1. a Department of Mechanical Engineering and Intelligent Systems , The University of Electro-Communications , 1-5-1 Chofugaoka Chofu-shi, Tokyo , 182-8585 , Japan
2. b Honda Research Institute Japan Co., Ltd. , 8-1 Honcho, Wako-shi, Saitama , 351-0188 , Japan
3. c NICT Knowledge Creating Communication Research Center , 2-2-2 Hikaridai, Seika-cho Souraku-gun, Kyoto , 619-0288 , Japan