1. Toshiba Corporate Research & Development Center, 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan
2. Toshiba Corporation, Power Systems Company, 1-1, Shibaura 1-Chome, Minato-ku, Tokyo 105-8001, Japan
3. Department of Engineering Science and Mechanics, Shibaura Institute of Technology, 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548, Japan
4. Graduate School of Informatics, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan