1. Foster Electric Company, Limited, 1-1-109 Tsutsujigaoka, Akishima, Tokyo 196-8550, Japan
2. College of Mechanical Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japan
3. Department of Mechanical Science and Technology, Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan