1. Department of Mechanical and Manufacturing Engineering, Aalborg University, Fibigerstraede 16, DK-9220 Aalborg East, Denmark;
2. Siemens Wind Power, Assensvej 11, Aalborg East DK-9220, Denmark e-mail:
3. AMADE, Polytechnic School, University of Girona, Campus Montilivi s/n, Girona 17071, Spain e-mail: