Application of Fiber Optic Strain Gauging in Wind Turbine Blades

1. Application of Fiber Optic Strain Gauging in Wind Turbine Blades Prof. D.S. Cairns J.C. Blockey Picture:http://www.leica-geosystems.com/news/images/windmill.jpg

2. Some Definitions • Safe Life – a structure is guaranteed to have no failures over its lifetime (usually several lifetimes for conservatism) • Durability – the ability of a material/structure to sustain an event or sequence of events without damage (fatigue is a subset of durability) • Damage Tolerance – the ability of a material/structure to maintain performance with damage present

3. Key Elements of Damage Tolerance Residual Strength Analysis + Progressive Damage Analysis (requires a damage growth model and accurate loads data) + Inspection Program = No in-service failures

4. Reliability with Damage Tolerant Design Design, Analysis, Materials and Manufacturing Maintenance and Inspection Certification and Regulatory Actions

7. Basics • Fiber optic cables transmit light • Gratings created in the cable reflect different wavelengths of light • Strains change the grating spacing altering reflected wave lengths • Grating referred to as Fiber Bragg Gratings • Introduced to public market it 19951

8. Fiber Optic Construction

9. Polarized Fiber Optics • Core-Shell Construction Polarization at different angles, strain causes shift

10. Polarized Light

11. Polarized Light Source (Laser LED)

12. Get Yours Now on eBay • The PFOSS is a polarized fiber optic source. The unit incorporates a 1550 nm, 1 mW polarized fiber optic source with a super/ultra FC receptacle and rotatable polarizer achieving up to 35 dB extinction ratio. The rotatable polarizer allows adjustment of the polarization axis to any desired angle. Unit has been tested for basic functionality. Includes carrying case, power supply, and user manual. • Save 10% on this item when you make a purchase at • ILXLightwave.com, Bozeman, Montana • Select a picture

13. Get Yours Now on eBay(cont.)

14. Fiber Bragg Grating • First demonstrated by Kenneth Hill in 19781 • Created by illuminating a fiber with a spatially-varying pattern of intense UV laser light2 • Gratings reflect a specific wavelength of light • Strains result in a change in the grating patter • Reflected wave length slightly shifted by change in pattern enabling use as sensor

15. Reflected Light http://www.bayertechnology.com/img_basis/de_Faser-Bragg-Gitter.gif http://www.sensorsmag.com/sensors/data/articlestandard/sensors/162006/321349/fig1b.gif • Each grating reflects only a small wavelength3 • Non-reflected waves allowed to continue on • Shift in reflected wavelength can be read

16. Use as Gauges http://www.specialtyphotonics.com/img/newsletter/tdc_gratings.jpg • By selecting only a small wavelength at each grating, many gauges can be put on one cable • Over one hundred gauges on one cable4 • Sensing abilities include strain, temperature and pressure5

17. The Latest Twist, Polarization with Bragg Grating • Linearly polarized fiber-optic laser • Document Type and Number: • United States Patent 5561675 • Link to this page: • http://www.freepatentsonline.com/5561675.html • Abstract: • A fiber-optic laser comprising a birefringent optic fiber possessing a Bragg grating at each of its ends,. A light source emits a light beam having two modes of polarization in the fiber. The birefringence of the fiber makes it possible to keep the two polarization modes separate. The two Bragg gratings are photo-recorded in the fiber and are made in such a way that their resonance wavelength is matched for one polarization. The wave emitted by the fiber is then polarized linearly along P1. Applications to linearly polarized lasers for optical transmission, instrumentation, spectroscopy, medicine, the detection of chemical species and telemetry.

18. Advantages • Much cleaner than electrical strain gauges • One cable can have over one hundred individual sensors of varying types4 • Can be attached to surface or embedded in laminates http://www.smartfibres.com/images/FBG_vs_Electrical_Strain.jpg http://www.smartfibres.com/images/Optical_Fiber_Sensor_in%20Comp.jpg

19. Advantages • Simple to install • One wire to run and track • History and research in wind energy6 • Stability and reliability4 • Only one wire that must be considered compared to 3 per gauge for foil gauges • No soldered joints to worry about • Fatigue tests comparable to 25 year service life on wind turbine • No signal degradation with time

20. Advantages • No need for recalibration after installation4 • Signal doesn’t degrade when transmitted over long distances • Passive sensors with no electrical power required • No concern of electrical interference from outside sources

21. Disadvantages • Sensors and interrogation units are expensive • Relatively new technology that doesn’t have the history of other systems • Limited number of manufacturers http://techfreep.com/images/fiberoptics3.jpg

