducer showing underside and overhead views (left) and two different types of piezoelectric transducer sets: disk and rings (right).
FIG. 2. The PAMELA HW with all electronic components inside the composite box placed over the PhA transducer adapter and bonded to the structure to perform SHM testing.
a full-duplex connection between them with electric isolation and protection from short-duration, high-voltage signals. The Acquisition Module amplifies, filters, samples, and converts the analog waveforms received by the transducers into digital numeric values. The FPGA stores the samples in the Data Storage Module using a direct memory access (DMA) controller. The Communications Module communicates with the architecture and the external equipment through an Ethernet link or wireless connectivity link. The option for a wireless connectivity offers different possibilities.16 The electronic system is designed, manufactured, tested, and enclosed in a composite box. The fully assembled system is shown in Figure 2. The printed circuit board has 12 layers and over 800 components when fully assembled. The overall dimensions are 161 × 161 × 24 mm. PAMELA HW weighs 283 g (box included). PAMELA HW has been designed to carefully avoid any working issues. All of the chosen components and assembly process have been selected to be lead free and obey Restriction of Hazardous Substances (RoSH) regulations. There is a patent pending on the system.17 III. INTEGRATED PHASED ARRAY TRANSDUCER
The other key component required for the SHM instrumentation is the piezoelectric transducer. This critical component is responsible for converting the electrical signals into ultrasonic waves. Piezoelectric transducers are also reversible and can convert ultrasonic waves from/into electrical signals. There are many types of piezoelectric transducers18 in many sizes19 that can be dispersed in a structure in many ways.20 They can be used with both metallic and composite materials.21 In this project, the authors designed and manufactured their own phased array with piezoelectric transducers. The resulting integrated PhA transducer consists of a set of 12 aligned piezoelectric discs with a wrapping around the electrodes for receiving and transmitting elastic ultrasonic waves. The piezoelectric discs are assembled over a four-layer, halfrigid printed circuit board (PCB). Electromagnetic shielding layers are introduced into the PCB to separate the channels,
and one electromechanical multi-pinned connector with stiffening ring is mounted over the PCB. The PhA transducer is a single, small unit that can be easily installed on a structure’s surface by conformed bonding. The final exterior aspects of the PhA are shown in Figure 3. There is a patent pending on the design of the transducer.22 PAMELA HW connects to the array of 12 piezoelectric discs mounted on the integrated PhA transducer. The transducer is bonded to the structure using an appropriate epoxy adhesive. PAMELA HW is then installed on a structure connecting the hardware to the transducer adapter and attaching the hardware without any cables. Figure 4 shows an example of the transducer surface with the integrated PAMELA HW bonded onto an actual aircraft structure. The hardware is electromechanically coupled to and supported by the transducer without requiring additional fixtures on the structure. Despite the advantages of piezoelectric transducers, there are two major lead zirconate titanate (PZT) transducer failure modes that cause concern in field applications, transducer debonding and cracking,23 both of which can change the response and shift the resonant frequency of the PZT. A previous analysis based on the measurement of the admittance of each transducer in a wide range of frequencies and its comparison with a healthy admittance allows the transducer integrity to be verified automatically. IV. ANALYSIS TECHNIQUES IMPLEMENTED IN THE SYSTEM
The diverse number of SHM analysis techniques allows for the detection of many different types of defects. In metallic structures, it is possible to detect fatigue crack development, corrosion, joint debonding, stress corrosion cracking, or impact damage. In composite structures it is possible to detect
FIG. 4. The PhA transducer bonded onto a CFRP leading edge rib (left) and the PAMELA HW sensor electromechanically coupled directly onto the PhA transducer and avoiding contact with structure (right).
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FIG. 5. The PAMELA SW system Human Machine Interface (HMI) and some of the configurable parameters, including types of excitation signals, parameters of the selected excitation signal, windowing options, types of texts, and selected test parameters.
