1. Department of Mechanical and Electrical Control Engineering, School of Mechanical Engineering, Yanshan University, Qinhuangdao, P. R. China;
2. Basic Education Department, Liren College, Yanshan University, Qinhuangdao, P. R. China;
3. Heibei Provincial Key Laboratory of Heavy Machinery Fluid Power Transmission and Control, Yanshan University, Qinhuangdao, P. R. China