Two Tiered Wi Sensor Networks

Wireless Sensor Network Wireless Sensor Network Architecture for Architecture for

Structural Health Structural Health MonitoringMonitoring

Michael SirivianosMichael Sirivianos

April 17, 2003April 17, 2003

PaperPaper

““Two-tiered wireless sensor network Two-tiered wireless sensor network architecture for structural health architecture for structural health monitoring”monitoring”

Appeared in SPIE’s 10Appeared in SPIE’s 10thth Annual Annual International Symposium on Smart International Symposium on Smart Structures and Materials, San Diego, March Structures and Materials, San Diego, March 20032003

V. A. Kottapalli, A S. Kiremidjian, J.P Lynch, V. A. Kottapalli, A S. Kiremidjian, J.P Lynch, Ed Carryer, T. W. Kenny, K. H Law, Ying LeiEd Carryer, T. W. Kenny, K. H Law, Ying Lei

John A. Blume Earthquake Engineering John A. Blume Earthquake Engineering Center, Stanford UniversityCenter, Stanford University

Structural Health Structural Health MonitoringMonitoring Recent advances and technologies Recent advances and technologies

assist structural engineers in their assist structural engineers in their attempts to ensure the safety and attempts to ensure the safety and reliability of structures over their life reliability of structures over their life spans through monitoring systemsspans through monitoring systems

Structural Health monitoring an Structural Health monitoring an application of advanced monitoring application of advanced monitoring systemssystems

The technology employs smart The technology employs smart sensors in a configuration that sensors in a configuration that provides materials monitoring provides materials monitoring needed to detect and remotely needed to detect and remotely address any compromise in material address any compromise in material structural integrity.structural integrity.

Traditional Structural Traditional Structural MonitoringMonitoring

Employs conventional cables to allow Employs conventional cables to allow sensors deployed in a few critical locations sensors deployed in a few critical locations of the structure to communicate their of the structure to communicate their measurements to a central Data measurements to a central Data Acquisition System module. Acquisition System module.

Older systems had analog sensors and A/D Older systems had analog sensors and A/D converters at the DAQ to convert the converters at the DAQ to convert the analog vibration signal into a digital formatanalog vibration signal into a digital format

Newer systems incorporate digital sensors Newer systems incorporate digital sensors to avoid A/D conversion at the central point to avoid A/D conversion at the central point to enable more reliable communication and to enable more reliable communication and relieve the central DAQ from the conversion relieve the central DAQ from the conversion loadload

Traditional Structural Traditional Structural Monitoring (2)Monitoring (2)

Cons :Cons : Cabled based sensing systems for structures Cabled based sensing systems for structures

have high installation and maintenance have high installation and maintenance costscosts

Wires vulnerable to ambient signal noise Wires vulnerable to ambient signal noise corruption.corruption.

Wired links are prone to breakage and Wired links are prone to breakage and environmental wear.environmental wear.

centralized approach with all system sensors centralized approach with all system sensors sending measurement data to one data sending measurement data to one data server. Such an approach adds latency to server. Such an approach adds latency to the system during real-time data processing the system during real-time data processing and represents a single point of failure. and represents a single point of failure.

Wireless Monitoring Wireless Monitoring SystemsSystems

Research effort has been Research effort has been initiated towards the initiated towards the development of a wireless development of a wireless modular modular monitoringmonitoring system. system.

Lower installation and Lower installation and maintenance cost.maintenance cost.

More reliability in the More reliability in the communication of sensor communication of sensor measurementsmeasurements

Wireless Monitoring Wireless Monitoring Systems (2)Systems (2)

Areas of innovations :Areas of innovations : Use of a wireless communication system for Use of a wireless communication system for

inter-sensor communication. Low cost inter-sensor communication. Low cost wireless technologies have significantly wireless technologies have significantly contributed towards this direction.contributed towards this direction.

BBluetoothluetooth. Operate . Operate in the unlicensed, 2.4 in the unlicensed, 2.4 GHz radio spectrum. These radios use a GHz radio spectrum. These radios use a spread spectrum, frequency hopping, full-spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz signal hops among 79 frequencies at 1 MHz intervals to give a high degree of intervals to give a high degree of interference immunity.interference immunity.


The 802.11The 802.11 standard standard specifies a single Medium specifies a single Medium Access Control (MAC) sub layer and 2 radio and Access Control (MAC) sub layer and 2 radio and one infrared Physical Layer one infrared Physical Layer ( PHY ) ( PHY ) SpecificationsSpecifications..

