university of southern california embedded networks laboratory a programmable sensor network based...
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![Page 1: UNIVERSITY OF SOUTHERN CALIFORNIA Embedded Networks Laboratory A Programmable Sensor Network Based Structural Health Monitoring System Krishna Kant Chintalapudi](https://reader035.vdocuments.net/reader035/viewer/2022070409/56649e905503460f94b957f6/html5/thumbnails/1.jpg)
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
A Programmable Sensor Network A Programmable Sensor Network Based Structural Health Monitoring Based Structural Health Monitoring
SystemSystem
Krishna Kant Chintalapudi
Embedded Networking Laboratory,
University of Southern California, Los Angeles, USA
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for sensor network based SHM
• A prototype – implementation and deployment
• What next?
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What’s the talk about?What’s the talk about?
• A programmable sensor network based system for structural health monitoring
• What are the requirements of SHM applications?
• How do we architect a sensor network system to satisfy these requirements?
• A prototype and its performance
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for sensor network based SHM
• A prototype – implementation and deployment
• What next?
![Page 5: UNIVERSITY OF SOUTHERN CALIFORNIA Embedded Networks Laboratory A Programmable Sensor Network Based Structural Health Monitoring System Krishna Kant Chintalapudi](https://reader035.vdocuments.net/reader035/viewer/2022070409/56649e905503460f94b957f6/html5/thumbnails/5.jpg)
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What Is Structural Health Monitoring What Is Structural Health Monitoring (SHM)?(SHM)?
• Structural integrity assessment for buildings, bridges, offshore rigs, vehicles, aerospace structures etc.
• Goals of SHM are:
– damage detection “is there damage?”
– damage localization “where is the damage?”
– damage quantification “how severe?”
– damage prognosis “future prediction”
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
How Are Damages Caused?How Are Damages Caused?
• Extreme stress leading to fatigue in elements
– several freeway bridges today bear traffic far exceeding tolerance levels they were originally designed to bear.
• Rusting and degradation of material properties
– leads to change in stress distribution and overloading of certain elements more than others
• Continuous vibrations/cyclic stresses in the structure
– waves shaking offshore oil-rigs, gales shaking bridges.
• Catastrophes (earthquakes)
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
How Do Damages Evolve?How Do Damages Evolve?
• Most damages start as tiny cracks caused by metal fatigue (microns-mm).
• If unattended the cracks creep and grow in size leading to deterioration of the material.
• If unchecked, it eventually results in an unpredictable, sudden and catastrophic failure.• SHM techniques focus on detection and localization of damages as early as possible.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
SHM TodaySHM Today•Today SHM is carried out by
– collecting sensor data from several locations in the structure and analyzing it on a high end platform
– periodic (bi-annual) human inspections (visual/using portable devices),
– expensive and dedicated data-acquisition systems (for structures where monitoring is critical) .
• SHM suffers from
– human error and inaccessibility of locations within the structure
– expensive labor (for inspection), cabling and installation (for data-acquisition systems)
– possibility of catastrophic failure between inspections
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for sensor network based SHM
• A prototype – implementation and deployment
• What next?
![Page 10: UNIVERSITY OF SOUTHERN CALIFORNIA Embedded Networks Laboratory A Programmable Sensor Network Based Structural Health Monitoring System Krishna Kant Chintalapudi](https://reader035.vdocuments.net/reader035/viewer/2022070409/56649e905503460f94b957f6/html5/thumbnails/10.jpg)
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Local vs. Global TechniquesLocal vs. Global Techniques
• Use sophisticated imaging techniques – 250KHz ultrasound, x-ray, thermal, magnetic etc.
• Use accelerometers to collect structural response.
LOCAL GLOBAL
• Detect tiny cracks (mm/cm) and small corroded patches.
