1. State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400030, China e-mail:
2. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208
3. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208; State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400030, China