1. Hitachi, Ltd, R&D Group, 7-1-1 Oomika-cho, Hitachi-shi, Ibaraki-ken, Japan
2. Department of Electrical and Electronic Engineering, Daido University, Nagoya-shi, Aichi, Japan
3. Hitachi Industrial Equipment Systems Company Ltd., Hitachinaka-shi, Ibaraki-ken, Japan
4. Department of Electrical Engineering, Nagoya University, Nagoya-shi, Aichi-ken, Japan