impact damage, delaminations (including growth), and bonding and debonding. There are many techniques used for damage detection; PAMELA HW can implement most of these damage detection techniques in any structure that has piezoelectric transducers. The flexibility and capacities of the FPGA, which constitutes the core of the electronic device, allow the system to adapt itself to different working modes and to perform several tasks in parallel. PAMELA HW is designed to be able to produce all types of excitation signals and transmit them to the piezoelectric transducers. Using the PAMELA control and configuration software (shown in Figure 5), the signal characteristics and parameters to be sent to the hardware can be selected. Traditionally, the most basic SHM technique is the simple test, which uses only one excitation and one acquisition channel. PAMELA HW can perform a combination of simple tests efficiently and consecutively. Due to this capability, PAMELA HW can carry out a Round-Robin test (both in simple and fast modes). A Round-Robin test consists of performing simple tests consecutively in all available channels, alternately changing the excitation channel between the 12 available transducers, starting in the first transducer and cycling through the rest, and acquiring their signals. Each single-channel simple test takes approximately 3 ms. Using advanced signal processing techniques and reconstruction algorithms, 2D or 3D images of the structure can be generated that present any changes found in the structure. Only 12 steps are necessary to acquire the entire 12 × 12 matrix in the fast Round-Robin test mode, whereas 144 steps are necessary in the simple Round-Robin test. In Figure 6, a screen-capture of a Round-Robin test is shown. Twelve single tests are superimposed, and the 144 acquired signals are shown. PAMELA
SW also allows the user to select and view any single test in particular. Transmitter beamforming24 is a technique based on the directional transmission of ultrasonic signals using an array of transducers. This spatial selectivity is achieved by exciting all of the array elements with an excitation signal but delaying each signal for a certain length of time from one transducer to another, increasing the damage detection ability. Placing the PhA transducer close to the edge of the structure and using a 10 ns timing between sampling the excitation signals allow PAMELA SHMTM to perform several tests to cover 180◦ of the structure being tested with high precision. The generated signals from a beamforming analysis are shown in Figure 7. The angle of action is determined as a function of the
FIG. 6. Signals from a Round-Robin example. Both the excitation signals (eight-cycle sinusoidal signals) and the resulting reflected signals are acquired and shown.
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FIG. 7. Excitation signals for the 12 channels (each color represents a different channel) corresponding to a test performed at an actuation angle of 68◦ 59 .
separation of the transducers and the propagation speed of the ultrasonic waves for the respective frequency of the excitation signal. For example, considering the propagation speed of Lamb waves in aluminum plates, it is possible to cover 180◦ with 400 different angles. This wide coverage also allows the user to perform a receiver beamforming analysis, which consists of acquiring the signals while changing the delay. The generated and received signals corresponding to a beamforming analysis with different angles are partially visualized in Figure 8. This figure illustrates how the delay between each new test changes the signal with respect to the previous one. In simple or fast Round-Robin tests, time reversal is a technique for focusing waves in an inhomogeneous medium.25 During the time reversal process, the elements of the array are excited and the ultrasonic signals are acquired and sent inverted to the medium. According to the time reversal concept, the signal that is acquired after the second test must be identical to the reversed signal that originated from the first transmission. There is likely a nonlinearity in the structure (i.e., a defect) when this criterion is not met.
FIG. 8. Acquired time-domain signals obtained from a transmitter beamforming analysis, shown in the PAMELA SW HMI.
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FIG. 9. Generated and acquired signals from a transmitter focusing test. These tests can last up to 2.5 ms.
The flexibility of the electronic system makes it suitable to generate 12 different signals for each piezoelectric transducer simultaneously. The technique of transmitter focusing26 first measures the backscattered signals from the first signal transmitted by a single element of the linear array of piezoelectric transducers. By changing the time delays between the excitation signals, the transmitted signal can be focused on a specific defect or structure section of interest to improve the response and final image resolution in successive iterations. An example of this test is provided in Figure 9. The previous algorithms are normally used for far-field damage detection. However, such techniques as a variation in the beamforming technique, known as adaptive beamforming, or triangulation algorithms can be successfully used for near-field analysis. The flexibility and independence of the 12 excitation signals allow the user to divide the PhA transducer into a pair of sub-arrays and calculate the distance to the defect in the near-field range. SHM systems often include an impact detection capability, as sensors must be able to measure the energy generated by an impact to estimate the damage in the structure. When using ultrasonic waves, the previous requirement is no longer needed, as it is possible to measure damage in the structure afterwards. In passive mode, PAMELA HW listens on all the transducers for a signal with an amplitude that is greater than a previously set threshold level. After an impact, any of the aforementioned SHM techniques can be used to determine the severity of the defect. It is also possible to synchronize two or more transducers for the inspection of rivet lines, structure unions, incidents of stiffener debonding, analysis of large dimensioned structures, and soldered junctions. This synchronization is called pitch-catch mode and uses multiple PAMELA systems. A PAMELA sensor can be used to excite and sense the structure, while other PAMELA sensor acquires the signals and senses the structural response, allowing combined and synchronized tests to be performed. Additionally, the flexibility in the hardware allows the user to filter acquired signals, change the sampling frequency, and use other techniques. All of the data from the acquired
