1. Division of Mechanical and Aerospace Engineering, Hokkaido University, North 13, West 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
2. Department of Transdisciplinary Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
3. LINTEC Corporation, 5-14-42 Nishiki-cho, Warabi-shi, Saitama 335-0005, Japan