1. FUKOKU Co., Ltd., 6 Showa Chiyoda-machi, Oura-gun, Gunma 370-0723, Japan
2. Department of Engineering, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku, Niigata 950-2181, Japan
3. Graduate School of Science and Technology, Niigata University, Ikarashi 2-nocho 8050, Nishi-ku, Niigata 950-2181, Japan