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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
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?
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
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?
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”
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)
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.
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
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?
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
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
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.
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.
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”
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
Structural Dynamics 101Structural Dynamics 101
Structures are no different from strings!!
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 ,
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 ,
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.
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.
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
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
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
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)
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
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
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?
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
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
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
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
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?
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
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
The StacksThe Stacks
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.
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
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)
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
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
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
UNIVERSITY OFSOUTHERN CALIFORNIA
Embedded Networks Laboratory
DeploymentDeployment
Seismic Test Structure
Scaled Building Model
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
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
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?
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