• Target larger damages e.g. undermined cables, braces or columns
• Can detect damages within a few inches of the equipment
• Detect structural damages in the entire structure
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Feasibility of Local SHM Feasibility of Local SHM Techniques Techniques
• They are expensive, require a lot of power and bulky
• Demand extremely dense deployments
• Local SHM techniques are not amenable to sensor network deployments
• So let us focus on global schemes henceforth
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Ambient vs. Forced Excitation Ambient vs. Forced Excitation
• Low signal-to-noise ratio. • Much higher signal-to-noise ratio.
AMBIENT FORCED
• Rely on ambient sources (wind, passing vehicles, earthquakes)
• Rely on induced excitation (impact hammer, rotating mass etc.)
• Unpredictable in nature and timing
• Pre-meditated and precise.
• Require continuous monitoring; hard to implement duty cycles.
• Amenable to extremely low duty cycle functioning.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Recall Our Goal…Recall Our Goal…
• We want a system that SHM engineers can program … not experts in TinyOS
• We explore existing SHM schemes to find what SHM engineers want?
• We design our system based on requirements of SHM schemes.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What SHM Engineers want?What SHM Engineers want?
• Structural integrity assessment for buildings, bridges, offshore rigs, vehicles, aerospace structures etc.
• Today SHM engineers want:
– damage detection “is there damage?”
– damage localization “where is the damage?”
– damage quantification “how severe?”
– damage prognosis “future prediction”
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Structural Dynamics 101Structural Dynamics 101
Structures are no different from strings!!
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Structural Dynamics 101…Structural Dynamics 101…• Structural response is the spatio-temporal deformation induced in the structure.
• The dynamics of a structure are often expressed as,
• The impulse response is given by
• vl are mode shapes – normalized structural deformation patterns
• are modal/resonant frequencies of the structure
• are the amplitude and phase of the mode induced in the structure
)(''' tfKyyCyM
ml
lll
tll tevaty l
1
cos)(
l
lla ,
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Structural Dynamics 101…Structural Dynamics 101…
• mode shapes and frequencies are fundamental to the structure
• material properties, geometry and assemblage of elements
• depend on both the sensing and actuating locations
• mode are global phenomena – may span the entire structure
ml
lll
tll tevaty l
1
cos)(
lla ,
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
How Does Damage Affect Modes?How Does Damage Affect Modes?
• Modal (resonant) frequencies and mode shapes change
• Modal frequencies decrease
• Break in symmetry of the structure may lead to splitting of overlapping modes and cause extra modes to appear
• Non-linearities may introduce new modes.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Some practical aspectsSome practical aspects
• Modal frequencies are typically in the range of few tens of Hz
• Real structures are often heavily damped and decay within a second
• Most SHM engineers prefer 10 times oversampling
• Sampling rates desired are around 200-500Hz for most structures.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Literature Review – Damage DetectionLiterature Review – Damage Detection
• Model the structural response using ARMA/AR based linear predictors and look for a significant change in coefficients.
• Look for shifts/changes in modal frequencies through spectral analysis.
• Look for changes in mode shapes.
• Use non-linear techniques such as neural networks.
• Literature is very vast
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Literature Review – Damage LocalizationLiterature Review – Damage Localization
• Significantly more challenging and still a very hot research topic.
.
• Time domain methods, model structure as a LTI system
• try to solve for A,B,C and D using response from all sensors
• compute stiffness of elements using A, B, C and D
• loss of stiffness indicates damage in an element
)()()(
)()()('
tDutCxty
tButAxtx
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Damage Localization Techniques…Damage Localization Techniques…
• Frequency domain - estimate mode shapes using structural response from all sensors and use mode shapes to estimate stiffness of members
• ERA (Eigenvalue Realization Algorithm) – perform SVD on the Hankel matrix
y is the impulse response
vector
• Select modes corresponding to the high singular values to forma reduced order system, and calculate the modal vector matrix V using,
PEQH
rpkyrkyrky
pkykyky
pkykyky
kH
)0(,
)(...)1()(
............
