1. State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, P. R. China
2. Department of Economic and Trade, School of Business Administration, Zhongnan University of Economics and Law, Wuhan 430000, P. R. China
3. Department of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China
4. Robotics and Rehabilitation (ROAR) Laboratory, Department of Mechanical Engineering, Columbia University, New York 10027, USA