The standard provides multiple data rates and The standard provides multiple data rates and power management (stations can switch off their power management (stations can switch off their transceivers to conserve power). transceivers to conserve power).

The MAC protocol is Carrier Sense Multiple Access The MAC protocol is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). with Collision Avoidance (CSMA/CA).

2 Physical layer specifications for radio, operating in 2 Physical layer specifications for radio, operating in the 2 400 - 2 483.5 MHz band and one for infrared. 1 the 2 400 - 2 483.5 MHz band and one for infrared. 1 or 2 Mbps data rates or 2 Mbps data rates

(1) (1) Frequency Hopping Spread Spectrum RadioFrequency Hopping Spread Spectrum Radio

PHY. PHY. (2) (2) Direct Sequence Spread Spectrum Radio PHYDirect Sequence Spread Spectrum Radio PHY.. (3) (3) Infrared PHYInfrared PHY. .


Utilization of micro-electro mechanical Utilization of micro-electro mechanical (MEMs) sensing elements and(MEMs) sensing elements and

UUse of advanced microprocessor se of advanced microprocessor architectures for computationally architectures for computationally expensive real-time damageexpensive real-time damage detection detection and assessment methodsand assessment methods..

Wireless sensing units have the flexibility Wireless sensing units have the flexibility to communicate peer to peer to communicate peer to peer (decentralized P2P) or in a traditional (decentralized P2P) or in a traditional centralized fashion.centralized fashion.

Problem StatementProblem Statement

The nature of monitoring systems The nature of monitoring systems dictates a specialized network dictates a specialized network architecture that would specify architecture that would specify

optimum network topology, optimum network topology, the best suitable wirelessthe best suitable wireless technology and technology and the appropriate protocol stack.the appropriate protocol stack.

RoadmapRoadmap

– Study of the features and requirements of Study of the features and requirements of the structural monitoring application.the structural monitoring application.

– The two-tiered architecture as the answer The two-tiered architecture as the answer to these issues.to these issues.

– Network and System Components Network and System Components ArchitectureArchitecture

– Description of a communication protocol Description of a communication protocol – Power saving techniques incorporated in Power saving techniques incorporated in

the sensor unit architecturethe sensor unit architecture

RoadMap (Cont.)RoadMap (Cont.)

– System AnalysisSystem Analysis Conclusions on the trade-offs Conclusions on the trade-offs Estimation of the maintenance Estimation of the maintenance

cycle based on the power cycle based on the power consumption of the sensor unit consumption of the sensor unit

– A basic laboratory A basic laboratory implementation of the implementation of the suggested work. suggested work.

Characteristics and Characteristics and RequirementsRequirements

Two modes of operationTwo modes of operation Extreme Event MonitoringExtreme Event Monitoring Long Term Periodic MonitoringLong Term Periodic Monitoring

Size of monitored infrastructureSize of monitored infrastructure Generally large, miles long in case Generally large, miles long in case

of bridgesof bridges Measured parametersMeasured parameters

AccelerationAcceleration Linear and angular displacementLinear and angular displacement Environmental variables as Environmental variables as

temperature and humiditytemperature and humidity

Characteristics and Characteristics and Requirements (Cont.)Requirements (Cont.) Data generation rateData generation rate

Depends on the sampling rateDepends on the sampling rate

Vibration Data SynchronizationVibration Data Synchronization MaintenanceMaintenance

Long maintenance cycles-yearsLong maintenance cycles-years Long lasting batteriesLong lasting batteries Environmental variables as Environmental variables as

temperature and humiditytemperature and humidity

Characteristics and Characteristics and Requirements-Requirements-

SummarySummary High throughput for real time High throughput for real time

performanceperformance Synchronization of Distributed Synchronization of Distributed

Sensor’s DataSensor’s Data Large transmission rangeLarge transmission range Minimum power consumptionMinimum power consumption Large transmission range and data Large transmission range and data

rates requirement in conflict with rates requirement in conflict with power consumptionpower consumption Solution: two tiered wireless Solution: two tiered wireless

sensor networksensor network

Two-tiered Wireless Two-tiered Wireless Sensor NetworkSensor Network

First subsystem. First subsystem. Sensor Units.Sensor Units. Low data rate, low Low data rate, low

transmission range, transmission range, low consumptionlow consumption

Second Second subsystem. Large subsystem. Large Coordinator Units.Coordinator Units. High data rate, large High data rate, large

transmission range, transmission range, not energy not energy constrainedconstrained

Two-tiered Two-tiered ArchitectureArchitecture

Network Architecture and Topology.Network Architecture and Topology. Clustering of distributed sensor units Clustering of distributed sensor units

(SUs)(SUs) similar to the structure of a similar to the structure of a cellular networkcellular network

A Local Site Master LSM assigned in A Local Site Master LSM assigned in each cluster to coordinate SUs each cluster to coordinate SUs and and collect their datacollect their data..

