1. a U-Robot Research Division, ETRI, 138 Gajeongno, Yuseong-gu, Daejeon 305-700, South Korea
2. b Major of Mechatronics Engineering, Cheju National University, 1 Ara-dong, Jeju 690-756, South Korea
3. c Department of Mechanical Engineering, POSTECH, San 31 Hyoja-dong, Pohang 790-784, South Korea
4. d Department of Mechanical Engineering, POSTECH, San 31 Hyoja-dong, Pohang 790-784, South Korea