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Pres110811
1. Wireless Sensor Networks for Online Health
Monitoring
Gerges Dib Lalita Udpa
Michigan State University
Nondestructive Evaluation Lab
November 10, 2011
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2. Objectives
Develop a wireless multi-modal sensor network system for
real-time monitoring of structural health.
Investigate damage detection techniques for use in Wireless
Sensor Networks, including:
Acoustic Emission
Ultrasound Testing
Sensor node development, including data acquisition and
preprocessing, network control, and wireless communication.
Signal processing algorithms for impact and damage detection
and characterization.
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3. Project Schedule
Objectives completed and in progress
Sensor node development and interfacing with transducers for NDE using
Lamb wave method. (completed)
A wireless networking protocol for sensor nodes control and data
acquisition using Lamb wave method. (completed)
Development and validation of a Finite Element Model for Lamb wave
propagation in isotropic media. (completed)
Signal processing algorithms for damage localization. (In progress)
Future plans
Lamb wave method in complex geometries.
Investigate Acoustic Emission Testing, and interfacing this method with
the sensor nodes.
Development of multimodal networking.
Signal processing algorithms for damage characterization.
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4. Non-Destructive Evaluation
Nondestructive Evaluation (NDE) techniques are used for
damage detection and characterization
Evaluate fatigue and impact damage,
Assess the integrity and remaining life of component,
Ensure reliability and safety of component.
Shortcomings of Traditional NDE methods
Require schedules for inspection, hence interrupting service
time.
Structure operating condition cannot be instantly known.
Testing interpretation may be subjective to the operator.
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5. Wireless Sensor Networks
Advantages and challenges
Advantages
Solves the problem of impractical wiring and high deployment
expenses of an in-situ system for Continuous Monitoring.
Requires little or no infrastructure.
Composed of low profile sensor nodes, installed
non-obtrusively.
Fully automated, self-configuring, and self healing.
Challenges
Limited power supply is available for conducting NDT and for
wireless communication.
Limited data acquisition and data processing capabilities at
the sensor nodes.
Wireless communication interference in a noisy environment.
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12. Acoustic Emission Testing
Passive technique that just listens to the structure.
Detects the release of energy in a material when a
fracture/crack takes place.
Can be used to monitor crack growth and load change (Kaiser
effect).
Detection of sudden acoustic events to classify impact
damages.
Acoustic Emission signal parameters used for detecting Load
changes.
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13. Lamb wave Inspection: Finite Element Modeling
Derivation of the wave equation
Newton’s 2nd law:
σij,j = ρ¨i
u (1)
Stress-strain relation:
1
kl = (uk,l + ul,k ) (2)
2
Generalized Hooke’s law:
σij = Cijkl kl (3)
In isotropic media, Hooke’s law becomes:
σij = λ kk δij + 2µ ij (4)
Substitute (2) in (4), and then in (1), we get Navier equation:
µui,jj + (λ + µ)uj,ij = ρ¨i
u (5)
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14. Lamb wave Inspection: Finite Element Modeling
Potential form of the wave equation
Express the Navier equation in vector form:
2 ∂2u
(λ + µ) .u + µ u=ρ (6)
∂t2
Representing the displacement vector u = φ + × ψ, we
get the potential forms,
The scalar potential φ governing the propagation of
logitudinal waves:
2 1 ∂2φ
φ= 2 2 (7)
cL ∂t
The vector potential ψ governing the propagation of shear
waves:
2 1 ∂2ψ
ψ= 2 (8)
cT ∂t2
where
λ+µ µ
cL = and cT =
ρ ρ 14 / 35
15. Lamb wave Inspection: Finite Element Modeling
Assumptions and simplifications
Consider plane harmonic waves propagating in plate of
thickness 2d in positive x direction.
Plane strain conditions: Strain components in z-direction are
zero: zz = zx = zy = 0
The vector potential only non-zero value is along the
z-direction, will be denoted by ψ.
∂2φ ∂2φ 1 ∂2φ
+ 2 = 2 2 (9)
∂x2 ∂y cL ∂t
∂2ψ ∂2ψ 1 ∂2ψ
+ = 2 (10)
∂x2 ∂y 2 cT ∂t2
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16. Lamb wave Inspection: Finite Element Modeling
Wave Equation solution
The PDE solutions for equations (9) and (10) respectively
have the form:
φ = φ(y)ei(kx−ωt) (11)
ψ = ψ(y)ei(kx−ωt) (12)
Substitute back into the wave equation, we get:
φ(y) = A1 sinpy + A2 cospy (13)
ψ(y) = B1 sinqy + B2 cosqy (14)
where,
ω2 ω2
p= − k 2 and q = − k2
c2
L c2
T
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17. Lamb wave Inspection: Finite Element Modeling
Symmetric and Anti-symmetric modes
Recall that u = φ + × ψ, we get the displacement vector
which will be a function of sines and cosines, and hence it can
be split into two sets of mode:
Symmetric modes:
ux = ikA2 cospy + qB1 cosqy (15)
uy = −pA2 sinpy − ikB1 sinqy (16)
Anti-symmetric modes:
ux = ikA1 sinpy − qB2 sinqy (17)
uy = pA1 cospy + ikB2 cosqy (18)
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18. Lamb wave Inspection: Finite Element Modeling
Boundary conditions
The constants A1 , A2 , B1 , B2 , and the dispersion equations
are still unknown.
Apply traction free boundary conditions at the surface of the
plate: σxy = σyy = 0 at y = ±d.
