In the last decade, wind turbines have developed
into huge pieces of machinery with ever-increasing
efficiency and power output. To enhance uptime,
different types of monitoring systems have been
proposed to reduce the use of construction material,
and make the maintenance process more efficient. The
need for load and structural monitoring of wind turbine
blades is related to the turbine size and the subsequent
investment. Monitoring wind turbine blades reduces
material usage by actively checking the load, deducing
the structural integrity (blade damage), and detecting
ice buildup so that correct countermeasures can be
Because wind turbine blades are subjected to an increased risk of lightning strikes, equipment used to monitor turbines is required to be immune to EMC and EMI. For this reason, fiber-optic sensors are an optimal choice, because the sensors to be interrogated can be placed far away from the actual sensor point. This article describes examples of using the OPI (optical phase interrogation) technology in a monitoring system in wind turbine applications.
SENSOR DESIGN AND INSTALLATION
For the purpose of interrogation, the sensor system was
embedded in a two-bladed turbine WES 30/250. The
two blades, 14m long, were manufactured specifically
for testing the integration of the OPI technology. The
sensor technology was also used in a static test of a
52m wind turbine blade, designed and manufactured
by Skywind. For the sensor transducer, POF (polymer
optical fiber) was installed according to two different
designs; see Figures 1 through 3.
• The first design was a single POF loop running along the complete length of the blade and was integrated into the composite material of the blade during production. The complete length of the integrated fiber loops was 29.6m.
• The second design was a confined sensor patch, which was installed at post-blade production. The sensor patch used 24m of POF that was integrated in the same composite material as the blade. The confined format was achieved by meandering the fiber back and forth (analogous to copper strain gauges).
The POF has a diameter of 2.2mm, including jacket material (PE) of approximately 0.6mm thickness. The core of the POF is made of PMMA and has a core diameter of 1mm. Using POF in this experimental setup is related to the extraordinary elastic characteristic (greater than 5% strain) of the POF material. Due to this characteristic, the integration of the long fiber into the composite material was possible without breaking the fiber during the infusion process and the post-curing compression forces .
For interrogation purposes, we used the OPI Interrogator, which generated a sinusoidal signal modulated at 240MHz, with a robust LED light source (650nm), see Figure 4. The POF fibers made the sensor implementation straightforward and insensitive to harsh production processes [1,2].
To achieve a phase shift between the reference fiber and the sensor fiber, the sensor fiber was induced in the direction of the strain that we wanted to measure. The reference fiber can be included in the sensor setup, floating (unstrained), or perpendicular to the strain direction and, thereby, achieve an intrinsic temperature compensation of the resulting strain sensor. This action was done in the case of the sensor patches by using the same composite material for the sensor patches as used for the blade itself.
In the case of the integrated POF loop, the reference loop was also integrated in the blade on the opposite side (edge-wise and flap-wise) of the blade. This configuration did not achieve a temperature-compensated signal (the sunny side will expand more), and the signal output was doubled by exerting compression forces on one side and proportional tensile forces on the other, which doubled the measured phase shift (see Figure 5). Along with the POF-based sensors, we integrated copper strain gauges for reference. The static test of the POF-based sensor was made in two different experiments.
STATIC TEST 1 – 14M BLADE
The first static test benchmarked and compared the optical strain sensor behavior to a copper strain gauge as shown in Figure 5. The signal output correlated between the OPI sensor made out of POF and the copper strain gauge.
The loop sensor compared to the more compact sensor patch showed similar results. The lead foils along the leading edge of the blade were then tested to simulate ice buildup. Each lead foil weighed 10kg and, in subsequent steps, additional weight was attached to the blade, and the blade was measured, until 30kg of total weight was achieved (see Figure 6). This weight represented 1cm of ice along the leading edge. As shown in Figure 7, even a weight change as low as 1kg was detectable on the shift of the blade Eigen frequency.
STATIC TEST 2 - 52M BLADE
Static test 2 evaluated the ability of the OPI strain sensor to detect flap-wise strain at the blade root during full-scale destructive testing of the blade. During these tests, the blade was subjected to a series of increasing loads in which the entire blade was monitored with many strain gauges. The blade was evaluated for compliance to maximum design load conditions. This situation presented an excellent opportunity to evaluate the use of POF and the OPI technology for measuring extreme load conditions. The POF-based strain sensor, in the pre-manufactured format of a sensing patch, was installed at the root of the blade and measured throughout the execution of the complete static certification test .
The results indicated that the POF-based sensor could easily measure the different load cycles of the blade up to the maximum design load conditions (see Figure 11). An additional benefit of the sensor patch, compared to the classical strain gauge, is that the strain measurement is integrated over a larger measurement area, which eliminates the effects of localized material non-uniformities.
The turbine integration test integrated the 14m blades (used in static test 1) for operational testing on an actual turbine. The blades were installed in a wind turbine to evaluate the sensor system in the field. The sensor system was exposed to turbine operational conditions during the scope of 3 consecutive months. Data was collected on a remote hard drive over a wireless connection. During the field test, the following data was recorded: wind speed, temperature, rotational speed, strain values from copper strain gauges, and POF-based OPI sensors. All data was properly synchronized and time stamped .
In the operational test, we found a good correlation between copper strain gauges and the POF-based OPI sensor. Various events were recorded, such as start conditions, stop conditions, and wind speed change conditions. The sensor system easily recognized these different events. As the blade rotated, a characteristic vibration pattern was induced in the blade. The strain experienced due to applied load changes was different for the various rotational positions of the blade. The strain loading pattern of the blade was easily measured by performing a Fourier transform of this time signal. During normal blade rotation, a specific frequency fingerprint was identified for the blade. This spectral pattern shifted in relation to the wind speed. A higher wind speed related to a higher rotor speed and higher measured frequency. These operational tests in the turbine integration show the correlation to copper strain gauges.
Through a number of tests, static and dynamic, the OPI technology was shown to work reliably in the wind turbine blade. The integration of the rugged POF fiber directly into the blade, or post-production in the format of a sensor patch, is possible. The ruggedness of fiber material and the optoelectronic interrogation approach allow a qualitative and cost-efficient technology tool with which system and sensor integrators can work.