1. Department of Mechanical Engineering, Keio University, Kanagawa, Yokohama 223-8522, Japan
2. Department of Electronics and Electrical Engineering, Keio University, Kanagawa, Yokohama 223-8522, Japan
3. JST PRESTO, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
4. Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan