To meet the growing demand for sustainable energy, existing wind turbines have to be made more powerful and efficient. One of the research topics to achieve this, is the aero-elastic stability limit of the blades. This can be analyzed by the vibrations that occur on the blades during normal operational conditions. In this study high speed photogrammetry is used to measure blade vibrations of a model wind turbine.
By placing targets on the blades of the wind turbine and taking images with two synchronized high speed cameras, the motion of the blades is captured. High speed cameras produce a large number of images, making manual target measurements impractical. In this study, it is investigated how target
measurements in high speed images can be automated and what accuracy can be obtained for the reconstructed object coordinates.
To automate the target measurements, a method is developed based on target detection and tracking. The targets in the images are detected using a threshold histogram segmentation. Since the targets describe a circular motion a circle is used as model to track its position in the sequence of frames. To measure the targets with subpixel precision, the edges of the targets are detected and a circle is fitted to the edges.
To reconstruct the object coordinates a Direct Linear Transformation (DLT) is used. The DLT equations are linear and can easily be solved with standard Least Squares Estimation.
The developed methods were used for an experiment in a wind tunnel, whereby images of a model wind turbine were made with an acquisition frequency of 500 Hz and a rotational speed of 260 rpm. Retro-reflective targets were put on the blades to identify the measuring points.
Using the developed methods, the targets were successfully measured and tracked in the images of the experiment. Using the targets measured in the images, 3D object coordinates were calculated with an accuracy of 1.32 millimeter.
The vibrations of a target were obtained by means of Principal Component Analyses (PCA). Using PCA, the track of coordinates of a target was transformed to a new coordinate system. The xy plane of this new coordinate system coincided with the rotation plane of the target and the z axis is perpendicular to it, containing the vibrations. The measured vibration had a maximum amplitude of 6 mm, with an accuracy of 0.56 mm in the vibration component.
From the performed experiment with the model wind turbine, it can be concluded that the blade vibrations can be measured using high speed photogrammetry.