1. College of Mechanical Engineering and Automation, Northeastern University, Shenyang 110004, China
2. State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China e-mail:
3. Center for Advanced Mechanisms and Robotics, MOE Key Laboratory for Mechanism Theory and Equipment Design, Tianjin University, Tianjin 300072, China
4. Center for Robotics Research, King's College, University of London, Strand, London WC2R 2LS, UK e-mail: