1. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, China
2. School of Mathematics, Anhui University, Hefei 230039, China
3. Department of Electrical Engineering, Institute of Engineering, Polytechnic of Porto, R. Dr. António Bernardino de Almeida, 431, 4249-015 Porto, Portugal
4. UISPA–LAETA/INEGI, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal