1. School of Aerospace Engineering and Applied Mechanics; Tongji University; Shanghai 200092 P R China
2. State Key Laboratory of Structural Analysis for Industrial Equipment; Department of Engineering Mechanics; Dalian University of Technology; Dalian 116024 P R China
3. School of Mechanical Engineering; Yonsei University; Seoul 120-749 Republic of Korea
4. Beijing Key Laboratory of Rehabilitation Technical Aids for Old-Age and Disability and Key Laboratory of Rehabilitation Aids Technology and System of the Ministry of Civil Affairs; National Research Center for Rehabilitation Technical Aids; Beijing 100176 P R China