1. Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
2. Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
3. LIMMS/CNRS-IIS, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan