1. Graduate School of Mechanical EngineeringKyung Hee University, Yongin 449‐701, Korea
2. Industrial Liaison Research InstituteDepartment of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 449‐701, Korea
3. Gumi Electronics and Information Technology Research InstituteGumi 730‐853, Korea
4. Department of Mechanical Engineering and Applied MechanicsUniversity of Pennsylvania, Philadelphia, PA 19104, USA