Short answer
Wind turbine blade loads are measured with Fiber Bragg Grating (FBG) sensors embedded in or bonded to the blade root and shell, which convert strain into a shift in reflected wavelength. With no metal or electrical power in the blade, the measurement is immune to lightning and supports root bending moment, individual pitch control and damage detection. DuyuSense develops FBG and DAS solutions for wind energy.
How is blade root bending moment measured with FBG?
Root bending moment is calculated from FBG strain sensors placed around the blade root, typically at 90 degree intervals. Opposing sensor pairs separate flapwise and edgewise moments, while an additional reference grating compensates for temperature.
Near 1550 nm an FBG typically shifts by about 1.2 pm per microstrain and about 10 pm per °C. An interrogator in the hub reads every sensor on all three blades over optical fiber at a high sampling rate.
- Flapwise and edgewise moment at the blade root
- Multi-point strain profile along the blade span
- Dozens of FBG points in series on a single fiber
- Embedded in the composite laminate during manufacture or surface mounted later
Why use FBG instead of electrical strain gauges in wind turbine blades?
The main reason is lightning: the blade is the part of the turbine most often struck, and an FBG sensor with its fiber is fully dielectric. With no copper leads, power supply or electronics in the blade, there is no new path for lightning current and the measurement chain is immune to electromagnetic interference.
The second reason is fatigue life. A blade sees hundreds of millions of load cycles over a service life of more than twenty years; electrical gauges often suffer from drift and broken leads, while FBG is compatible with glass and carbon fiber composites and remains stable over the long term.
How does blade strain data feed individual pitch control?
Individual pitch control (IPC) adjusts each blade's pitch angle separately during rotation to reduce asymmetric loads, and it needs the instantaneous root moment of every blade. FBG sensors deliver that measurement directly to the turbine controller with low latency.
Periodic loads from wind shear, turbulence and wake effects are damped as a result. The outcome is lower fatigue loading on the hub, main shaft and tower, and longer component life.
Can fiber optic sensors detect blade icing and damage?
Yes. Ice on a blade changes its mass and natural frequencies, and FBG strain and vibration data reveal this as imbalance between blades and a frequency shift. The result can trigger a de-icing system or a safety stop.
The same data carries early signs of structural damage. Bond line separation, delamination within the laminate or leading edge erosion gradually change how the blade responds under load and alter its modal properties.
- Mass imbalance and frequency drop caused by ice
- Early indications of bond line failure and delamination
- Changes in structural response after a lightning strike
- Anomaly detection by comparing blades against each other
How are towers, monopiles and bolted flanges monitored with fiber optics?
Towers and foundations are monitored continuously with FBG strain, tilt and temperature sensors as part of Structural Health Monitoring (SHM). On offshore monopiles, sensors measure wave and wind induced fatigue loads, strain near the mudline and foundation tilt; the measured load history feeds remaining fatigue life calculations.
Loss of preload in the bolted flanges joining tower sections is a critical risk. FBG sensors integrated into bolts or flanges track loosening and load changes, so maintenance can be scheduled on data rather than fixed torque check intervals.
- Bending strain and natural frequency tracking at the tower base
- Fatigue load measurement on monopile and transition piece
- Preload loss monitoring on bolted flanges
- Foundation tilt and settlement changes
How are offshore wind export and array cables monitored with DAS and DTS?
The optical fiber usually already present in offshore wind export and array cables is turned into a sensor with Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS). DTS tracks conductor temperature and loss of burial, while DAS locates anchor drag, trawling and cable movement.
This follows the same principle as onshore and subsea power cable monitoring. A single DAS channel typically covers tens of kilometres with a spatial resolution of a few metres, so fault locations can be narrowed down quickly.
Can gearbox and bearing temperatures be monitored with FBG?
Yes. FBG temperature sensors in series on a single fiber monitor many points, such as the main bearing, gearbox bearings and generator windings, through one interrogator channel. In the electromagnetically noisy nacelle the measurement is unaffected by electrical interference.
Temperature trends, read together with vibration data, give early warning of lubrication problems and bearing wear. In markets like Turkey, with a large onshore wind fleet and planned offshore capacity, this kind of condition monitoring is a practical way to reduce unplanned downtime.
| Feature | FBG sensor | Electrical strain gauge | Accelerometer |
|---|---|---|---|
| Measurand | Strain, temperature, vibration | Strain | Acceleration and vibration |
| Lightning and EMI | Fully dielectric, immune | Copper leads, needs protection | Electronics and cabling, needs protection |
| Metal and power in blade | None | Yes | Yes |
| Fatigue durability | High, stable long term | Risk of drift and broken leads | Depends on mounting and cabling |
| Multiplexing | Dozens of points on one fiber | Separate leads per point | Separate channel per sensor |
| Typical use | Blade loads, IPC, tower, flanges | Testing and short-term campaigns | Nacelle and tower vibration |
Frequently asked questions
Are FBG sensors in wind turbine blades affected by lightning?
No. FBG sensors and their fiber are glass based, contain no metal and carry no electrical current. They create no new conductive path for lightning in the blade, and the measurement is immune to electromagnetic interference. The interrogator sits in a protected zone in the hub or nacelle, and only fiber enters the blade.
Why measure blade root bending moment?
Root bending moment is the most direct indicator of the aerodynamic load a blade carries. It feeds individual pitch control, records the fatigue load history and allows comparison with design loads. The data supports both load reduction in operation and remaining life assessment.
Can FBG sensors be retrofitted to an existing turbine?
Yes. FBG sensors can be embedded in the laminate during blade manufacture or bonded to the inner surface of blades already in service. Tower, flange and foundation sensors are generally well suited to retrofit. The installation method is engineered around the blade structure and access.
Can fiber optics monitor monopile fatigue?
Yes. FBG strain sensors installed on the monopile and transition piece continuously measure cyclic loads from waves and wind. The measured load history is used to calculate accumulated fatigue damage and, compared with design assumptions, supports decisions on remaining life and inspection intervals.
How are offshore wind export cables monitored?
The optical fiber inside the export cable is used as a sensor: DTS for temperature and changes in burial depth, DAS for anchor drag, trawling and cable movement. An onshore interrogator reports events with their position along the cable, speeding up damage prevention and fault location.
Can DAS detect cable faults in offshore wind farms?
DAS locates mechanical events acting on the cable, such as anchor drag, trawl impact or vibration of free spans. For the electrical fault itself it is used together with DTS temperature data and protection systems. Combined, they give fast information on pre-failure risks and fault location.
What does DuyuSense offer for wind energy?
DuyuSense offers custom sensor design, prototyping and system integration for blade, tower and flange monitoring, based on its proprietary FBG sensors and patented FBG interrogation system. For cable monitoring it applies its patented DAS signal-processing approach. Pilots and collaborations can be discussed at contact@duyusense.com.