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Optical Blade Position 

Tracking System Test 

L.J. Fingersh 

Technical Report 

NREL/TP-500-39253 

January 2006 
Optical Blade Position 

Tracking System Test 

L.J. Fingersh 

Prepared under Task No(s). CP05.3001 

Technical Report 

NREL/TP-500-39253 

January 2006 

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Contract No. DE-AC36-99-GO10337
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Introduction 

As wind turbines have grown larger, their structures have grown softer and more highly 

optimized. As a result, wind turbine controls are now seen as a viable way of reducing 

structural loads on wind turbines and therefore reducing the amount of material needed to 

withstand the resulting stresses. One problem with using such advanced controls is that 

they need inputs. The inputs that work best are those closest to the forces applying the 

loads to the structure. On a wind turbine, those are aerodynamic forces applied to the 

blades. Unfortunately, systems designed to measure those forces directly are expensive 

and unreliable and are therefore very unlikely to be deployed onto commercial machines. 

The alternative described in this paper is to measure the blade deflection along the span 

of the blade. It has long been known that this is a very good input source for advanced 

controls. The problem is in obtaining a blade deflection signal in a cost-effective and 

reliable manner. This paper proposes the use of simple off-the-shelf infrared security 

cameras along with blade-mounted retro-reflective tape and video image processing 

hardware and software to obtain these measurements. 

System 

The system proposed is extremely simple and easy to implement. All one must do is to 

mount small tabs along the blade, preferably 

along the trailing edge, each with a small 

amount of highly-selective retro-reflective 

tape facing the blade root. A security 

camera containing infrared emitters (Figure 

1) is then mounted at a position near the 

root of the blade such that it can see the 

tabs. The infrared light from the emitters is 

bounced straight back to the camera via the 

tape. The camera can observe the location 

of the tape and the image can be captured by 

video capture hardware. Through 

calibration, the location of the tape, and 

hence the blade, can be determined. 

The camera used has been modified with the 

addition of an infrared filter over the lens. 

This filter has its center frequency at the 

frequency of the IR LEDs. This filter is 

designed to filter out all other wavelength 

except those that come from the infrared LED light reflected back by the tape. 

Unfortunately, clouds also emit this frequency when illuminated by sunlight. It is hoped 

that the IR tape is brighter than the clouds and the clouds can be filtered out by luminance 

Figure 1. Security camera with infrared 

emitter and IR filter 

1 
value at the processing stage. Initial testing indicated this might be the case, but this 

result has yet to be validated. 

Testing 

In order to determine the feasibility of this system, a simple test was performed. A static 

blade test was performed (Figure 2) and the testing involved string-potentiometers 

designed to measure deflection at three locations along the blade. This represented an 

opportunity to check the performance of the optical system against a known reference. 

Figure 2. Blade static test 

2 
The camera was set up and tape was applied to the three load saddles (Figure 3). The 

bolt will be addressed later. One complication of the test was that the stringpotentiometers were not located at the saddle locations but were instead located nearby. 

This resulted in the need to adjust one data set to the other in order to determine if the 

data were consistent. 

Figure 3. Camera, tape, and bolt locations 

3 
Calibrations were performed very simply by holding a large ruler next to the 

measurement locations. Video was captured during this process and frames were 

analyzed later. One such frame is in Figure 4. The frame was analyzed to determine the 

length of the ruler in pixels. Combining this information with the known length of the 

ruler (18 inches in this case) led to a calibration of pixels/inch at each station. The bright 

squares at the top are lens flare and the speckles around the frame are hot pixels. Both 

can be eliminated quite easily in several ways. We intend to remove most of the flare by 

reducing the image exposure and the hot pixels by a method called “dark-frame 

subtraction” in which a frame is captured in the dark and that frame is subtracted from all 

future frames. 

Figure 4. Example calibration frame 

4 
Once calibrations were complete, the test was begun. At the completion of the test, 

frames were captured from the video at each load point and analyzed. Once such frame 

is in Figure 5. 

Figure 5. Example load frame 

The overexposure of the first tape location can be avoided using one of several 

approaches (reducing gain, masking, etc.) but, for this test, the bolt to the left of the tape 

was used instead. For the other two locations, the center of the tape was used as the 

measurement location. All of the data were analyzed and compared to the output of the 

string-potentiometers. 

Results 

The test results are shown graphically in Figure 6. The results of the test indicated the 

optical system had a mean error of 1.1%, and a median error of 1.6%. The standard 

deviation of the errors was 3%, and the errors of the extremes were –6.2% and +4.1%. 

Some of these errors were caused because the string potentiometers were not measuring 

the same thing as the optical system. The string system measured deflection in two 

directions while the optical system only measured in one. Also, the measurements were 

not in the same radial or chord-wise location but were only adjusted for radial location. 

Overall the optical system performed very well. 

5 
Load

Displacement 

Station 1 - String pot

Station 2 - String pot

Station 3 - String pot

Station 1 - Optical

Station 2 - Optical

Station 3 - Optical

Figure 6. Test results 

One big benefit to this system over other systems for measuring blade deflection is that it 

is inherently continuous and multi-point. In fact, with an interleaved camera, it is 

possible to extract 60 new frames per second for analysis at three or more points along 

the blade span. The system is also very inexpensive, highly reliable, and readily 

available. Possible drawbacks include the need for video capture hardware and 

processing software, though, in many cases these can also be obtained quite 

inexpensively. Another possible drawback is interference from the sun or from clouds. 

Finally, it is not known how well this system will perform in snow, fog, or icing 

conditions. 

In the event there are dropouts from the system, it may be possible to control the turbine 

loads anyway. With estimators, there is likely to be a loss of performance but it is 

equally likely that turbine shutdown will not be required. There are many approaches to 

increasing robustness in control design and many of them are applicable to this system. 

Conclusion and Future Plans 

The optical system was set up very easily and quickly and yet performed very well. We 

now intend to put the system on the CART 3 turbine that is currently being installed at 

the National Wind Technology Center for controls research. The cameras will be 

interfaced with real-time video processing hardware and software and integrated into the 

control system. 

6 
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January 2006 

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technical report 

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5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

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4. TITLE AND SUBTITLE

Optical Blade Position Tracking System Test 

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NREL/TP-500-39253 

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WER6.2106 

6. AUTHOR(S)

L.J. Fingersh 

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National Renewable Energy Laboratory 

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Golden, CO 80401-3393 

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14. ABSTRACT (Maximum 200 Words)

Wind turbine controls are a viable way of reducing structural loads on large wind turbines, therefore reducing the 

amount of material needed to withstand the resulting stresses. Advanced controls need inputs on the forces applying 

the loads to the structure. On a wind turbine, those are aerodynamic forces applied to the blades. Systems 

designed to measure those forces directly are expensive and unreliable. This paper proposes the use of simple offthe-shelf infrared security cameras along with blade-mounted retro-reflective tape and video image processing 

hardware and software to obtain wind turbine blade deflection measurements along the span of the blade. 

15. SUBJECT TERMS

wind turbine control systems; load measurement; blade deflection measurements; infrared camera 

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