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Embedded Networks Laboratory
Embedded Sensing of Structures :
A Reality Check
Jeongyeup Paek
Krishna Kant Chintalapudi, Jeongyeup Paek, Nupur Kothari,
Sumit Rangwala, Ramesh Govindan, Erik Johnson
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Embedded Networks Laboratory
Goals of the Talk
• Original vision
“ Millions of tiny sensors embedded in concrete
detect damages in buildings and bridges ”
• Where are we today?
• Where are we heading?
– Is the original vision still meaningful?
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Embedded Networks Laboratory
Agenda
• Introduction to Structural Health Monitoring
• Requirements of SHM Applications
• WISDEN - a wireless sensor network data acquisition
system
• NetSHM – a programmable sensor network for SHM
applications
• Speculations about the future
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Embedded Networks Laboratory
What Is Structural Health
Monitoring (SHM)?
• Structural integrity assessment for
buildings, bridges, offshore oil rigs,
aerospace structures etc.
• Goals of SHM:
– Detection “is there damage?”
– Localization “where is the damage?”
– Quantification “how severe?”
– Prognosis “future prediction”
• Why SHM?
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Embedded Networks Laboratory
SHM Basics
•
Measure and analyze structural
vibrations induced due to heavy
winds or earthquakes, etc
•
Principles behind structural algorithms
can be illustrated by strings
– Structural response is composed of
several harmonics - modes
– Mode = < Frequency, Mode Shape>
•
Damages alter the structural
properties and hence the modes
•
Structural response is measured by
using sensors (accelerometers, strain
gauges) at several locations in the
structure
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Embedded Networks Laboratory
Sensor Networks for SHM
• Current SHM
– Bi-annual visual inspections (most common)
• Limitations of human accessibility and error
• Catastrophic failure between inspections
– Expensive wired data acquisition systems
• Extremely high installation, cabling, and maintenance cost
• Wireless Sensor Network based SHM system
– Flexible, fast and low cost deployments
– No cabling cost!!
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Embedded Networks Laboratory
Agenda
• Introduction to Structural Health Monitoring
• Requirements of SHM Applications
• WISDEN - a wireless sensor network data acquisition
system
• NetSHM – a programmable sensor network for SHM
applications
• Speculations about the future
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Embedded Networks Laboratory
Existing SHM Techniques
Damage Detection
Time
Series
Modal
Frequency
Mode
Shape
Changes in
ARMA
coefficients
Changes in
modal
frequencies
Changes
in mode
shape
Damage Localization
Neural
Networks
Train
neural
networks
with data
Time
Domain
Frequency
Domain
Reconstruct
a structural
model from
data
Reconstruct
structural
model using
mode
shapes
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Embedded Networks Laboratory
Basic Requirements for
SHM Applications
• Reliable Delivery
– SHM applications are loss-intolerant, sensors need to transmit data
reliably
• Time Synchronization
– Data from various sensors should be time-synchronized to within 100
micro-sec for damage localization.
• High Data Rates
– A hundred tri-axial sensors sampling at 500Hz can generate a data rate
of 5Mbps.
• Dense Sensing
– The larger the number of sensors the better the performance
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Embedded Networks Laboratory
Importance of In-Network
Processing
• Sensor networks are expected to last for several months or even a
year without human intervention
• With high data rate radio communication and sensing, nodes will
typically not last more than few days.
• In-network processing can lead to long lived SHM systems by
reducing communication overhead
• Most SHM techniques can leverage local computation at node to
minimize radio communication
– ARMA coefficient for time series based damage detection
– FFT for modal frequency shift based damage detection
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Embedded Networks Laboratory
Agenda
• Introduction to Structural Health Monitoring
• Requirements of SHM Applications
• WISDEN - a wireless sensor network data acquisition
system
• NetSHM – a programmable sensor network for SHM
applications
• Speculations about the future
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Embedded Networks Laboratory
Wisden
• First step
– Replace the existing wired data acquisition system
• Wisden
– Wireless sensor network based data acquisition system
– Allows continuous sampling and reliable logging of time-synchronized
structural response data
• Advantages
– Flexibility
• Nodes self-organize into a multi-hop network.
• Nodes can be inserted in and out of the network dynamically
– Low time and cost of installation
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Embedded Networks Laboratory
Wisden overview
• Three components of Wisden
– Reliability
• Over multiple hops with end-to-end and hop-by-hop recovery
– Time-synchronization
• Novel low-overhead residence time based approach
– Data compression
• Necessary at the source nodes to relieve bandwidth bottleneck and reduce
communication overhead.
• Onset detection – transmit only relevant data
“A Wireless Sensor Network for Structural Monitoring”, Ning Xu, Sumit Rangwala, Krishna
Chintalapudi, Deepak Ganesan, Alan Broad, Ramesh Govindan, Deborah Estrin, In Proceedings
of the ACM Conference on Embedded Networked Sensor Systems, Nov.2004
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Embedded Networks Laboratory
Onset Detection
• Why transmit data when nothing is happening?