SU clusters form lower tier. LSM SU clusters form lower tier. LSM network and Central Site Master network and Central Site Master form upper tierform upper tier

Two-tiered Two-tiered Architecture (Cont.)Architecture (Cont.)

Lower TierLower Tier R/F within cluster is over the 915 MHz ISM band. R/F within cluster is over the 915 MHz ISM band.

Spread Spectrum Spread Spectrum Signal is modulated with Signal is modulated with

sequence of digits generated by pseudo random sequence of digits generated by pseudo random number generator, so that signal to be number generator, so that signal to be transmitted has wider bandwidth. transmitted has wider bandwidth.

FreFrequency Hopping. Signal hops from quency Hopping. Signal hops from frequency to frequency frequency to frequency at fixed intervals. at fixed intervals. Receiver hopping ,in synchronization with Receiver hopping ,in synchronization with transmitte,r picks up message.transmitte,r picks up message.

26 MHz available band divided into 250KHz 26 MHz available band divided into 250KHz channels => 104 channels that can be divided channels => 104 channels that can be divided into chunks of 52 frequencies so that adjacent into chunks of 52 frequencies so that adjacent cells can use a different chunk.cells can use a different chunk.

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

Upper TierUpper Tier LSM transmits at 915 MHz ISM for LSM transmits at 915 MHz ISM for

communication with SUs and at 2.4 GHz ISM communication with SUs and at 2.4 GHz ISM for communication with adjacent clusters.for communication with adjacent clusters.

Clock synchronization among LSMs also at Clock synchronization among LSMs also at 915 MHz 915 MHz

LSM routes received data to the CSM LSM routes received data to the CSM through the other LSMsthrough the other LSMs..

IIn the absence of energy constraints IEEE n the absence of energy constraints IEEE 802.11b appears a802.11b appears ass good choice of wireless good choice of wireless network standard preferred over expensive network standard preferred over expensive custom designed ones custom designed ones

Sensor UnitSensor Unit ComponentsComponents

SU controllerSU controller 915 MHz radio transceiver915 MHz radio transceiver SRAM MemorySRAM Memory Low sensitivity, high g accelerometer ( ADLXL120 ) for Low sensitivity, high g accelerometer ( ADLXL120 ) for

extreme event responses extreme event responses Sensor module with a high sensitivity low noise Sensor module with a high sensitivity low noise

accelerometer ( 1221 accmtr ) for ambient responsesaccelerometer ( 1221 accmtr ) for ambient responses High resolution low speed A/D converterHigh resolution low speed A/D converter Various other sensors Various other sensors i.e thermometeri.e thermometer

Sensor Unit (2)Sensor Unit (2) Operation StatesOperation States

SleepSleep. Sensor module and R/F transceiver in . Sensor module and R/F transceiver in sleep mode. sleep mode. SU controller and the low sensitivity SU controller and the low sensitivity accelerator ( Acc-Low ) intermittently powered. accelerator ( Acc-Low ) intermittently powered. Right After Acc-Low startup time elapsed switches Right After Acc-Low startup time elapsed switches on the A/D converter and samples the Acc-Low on the A/D converter and samples the Acc-Low output. If the data do not indicate an extreme output. If the data do not indicate an extreme seismic event Acc-Low and A/D are powered off.seismic event Acc-Low and A/D are powered off.

If an event ( 5mg and above ) is detected unit If an event ( 5mg and above ) is detected unit enters enters Awake Awake state. Sampling rate is increased state. Sampling rate is increased accordingly and data are saved in SRAM. Event accordingly and data are saved in SRAM. Event time is noted. Synchronizes with LSM andtime is noted. Synchronizes with LSM and adjust adjustss event time accordingly. event time accordingly. Awake state is Awake state is synchronized with TDMA slot.synchronized with TDMA slot.


