1. Robust Control and Nonlinear Dynamics Laboratory, Department of Electrical Engineering, Yeungnam University, 214-1 Dae-Dong, Kyongsan 712-749, Republic of Korea
2. School of Electrical & Computer Engineering, 410 SungBong-Ro, Heungduk-Gu, Chungbuk National University, Cheongju 361-763, Republic of Korea