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signals are sent to a computer using an Ethernet link. The developer can perform any required algorithms on the computer. In fact, algorithms can be included inside the processor of PAMELA HW to perform calculations in real time. These algorithms can be embedded in the system and made to work autonomously, perform the tests on their own, analyze data, and send only corresponding alerts back to the external computer. V. PERFORMANCE VERIFICATION TESTS
An extensive test campaign was performed to verify and validate PAMELA SHMTM and transducer performance and fulfill the design requirements. An overview of the performed tests, objectives, set-up, and results is presented herein. A. Static mechanical loading tests
The initial tests serve to verify the stiffness, durability, and resistance of the adhesives used for transducer bonding, to find the optimum adhesive or combination of several adhesives, and to find the optimum excitation frequencies. Ultrasonic structural responses were acquired for each load level step up to 70 kN and different test set-up conditions. All of the results were compared to verify the structural and signal integrity of the PhA. Figure 10 presents the laboratory environment in which these tests were conducted. B. Dynamic mechanical loading tests
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capacity to distinguish wave changes due to dynamic loading effects from the damage effects, and survivability of the PhA transducers (with and without coupling) based on RTCA/DO160F standards for categories S and R. Signal quality and signal integrity loss or distortion were measured for all tests. C. Thermal loading (uniform temperatures)
The PhA transducer performance, compliance, and resistance to thermal loading are of utmost importance to ensure efficient and reliable SHM. The thermal loading tests consisted of cooling the transducers to −60 ◦ C, heating to 150 ◦ C, and then checking for changes in the amplitude and phase of the resulting ultrasonic waves. D. Thermal cycling loading
Both the adhesive resistance and transducer/adhesive survivability at very high temperature gradients are of great importance for space applications. Gradients up to 35 ◦ C/min during 10 full cycles were applied simultaneously on the aluminum and carbon-fiber-reinforced polymer (CFRP) specimens. E. Thermal loading (non-uniform temperatures)
Although thermal loading is not traditionally performed, it was considered of interest to acquire knowledge on ultrasonic wave propagation when experiencing random temperature gradients. A lamp was used as a heat source for this application. Other tests of importance that were performed on/with PAMELA HW and PhA transducers were impact resistance loading tests (near field), electromagnetic interference tests, combined thermo-mechanical loading tests, SHM tests during the assembly of the aerospace structure, and SHM tests with the structure subjected to electrical currents.
Dynamic mechanical loading tests were performed to demonstrate that the coupled PhA transducer and PAMELA HW comply with the applicable aerospace standards (including durability requirements) when subjected to vibration levels specified for the appropriate installation. These tests were performed to demonstrate and verify the reliability of all transducer constituent components after the tests, the stiffness and structural integrity of the electromechanical connection of PAMELA electronic PCBs in a vibration environment, damage detection abilities on dynamically loaded structures, the
VI. SUMMARY
FIG. 10. Laboratory setup for PAMELA SHMTM loading tests: two PhA transducers are attached to the aluminum specimen (left), two PAMELA HW systems are attached on top of the PhA transducers (middle), and the two computers are connected through Internet links to the respective PAMELA HW (right).
This paper presents an overview of a new SHM system. This embedded SHM instrument is composed of a piezoelectric PhA transducer, compact hardware with several excitation signal generators, signal acquisition circuits, and configuration software. The system allows a number of diverse SHM analyses to be performed. An electronic system oriented to SHM analysis, called PAMELA, has been designed and developed. This instrument requires a high level of parallel operation, as it must perform numerous functions simultaneously. To meet these requirements, the system uses a powerful FPGA with an embedded microprocessor. Additionally, an easy-to-install, 12-element piezoelectric transducer array has been presented in this work. This array is a key part of the complete system, as it converts the electrical signals into ultrasonic signals and vice versa. The choice of different piezoelectric transducers will depend on the material of the structures and the structure location to be tested.
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This system instrumentation can incorporate specific software to configure any type of excitation signal or test mode and perform treatment on the acquired signals to detect and alert the user of any failure in the structure being scanned. The dimensions and weight of the instrument make it suitable for laboratory testing and on-aircraft installation in small quantities. The authors are currently working on an updated version that will be lighter and smaller and have lower power consumption, allowing for deployment on a large scale inside an aircraft or any other structure. ACKNOWLEDGMENTS
The authors are grateful to The Basque Government under the research ETORTEK and AIRHEM II and to the Centre for Industrial Technological Development (CDTI) under the research projects TARGET and PROSAVE2 for funding this research. 1 M.
