1. Department of Product Design, Tianjin University of Science and Technology, Tianjin 300457, P. R. China
2. School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, P. R. China
3. School of Electronic Information and Automation, Tianjin University of Science and Technology, Tianjin 300222, P. R. China
4. Department of Electrical and Mining Engineering, University of South Africa, Florida 1710, South Africa