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Flg.4 Flow of boom cylinder under electro-hydeaulic proportional value controlled by step current
5 Conclusions
（1）The mathematic model of electro—hydraulic system is developed according to the characteristics of excavator．It is assumed that the flow across the valve is directly proportional to the size of valve orifice，and the influence of intemal and extemal leakage of hydraulic system is ignored．The simplified model can be obtained：
Y(s)?[b1Xv(s)?b1sFc(s)]/[s(a0s2?a1s?a2)]
where represent the displacement of piston and the displacement of spool．
（2）From the model of electro—hydraulic system，we can obtain the equivalent mass
m?J/l2O1B，bearing forceFl?(m1glOG??m2glO1G??m3lOG?) , flow gain coefficient of
11213value KqKl=2.825×10-4m3/(s·A) ，where KI is the current gain of electro—hydraulic proportional valves．
From: Journal of Central South University (English) 2008 Vol 15 No. 3 pages 382-386