22. Considerations • While initial cost is expensive costs are reduced by several measures • Lower cost of installation • Reliability reduces long term costs • One interrogator can handle hundreds of sensors • A great deal of research has been done in FBG and fiber optics in general are very common in broad range of applications • Manufacturers are working with and doing research in wind energy in Europe

23. Current Research • Some of the research projects currently ongoing with FBGs and FBG strain gauging • Wind turbine monitoring and control7 • Enercon GmbH-Emden, Germany • Increasing strain sensitivity without increasing temperature sensitivity8 • Hong Kong Polytechnic University • Simultaneous measurement of strain and temperature9 • INESC Porto, ISEP and University of Porto and the University of Madeira in Funchal

24. Manufacturers • While limited number of manufacturers compared to other systems, there are still many, including: • Smart Fibre Ltd. • FiberSensing • Luna Inovations Inc. • Micron Optics • Geokon Inc. • Fiso Technologies Inc. http://www.fibersensing.com/files/3/imagens/20060601142810508118.jpg

25. Equipment • Many types of gauges are available • Can be installed on surface of existing blades • Can be embedded in new laminates • Temperature • Pressure • Several types of interrogators for different applications and magnitudes of projects • Includes computer hardware • Can be used in conjunction with LabView10

26. Smart Fibre Ltd. • Bracknell, UK • Currently providing better information than other manufacturers • Wide selection of products • Have done work with wind energy http://www.sandia.gov/news/resources/releases/2006/images/turbine-land.jpg

27. Sensors • SmartPatch • Can be attached to surfaces temporarily or permanently • Available as a strain gauge rosette • SmartTemp • Allows for temperature compensation http://www.smartfibres.com http://www.smartfibres.com

28. Interrogator Units • Wx Interrogator • Small Unit • Capable of 16 sensors per channel • Excellent for research • W5 Interrogator • Capable of 125 sensors per channel • Up to 16 channels • Large field use system http://www.smartfibres.com http://www.smartfibres.com

29. Costs and Goals http://www.smartfibres.com http://www.smartfibres.com • For the purposes of laboratory research • Wx Interrogator Unit: $32,714 for 4-Channel • 0, 45, 90, 3-Axis Rosette: $900 each • Order and assemble system and hardware • Run tests on the effective area of different size gauges • Compare test results to ANSYS models • Build specimens and start running tests on the effects of embedding optical fibers in laminates

30. Why it’s Worth the Money • It’s going to take new technology to make an ancient concept revolutionary • A true opportunity for health monitoring • It is the best system for wind energy • Some scenario • Patches at critical areas • Proof testing and monitor strains • Limited complete installations to understand usage and loads for better reliability analysis http://msnbcmedia4.msn.com/j/msnbc/Components/Photos/060703/060703_shell_wind_vlrg_8a.widec.jpg

31. References • Communications Research Centre Canada, Licensing Fiber Bragg Gratings. Viewed 10/09/2007, http://www.crc.ca/en/html/crc/home/tech_transfer/bragg • Doyle, Dr. Crispin. Fibre Bragg Grating Sensors An Introduction to Bragg Gratings and Interrogation Techniques. Smart Fibres Ltd, 2003. • Sensors, Tunable Lasers for Multichannel Fiber-Optic Sensors. Viewed 10/18/2007 http://www.sensorsmag.com/sensors/Technology+Tutorials%2FSensors%2FAccele ration%2FVibration/Tunable-Lasers-for-Multichannel-Fiber-Optic- Sensor/ArticleStandard/Article/detail/321349?contextCategoryId=33139 • Smart Fibres, Fibre Bragg Gratings. Viewed 08/07/2007, http://www.smartfibres.com/Fiber_Bragg_Grating.htm • Sensors, Measuring with Light Part 2: Fiber-Optic Sensing—From Theory to Practice. Viewed 09/04/2007, http://www.sensorsmag.com/articles/0500/26/ • Smart Fibres, Wind Energy. Viewed 08/07/2007, http://www.smartfibres.com/Wind_Energy.htm • Smart Fibres, Wind Turbine Blade Monitoring and Control. Viewed 08/07/2007, http://www.smartfibres.com/Attachments/SFref261.pdf • Hitz, Breck. Wrinkles Improve Fiber Optic Strain Sensor. Technology World, January 2007 • Hitz, Breck. Birefringent Fiber Enhances Fiber Optic Strain Sensor. Technology World, January 2007 • Smart Fibres, SmartSoft. Viewed 10/17/2007, http://www.smartfibres.com/SmartSoft.htm