A WIRELESS ULTRASONIC GUIDED WAVE STRUCTURAL HEALTH MONITORING SYSTEM FOR AIRCRAFT WING INSPECTION X. Zhao1, T. Qian1, Z. Popovic2, R. Zane2, G. Mei1, C. Walsh2, T. Paing2, and C. Kwan1 intelligent Automation, Inc., 15400 Calhoun Drive, Rockville, MD 20855, U.S.A department of Electrical and Computer Engineering, University of Colorado at Boulder, Boulder, Colorado 80309, U.S.A ABSTRACT. A wireless, in-situ ultrasonic guided wave structural health monitoring (SHM) system was developed and tested for aircraft wing inspection. It applies small, low cost and light weight piezoelectric (PZT) disc transducer network bonded to the surface of a structure, and an embedded miniature diagnosis device that can generate 350 kHz, 70 V peak-to-peak tone-burst signal; collect, amplify and digitize multiple channel ultrasonic signals; and process the data on-board and transfer them wirelessly to a ground station. The whole system could be powered by an X-band microwave rectenna that converts illuminating microwave energy into DC. The data collected with this device are almost identical with those collected through a direct-wire connection. Keywords: PZT Transducer Network, Structural Health Monitoring, Wireless, Aircraft Wing PACS: 81.40.Np, 84.30.-r, 84.40.-x INTRODUCTION A significant number of civil and military aircraft have exceeded their design lives. In 2000, 75% of US Air force aircrafts were more than 25 years old [1]. A major sustainment cost for these aging air vehicles is unscheduled maintenance. Structure health monitoring (SHM) technologies, by embedding smart sensors/actuators to the structures and responding/adapting to conditions changes, can help the transition from a schedulebased maintenance practice to condition-based maintenance (CBM). It will improve the maintenance agility and responsiveness for increased fleet availability, quicker turnaround time, and reduce the life cycle total ownership cost [2]. As an example, inspection of a frequent structural problem or a defect-prone area (hot spots) at an inaccessible location desires a fast, low cost and reliable integrity monitoring system to ensure the safety and functionality of the structure. One promising solution is an SHM system that could be embedded into the structure, inspect the structural hot spots and download data or diagnostic results wirelessly to a remote station. It will also reduce the wire problems and facilitate the inspection process. Many researchers have been developing or implementing wireless sensors or sensor networks for structural health monitoring applications. Straser and Kiremidjian [3] were CP894, Review of Quantitative Nondestructive Evaluation Vol. 26, ed. by D. O. Thompson and D. E. Chimenti © 2007 American Institute of Physics 978-0-7354-0399-4/07/$23.00 1548 among the first that proposed the integration of wireless communications with sensors based on a low-power 8-bit Motorola 68HC11 microcontroller. Lynch [4] has a good review article on various wireless sensors platforms for real-time health monitoring of civil structures. The Berkeley Mote Mica and Mica 2 are among the most popular platforms for implementing wireless sensor networks [5, 6], however, it only operates in a passive mode and is limited in data sampling rate. Liu and Yuan [7] developed a high frequency Wireless Intelligent Sensor Platform (WISP) for ultrasonic applications, but they did not address the signal generation and powering issues. Ultrasonic Lamb waves have recently been widely used for SHM applications by embedding PZT ceramics or wafer sensors to the structures. A wireless means of interfacing with those guided wave sensor/actuators will greatly benefit SHM. In this paper, the concept of integrating wireless communications with sensors was adopted. A miniaturized diagnostic device with an on-board ultrasonic pulser and A/D was developed so that it can be embedded into the wing and integrated with the PZT arrays. The on-board pulser can generate 350 kHz, 70 V peak-to-peak tone-burst signal; a multiplexer made of mechanical relays and laser-diode switches and an 8-bit 10 Ms/s A/D chip was used for multi-channel data acquisition; a microprocessor and a wireless module were used to process the data and send them out through a 915 MHz FM radio. To power the device wirelessly, a 10 GHz microwave rectenna was designed for converting microwave energy into DC. The rectenna can effectively deliver lOOmW of DC power continuously with 8 W of microwave power input into an illuminating horn antenna 0.6 meters away. The wireless system was tested with the PZT sensor array on an aircraft wing panel. It can effectively monitor a large panel area for rivet cracks with size comparable to the wire connection cases. EMBEDDED ULTRASONIC GUIDED WAVE TRANSDUCER NETWORK Aircraft wing panel is a complex structure. Figure 1(a) shows an example of a piece of E-2c surveillance wing panel and its rivet distribution. Those rivets are used to fasten the wing panel with the inner structures like spars, ribs, and stiffeners. Figure 1(b) shows a detailed sketch of a stiffener connection to the wing skin. As shown in Figure, there are stiffener rivet rows at least every 50 mm on the panel; within each row, the rivets are 8.8 mm apart. The wing skin is not uniform in thickness. The section under the stiffeners is 0.5000 0,2350 1,1000 (a) (b) FIGURE 1. Wing skin structure (a) overview of the rivet row sketch (b) typical geometry of a stiffener riveted to the skin (all the scales are in cm). 