Vicoter (www.vicoter.it) is a consulting company that has been operating in the field of mechanical structures and measurements for more than ten years, mainly in the dynamics sector. Its main activities concern the experimental survey of vibrations and noise, the identification of modes, the study of isolation equipment and the correlation of finite element models. Thanks to its transversal skills and to the available instrumentation, Vicoter is able to work in several areas, such as aeronautics, automotive, electronics or operating machines.
In a consolidated collaboration with SEVA S.r.l, (http://www.sevasrl.it/index.php/it/) a company that operates in the renewable energy sector, Vicoter has just concluded a test campaign for the characterization of two types of blades for wind turbines. On both the blades, which are about 20 meters long, modal analysis tests were conducted to investigate their dynamic behaviour. The study was carried out at the client’s where the blade root was fixed to the ground by means of a support structure.
Using a sensor configuration already verified in previous similar activity, it was decided to measure 16 locations on the structure, divided into 8 sections, and 6 locations on the constraint. At each point, two uniaxial accelerometers were placed, oriented respectively in the normal and in the aligned-to-the-chord directions, in order to identify the flap, the torsional and delay modes. A total of 44 accelerometers plus a load cell were synchronously acquired in each test. The blades were forced with a long stroke electro-dynamic shaker, which is necessary in tests involving the acquisition of low frequency modes of very flexible structures. A stepped-sine excitation technique was adopted to obtain clean FRFs (Frequency Response Functions), even in the low frequency band.
This installation made it possible to identify the first eleven modes of both structures, in terms of frequency, modal shape, damping and generalized mass with a high degree of reliability of the results obtained. The entire band of interest, about 25 Hz, was thus covered. The data obtained will be used by SEVA S.r.l. to develop a dynamic analytical model of the blade.