1. School of Mechanical Engineering, Southwest Jiaotong University, Chengdu, Sichuan, PR China
2. School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan, PR China
3. State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu, Sichuan, PR China
4. School of Mathematics, Southwest Jiaotong University, Chengdu, Sichuan, PR China