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 125106 (2013)
Ultrasonic wave-based structural health monitoring embedded instrument G. Aranguren,1 P. M. Monje,1,a) Valerijan Cokonaj,2 Eduardo Barrera,3 and Mariano Ruiz3 1
Electronic Design Group, Faculty of Engineering of Bilbao, University of the Basque Country, Bilbao, Spain AERnnova Engineering Solutions Ibérica S.A., Madrid, Spain 3 Instrumentation and Applied Acoustic Research Group of the Technical University of Madrid, Madrid, Spain 2
(Received 24 July 2013; accepted 12 November 2013; published online 17 December 2013) Piezoelectric sensors and actuators are the bridge between electronic and mechanical systems in structures. This type of sensor is a key element in the integrity monitoring of aeronautic structures, bridges, pressure vessels, wind turbine blades, and gas pipelines. In this paper, an all-in-one system for Structural Health Monitoring (SHM) based on ultrasonic waves is presented, called Phased Array Monitoring for Enhanced Life Assessment. This integrated instrument is able to generate excitation signals that are sent through piezoelectric actuators, acquire the received signals in the piezoelectric sensors, and carry out signal processing to check the health of structures. To accomplish this task, the instrument uses a piezoelectric phased-array transducer that performs the actuation and sensing of the signals. The flexibility and strength of the instrument allow the user to develop and implement a substantial part of the SHM technique using Lamb waves. The entire system is controlled using configuration software and has been validated through functional, electrical loading, mechanical loading, and thermal loading resistance tests. © 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4834175] I. INTRODUCTION
The growth of the electronic systems is contributing to the appearance of many diverse applications for these devices in transport environments, industry, and homes. Even mechanical structures, which are seemingly far removed from electronic technologies, have led to the creation of a field of study known as structural health monitoring (SHM), which collects all of the technologies and techniques developed to monitor aeronautical structures, bridges, gas pipelines, wind turbine blades, and other large structures. Many advantages are observed using this technology compared to more conventional but less effective visual inspection techniques that are often used. When SHM is used, operation costs and inspection periods are reduced and measurement precision is increased considerably. The maintenance costs of an aeronautic firm represent up to 11% of the total company costs,1 and a meaningful portion of this percentage is the result of necessary structural maintenance. Maintenance is typically carried out at pre-arranged technical shops, where most of the panels on the structures are removed to gain access to the unreachable areas for visual inspection without specialized instrumentation. New SHM techniques can improve the security of airline travelers, detect failures before they become threatening problems, and greatly reduce structural maintenance costs.2 Staszewski et al.3 presented several non-destructive testing (NDT) techniques to ensure the structural integrity of aircraft. Apart from the above mentioned (and very extended) periodic visual inspections, many techniques have been described as promising solutions, such as eddy current foil sensors4 based on currents induced in conducting materials a) Electronic mail: [email protected]
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by time-varying magnetic fields, comparative vacuum monitoring (CVM),5 microwave antennas,6 fiber Bragg gratings (FBGs)7 based on optical fibers that reflect certain light wavelengths, magnetic particle inspection, acoustic-emission inspection, acoustic-ultrasonic inspection, and electromechanical impedance8 or radiographic inspections. Ultrasonic inspection methods are based on the waves described by the English mathematician Horace Lamb. Lamb waves are ultrasonic elastic waves that propagate along the surface of solid and thick plates and can be generated and acquired using piezoelectric transducers via the following steps: 1. Piezoelectric transducers are excited using a voltage from a signal generator to produce ultrasonic waves. 2. The ultrasonic waves propagate throughout the structure, interact with obstacles/flaws/borders in the structure, and reflect back to the transducer. 3. The piezoelectric transducers acquire the reflected signals containing information about the dimensions of the structure and the obstacles found that modify the trajectory of the waves. 4. Using appropriate post-processing, a comparison between the signals obtained before and after the appearance of any defect can determine its position and severity. This technique has many advantages compared to other SHM techniques. The propagation scope of Lamb waves is on the order of many dozens of meters, making it possible to study a large structure using a series of piezoelectric transducers placed in one of its corners. The other techniques require the installation of several sensors distributed through the entire structure, not on only one of its sides. Moreover, most of the previous techniques are only capable of detecting damage
