Health Monitoring: from sensor to data collect for Proceedings of .... server to Android pads via Wi-Fi or 3G systems, so the technicians can see the evolution of ...
COVER SHEET
Title: Energy Efficient Wireless Sensor Network Architecture for Aircraft Structure Health Monitoring: from sensor to data collect for Proceedings of the 9th International Workshop on Structural Health Monitoring 2013
Authors (names are for example only): Daniela Dragomirescu Florian Perget Frederic Camps Robert Plana Andrea De Luca Florin Udrea
(FIRST PAGE OF ARTICLE)
ABSTRACT Aerospace testing and monitoring systems are based nowadays on point-to-point wiring, which results in heavy cables, difficult installation and maintenance and a limited number of sensors. Switching to wireless communication will allow to respond to these constraints but will create new challenges due to transfer reliability requirements, high data rate and energy constraints. In this context, there is a strong need for energy efficient wireless sensor communications as each node is powered from a battery or from energy harvesting. This paper describes a wireless communications architecture tailored for this purpose, based on a low power Impulse Radio Ultra Wide Band (IR-UWB) physical layer and an energy efficient TDMA MAC layer. The whole measurement system for aircraft Structure Health Monitoring was implemented and tested on a G500 aircraft mock-up with good results. The complete system architecture from sensors to data collect is presented. INTRODUCTION The aerospace industry is subjected to stringent requirements regarding aircraft safety and performance. This translates into both thorough testing during the development stages and regular monitoring of the aircraft’s systems wear during its operational lifetime. Traditionally, the latter is accomplished by regularly scheduled manual inspection while the former is performed using analog wired sensors systems. Wired sensor systems suffer from a time consuming installation and removal process which thus increase development costs and reduces availability of scarce prototype during the development schedule. Wireless Sensor Networks (WSN) promise to lift all these drawbacks through quick and easy deployment and removal during development, but also by allowing constant monitoring of the aircraft during operation. This will lead to increased safety and reduced development and operating cost. __________ D.Dragomirescu, CNRS, LAAS, 7 avenue du colonel Roche, F-31400 Toulouse, France and Univ de Toulouse, INSA, LAAS, F-31400 Toulouse, France F.Perget, F.Camps CNRS, LAAS, 7 avenue du colonel Roche, F-31400 Toulouse, France R.Plana, Alstom Corporate, 3 avenue André Malraux, 92300 Levallois Perret, France A. De Luca, F.Udrea, University of Cambridge, Department of Engineering, Cambridge CB2 1PZ, United Kingdom
WSN must provide the same data transfer reliability and endurance as their wired counterparts. Developing energy efficient communication systems is critical to the successful development of WSN for aerospace monitoring applications. Recent advances in microelectronics processes and digital communications have enabled new possibilities in this field. Wireless Sensor Networks have been proposed as a mean to monitor aircraft aging and improve maintenance operational burden and costs [1], [2]. However, the proposed systems are all self-reliant for data analysis and are not connected to other systems, such as the airline or the aircraft manufacturer’s facilities. We are proposing a wireless sensor network architecture, which employs existing cellular networks and a highly energy efficient wireless communication nodes based on UWB-IR communication. APPLICATION: AIRCRAFT STRUCTURE HEALTH MONITORING The main application for such a system is an aircraft monitoring and maintenance scenario. Under a typical scenario, an airplane would be fitted with wireless sensors during manufacturing. These sensors would be placed at key location to monitor aircraft parts and systems subjected to aging or critical for the aircraft safety or performance. Relay nodes may be employed to make all the sensor nodes reachable from the gateway node by relaying data transmissions while also improving energy efficiency. Depending on the flight profile, network availability and the data itself, the gateway node could buffer data until arrival at the parking spot, or send data mid-flight if required. The location of the sensor nodes will severely constrain their energy budget, because of the difficulty of accessing and recharging a battery system, and because powering them by wires would defeat the benefit of wireless communications.The most promising technology for powering these wireless nodes is advanced battery concepts with extended lifetimes coupled with energy harvesting [3]. This paper details the design of this energy efficient wireless communication system and present performance figures simulated and measured on the two hardware prototypes developed by the team. WIRELESS SENSOR NETWORK ARCHITECTURE The Structure Health Monitoring application intends a large number of nodes sending unidirectional traffic towards a single or limited number of nodes, the master/concentrator nodes. The WSN architecture proposed to respond to this application is cluster-tree architecture as presented in Figure 1. In this architecture are three kinds of nodes: • Wireless sensor nodes placed on the aircraft structure. In this application, little intelligence and processing power is needed at the sensor nodes. The most consuming part on these nodes will be the wireless communication. As these nodes are only powered by battery or energy harvesting, they have to be very low power consuming. • Concentrator nodes - gather information from wireless sensor nodes and transmitted it to the gateway nodes. If necessary, some data processing can be done on the concentrator nodes depending on the energy available. These concentrator nodes can be powered by the aircraft system.
