Wireless Sensor Network Architecture for Structural Health Monitoring

1. Wireless Sensor Network Architecture for Structural Health Monitoring Michael Sirivianos April 17, 2003

2. Paper • “Two-tiered wireless sensor network architecture for structural health monitoring” • Appeared in SPIE’s 10th Annual International Symposium on Smart Structures and Materials, San Diego, March 2003 • V. A. Kottapalli, A S. Kiremidjian, J.P Lynch, Ed Carryer, T. W. Kenny, K. H Law, Ying Lei • John A. Blume Earthquake Engineering Center, Stanford University

3. Structural Health Monitoring • Recent advances and technologies assist structural engineers in their attempts to ensure the safety and reliability of structures over their life spans through monitoring systems • Structural Health monitoring an application of advanced monitoring systems • The technology employs smart sensors in a configuration that provides materials monitoring needed to detect and remotely address any compromise in material structural integrity.

4. Traditional Structural Monitoring • Employs conventional cables to allow sensors deployed in a few critical locations of the structure to communicate their measurements to a central Data Acquisition System module. • Older systems had analog sensors and A/D converters at the DAQ to convert the analog vibration signal into a digital format • Newer systems incorporate digital sensors to avoid A/D conversion at the central point to enable more reliable communication and relieve the central DAQ from the conversion load

5. Traditional Structural Monitoring (2) • Cons : • Cabled based sensing systems for structures have high installation and maintenance costs • Wires vulnerable to ambient signal noise corruption. • Wired links are prone to breakage and environmental wear. • centralized approach with all system sensors sending measurement data to one data server.  Such an approach adds latency to the system during real-time data processing and represents a single point of failure. 

6. Wireless Monitoring Systems • Research effort has been initiated towards the development of a wireless modular monitoring system. • Lower installation and maintenance cost. • More reliability in the communication of sensor measurements

7. Wireless Monitoring Systems (2) • Areas of innovations : • Use of a wireless communication system for inter-sensor communication. Low cost wireless technologies have significantly contributed towards this direction. • Bluetooth. Operate in the unlicensed, 2.4 GHz radio spectrum. These radios use a spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz intervals to give a high degree of interference immunity.

8. Wireless Monitoring Systems (3) • The 802.11 standard specifies a single Medium Access Control (MAC) sub layer and 2 radio and one infrared Physical Layer ( PHY ) Specifications. • The standard provides multiple data rates and power management (stations can switch off their transceivers to conserve power). • The MAC protocol is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). • 2 Physical layer specifications for radio, operating in the 2 400 - 2 483.5 MHz band and one for infrared. 1 or 2 Mbps data rates • (1) Frequency Hopping Spread Spectrum Radio PHY. • (2) Direct Sequence Spread Spectrum Radio PHY. • (3) Infrared PHY.

9. Wireless Monitoring Systems (4) • Utilization of micro-electro mechanical (MEMs) sensing elements and • Use of advanced microprocessor architectures for computationally expensive real-time damagedetection and assessment methods. Wireless sensing units have the flexibility to communicate peer to peer (decentralized P2P) or in a traditional centralized fashion.

10. Problem Statement The nature of monitoring systems dictates a specialized network architecture that would specify • optimum network topology, • the best suitable wireless technology and • the appropriate protocol stack.

11. Roadmap • Study of the features and requirements of the structural monitoring application. • The two-tiered architecture as the answer to these issues. • Network and System Components Architecture • Description of a communication protocol • Power saving techniques incorporated in the sensor unit architecture

12. RoadMap (Cont.) • System Analysis • Conclusions on the trade-offs • Estimation of the maintenance cycle based on the power consumption of the sensor unit • A basic laboratory implementation of the suggested work.