It passes in It passes in Semi-AwakeSemi-Awake state all modules but state all modules but transceiver are on. Active sampling of both transceiver are on. Active sampling of both accelerators output. Semi-Awake periodically accelerators output. Semi-Awake periodically alternates with alternates with Awake state were radio Awake state were radio transceiver is also ontransceiver is also on, , allowing transmission of allowing transmission of sensor data to LSMsensor data to LSM along with activealong with active samplingsampling

After event has passed and all collected data are After event has passed and all collected data are sent SU enters sleep state.sent SU enters sleep state.

CSM determines at which instances ambient CSM determines at which instances ambient vibration info is to be recorded to enable periodic vibration info is to be recorded to enable periodic monitoring. SUs usually in sleep states are waken monitoring. SUs usually in sleep states are waken up in fixed number of times per day and enter the up in fixed number of times per day and enter the Update State. Update State.


In the In the Update stateUpdate state state sensor module state sensor module is off, SU ctlr and transceiver on and Ac-is off, SU ctlr and transceiver on and Ac-Low cycle powered. It synchronizes with Low cycle powered. It synchronizes with LSM and sets a wake up timer for the LSM and sets a wake up timer for the next scheduled monitoring phase. After next scheduled monitoring phase. After that enters Awake State alternating with that enters Awake State alternating with Semi-Awake transmiting data Semi-Awake transmiting data

After a predefined time interval it goes After a predefined time interval it goes back to sleep.back to sleep.

Local Site MasterLocal Site Master ComponentsComponents

915 MHz radio transceiver for upper tier communication915 MHz radio transceiver for upper tier communication 2.4 GHz radio transceiver for lower tier communication2.4 GHz radio transceiver for lower tier communication LSM ControllerLSM Controller Memory to store the received data to be routed.Memory to store the received data to be routed.

Operates continuously throughout the life of theOperates continuously throughout the life of the

networknetwork

Communication Communication ProtocolProtocol

A special purpose customized TDMA protocol. For N sensor A special purpose customized TDMA protocol. For N sensor units per cluster we have a round of N+2 slots. N for data, units per cluster we have a round of N+2 slots. N for data, 2 for control. A data slot assigned per SU. 12 for control. A data slot assigned per SU. 1stst control slot for control slot for Synch-Ack and second for Global-Synch signal. One packet Synch-Ack and second for Global-Synch signal. One packet per data slot. Frequency hopping at every slot.per data slot. Frequency hopping at every slot.

LSM acknowledges SU packets broadcasting a Synch-Ack LSM acknowledges SU packets broadcasting a Synch-Ack signal containing ack bits for all packets received in the signal containing ack bits for all packets received in the current and the previous round. Of course contains the current and the previous round. Of course contains the local clock information for synchronization of SU clocks with local clock information for synchronization of SU clocks with the global clock.the global clock.

SU notifies LSM before entering the sleep state at the end SU notifies LSM before entering the sleep state at the end of a monitoring or update state. Uppon notification LSM is of a monitoring or update state. Uppon notification LSM is able to use the SU slot for startup packet.able to use the SU slot for startup packet.

Synch-acks are appended with the ambient monitoring Synch-acks are appended with the ambient monitoring schedules enabling schedule updating upon schedules enabling schedule updating upon synchronization. synchronization.

Analysis of the Analysis of the proposed monitoring proposed monitoring

systemsystem Trade Off conclusionsTrade Off conclusions

Best transmission rangeBest transmission range,, possible with low power possible with low power resultsresults,, to low transmission rates. Low to low transmission rates. Low transmission rates support less sensors per cluster transmission rates support less sensors per cluster

When clusters are in a straight chain then one When clusters are in a straight chain then one single LSM is the only access point to the CSM and single LSM is the only access point to the CSM and therefore receives all the routed data. Worst case therefore receives all the routed data. Worst case upper tier data rate requirement is set toupper tier data rate requirement is set to

(2N-n) SU data(2N-n) SU data __________________________ link throughput link throughput where N is total number of SUs and n SUS per cluster.where N is total number of SUs and n SUS per cluster.

Analysis of the Analysis of the proposed monitoring proposed monitoring

system ( 2 )system ( 2 ) Trade Off conclusionsTrade Off conclusions

CSM receives data generated from all Nodes CSM receives data generated from all Nodes ( N) SU data. Latest 2.4GHz radio ( N) SU data. Latest 2.4GHz radio transceivers can support 10Mbps => 2200 transceivers can support 10Mbps => 2200 nodes worst case. But if multiple LSMs have nodes worst case. But if multiple LSMs have access to CSM => 4400 sensorsaccess to CSM => 4400 sensors

High degree of scalability but it is based on High degree of scalability but it is based on veryvery

optimistic assumptions on the supported bit optimistic assumptions on the supported bit rate.rate.

