1. Graduate Student, 1ga, Anam-dong, Seongbuk-gu, Korea Univ., Seoul 136-701, Republic of Korea.
2. Professor, 1ga, Anam-dong, Seongbuk-gu, Korea Univ., Seoul 136-701, Republic of Korea (corresponding author).
3. Professor, Dept. of Mechatronics Engineering, Siheung-Si, Korea Polytechnic Univ., Gyunggi-Do 429-793, Republic of Korea.
4. Associate Professor, Mechanical and Aeronautical Engineering Dept., Clarkson Univ., 8 Clarkson Ave., Potsdam, NY 13699-5725.