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【正文】 ameters in the simplified model are related to the structure。the motional situation and the posture of excavator’s arm．Moreover，these parameters are time variable. So it is quite difficult to get accurate values and mathematic equations of these parameters. To solve this problem，those important parameters of model were estimated approximately by the estimation equation and method proposed in this work． Equivalent mass estimation for load on hydraulic cylinderThe load of boom hydraulic cylinder(it is assumed there is no external load)consists of boom，dipper and bucket．In ，boom，dipper and bucket rotate around points O1，O2 and O3，respectively．So their motions are not straight line motions about the cylinders, that is to say, their motion directions are different from Y in Eqn.(5)．So，m in Eqn.(6)cannot be simply regarded as the sum mass of boom，dipper and bucket．Considering O1 at an axis of manipulator, the torque and angular acceleration can begiven as follows：(8)where M and are the torque and angular acceleration of manipulator to O1，respectively。 is the length from point O1 to point B．According to the rotating law：M=J,we get that is （9） where J is the equivalent moment inertia of manipulator to point O1,kgm2，and it can be written as follows： (10)J1, J2 and J3 are the moment inertia of boom，dipper and bucket to their own bary center respectively．The values of them can be obtained by dynamic simulation based on the dynamic mode, J1=m, J2=m, J3=m.Comparing Eqn．(9)with Fc=mac，the equivalent mass at point B can be given： (11) Estimation for load on hydraulic cylinderThe equivalent moment equation of manipulator to O1 is (12)where and are the length from pointO1 to point G1180。 ，G2180。and G3180。；，respectively．Then，the counter force of load is (13) Estimation for flow gain coefficient of valveThe flow of pump can be measured by flow transducer． The instrument used in this work was Multi—system 5050．The step response of flow of boom cylinder under the electro—hydraulic proportional valve controlled by the step curent is shown in ．At the same time，the curve verifies Eqn．[11]．Based on the experiment curve，the range of KqKl can be identified according to Eqns.(1)and(4)．And then，according to data in ，we can get：KqKl=104m3/(sA)． Flow of boom cylinder under electrohydeaulic proportional value controlled by step current5 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：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 ，bearing force , flow gain coefficient of value KqKl=104m3/(sA) ，where KI is the current gain of electro—hydraulic proportional valves．From: Journal of Central South University (English) 2008 Vol 15 No. 3 pages 382386