1549 FIGURE 2. Eight element PZT array mounted on the wing panel. The image on the top right corner shows the transducer size compared with a coin. 0.5mm thicker than the rest of the panel. The wing panel is also painted with at least two layers of paints on the outer surface and one on the inner side. All those structural properties cause strong wave scattering or attenuation when ultrasonic guided waves are used for defect inspection. A circular configuration of a PZT transducer network was implemented on the wing panel for a relatively large area inspection. Figure 2 shows the example array. In this study, eight piezo-ceramic discs were attached to the inner surface about 10 inches in diameter and evenly spaced. They can take turns generating ultrasonic signals while the others are listening. As an example, when transducer 1 is sending a signal, transducers 2 to 8 are in a reception mode. After the guided wave data are collected, transducer 2 generates the ultrasound and transducer 1, 3-8 are listening, etc. A total of 56 sets of data can be collected this way. EMBEDDED DIAGNOSIS DEVICE DEVELOPMENT The diagram of the miniature ultrasonic diagnosis device developed is shown in Fig. 3. It is targeted to be embedded into the structure along with the transducer network for insitu data acquisition and processing. The embedded SHM device has an on-board toneburst pulser that can generate 350 kHz, 70 V peak-to-peak tone-burst signal, a multiplexed A/D board with a programmable gain amplifier for multi-channel data acquisition, a low power consumption microprocessor for circuit control and data processing, and a wireless module for download the data or processing results. On the ground or a remote location, a wireless receiver and a computer console can pick up the wireless data from the embedded SHM device and perform the necessary post data-processing or display the results. In an ideal situation, an energy harvester would be able to collect ambient energy such as vibration or electromagnetic waves and convert them into electrical energy to power the embedded electronics and the sensor array. In this work, the microwave rectenna developed [8] was used to convert microwave energy into DC. It can effectively deliver at least lOOmW of power continuously with an illuminating microwave source. 1550 Tone burst signal to one sensp •/ Embedded Transducer Array Power harvesteif Serial data through UARt f Wireless Module Embedded SHM device FIGURE 3. Diagram of the embedded ultrasonic structural health monitoring system. The ultrasonic pulser produced for the SHM system can generate tone-burst pulses every 10 ms, with the center frequency tunable from 300 kHz to 400 kHz. It is designed based on a series resonant inverter. Its efficiency of power stage is realized by adjusting the resonant frequency of a resonant tank closer to the operating frequency. The total power consumption of this pulser is around 30mW for a 0.2% duty cycle operation, which is typically 7-8 cycles of a sinusoidal wave. The communication of the SHM device with the on-board transducer network is realized through a multiplexer so that each transducer element of the network can be interrogated individually. The multiplexer developed currently has eight-channel capability, but is expandable to 16 channels or more. It can pass 200 V peak-to-peak ultrasonic signals. The A/D converter has 8 bits resolution and 10MSPS sampling rate. A programmable-gain amplifier was incorporated so that signals from different channels can be compensated to the same amplitude level for fully utilizing the digitization resolution. The wireless modules used for data transfer from the SHM device to a local PC are LINX ES Series RF modules. Firmware communication protocol was developed to handle packet generation, processing, synchronization, and error detection and correction, etc. All the electronics boards, including the tone-burst pulser, multiplexer, A/D converter, microprocessor, and the wireless module, were put into a metal box of 6.75"x4.75"x2" in dimension. It could be further miniaturized with careful circuit layout and boards integration. The overall system requires only a +15V voltage input, and the total power consumption currently is less than 500 mW. As mentioned earlier, to power the electronics wirelessly, an X-band microwave rectenna was design and used for converting illuminating microwave energy into DC power. The specific rectenna is shown in Fig. 4. The antenna element is a narrowband, linearly polarized patch antenna at 10 GHz designed for a 0.25-mm thick Rogers Duroid substrate with a permittivity of 2.2. The gain of the patch calculated from its physical area is 1.39 (1.45dB). In a rectenna element, the rectifying diode is connected directly to the antenna so that DC output is obtained with incident microwave. The thin substrate allows the rectenna array to be flexible enough to conform to the moderate curvature of the 1551 FIGURE 4. X-band microwave rectenna for wireless powering the on-board electronics. airframe while desirable microwave properties are maintained. The rectenna can deliver lOOmW of DC power continuously with a 15V output at a 10mW/cm2 incident RF power. A single rectenna element at this incident power density has an output power of 5 mW and an estimated efficiency of 50%. Each of the 25 antenna elements has an integrated rectifier, the outputs of which are combined in series to achieve the total required voltage and power at an estimated efficiency of 40%. SYSTEM TEST RESULTS The test setup for an aircraft wing panel inspection is shown in Fig. 5. An eightelement 350 kHz PZT transducer array was installed on the inner surface of the E-2 wing panel. The transducer input/output was connected to the embedded SHM device that can generate tone-burst signal and collect sensor data. With the LINX RF module and receiver FIGURE 5. Embedded ultrasonic data acquisition and wireless transfer system for an E-2 wing panel inspection. 