1. a Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
2. b Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
3. c School of Mechatronics Engineering and Automation, Shanghai University, Shanghai, PRC
4. d Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576
5. e Department of Mechanical and Control Engineering, University of Electro-Communications, Tokyo, Japan