1. NGV R&D Center, Gwangju Technopark, Chum-Dan Gwagiro 333, Buk-gu, Gwangju 500-706, South Korea
2. Department of Advanced Parts and Materials Engineering, Chosun University Graduate School, 375 Seosuk-dong, Dong-gu, Gwangju 501-750, South Korea
3. Department of Mechatronics Engineering, Chosun University, 375 Seosuk-dong, Dong-gu, Gwangju 501-750, South Korea