1. Dept. of Mechanics, Tianjin Polytechnic Univ., Tianjin 300160, P. R. China;
2. Information Technology Science Coll., Nankai Univ., Tianjin 300071, P. R. China;
3. Information & Communitiaon of Engineering School, Tianjin Polytechnic Univ., Tianjin 300160, P. R. China