1. School of Information Science and Technology, Qingdao University of Science and Technology, Qingdao 266061, P. R. China
2. Development Planning Office, Qingdao University of Science and Technology, Qingdao 266061, P. R. China
3. School of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao 266061, P. R. China