1. College of Electrical Engineering, Yanshan University, Qinhuangdao, Hebei 066004, P. R. China
2. College of Electrical Engineering, Engineering Research, Center of Intelligent Control System and Intelligent Equipment Ministry of Education, Key Laboratory of Intelligent Rehabilitation and Neromodulation of Hebei Province, Yanshan University, Qinhuangdao, Hebei 066004, P. R. China