)1(...)2()1(
)(...)1()(
)(
CV
IFVDPFC
EQHPEA
pTpnn
Tn
nnT
nn
]0[,
)1(
2
1
2
1
2
1
![Page 23: UNIVERSITY OF SOUTHERN CALIFORNIA Embedded Networks Laboratory A Programmable Sensor Network Based Structural Health Monitoring System Krishna Kant Chintalapudi](https://reader035.vdocuments.net/reader035/viewer/2022070409/56649e905503460f94b957f6/html5/thumbnails/23.jpg)
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What’s common to SHM schemes?What’s common to SHM schemes?
• Inherently Centralized – Global nature of modes naturally leads to centralized algorithms for detection and localization.
• Can leverage local computation – Almost none of the schemes uses data in its raw form
• ARMA/AR models need coefficients
• Modal frequency based schemes need to use the estimated spectrum
• Compute these quantities locally and transmit instead of raw data.
• 40 ARMA coefficients instead of 5000 samples (over 99% savings!!!)
• Little or no collaboration/aggregation – most algorithms do not require inter-node collaboration (eg SVD is hard to decentralize)
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
How many sensors would a typical How many sensors would a typical structure need?structure need?
• Strategies for deploying sensors
• Deploy a tri-axial sensor at the end of every member (damage localization/member)
• Divide the structure into sections and deploy a tri-axial sensor at every corner (damage localization/section)
• Number of sensors determines the granularity of localization (per floor? Per column?)
• A real structure can have several 100s of members/sections
• Local computation is absolutely critical
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What are the requirements of What are the requirements of SHM schemes?SHM schemes?
• High data rates – 100 sensor will generate a few Mbps of data
• Reliable Delivery – SHM algorithms do not tolerate sample losses
• Time Synchronization - Required by most schemes
• error in time-synchronization manifests as phase error in modes
• error ~ , the higher the modal frequency the more accuracy one needs
• For 1% error in a 20Hz mode, an accuracy of about 100
• Local computation – data acquisition system based solutions will not scale
tf2
s
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for a programmable sensor network based SHM system
• A prototype – implementation and deployment
• What next?
![Page 27: UNIVERSITY OF SOUTHERN CALIFORNIA Embedded Networks Laboratory A Programmable Sensor Network Based Structural Health Monitoring System Krishna Kant Chintalapudi](https://reader035.vdocuments.net/reader035/viewer/2022070409/56649e905503460f94b957f6/html5/thumbnails/27.jpg)
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Recall Our Goal…Recall Our Goal…
• We want a system that SHM engineers can program … in Matlab/C
• An SHM engineer should be able to write and test variety of algorithms without having to re-program the motes
• The system should be evolvable – a if better mote platform come, the SHM engineer should not need to rewrite his code
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Typical operation of an SHM systemTypical operation of an SHM system
• Sensors collect noise unless the structure is shaking!!!
• Ambient Schemes – rely on significant event (heavy wind, passing truck)
• Forced Schemes – rely on actuators (impact hammers)
• Structural Response lasts a few seconds!!!
• Sensors sleep unless an event occurs or the users requests actuators to test
• Sleep --- test/significant event ---- collect data and locally process --- transmit to central location --- sleep (wake once a day/ once a few hrs)
• SHM systems will be Triggered Systems
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Architecture Design DecisionsArchitecture Design Decisions
• Two-level Hierarchy – A higher more endowed layer is required to manage the aggregate data rates generated by the motes.
• Isolate Application code from mote code – Mote class devices provide a generic task interface but no application specific code
• getSamples(startTime, noSamples, sampFreq, axis)
•getFFTSamples(startTime,noSamples,sampFreq,axis,fftSize)
• actuateStructure(startTime,type, parameters)
• conveyed to motes as tasking packets by gateway-class devices
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What does code isolation buy us?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 me 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.
• Gateway class nodes translate C/Maltab application code into mote tasking commands
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for a programmable sensor network based SHM system
• A prototype – implementation and deployment• What next?
