1. Department of Mechanical and Power Engineering, Northwestern Polytechnical University, P.O. Box 24, Xi'an 710072, Shaanxi, China e-mail:
2. School of Mechanical Engineering, Northwestern Polytechnical University, P.O. Box 552, Xi'an 710072, Shaanxi, China e-mail:
3. Dipartimento di Ingegneria Industriale e dell'Informazione, Seconda Universita' degli Studi di Napoli, Real Casa dell'Annunziata, Via Roma 29, Aversa (CE) 81031, Italy e-mail: