1. a Mechanism Division, Robotics Department, Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan
2. b Mechanism Division, Robotics Department, Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan
3. c Mechanism Division, Robotics Department, Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba, Ibaraki 305-8564, Japan
4. d Department of' Information Science, Gifu University 1-1 Yanagido, Gifu 501-1193, Japan