1. College of Mechatronics Engineering, North University of China, Shanxi Province, Taiyuan 030051, P. R. China
2. No. 3 Xueyuan Road, Jiancaoping District, North University of China, Taiyuan 030051, P. R. China
3. School of Precision Instruments and Optoelectronic Engineering, Tianjin University, Tianjin 300072, P. R. China