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the moment it is being generated and therefore require continuous operation to look for changes over a preconfigured threshold. Lamb waves can detect damage in the structure long after they have appeared; the evolution of the damage can even be determined from its first appearance. Rose presented an improved understanding of the fundamental principles of ultrasonic Lamb waves and the main areas of focus of this technology.9 In conclusion, Lamb wave-based techniques require less equipment and thus less weight and lower power consumption. Weight and power consumption savings are two essential requirements that must always be kept in mind when designing new systems for deployment on an airplane for SHM. A wide variety of SHM instrumentation is available. CompactRIO from National Instruments is an extended PC-based platform for testing, measurements, and controls with many possibilities for laboratory tests.10 Scan Genie from Acellent is a FBG interrogation system used to detect Lamb waves and acoustic emission events in both aluminum and composite substrates.11 Another example is DiAMon Plus, from Inasco Hellas, a dielectric and electrical monitoring system for real-time sensing of composites in manufacturing.12 Other equipment that uses ultrasonic techniques are USPC from Dr. Hilger,13 which provides non-contact ultrasonic testing with air using a single-shot technique, and the SMART platform from Alenia,14 a hybrid system that combines piezoelectric transducers and optical fibers. Apart from these systems, there are numerous experimental solutions that have been built using commercial signal generators and oscilloscopes.15 However, most of this equipment cannot be automated or used in real time because it is necessary to disassemble parts of the aircraft to perform the structural inspections. With the objective of improving the operational safety of aerospace structures and reducing an aircraft’s structural maintenance costs, the authors started a research and development line termed the Phased Array Monitoring for Enhanced Life Assessment (PAMELA). The result of this project is the PAMELA SHMTM system, which integrates the necessary hardware (PAMELA HW), including the integrated phased-array (PhA) of piezoelectric transducers and software (PAMELA SW). This SHM system has been designed for laboratory use and on-aircraft experiments. Moreover, PAMELA SHMTM can also be used to test other types of structures, including ships, buildings, bridges, pressure vessels, oil tanks, and pipelines. In this paper, the design of PAMELA HW is presented in Sec. II, the design of the PhA transducer in Sec. III, the analysis techniques available for the system and PAMELA SW in Sec. IV, and the laboratory tests performed to validate the system in Sec. V.
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FIG. 1. General diagram of the PAMELA HW system.
nents (generation, acquisition, and processing) have been developed and integrated into the same electronic instrumentation. The PAMELA HW can generate any type of excitation signal for up to 12 piezoelectric transducers. Concurrently, it can acquire the response signals that propagate throughout the structure being tested and also perform the signal processing for damage detection in situ. The general diagram of the integrated electronic architecture inside PAMELA HW is presented in Figure 1. The Processor Module configures and controls the remaining modules in PAMELA HW and performs the signal processing required for each analysis algorithm. This module is based on an embedded PowerPC processor, which is inside a Virtex 5 field programmable gate array (FPGA) device from Xilinx Inc. The Excitation Module generates the excitation signals for each of the 12 output channels. The shape of the excitation signals is digitally synthesized for any desired signal inside the FPGA. An analog circuit converts the signals into analog voltages up to 40 V peak-to-peak. This module allows each channel to be excited independently of each other. The main characteristics of the excitation signals are shown in Table I. The Adaptation Module receives the excitation signals and adapts each of them to the corresponding piezoelectric transducer. It connects the Excitation Module and Acquisition Module with the piezoelectric transducers by providing
TABLE I. Configurable characteristics of the excitation signal. Characteristic
II. HARDWARE INSTRUMENTATION OVERVIEW
An ultrasonic wave-based SHM system consists of many piezoelectric transducers, one or more circuits to generate the actuator excitation signals, one or more circuits that acquire the ultrasonic signals from the transducers, and a processing system. In the system presented in this paper, these compo-
Number of channels Frequency range Voltage range Shape of signal Delay between signals
Value 12 independent or combined 30 kHz to 1 MHz 0–40 V peak-to-peak Sinusoidal, triangular, Hanning-windowed signal, white noise, sine sweep. . . 0–2500 ns (10 ns step)
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FIG. 3. Integrated PhA transducer showing underside and overhead views (left) and two different types of piezoelectric transducer sets: disk and rings (right).