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Gateway nodes: main nodes of the system doing data processing and storage. They are powered by the aircraft system.
Figure 1. WSN Cluster tree architecture for aircraft SHM Each wireless sensor node can have different number of sensors of different kind upon the application requirements. The global architecture of the wireless sensor communication node is given in Figure 2.
Figure 2. Wireless sensor node architecture The architecture of wireless sensor node includes: • interfaces to sensors : SPI or direct ADC connection • physical wireless layer based on UWB Impulse Radio (UWB-IR) • deterministic MAC layer based on TDMA • connection to RF front-end : fast DAC (Digital Analog Converter) • interfaces control, task scheduling and connection to the application layer (called CPU in the Figure 2)
SENSORS For the demonstration use, four types of sensors were connected to the demonstrator: pressure sensor, accelerometer sensors, gyroscope sensor and flow sensor. The first three sensors are commercially available and can be connected via SPI interfaces to the wireless communicating node. The flow sensor was design by Cambridge University especially to be ultra-low power and resistant to the harsh environment. The detailed flow sensor design and its performances measured in the wind tunnel are presented in [4]. The sensors have proven to be robust at highspeed flow (Mach 1.2) and are therefore equally suitable for flow transition studies in aeronautics flows. Full CMOS compatibility of these sensors makes them low cost and brings in the possibility of on-chip circuit integration. Figure 3 presents the packaged flow sensor on the PCB with the signal processing circuit.
Figure 3. Low power CMOS compatible flow sensor VERY LOW POWER WIRELESS COMMUNICATION Frequency choice An important constraint for the aircraft SHM application is the required non-interference with avionics and the successful data transmission of hundreds or thousands of nodes. This has led us to target the 60 GHz band which provides large bandwidth, reduced size antennas and short range transmission. Short range transmission will lead to low nodes interferences and so the possibility to deploy hundreds up to thousands wireless sensor nodes in the aircraft. Physical layer The physical layer design focuses on very low power and high data rate physical layer. We propose the use of Ultra Wide Band Impulse Radio (UWB-IR), which transmits information by modulating a single symbol period and then going silent for a certain amount of time [5]. The transmitter design at its core is quite simple and features FEC encoding, a digital modulator and an analog pulse generator (see Figure 4). The low complexity of this design results in low power consumption for data transmission which is beneficial in the typical use case where sensor nodes which are the most power constrained send the most data.
Figure 4. UWB-IR transmitter
The received signal is down-converted to baseband frequency by the receiver RF front-end, then fed to a 4Gsps 6-bit flash Analog to Digital Converter. A rake receiver topology is employed to increase receiver sensitivity [6]. A digital Schimtt trigger converts the samples into bits, with noise filtering controlled by its hysteresis curve threshold. Power consumption is reduced compared to a traditionally used correlator or matched filter. Demodulated data is then error corrected. Figure 5 presents the UWB-IR receiver schema.
Figure 5. UWB-IR receiver schema Media Control Access layer Thanks to the cluster tree network architecture, each cluster can be controlled separately. Thus each cluster is organized in an autonomous wireless network we call a piconet. To satisfy data transfer reliability, real-time transmission if needed and most important, prevent data collision which means prevent energy wasting, we propose to use a TDMA (Time Division Multiplex Access) MAC layer. In this case each wireless sensor node in a piconet will have its own time slot to communicate. Clock synchronization Wireless clock synchronization of sensor nodes is absolutely necessary to be able to interpolate data and locate the default in the structure. For the wireless sensor nodes, the reference will be the concentrators' node clock which it is itself synchronized with the gateways node clocks. On our prototypes we developed a new protocol called WIDECS [7] (Wireless Deterministic Clock Synchronization) based mainly on the measurement of ToF (time of flight) and a precise connection between MAC and Physical layer. This protocol is able to assure less than 10 ns clock synchronization precision on the FPGA developed prototype and less than 1 ns on our ASIC prototype. This will lead at very precise default localization in the structure.
HARDWARE PROTOTYPES The first prototype has been implemented on a Xilinx Virtex-5 FPGA coupled with 500Msps ADC and DAC devices (see Figure 6), and a 5-10 GHz RF front-end made of assembled discrete RF components. A 5-10GHz RF front-end was chosen for the first tests as the European authorized band for UWB communications is between 6 to 8.5 GHz and RF discrete components are available at this frequency. Our testing shows that a 62.5 Mbps data rate can be accommodated over a 6 meters link while the RF front-end provides only a moderate amplification of 45dB, no gain control and a relatively high noise figure of 6.15dB. A global demonstrator was build including several sensors connected to our wireless communication hardware prototype, a router-gateway communicating to a data server for advanced processing. The data will be transmitted from the data server to Android pads via Wi-Fi or 3G systems, so the technicians can see the evolution of each sensor. The demonstrator was set up on a small airplane for real measurements as shown in Fig. 7.