13. Characteristics and Requirements • Two modes of operation • Extreme Event Monitoring • Long Term Periodic Monitoring • Size of monitored infrastructure • Generally large, miles long in case of bridges • Measured parameters • Acceleration • Linear and angular displacement • Environmental variables as temperature and humidity

14. Characteristics and Requirements (Cont.) • Data generation rate • Depends on the sampling rate • Vibration Data Synchronization • Maintenance • Long maintenance cycles-years • Long lasting batteries • Environmental variables as temperature and humidity

15. Characteristics and Requirements-Summary • High throughput for real time performance • Synchronization of Distributed Sensor’s Data • Large transmission range • Minimum power consumption • Large transmission range and data rates requirement in conflict with power consumption • Solution: two tiered wireless sensor network

16. Two-tiered Wireless Sensor Network • First subsystem. Sensor Units. • Low data rate, low transmission range, low consumption • Second subsystem. Large Coordinator Units. • High data rate, large transmission range, not energy constrained

17. Two-tiered Architecture • Network Architecture and Topology. • Clustering of distributed sensor units (SUs)similar to the structure of a cellular network • A Local Site Master LSM assigned in each cluster to coordinate SUs and collect their data. • SU clusters form lower tier. LSM network and Central Site Master form upper tier

18. Two-tiered Architecture (Cont.)

19. Lower Tier • R/F within cluster is over the 915 MHz ISM band. • Spread Spectrum Signal is modulated with sequence of digits generated by pseudo random number generator, so that signal to be transmitted has wider bandwidth. • Frequency Hopping. Signal hops from frequency to frequency at fixed intervals. Receiver hopping ,in synchronization with transmitte,r picks up message. • 26 MHz available band divided into 250KHz channels => 104 channels that can be divided into chunks of 52 frequencies so that adjacent cells can use a different chunk.

20. Lower Tier (2) • Chunking further increases S/I ratio. This optimization does not cover the case of three clusters that are each other’s neighbors • TDMA scheme followed for the communication between SUs and LSM each other’s neighbors • Minimal Handshaking protocol to maximize power savings

21. Upper Tier • LSM transmits at 915 MHz ISM for communication with SUs and at 2.4 GHz ISM for communication with adjacent clusters. • Clock synchronization among LSMs also at 915 MHz • LSM routes received data to the CSM through the other LSMs. • In the absence of energy constraints IEEE 802.11b appears as good choice of wireless network standard preferred over expensive custom designed ones

22. Sensor Unit • Components • SU controller • 915 MHz radio transceiver • SRAM Memory • Low sensitivity, high g accelerometer ( ADLXL120 ) for extreme event responses • Sensor module with a high sensitivity low noise accelerometer ( 1221 accmtr ) for ambient responses • High resolution low speed A/D converter • Various other sensors i.e thermometer

23. Sensor Unit (2) • Operation States • Sleep. Sensor module and R/F transceiver in sleep mode. SU controller and the low sensitivity accelerator ( Acc-Low ) intermittently powered. Right After Acc-Low startup time elapsed switches on the A/D converter and samples the Acc-Low output. If the data do not indicate an extreme seismic event Acc-Low and A/D are powered off. • If an event ( 5mg and above ) is detected unit enters Awake state. Sampling rate is increased accordingly and data are saved in SRAM. Event time is noted. Synchronizes with LSM and adjusts event time accordingly. Awake state is synchronized with TDMA slot.

24. Sensor Unit (3) • Operation States • It passes in Semi-Awake state all modules but transceiver are on. Active sampling of both accelerators output. Semi-Awake periodically alternates with Awake state were radio transceiver is also on, allowing transmission of sensor data to LSMalong with activesampling • After event has passed and all collected data are sent SU enters sleep state. • CSM determines at which instances ambient vibration info is to be recorded to enable periodic monitoring. SUs usually in sleep states are waken up in fixed number of times per day and enter the Update State.

25. Sensor Unit (4) • Operation States • In the Update state state sensor module is off, SU ctlr and transceiver on and Ac-Low cycle powered. It synchronizes with LSM and sets a wake up timer for the next scheduled monitoring phase. After that enters Awake State alternating with Semi-Awake transmiting data • After a predefined time interval it goes back to sleep.

