1. Department of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China; Department of Electrical and Biomedical Engineering, University of Nevada, Reno, Reno, NV 89557 e-mail:
2. Robotics and Biomimetics Laboratory, Department of Electrical and Biomedical Engineering, University of Nevada, Reno, Reno, NV 89557 e-mail:
3. Department of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China e-mail: