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- 1. Wireless Sensor Networks for Online Health Monitoring Gerges Dib Lalita Udpa Michigan State University Nondestructive Evaluation Lab November 10, 2011 1 / 35
- 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. 2 / 35
- 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. 3 / 35
- 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. 4 / 35
- 5. Wireless Sensor NetworksAdvantages 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. 5 / 35
- 6. System OverviewA centralized WSN architecture 6 / 35
- 7. Control Sequence in a Multi-modal WSN 7 / 35
- 8. Top-Down System Speciﬁcation 8 / 35
- 9. Top-Down System SpeciﬁcationTransducer Selection 9 / 35
- 10. Transducer SelectionPiezoelectric Wafer Active Sensors (PWAS) Small size (< 10x10x0.2mm), Light weight(< 100 mg), Low cost (< $10). Far-ﬁeld damage detection using pulse-echo and pitch-catch methods. Near-ﬁeld damage detection using impedance method. Acoustic emission monitoring of crack initiation and growth, and impact detection. 10 / 35
- 11. Top-Down System SpeciﬁcationNDE Techniques 11 / 35
- 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. 12 / 35
- 13. Lamb wave Inspection: Finite Element ModelingDerivation 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) 13 / 35
- 14. Lamb wave Inspection: Finite Element ModelingPotential 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 ModelingAssumptions 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 15 / 35
- 16. Lamb wave Inspection: Finite Element ModelingWave 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 16 / 35
- 17. Lamb wave Inspection: Finite Element ModelingSymmetric 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) 17 / 35
- 18. Lamb wave Inspection: Finite Element ModelingBoundary 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 18 / 35
- 19. Lamb wave Inspection: Finite Element ModelingPoint 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: 19 / 35
- 20. Lamb wave Inspection: Finite Element ModelingPoint 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. 20 / 35
- 21. Lamb wave Inspection: Finite Element ModelingMultiphysics 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 21 / 35
- 22. Lamb wave Inspection: Finite Element ModelingMultiphysics simulation validation Hanning windowed actuation electric ﬁeld applied at PZT A with frequency 175 KHz. 22 / 35
- 23. Ongoing work: FEM of waves in an anisotropic layerSolution 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) 23 / 35
- 24. Waves in an anisotropic layerSolution 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) 24 / 35
- 25. Waves in an anisotropic layerSolution 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. 25 / 35
- 26. Top-Down System Speciﬁcation 26 / 35
- 27. Top-Down System SpeciﬁcationData 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. 27 / 35
- 28. Data AcquisitionIris 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). 28 / 35
- 29. Data AcquisitionThe 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. 29 / 35
- 30. Data AcquisitionThe 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. 30 / 35
- 31. Data Control and NetworkingThe Sensor Nodes and Base Station State Machines 31 / 35
- 32. System Validation 32 / 35
- 33. Top-Down System SpeciﬁcationSignal processing for defect localization 33 / 35
- 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. 34 / 35
- 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. 35 / 35

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