FIG. 2. The PAMELA HW with all electronic components inside the composite box placed over the PhA transducer adapter and bonded to the structure to perform SHM testing.
a full-duplex connection between them with electric isolation and protection from short-duration, high-voltage signals. The Acquisition Module amplifies, filters, samples, and converts the analog waveforms received by the transducers into digital numeric values. The FPGA stores the samples in the Data Storage Module using a direct memory access (DMA) controller. The Communications Module communicates with the architecture and the external equipment through an Ethernet link or wireless connectivity link. The option for a wireless connectivity offers different possibilities.16 The electronic system is designed, manufactured, tested, and enclosed in a composite box. The fully assembled system is shown in Figure 2. The printed circuit board has 12 layers and over 800 components when fully assembled. The overall dimensions are 161 × 161 × 24 mm. PAMELA HW weighs 283 g (box included). PAMELA HW has been designed to carefully avoid any working issues. All of the chosen components and assembly process have been selected to be lead free and obey Restriction of Hazardous Substances (RoSH) regulations. There is a patent pending on the system.17 III. INTEGRATED PHASED ARRAY TRANSDUCER
The other key component required for the SHM instrumentation is the piezoelectric transducer. This critical component is responsible for converting the electrical signals into ultrasonic waves. Piezoelectric transducers are also reversible and can convert ultrasonic waves from/into electrical signals. There are many types of piezoelectric transducers18 in many sizes19 that can be dispersed in a structure in many ways.20 They can be used with both metallic and composite materials.21 In this project, the authors designed and manufactured their own phased array with piezoelectric transducers. The resulting integrated PhA transducer consists of a set of 12 aligned piezoelectric discs with a wrapping around the electrodes for receiving and transmitting elastic ultrasonic waves. The piezoelectric discs are assembled over a four-layer, halfrigid printed circuit board (PCB). Electromagnetic shielding layers are introduced into the PCB to separate the channels,
and one electromechanical multi-pinned connector with stiffening ring is mounted over the PCB. The PhA transducer is a single, small unit that can be easily installed on a structure’s surface by conformed bonding. The final exterior aspects of the PhA are shown in Figure 3. There is a patent pending on the design of the transducer.22 PAMELA HW connects to the array of 12 piezoelectric discs mounted on the integrated PhA transducer. The transducer is bonded to the structure using an appropriate epoxy adhesive. PAMELA HW is then installed on a structure connecting the hardware to the transducer adapter and attaching the hardware without any cables. Figure 4 shows an example of the transducer surface with the integrated PAMELA HW bonded onto an actual aircraft structure. The hardware is electromechanically coupled to and supported by the transducer without requiring additional fixtures on the structure. Despite the advantages of piezoelectric transducers, there are two major lead zirconate titanate (PZT) transducer failure modes that cause concern in field applications, transducer debonding and cracking,23 both of which can change the response and shift the resonant frequency of the PZT. A previous analysis based on the measurement of the admittance of each transducer in a wide range of frequencies and its comparison with a healthy admittance allows the transducer integrity to be verified automatically. IV. ANALYSIS TECHNIQUES IMPLEMENTED IN THE SYSTEM
The diverse number of SHM analysis techniques allows for the detection of many different types of defects. In metallic structures, it is possible to detect fatigue crack development, corrosion, joint debonding, stress corrosion cracking, or impact damage. In composite structures it is possible to detect
FIG. 4. The PhA transducer bonded onto a CFRP leading edge rib (left) and the PAMELA HW sensor electromechanically coupled directly onto the PhA transducer and avoiding contact with structure (right).
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FIG. 5. The PAMELA SW system Human Machine Interface (HMI) and some of the configurable parameters, including types of excitation signals, parameters of the selected excitation signal, windowing options, types of texts, and selected test parameters.