Figure 6. Wireless sensor node prototype implemented on FPGA and wireless transmission measured using this prototype
Figure 7. WSN demonstrator for SHM on a G500 aircraft A video demonstration of the functioning of the whole system can be seen on our WEB site http://www.laas.fr/~daniela.
A completely integrated prototype in 65nm CMOS technology with RF front-end at 60GHz [8], [9] and a 65nm Low Power CMOS MAC/PHY ASIC chip was developed [7] to demonstrate low power consumption. Power efficiency was also a major design goal in this case which led to a state of the art 53mW for the RF transmitter and 43mW for the RF receiver. The digital chip power consumption is 30mW for the analog to digital converter, 10mW for the pulse generator and 10mW for the digital circuit. When in standby mode, the power consumption of the digital chip is reduced to 2mW. We measured an energy/bit around 150pJ/bit for the transmitter and 170 pj/bit for the receiver. CONCLUSION An energy efficient wireless sensor network for aircraft SHM was presented using a system approach: from sensor to data collect. The major driver for designing and dimensioning this WSN was the very low power consumption at each part of the system: sensors, wireless communication at physical layer but also from protocol point of view. Hardware prototypes were developed and the complete SHM system was tested on a G500 small airplane. Complete integrated prototypes in CMOS 65nm technology, including MAC layer, physical layer based on UWBIR (complete UWB-IR transceiver) and clock synchronization were for the first time developed and reported here, at the best of authors' knowledge. ASIC prototypes were also developed for 60GHz front-end. The global wireless communication system integrated on ASIC (MAC and PHY ASIC and 60GHz front-end) achieve very low power consumption. REFERENCES 1. S. Arms, J. Galbreath, C. Townsend, D. Churchill, B. Corneau, R. Ketcham, and N. Phan, “Energy harvesting wireless sensors and networked timing synchronization for aircraft structural health monitoring,” in 1st International Conference on Wireless Communication, Vehicular Technology, Information Theory and Aerospace Electronic Systems Technology, 2009. Wireless VITAE 2009, pp. 16 –20. 2. J. Demo, A. Steiner, F. Friedersdorf, and M. Putic, “Development of a wireless miniaturized smart sensor network for aircraft corrosion monitoring,” in IEEE Aerospace Conference, Mar. 2010, pp. 1–9. 3. H. A. Sodano, G. E. Simmers, R. Dereux, and D. J. Inman, “Recharging batteries using energy harvested from thermal gradients,” Journal of Intelligent Material Systems and Structures, vol. 18, no. 1, pp. 3–10, Jan. 2007. 4. Ibraheem Haneef, John D. Coull, Syed Zeeshan Ali, Florin Udrea, Howard P. Hodson "Laminar to Turbulent Flow Transition Measurements Using an Array of SOI-CMOS MEMS Wall Shear Stress Sensors" Proceedings of IEEE Sensors 2008, pp. 57-61 5. A.Lecointre, D.Dragomirescu, R.Plana " System architecture modeling of an UWB receiver for wireless sensor network", International Conference on Systems, Architectures, Modeling and Simulation (SAMOS VII), Samos (Grèce), 16-19 Juillet 2007,Lectures Notes on computer Science, pp.408-420 6. A. Lecointre, D. Dragomirescu, and R. Plana, “Largely reconfigurable impulse radio UWB transceiver,” Electronics Letters, vol. 46, no. 6, p. 453, 2010 7. T.Beluch, D.Dragomirescu, R.Plana, "A sub-nanosecond synchronized MAC-PHY cross-layer design for wireless sensor networks", Ad-Hoc Networks Vol.11, N°3, pp.833-845, Mai 2013 8. M. Ercoli, D. Dragomirescu, D. Belot, and R. Plana, “An extremely low consumption, 53mW, 65nm CMOS transmitter for 60 GHz UWB applications,” in 2012 IEEE Radio Frequency Integrated Circuits Symposium (RFIC), Jun. 2012, pp. 463 –466. 9. M. Kraemer, D. Dragomirescu, and R. Plana, “Design of a very low-power, low-cost 60 GHz receiver front-end implemented in 65nm CMOS technology,” International Journal of Microwave and Wireless Technologies, vol. 3, no. 02, pp. 131–138, Mar. 2011.
