To whom correspondence should be addressed. E-mail: lcq08@ 163. com Liu Chengqiang, born in 1982. He has been working for his Ph. D degree since 2008 in Department of Mechanical and Electronical Engineering, Harbin Institute of Technology. He received his B. S. and M.S. degrees from Shangdong University of Science and Technology and Liaoning University of Science and Technology in 2005 and 2008 respectively. His research interests include the fluid power transmission and control, optimization of hydraulic components.