1. Key Lab for Mechanism Theory and Equipment Design, International Centre for Advanced Mechanisms and Robotics, Tianjin University, Centre for Robotics Research, King's College London, London WC2R 2 LS, UK e-mail:
2. Centre for Robotics Research, King's College London, London WC2R 2 LS, UK e-mail:
3. Chair of Mechanisms and Robotics International Centre for Advanced Mechanisms and Robotics, Tianjin University, Centre for Robotics Research, King's College London, London WC2R 2 LS, UK e-mail: