1. School of Mechano-Electronic Engineering, Gansu Agricultural University, Lanzhou, Gansu 730070, P. R. China
2. School of Mechanical and Electronic Engineering, Lanzhou University of Technology, Lanzhou, Gansu 730050, P. R. China
3. School of Mechano-Electronic Engineering, Lanzhou City College, Lanzhou, Gansu 730070, P. R. China