Static test measurements on a wind turbine blade using a photogrammetry technique

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Vicoter (www.vicoter.it) is a consulting company founded in 2009 which operates in the sector of static and dynamic measurements of mechanical structures. Its organizational flexibility, its instrumentation as well as its wide-ranging skills make Vicoter the most suitable partner for solving unusual problems and research projects for many larger companies.

On behalf of SEVA S.r.l. (www.sevasrl.it), a company operating in the renewable energy sector, Vicoter engineers carried out a test campaign on two wind turbines, more than 20 meters long, in order to obtain information for the development of reliable finite element models to be used in estimating the generators performance.

Wind turbine blade in static tests.

The purpose of the test campaign was twofold: to determine the stiffness of the structures and to determine their aerodynamic profiles.

In agreement with SEVA S.r.l., it was decided to use the photogrammetric technique to satisfy both requests. Photogrammetry is a test methodology more and more adopted in recent years in the field of displacement measurements thanks to its relative simplicity and low cost. The structure is photographed from multiple angles and the images are processed using special software capable of determining the coordinates of selected points. Using ad-hoc printed markers placed on the structure, the number of identified points can be increased as well as the accuracy of their position.

Tests were carried out at the SEVA S.r.l. site, with the blades fixed to the ground by means of a support structure. The blades were scanned before and after the application of a static load. Load was applied in four stations along the blades, for a total of more than one ton. Thanks to this technique and to the comparison between the measurements obtained on the undeformed and on the deformed structure, it was possible to measure both the displacement and the rotation of the six investigated sections. It should be noted that, by appropriately positioning the reference markers, it is possible to purify the measurements by the intrusiveness of the support structure, i.e., to obtain the structural deformations free from the constraint ones.

Static test deformation.

Both flap and lag stiffness were identified by repeating the acquisitions after rotating each blade 90 ┬░.
Moreover, the measurements carried out on the unloaded blade also allowed the measurement of the profile of each section.
An example of the obtained results is shown in the following figure.

Identification by photogrammetry. Unloaded blade with red points and deflected blade with blue points.