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
We have a prototypeWe have a prototypefunction shifts = getModalShiftsFromBuilding()
% create a group for sensorsgidSensors = NetSHMCreateGroup([1,2,3,4]);
%create a group for actuatorsgidActuators = NetSHMCreateGroup([5]);
%actuate after 22 secondsNetSHMCmdActuate(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 frequenciesmodes = findModes(samples);%read original modesload OriginalModes;shifts = findModalFreqShifts(modes,OriginalModes);
• A complete SHM test
•Matlab API
• Matlab functions implemented as wrappers over C functions
• Platform MicaZ and starGates
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
The StacksThe Stacks
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
The API The API
• Groups – Every task is addressed to a group of sensors/actuators
• Create, AddNodes, DeleteNodes, ClearGroup etc
• Create returns a handle to the group
• Tasks – task(groupId, parameters)
• getSamples, getFFTSamples, getXCorrSamples, getModalFreqs, actuate etc.
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Mote Tasking LibraryMote Tasking Library
• Translates API commands into command packets to motes
• Uses TimeSynch Module to translate global time to sensor network time
• Dispatches command packets using the Reliability Layer
• Delivers results to applications according to API specifications
• A collection of C and Matlab Mex files
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Reliability LayerReliability Layer
• Transactional Delivery – Application expects results asynchronously
• Application issues a task
• Mote Tasking library breaks it up into commands
• Opens a connection to Reliability layer and sends command packet
• Reliability layer keeps connection open and forwards result packets to Mote Tasking Lib
• Mote Tasking Library aggregates results and returns to applications
• Takes care of out of order delivery
• Can handle several applications simultaneously
• OneShot Delivery – Application does not expect any results (e.g. Actuate)
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Time SynchronizationTime Synchronization
• Use FTSP
• Small modifications for compatibility with our code
• We use 28.8Khz timer and get accuracy to a few 100micro-sec
• All motes are synchronized to a single mote
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
RoutingRouting
• Does not require any-to-any routing
• starGates to motes
• mote to starGates
• starGates to starGates
• both communication end points are never motes
• Routing Modules used
• starGate to starGate - a distance vector routing scheme, also passes on routes to motes
• motes to starGates – CENS Extensible Sensing System,
• starGates to motes – each node periodically transmits list of nodes in its sub-tree to its
parent, the parent keeps a pointer on the reverse path
• We are still investigating better choices for routing
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Sensing HardwareSensing Hardware
• MDA400 vibration cards from Crossbow
• high quality low power vibration sensing
• 16-bit samples, on board storage (64k)
• 0-20000Hz sensing
• 4 simultaneous channels
• driven by a micaZ
• Accelerometers
• high sensitivity (1v/g)
• low noise
• Actuators
• off-the shelf door latch devices
• motor control board interfaced to micaZ
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
DeploymentDeployment
Seismic Test Structure
Scaled Building Model
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Damage Detection and LocalizationDamage Detection and Localizationon scaled modelon scaled model
• 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
• 4 Test Cases
• braces from floor 4 removed
• braces from floor 3 removed
• braces from floor 2 removed
• braces from floor 2 and 4 removed
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Performance Analysis on Performance Analysis on Seismic Test StructureSeismic Test Structure
• Structure details
• Full scale imitation of a hospital ceiling (28’ by 48’)
• electric lights, drop ceiling, water pipes, fire sprinklers
• 55,000 lb actuator, 10 inch stroke, manually operated right now
• 15 micaZ motes, 2 starGates, 200Hz
• Latency and robustness to failure
• One starGate carrying most motes killed
• all samples recovered
• 3000 samples in about 5 minutes
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
AgendaAgenda
• What’s the talk about ?
• What’s structural health monitoring (SHM)?
• SHM techniques and their impact on sensor network design
• Architecture design for a programmable sensor network based SHM system
• A prototype – implementation and deployment
• What next?
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UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
What next?What next?
• Develop schemes that allow aggressive local computation
for damage localization.
• Remotely actuate the Seismic Test Structure
• Developing local actuators for the Seismic Test Structure
• Damage Detection and Localization on the Seismic Test Structure
• Experiments on real bridges and structures with large scale deployments