1. School of Mechanical System Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul, 136-702, Republic of Korea
2. HUBO Laboratory, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 335 Gwahangno Yuseong-gu, Daejeon, 305-701, Republic of Korea