This gives an eigenvalue problem, with the eigenvalues
satisfying those equations:
Symmetric modes:
tanqd 4k 2 pq
= 2 (19)
tanpd (q − k 2 )2
Anti-symmetric modes:
tanqd q 2 − k 2 )2
=− (20)
tanpd 4k 2 pq
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19. Lamb wave Inspection: Finite Element Modeling
Point Force Simulation
Simulations in ABAQUS using the point force model where the actuation
signal due to attached PZT is modeled as a point force:
σa (x) = aτo [δ(x − a) − δ(x + a)] (21)
The actuation signal is the Hanning windowed signal.
Generating pure S0 mode:
Generating pure A0 mode:
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20. Lamb wave Inspection: Finite Element Modeling
Point force simulation results and validation
Conducted simulation for actuation frequencies from 50 KHz to
300 KHz, and compared obtained dispersion curves for phase
velocity with theoretical equations.
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21. Lamb wave Inspection: Finite Element Modeling
Multiphysics Simulation of Piezoelectric wafers
Plane strain model an Aluminum plate with 2 PZT-5A wafers
attached to it (strains in z-direction go to zero).
Piezoelectric constitutive equations:
Sij = sE Tkl + dkij Ek
ijkl (22)
Dj = djkl Tkl + εT E k
jk (23)
Where the mechanical compliance: Piezoelectric coupling:
16.4 −7.22 0 0 0 −171
sE = 10−12 −7.22 18.8 0 d = 10−12 0 0 374
0 0 47.5 584 0 0
Dielectric permittivity
1730 0 0
ε = εo 0 1730 0
0 0 1700
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22. Lamb wave Inspection: Finite Element Modeling
Multiphysics simulation validation
Hanning windowed actuation electric field applied at PZT A
with frequency 175 KHz.
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23. Ongoing work: FEM of waves in an anisotropic layer
Solution of the wave equation
The wave equation in anisotropic media:
Cijkl uk,jl = ρ¨i
u (24)
Assumptions
1 The coordinate system is chosen such that wave is
independent of the z-coordinate: k3 = kz = 0.
2 The x component of the wave number is known, kx = k.
3 k2 = ky = ly k.
4 Plane harmonic traveling waves:
ui = αi ei(k(x+ly y)−ωt) (25)
Substitute (25) back into the wave equation (24), we get the
Christoffel equation for anisotropic media:
ρω 2 δim − Ciklm kk kl αm = 0 (26)
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24. Waves in an anisotropic layer
Solution of the wave equation
For a nontrivial solution, we require that:
det[ρω 2 δim − Ciklm kk kl ] = 0 (27)
Expression (27) can be solved using the partial wave
technique: Take the superposition of three upward traveling
plane wave modes and three downward traveling plane wave
modes.
The expanded form of (27) can be written as:
6 5 4 3 2
Aly + Bly + Cly + Dly + Ely + F ly + G = 0 (28)
The six coefficients (A through G) are functions of density
and elastic constants.
For monoclinic or higher-symmetry materials, B = D = F =
0, and we have:
6 4 2
Aly + Cly + Ely + G = 0 (29)
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25. Waves in an anisotropic layer
Solution of the wave equation: Boundary conditions
The six values of ly , and the six polarization vectors α(n) are
obtained fom equation (28).
The displacement field would then be:
6
(n)
(n)
uj = Cn αj eik(x+ly y)
(30)
n=1
Boundary condition: Traction vanishes on the upper and lower
surfaces of the layer: Txy = Tyy = Tyz = 0 at y = ±d.
The Traction components are calculated by: Tij = Cijkl kl ,
1 ∂uk ∂ul
and kl = 2 ∂xl + ∂xk .
We therefore obtain the following homogeneous system:
Bij (ρ, Cijkl , kd)Cj = 0,
The Lamb wave dispersion curves are found by setting
det[B] = 0.
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27. Top-Down System Specification
Data acquisition, preprocessing, and wireless communication
Iris Mote
Commercial sensor nodes are available with
integrated microprocessors and RF radios
with IEEE 8.15.4 standard, running on
batteries.
They are intended for generic use such as
temperature and humidity sensing.
The sensing interface is not suitable for
specialized NDE techniques.
They do provide 51-pin extension connector,
enabling to extend their functionality.
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28. Data Acquisition
Iris mote extension sensor board design
A sensor board is designed to interface the Iris mote with the
PZT wafers for active sensing (data acquisition and
actuation).
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29. Data Acquisition
The sensing interface
Signal conditioning for reducing the required data sampling
frequency.
Evelop detector converts signal to base band frequency.
Full wave rectifier + Second order low pass filter.
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30. Data Acquisition
The actuation interface
Use Iris mote digital I/O to generate a square wave with
programmable number of cycles.
Filter square wave signal to obtain a sine tone burt.
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31. Data Control and Networking
The Sensor Nodes and Base Station State Machines
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34. Conclusion
Finite Element Modeling for Lamb wave propagation was
investigated and validated with experimental data.
A Sensor Board was designed for interfacing the Iris mote with
PZT wafers for Lamb wave inspection.
A wireless networking protocol was implemented for sensor
nodes control and data acquisition.
Signal processing algorithm for damage localization was
investigated.
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35. Future Work
Simulations and experiments for Lamb wave inspection in
anisotropic media.
Damage detection using Lamb wave in complex geometries
(cylinders, platelike-geometries with stiffners, etc).
Investigate Acoustic Emission testing, and interfacing this
method with the sensor nodes.
Update control sequence for multimodal inspection support.
Signal processing algorithms for damage characterization.
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