a wireless sensor network for structural monitoring (wisden)

UNIVERSITY OFSOUTHERN CALIFORNIA

Embedded Networks Laboratory

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A Wireless Sensor Network For Structural Monitoring

(Wisden)

Collaborators: Ning Xu, Krishna Kant Chintalapudi, Deepak Ganesan, Alan Broad, Ramesh Govindan, Deborah Estrin,

Jeongyeup Paek, Nupur Kothari

Sumit Rangwala



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BackgroundBackground• Structural health monitoring (SHM)

– Detection and localization of damages in structures» Structural response

• Ambient vibration (earthquake, wind etc)

• Forced vibration (large shaker)

• Current SHM systems– Sensors (accelerometers) placed at different structure location

– Connected to the centralized location » Wires (cables)

» Single hop wireless links

– Wired or single hop wireless data acquisition system



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MotivationMotivation• Are wireless sensor

networks an alternative?

• Why WSN?– Scalable

» Finer spatial sampling

– Rapid deployment

• Wisden– Wireless multi-hop data

acquisition system



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ChallengesChallenges• Reliable data delivery

– SHM intolerant to data losses

• High aggregate data rate– Each node sampling at 100 Hz or above

» About 48Kb/sec (10 node,16-bit sample, 100Hz, 3 axes)

• Data synchronization– Synchronizing samples from different sources at the base station

• Resource constraints– Limited bandwidth and memory

• Energy efficiency– Future work



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Wisden ArchitectureWisden Architecture

Challenges Architectural Component

Description

Reliable data delivery

Reliable Data Transport

Hybrid hop-by-hop and end-to-end error recovery

High data rate Compression Silence suppression

Wavelet based compression

Data Synchronization

Data Synchronization

Residence time calculation in the network



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Reliable Data TransportReliable Data Transport• Routing

– Nodes self-organize in a routing tree rooted at the base station

– Used Woo et al.’s work on routing tree construction

• Reliability – Hop-by-hop recovery

» How ?• NACK based• Piggybacking and

overhearing

» Why hop by hop? • High packet loss

NACK

Retransmission

NACK

Retransmission

NACK

Retransmission



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Reliable Data Transport (cont.)Reliable Data Transport (cont.)– End to End packet recovery

» How ?• Initiated by the base station (PC) • Same mechanism as hop-by-hop NACK

» Why ? • Topology changes leads to loss of missing packet information• Missing packet information may exceed the available memory

– Data Transmission rate» Rate at which a node inject data

• Currently pre-configured for each node at R/N– R = nominal radio bandwidth – N = total number of nodes

» Adaptive rate allocation part of future work.



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CompressionCompression• Sampled data significant

fraction of radio bandwidth• Event based compression

– Detect Event » Based on maximum

difference in sample value over a variable window size

– Quiescent period» Run length encoding

– Non-quiescent period» No compression

– Saving proportional to duty-cycle of vibration

• Drawback– High latency

Quiescent Period

Event Quiescent Period

Compression No Compression

Compression



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Compression For Low LatencyCompression For Low Latency• Progressive storage and

transmission– Event detection

– Wavelet decomposition and local storage

– Compression » Low – resolution components

are transmitted

– Raw data, if required available from local storage

• Current Status

– Evaluated on standalone implementation

– To be integrated into Wisden

Wavelet Decomposition

Quantization, Thresholding, Run length coding

Sink

Flash Storage

To sink on demand

Reliable Data Transport

Event

Low resolution components



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Data SynchronizationData Synchronization• Synchronize data samples at the

base station– Generation time of each sample in

terms of base station clock

– Network wide clock synchronization not necessary

• Light-weight approach– As each packet travels through the

network » Time spent at each node

calculated using local clock and added to the field “residence time”

» Base station subtracts residence time from current time to get sample generation time.

– Time spent in the network defines the level of accuracy

S

q AAq

A

q A +

q B

Bq

B

TA=T-(qA + qB) TC=T-(qC + qD)

qC C

qC

qC + q

D DqD



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ImplementationImplementation• Hardware

– Mica2 motes – Vibration card (MDA400CA

from Crossbow)» High frequency sampling (up

to 20KHz)» 16 bit samples» Programmable anti-aliasing

filter

• Software– TinyOS– Additional software

» 64-bit clock component» Modified vibration card

firmware



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Deployment ScenarioDeployment Scenario11

• Seismic test structure– Full scale model of an

actual hospital ceiling structure

• Four Seasons building – Damaged four-storey office

building subjected to forced-vibration

1Not presented in the paper



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Seismic Test Structure SetupSeismic Test Structure Setup• Setup

– 10 node deployment– Sampling at 50 Hz along three

axes– Transmission rate at 0.5

packets/sec– Impulse excitation using

hydraulic actuators• For validation

– A node sending data to PC over serial port (Wired node)

– A co-located node sending data to the PC over the wireless multihop network (Wisden node)



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Results: Frequency ResponseResults: Frequency Response

• Low frequency modes captured• High frequency modes lost

– Artifact of compression scheme we used

Power spectral density: Wisden node Power spectral density: Wired node



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Results: Packet Reception and LatencyResults: Packet Reception and Latency• Packet reception

– 99.87 % (cumulative over all nodes)

– 100 %, if we had waited longer

• Latency– 7 minutes to collect data for

1 minute of vibration



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Four Seasons BuildingFour Seasons Building• Setup

– 10 node deployment– Sampling at 50 Hz along

three axes– Transmission rate at 0.5

packets/sec– Excitation using eccentric

mass shakers

• For validation– Wisden nodes places

alongside floor mounted force-balance accelerometer (Wired node)



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Results: Frequency ResponseResults: Frequency Response

• Dominant frequency captured• Noise

– Sampling differences, force balanced accelerometer much more sophisticated, packet losses

Power spectral density: Wisden Node Power spectral density: Wired Node



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Results: Packet ReceptionResults: Packet Reception• Packet reception

– High data loss» Due to a bug



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Conclusions and Future WorkConclusions and Future Work• Wisden – A wireless data acquisition system that provides

– Reliable data collection– Supports high sampling rate– Data synchronization

• Future work– Adaptive rate allocation scheme– Integrating wavelet based compression– Power efficiency

• Wisden version 0.1 available at http://enl.usc.edu/

Thank you

a wireless sensor network for structural monitoring (wisden)

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