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Wireless Sensor Network Architecture for Structural Health Monitoring

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Title: 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 SPIEs 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 damage
    detection 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 Sensors 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 gt 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 others neighbors
  • TDMA scheme followed for the communication
    between SUs and LSM each others 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 LSM along with active sampling
  • 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 N2
    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 gt 2200 nodes worst case. But if
    multiple LSMs have access to CSM gt 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 code as 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?
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