• Detect onset of events at the sensor and transmit only when
something is happening
Data not transmitted
during quiescent period
“A Wireless Sensor Network for Structural Health Monitoring: Performance and Experience”,
Jeongyeup Paek, Krishna Chintalapudi, John Caffrey, Ramesh Govindan, Sami Masri, In
Proceedings of the IEEE Workshop on Embedded Networked Sensors, May.2005
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Embedded Networks Laboratory
Deployment of WISDEN
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Embedded Networks Laboratory
Deployment Experiences (1)
• Seismic Structure
– Structural vibrations are highly damped, last less than a second
• Higher sampling rates are needed to collect enough samples for analysis
(>200Hz)
– Platform limitations (such as EEPROM access latencies) proved to be
the obstacles for high sampling rates
– After the development of onset-detection and careful re-engineering,
Wisden was able to achieve 200Hz
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Embedded Networks Laboratory
Deployment Experiences (2)
• Four Seasons Building
– Communication environment was very lossy
• Avg. delivery rate 81% and worst case of 30%
• The path lengths were often 2-3 hops and sometimes even higher
• Frequent route changes occurred due to the variability of the wireless links
– Rate control and hop-by-hop retransmissions were required
– Does not scale
• As number of nodes grows, the bandwidth bottleneck becomes significant
• Leads to our next step hierarchical system: NetSHM
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Embedded Networks Laboratory
Agenda
• Introduction to Structural Health Monitoring
• Requirements of SHM Applications
• WISDEN - a wireless sensor network data acquisition
system
• NetSHM – a programmable sensor network for SHM
applications
• Speculations about the future
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Embedded Networks Laboratory
NetSHM
• NetSHM is the next step to WISDEN.
• A sensor network system that Structural engineers can program
in higher level language such as Matlab/C
• An SHM engineer should be able to write and test variety of
algorithms without having to understand the underlying sensor
network details
• The system should be evolvable – we should not need to rewrite
applications when the technology evolves
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Embedded Networks Laboratory
Architecture of NetSHM
• Two-level Hierarchy
– For scalability, a higher more
endowed layer is required to
manage the aggregate data rates
generated by the motes.
• Isolate application code from
wireless sensor network details
– Wireless sensor network provides
a generic task interface
• getSamples(startTime, noSamples,
sampFreq, axis)
• getFFTSamples(startTime,noSamp
les,sampFreq,axis,fftSize)
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Embedded Networks Laboratory
What does code isolation buy us?
•
Reusability
– Application programmers can use the generic task interface and
write many different SHM applications.
– Basic SHM library functions can be provided on motes: FFT,
auto-correlation, ARMA coefficient estimation, spectral estimation
etc.
• Evolvability
– If a new mote comes along with greater processing power, just
add new functionality, no need to rewrite application.
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Embedded Networks Laboratory
Application on NetSHM
function shifts = getModalShiftsFromBuilding()
% create a group for sensors
gidSensors = NetSHMCreateGroup([1,2,3,4]);
%create a group for actuators
gidActuators = NetSHMCreateGroup([5]);
%actuate after 22 seconds
NetSHMCmdActuate(gidActuators,22);
%collect structural response starting 20 seconds from now,
% 4000 samples at 200Hz,along x-axis only,
samples = NetSHMCmdGetSamples(gidSensors,20,200,1,4);
%find modal frequencies
modes = findModes(samples);
%read original modes
load OriginalModes;
shifts = findModalFreqShifts(modes,OriginalModes);
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Embedded Networks Laboratory
The Stacks
SHM Application
(in C or Matlab)
API in C
API in Matlab
Tasking Library
Reliable Communication
Routing
Tasking
Time
Sync.
Gateway node stack
Reliable
Communication
Routing
Time
Sync
Driver for
Sensing /
Actuation
Mote-class node stack
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Embedded Networks Laboratory
Sensors
Deployment
Actuators
•
Building Details
– 48 inches high, 4 floors, 60 lbs
– Floors –1/2 x 12 x 18 aluminum
plates
– steel 1/2 x 1/8 inch steel
columns
– 5.5 lb/inch spring braces
– 4 actuators on the top floor
– 8 motes, 2/floor
– dual axis, 200Hz, 2 starGates
Motes
•
4 Test Cases
–
–
–
–
braces from
braces from
braces from
braces from
removed
floor 4 removed
floor 3 removed
floor 2 removed
floor 2 and 4
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Embedded Networks Laboratory
Damage Detection and Localization
on scaled model
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Embedded Networks Laboratory
Agenda
• Introduction to Structural Health Monitoring
• Requirements of SHM Applications
• WISDEN - a wireless sensor network data acquisition
system
• NetSHM – a programmable sensor network for SHM
applications
• Speculations about the future
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Embedded Networks Laboratory
Limitations of Wireless Sensor Network
based SHM today
• Hundreds of nodes per structure
• Limited lifetime
– Couple of days with continuous sampling
– Up to couple of months with scheduled monitoring
• Limited in-network processing
– Platform limitations (eg. mica2, micaz)
• Memory (FFT, ARMA, etc)
• Processing (floating point, etc)
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Embedded Networks Laboratory
What next?
• Vision of millions of embedded sensors in concrete seems a bit
too farfetched
– Energy, form factor, communication, etc
• Within the next few years, NetSHM like systems will encourage
SHM engineers to migrate to sensor network systems
• Most of the data processing will migrate into the sensors within
the next five years with the advent of improved sensor platforms
• We believe that the wired sensing will be almost entirely
replaced by wireless networks within the next ten years
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Embedded Networks Laboratory
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