1. School of Software and Microelectronics, Northwestern Polytechnical University, 127 West Youyi Road, Xi’an, Shaanxi 710072, P. R. China
2. State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, 99 Yan Xiang Road, Xi’an, Shaanxi 710054, P. R. China
3. School of Education, Shaanxi Normal University, 199 South Chang’an Road, Xi’an, Shaanxi 710062, P. R. China