26. Local Site Master • Components • 915 MHz radio transceiver for upper tier communication • 2.4 GHz radio transceiver for lower tier communication • LSM Controller • Memory to store the received data to be routed. Operates continuously throughout the life of the network

27. Communication Protocol • A special purpose customized TDMA protocol. For N sensor units per cluster we have a round of N+2 slots. N for data, 2 for control. A data slot assigned per SU. 1st control slot for Synch-Ack and second for Global-Synch signal. One packet per data slot. Frequency hopping at every slot. • LSM acknowledges SU packets broadcasting a Synch-Ack signal containing ack bits for all packets received in the current and the previous round. Of course contains the local clock information for synchronization of SU clocks with the global clock. • SU notifies LSM before entering the sleep state at the end of a monitoring or update state. Uppon notification LSM is able to use the SU slot for startup packet. • Synch-acks are appended with the ambient monitoring schedules enabling schedule updating upon synchronization.

28. Analysis of the proposed monitoring system • Trade Off conclusions • Best transmission range, possible with low power results, to low transmission rates. Low transmission rates support less sensors per cluster • When clusters are in a straight chain then one single LSM is the only access point to the CSM and therefore receives all the routed data. Worst case upper tier data rate requirement is set to (2N-n) SU data _____________ link throughput where N is total number of SUs and n SUS per cluster.

29. Analysis of the proposed monitoring system ( 2 ) • Trade Off conclusions • CSM receives data generated from all Nodes ( N) SU data. Latest 2.4GHz radio transceivers can support 10Mbps => 2200 nodes worst case. But if multiple LSMs have access to CSM => 4400 sensors • High degree of scalability but it is based on very optimistic assumptions on the supported bit rate.

30. Estimation of the maintenance cycle 10 sensors per cluster, extreme event duration 3 mins, periodic monitoring phase 3 mins, and time needed by SU for startup synchronization 4 mins. • Power consumption components on a yearly basis • Energy consumed in radio transceiver 275mAh • Energy consumed in microcontroller unit and Low Sensitivity Accelerator 300mAh • Energy consumed by memory sensor modules and others 1000 mAh Total of ~ 1600 mAh

31. Implementation • Network • Two cluster network • 80Hz sampling • frequency • one channel per SU • sample resolution 16 • bits • 5 SUs per cluster • 0.75% throughput

32. Implementation – H/W • Sensor unit • An EVK915 module containing a 915 MHz Bluechip radio transceiver at 20kbps baud rate. Receiver current 12 mA and transceiver 50mA at its peak 50 db transmit power. • Interfaced with a lab made sensor board • PIC Microchip MCU used as the SU controller • ADXL Accelerometer on the sensor board  • LSM • An ProximRangelLAN radio modem operating at 2.4 GHz with 2 Mbps bit rate. Uses IEEE802.11b • Interfaced to an EVK915 radio module • PIC Microchip MCU used as the LSM controller

33. Implementation – S/W • SU and LSM controller were programmed in assembly and the code was loaded in the microcontrollers. • Local clock is a S/W clock based on a 16bit timer. • FHSS patterns obtained from a linear feedback shift register implemented in the codeas pseudo random generator. • CSM software written in C

34. Limitations • Applicable only in static structures with regular power supplies which are needed for the operation of the upper tier. • LSMs are single points of failure for the SUs in its cluster and CSM a single point of failure for the whole system!

35. Conclusions • Significant role of power efficiency on the viability of the system. • System throughput of hundreds or thousands kbps for real time performance. • Required data rate and range require higher power consumption. • The goal of power efficiency is achieved through partition in two tiers. • TDMA communication protocol contributes to reduced power consumption. • Lab and Field testing under realistic vibration and environmental conditions

36. Future work • Self reconfigurable SUs so that they are not dependant on a specific LSM. In case of failure it can be assigned to the closer LSM in range or to a backup one. • More robust sensor and Site Master Units

37. The end. Questions?