impact damage, delaminations (including growth), and bonding and debonding. There are many techniques used for damage detection; PAMELA HW can implement most of these damage detection techniques in any structure that has piezoelectric transducers. The flexibility and capacities of the FPGA, which constitutes the core of the electronic device, allow the system to adapt itself to different working modes and to perform several tasks in parallel. PAMELA HW is designed to be able to produce all types of excitation signals and transmit them to the piezoelectric transducers. Using the PAMELA control and configuration software (shown in Figure 5), the signal characteristics and parameters to be sent to the hardware can be selected. Traditionally, the most basic SHM technique is the simple test, which uses only one excitation and one acquisition channel. PAMELA HW can perform a combination of simple tests efficiently and consecutively. Due to this capability, PAMELA HW can carry out a Round-Robin test (both in simple and fast modes). A Round-Robin test consists of performing simple tests consecutively in all available channels, alternately changing the excitation channel between the 12 available transducers, starting in the first transducer and cycling through the rest, and acquiring their signals. Each single-channel simple test takes approximately 3 ms. Using advanced signal processing techniques and reconstruction algorithms, 2D or 3D images of the structure can be generated that present any changes found in the structure. Only 12 steps are necessary to acquire the entire 12 × 12 matrix in the fast Round-Robin test mode, whereas 144 steps are necessary in the simple Round-Robin test. In Figure 6, a screen-capture of a Round-Robin test is shown. Twelve single tests are superimposed, and the 144 acquired signals are shown. PAMELA
SW also allows the user to select and view any single test in particular. Transmitter beamforming24 is a technique based on the directional transmission of ultrasonic signals using an array of transducers. This spatial selectivity is achieved by exciting all of the array elements with an excitation signal but delaying each signal for a certain length of time from one transducer to another, increasing the damage detection ability. Placing the PhA transducer close to the edge of the structure and using a 10 ns timing between sampling the excitation signals allow PAMELA SHMTM to perform several tests to cover 180◦ of the structure being tested with high precision. The generated signals from a beamforming analysis are shown in Figure 7. The angle of action is determined as a function of the
FIG. 6. Signals from a Round-Robin example. Both the excitation signals (eight-cycle sinusoidal signals) and the resulting reflected signals are acquired and shown.
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FIG. 7. Excitation signals for the 12 channels (each color represents a different channel) corresponding to a test performed at an actuation angle of 68◦ 59 .
separation of the transducers and the propagation speed of the ultrasonic waves for the respective frequency of the excitation signal. For example, considering the propagation speed of Lamb waves in aluminum plates, it is possible to cover 180◦ with 400 different angles. This wide coverage also allows the user to perform a receiver beamforming analysis, which consists of acquiring the signals while changing the delay. The generated and received signals corresponding to a beamforming analysis with different angles are partially visualized in Figure 8. This figure illustrates how the delay between each new test changes the signal with respect to the previous one. In simple or fast Round-Robin tests, time reversal is a technique for focusing waves in an inhomogeneous medium.25 During the time reversal process, the elements of the array are excited and the ultrasonic signals are acquired and sent inverted to the medium. According to the time reversal concept, the signal that is acquired after the second test must be identical to the reversed signal that originated from the first transmission. There is likely a nonlinearity in the structure (i.e., a defect) when this criterion is not met.
FIG. 8. Acquired time-domain signals obtained from a transmitter beamforming analysis, shown in the PAMELA SW HMI.
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FIG. 9. Generated and acquired signals from a transmitter focusing test. These tests can last up to 2.5 ms.
The flexibility of the electronic system makes it suitable to generate 12 different signals for each piezoelectric transducer simultaneously. The technique of transmitter focusing26 first measures the backscattered signals from the first signal transmitted by a single element of the linear array of piezoelectric transducers. By changing the time delays between the excitation signals, the transmitted signal can be focused on a specific defect or structure section of interest to improve the response and final image resolution in successive iterations. An example of this test is provided in Figure 9. The previous algorithms are normally used for far-field damage detection. However, such techniques as a variation in the beamforming technique, known as adaptive beamforming, or triangulation algorithms can be successfully used for near-field analysis. The flexibility and independence of the 12 excitation signals allow the user to divide the PhA transducer into a pair of sub-arrays and calculate the distance to the defect in the near-field range. SHM systems often include an impact detection capability, as sensors must be able to measure the energy generated by an impact to estimate the damage in the structure. When using ultrasonic waves, the previous requirement is no longer needed, as it is possible to measure damage in the structure afterwards. In passive mode, PAMELA HW listens on all the transducers for a signal with an amplitude that is greater than a previously set threshold level. After an impact, any of the aforementioned SHM techniques can be used to determine the severity of the defect. It is also possible to synchronize two or more transducers for the inspection of rivet lines, structure unions, incidents of stiffener debonding, analysis of large dimensioned structures, and soldered junctions. This synchronization is called pitch-catch mode and uses multiple PAMELA systems. A PAMELA sensor can be used to excite and sense the structure, while other PAMELA sensor acquires the signals and senses the structural response, allowing combined and synchronized tests to be performed. Additionally, the flexibility in the hardware allows the user to filter acquired signals, change the sampling frequency, and use other techniques. All of the data from the acquired
