1. a Department of Systems Engineering, Faculty of Engineering, Ibaraki University, 4-12-1 Naka-Narusawa-Cho, Hitachi-Shi 316, Japan
2. b Department of Mechano-Aerospace Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan
3. c Research Division, Komatsu Ltd, Industrial Machinery R&D Center, 1200 Manda, Hiratsuka-Shi, Kanagawa-Ken 254, Japan