1. Mechanical Engineering Laboratory, Namiki 1-2, Tsukuba Science City, Ibaraki 305, Japan.
2. RCAST, The University of Tokyo, Komaba 4-6-l, Meguro, Tokyo 153, Japan.
3. Tsukuba Research Center, Yasukawa Electric Manufacturing Corp., Tohkohdai 5-9-10, Tsukuba Science City, Ibaraki 305, Japan.