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signals are sent to a computer using an Ethernet link. The developer can perform any required algorithms on the computer. In fact, algorithms can be included inside the processor of PAMELA HW to perform calculations in real time. These algorithms can be embedded in the system and made to work autonomously, perform the tests on their own, analyze data, and send only corresponding alerts back to the external computer. V. PERFORMANCE VERIFICATION TESTS
An extensive test campaign was performed to verify and validate PAMELA SHMTM and transducer performance and fulfill the design requirements. An overview of the performed tests, objectives, set-up, and results is presented herein. A. Static mechanical loading tests
The initial tests serve to verify the stiffness, durability, and resistance of the adhesives used for transducer bonding, to find the optimum adhesive or combination of several adhesives, and to find the optimum excitation frequencies. Ultrasonic structural responses were acquired for each load level step up to 70 kN and different test set-up conditions. All of the results were compared to verify the structural and signal integrity of the PhA. Figure 10 presents the laboratory environment in which these tests were conducted. B. Dynamic mechanical loading tests
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capacity to distinguish wave changes due to dynamic loading effects from the damage effects, and survivability of the PhA transducers (with and without coupling) based on RTCA/DO160F standards for categories S and R. Signal quality and signal integrity loss or distortion were measured for all tests. C. Thermal loading (uniform temperatures)
The PhA transducer performance, compliance, and resistance to thermal loading are of utmost importance to ensure efficient and reliable SHM. The thermal loading tests consisted of cooling the transducers to −60 ◦ C, heating to 150 ◦ C, and then checking for changes in the amplitude and phase of the resulting ultrasonic waves. D. Thermal cycling loading
Both the adhesive resistance and transducer/adhesive survivability at very high temperature gradients are of great importance for space applications. Gradients up to 35 ◦ C/min during 10 full cycles were applied simultaneously on the aluminum and carbon-fiber-reinforced polymer (CFRP) specimens. E. Thermal loading (non-uniform temperatures)
Although thermal loading is not traditionally performed, it was considered of interest to acquire knowledge on ultrasonic wave propagation when experiencing random temperature gradients. A lamp was used as a heat source for this application. Other tests of importance that were performed on/with PAMELA HW and PhA transducers were impact resistance loading tests (near field), electromagnetic interference tests, combined thermo-mechanical loading tests, SHM tests during the assembly of the aerospace structure, and SHM tests with the structure subjected to electrical currents.
Dynamic mechanical loading tests were performed to demonstrate that the coupled PhA transducer and PAMELA HW comply with the applicable aerospace standards (including durability requirements) when subjected to vibration levels specified for the appropriate installation. These tests were performed to demonstrate and verify the reliability of all transducer constituent components after the tests, the stiffness and structural integrity of the electromechanical connection of PAMELA electronic PCBs in a vibration environment, damage detection abilities on dynamically loaded structures, the
VI. SUMMARY
FIG. 10. Laboratory setup for PAMELA SHMTM loading tests: two PhA transducers are attached to the aluminum specimen (left), two PAMELA HW systems are attached on top of the PhA transducers (middle), and the two computers are connected through Internet links to the respective PAMELA HW (right).
This paper presents an overview of a new SHM system. This embedded SHM instrument is composed of a piezoelectric PhA transducer, compact hardware with several excitation signal generators, signal acquisition circuits, and configuration software. The system allows a number of diverse SHM analyses to be performed. An electronic system oriented to SHM analysis, called PAMELA, has been designed and developed. This instrument requires a high level of parallel operation, as it must perform numerous functions simultaneously. To meet these requirements, the system uses a powerful FPGA with an embedded microprocessor. Additionally, an easy-to-install, 12-element piezoelectric transducer array has been presented in this work. This array is a key part of the complete system, as it converts the electrical signals into ultrasonic signals and vice versa. The choice of different piezoelectric transducers will depend on the material of the structures and the structure location to be tested.
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This system instrumentation can incorporate specific software to configure any type of excitation signal or test mode and perform treatment on the acquired signals to detect and alert the user of any failure in the structure being scanned. The dimensions and weight of the instrument make it suitable for laboratory testing and on-aircraft installation in small quantities. The authors are currently working on an updated version that will be lighter and smaller and have lower power consumption, allowing for deployment on a large scale inside an aircraft or any other structure. ACKNOWLEDGMENTS
The authors are grateful to The Basque Government under the research ETORTEK and AIRHEM II and to the Centre for Industrial Technological Development (CDTI) under the research projects TARGET and PROSAVE2 for funding this research. 1 M.
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