1. Graduate School of Science and Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2. Department of Electronics and Mechanical Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan,
3. Yamaha Motor Co., Ltd, 2500 Shingai, Iwata, Shizuoka 438-8501, Japan