1. Application of Fiber Optic Strain Gauging in Wind Turbine Blades
Prof. D.S. Cairns
J.C. Blockey
Picture:

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)

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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 Each grating reflects only a small wavelength3
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
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
Link to this page:
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

19. Advantages Simple to install History and research in wind energy6
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

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.

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

27. Sensors SmartPatch SmartTemp
Can be attached to surfaces temporarily or permanently
Available as a strain gauge rosette
SmartTemp
Allows for temperature compensation

28. Interrogator Units Wx Interrogator W5 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

29. Costs and Goals For the purposes of laboratory research
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

31. References
Communications Research Centre Canada, Licensing Fiber Bragg Gratings. Viewed 10/09/2007,
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/ ration%2FVibration/Tunable-Lasers-for-Multichannel-Fiber-Optic- Sensor/ArticleStandard/Article/detail/321349?contextCategoryId=33139
Smart Fibres, Fibre Bragg Gratings. Viewed 08/07/2007,
Sensors, Measuring with Light Part 2: Fiber-Optic Sensing—From Theory to Practice. Viewed 09/04/2007,
Smart Fibres, Wind Energy. Viewed 08/07/2007,
Smart Fibres, Wind Turbine Blade Monitoring and Control. Viewed 08/07/2007,
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,