1. a Department of Mechatronics , Kwangju Institute of Science and Technology (K-J1ST) , 572 Sangam-dong, Kwangsan-ku, Kwangju, 506-303, South Korea.
2. b Department of Mechanical Engineering , KAIST , 373-1 Kusong-dong, Yusong-gu, Taejon, 305-701, South Korea.