1552 (a) Wireless waveform (b) Direct wire collected waveform FIGURE 6. Comparison of ultrasonic guided wave data collected by (a) the wireless SHM device, and (b) the direct wire connected instrument. device, the collected ultrasonic guided wave data was successfully transferred from the onboard SHM device to a remote PC station. A near real-time data collection and display was achieved with the 36.8 k baud rate LINX RF link with a range about 2-3 meters. Figure 6 shows example waveforms collected with the embedded device and downloaded to a PC, and collected with direct wire connection, respectively. It is seen that the wireless waveform agrees quite well with the direct wire-connection signal except at the very beginning where electromagnetic coupling effect is stronger than the wire-connection case. After we collected reference data when there is no defect on the wing panel, we then drilled a rivet out of the E-2 wing panel and used the wireless SHM system to collect pair-wise PZT ultrasonic guided wave data for defect detection and localization. The data collected were transferred wirelessly to a laptop PC and processed with the RAPID algorithm developed previously [9]. Figure 7 shows the defect distribution probability map and the estimation result after applying a proper threshold to the probability density map. The estimated defect location agrees very well with the true location. This test proves the feasibility of using embedded PZT transducer network and diagnosis device for in-situ structural integrity monitoring of aircraft wings. CONCLUSIONS The approach of using embedded ultrasonic guided wave transducer network and an wireless structural health monitoring device for aircraft wing inspection have been studied. The ultrasonic data can be collected on-board the wing and send out wirelessly to a local PC station. It shows good signal quality and can be used for defect detection and localization. The wireless range of the device could be further increased with a Zigbee radio whose broad-bandwidth feature can tolerate noise interferences. This approach requires power supply for the electronics and transducers to work. In environment where ambient energy is rich or an illumination source is available such as vibration and RF, an energy harvesting device may be able to scavenge the otherwise wasted energy into DC power to drive the circuit, however, when those energy source is not available, a battery or power cord will have to be installed for powering those embedded system. 1553 Locaizatio-r Result - 1 0 - 5 O mm S 10 (a) Defect distribution probability map Localization Result (b) Defect location estimation FIGURE 7. Defect detection and location estimation with the embedded ultrasonic data acquisition and wireless transfer system and RAPID algorithm [9]. (a) The defect probability density map for the loose rivet; (b) defect location estimation by setting a threshold to the probability density map. ACKNOWLEDGMENTS This work is partially supported by U. S. Naval Air Systems Command under contract N68335-02-C-3105 and NASA Glenn Research Center under contract NNC05CA40C. The authors would also like to thank Dr. Vinod Agarwala and James Stephenson of NAVAIR for providing the wing specimens and helpful discussions. 1554 REFERENCES 1. W. J. Staszewski, C. Boiler and G. R. Tomlinson, Health Monitoring of Aerospace Structures, John Wiley& Sons, Ltd., West Sussex 2004. 2. M. M. Derriso and S. E. Olson, "The future role of structural health monitoring for air vehicle applications," in Structural Health Monitoring, Proc. of the 5 th IWSHM, edited by Chang F K, 2005, ppl7-25. 3. E. G. Straser and A. S. Kiremidjian, "A modular, wireless damage monitoring system for structures" Technical Report 128, John A. Blume Earthquake Engineering Center, Stanford University, Stanford, CA. 1998. 4. J. P. Lynch and K. Loh, Shock and Vibration Digest 38, pp 91-128 (2005). 5. K. Chintalapudi et al, "Structural damage detection and localization using wireless sensors networks with low power consumption," in Structural Health Monitoring, Proc. of the 5 th IWSHM, edited by Chang F K, 2005, ppl251-1258. 6. Y. Nitta et al, "Rapid damage assessment for the structures utilizing smart sensor MICA2 MOTE," in Structural Health Monitoring, Proc. of the 5 th IWSHM, edited by Chang F K, 2005, pp283-290. 7. L. Liu and F. G. Yuan, "Design of wireless sensor for high frequency applications," in Structural Health Monitoring, Proc. of the 5 th IWSHM, edited by Chang F K, 2005, pp 1602-1609. 8. C. Walsh, S. Rondineu, M. Jankovic, X. Zhao and Z. Popovic, "A conformal 10 GHz rectenna for wireless powering of piezoelectric sensor electronics," in Microwave Symposium Digest, IEEE MTT-S IMS, Long Beach, CA. 2005. 9. X. Zhao et al, "Active health monitoring of an aircraft wing with embedded piezoelectric sensor/actuator network: I. Defect detection, localization and growth monitoring," submitted to Smart Mater. Struct. (2006) 1555