Estimation of the Estimation of the maintenance cyclemaintenance cycle

10 sensors per cluster, extreme event 10 sensors per cluster, extreme event duration 3 mins, periodic monitoring phase duration 3 mins, periodic monitoring phase 3 mins, and time needed by SU for startup 3 mins, and time needed by SU for startup synchronization 4 mins.synchronization 4 mins.

Power consumption components on a yearly basisPower consumption components on a yearly basis Energy consumed in radio transceiver 275mAh Energy consumed in radio transceiver 275mAh Energy consumed in microcontroller unit and Energy consumed in microcontroller unit and

Low Sensitivity Accelerator 300mAh Low Sensitivity Accelerator 300mAh Energy consumed by memory sensor modules Energy consumed by memory sensor modules

and others 1000 mAhand others 1000 mAh

Total of ~ 1600 mAhTotal of ~ 1600 mAh

ImplementationImplementation

NetworkNetwork Two cluster networkTwo cluster network 80Hz sampling 80Hz sampling frequencyfrequency one channel per SUone channel per SU sample resolution 16sample resolution 16 bitsbits 5 SUs per cluster5 SUs per cluster 0.75% throughput 0.75% throughput

Implementation – H/WImplementation – H/W

Sensor unitSensor unit An EVK915 module containing a 915 MHz An EVK915 module containing a 915 MHz

Bluechip radio transceiver at 20kbps baud rate. Bluechip radio transceiver at 20kbps baud rate. Receiver current 12 mA and transceiver 50mA at Receiver current 12 mA and transceiver 50mA at its peak 50 db transmit power.its peak 50 db transmit power.

Interfaced with a lab made sensor boardInterfaced with a lab made sensor board PIC Microchip MCU used as the SU controllerPIC Microchip MCU used as the SU controller ADXL Accelerometer on the sensor board ADXL Accelerometer on the sensor board

LSMLSM An ProximRangelLAN radio modem operating at An ProximRangelLAN radio modem operating at

2.4 GHz with 2 Mbps bit rate. Uses IEEE802.11b2.4 GHz with 2 Mbps bit rate. Uses IEEE802.11b Interfaced to an EVK915 radio moduleInterfaced to an EVK915 radio module PIC Microchip MCU used as the LSM controllerPIC Microchip MCU used as the LSM controller

Implementation – S/WImplementation – S/W SU and LSM controller were SU and LSM controller were

programmed in assembly and the code programmed in assembly and the code was loaded in the microcontrollers.was loaded in the microcontrollers.

Local clock is a S/W clock based on a Local clock is a S/W clock based on a 16bit timer.16bit timer.

FHSS patterns obtained from a linear FHSS patterns obtained from a linear feedback shift register implemented in feedback shift register implemented in the codethe code as pseudo random generator.as pseudo random generator.

CSM software written in CCSM software written in C

LimitationsLimitations

Applicable only in static structures Applicable only in static structures with regular power supplies with regular power supplies which are which are needed needed for the operation of thefor the operation of the upper upper tier. tier.

LSMLSMss are single points of failure for are single points of failure for the SUs in its cluster and CSM a single the SUs in its cluster and CSM a single point of failure for the whole system! point of failure for the whole system!

ConclusionsConclusions

Significant role of power efficiency on the Significant role of power efficiency on the viability of the system.viability of the system.

System throughput of hundreds or thousands System throughput of hundreds or thousands kbps for real time performance.kbps for real time performance.

Required data rate and range require higher Required data rate and range require higher power consumption.power consumption.

The goal of power efficiency is achieved The goal of power efficiency is achieved through partition in two tiers.through partition in two tiers.

TDMA communication protocol contributes to TDMA communication protocol contributes to reduced power consumption. reduced power consumption.

Lab and Field testing under realistic vibration Lab and Field testing under realistic vibration and environmental conditionsand environmental conditions

Future workFuture work

Self reconfigurable SUs so that they Self reconfigurable SUs so that they are not dependant on a specific LSM. are not dependant on a specific LSM. In case of failure it can be assigned to In case of failure it can be assigned to the closer LSM in range or to a backup the closer LSM in range or to a backup one.one.

More robust sensor and Site Master More robust sensor and Site Master UnitsUnits

The end. Questions?The end. Questions?

Technology

Two Tiered Wi Sensor Networks