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excavator’s arm
HE Qing-hua，HAO Peng，ZHANG Da-qing
Abstract
A retrofitted electro-hydraulic proportional system for hydraulic excavator was introduced firstly. According to the principle and characteristic of load independent flow distribution (LUDV) system, taking boom hydraulic system as an example and ignoring the leakage of hydraulic cylinder and the mass of oil in it ,a force equilibrium equation and a continuous equation of hydraulic cylinder were set up. Based on the flow equation of electro-hydraulic proportional valve, the pressure passing through the valve and the difference pressure were tested and analyzed. The results show that the difference of pressure does not change with load and it approximates to 2.0MPa. And then, assume the flow across the valve id directly proportional to spool displacement and is not influenced by load, a simplified model of electro-hydraulic system was put forward. At the same time, by analyzing the structure and load-bearing of boom instrument, and combining moment equivalent equation of manipulator with rotating law, the estimation methods and equations for such parameters as equivalent mass and bearing force of hydraulic- cylinder were set up. Finally, the step response of flow of boom cylinder was tested when the electro-hydraulic proportional valve was controlled by the step current. Based on the experiment curve, the flow gain coefficient of valve unidentified as 2.825×10-4m3/(s·A) and the mode is verified.
Key words: Excavator, Hydraulic-cylinder proportional system, Load independent flow distribution (LUDV) system, Modeling, Parameter estimation
1 Introduction
For its high efficiency and multifunction, hydraulic excavator is widely used in mines，road building, civil and military construction，and hazardous waste cleanup areas．The hydraulic excavator also plays an important role in construction machines．Nowadays, macaronis and mobilization have been the latest trend for the construction machines．So，the automatic excavator gradually becomes popular in many countries and is considered a focus．Many control methods can be used to automatically control the manipulator of excavator．Whichever method is used, the researchers must know the structure of manipulator and the dynamic and static characteristics of hydraulic system．That is, the exact mathematical models are helpful to design controller． However, it is difficult to model on time-variable parameters in mechanical structures and various nonlinearities in hydraulic actuators， and disturbance from outside．Researches on time delay control for excavator were carried out in Refs．NGUYE used fuzzy sliding mode control and impedance control to automate the motion of excavator’s manipulator． SHAHRAM et al adopted impedance control to the teleported excavator．Nonlinear models of hydraulic system were developed by some researchers. However, it is complicated and expensive to design controller, which 1imits its application．In this paper, based on the proposed model，the model of boom hydraulic system of excavator was simplified according to engineering and by considering the force equilibrium， continuous equation of hydraulic cylinder and flow equation of electro-hydraulic proportional valve；at the same time，the estimation methods and equations for the parameters of model were developed．
2 Overview of robotic excavator
The backhoe hydraulic excavator studied is shown in Fig.1．In Fig.1，Fc presents the resultant force of hydraulic cylinder, gravity of boom，dipper, bucket and so on at point B，whose direction is along cylinder AB； Fc can be decomposed into Fcl and Fc2，and their directions are vertical and parallel to that of O1B，respectively；ac is the acceleration whose direction is same to that of Fc，and ac can be decomposed into acl an d ac2 too；G1，G2 and G3 are the gravity centers of boom，dipper and bucket，respectively；ml，m2 and m3 are the masses of them，and their values can be given by experiment( m1=868.136kg，m2=357.115kg and m3=210.736kg)；Ol，O2 and O3 are the hinged points；G1′，G2′and G3′are projections of Gl，G2 and G3 on x axis，respectively．
The arm of excavator was considered a manipulator with three degrees of freedom (three inclinometers were set on the boom，dipper and bucket，respectively)．In tracking control experiment，the objective trajectories were planed based on the kinematic equation of excavator’s manipulator．Then，the motion of boom，dipper an d bucket was set by the controller．In order to suit for automatic contro1．the normal hydraulic control excavator should be retrofitted to electro-hydraulic controller．
Based on original hydraulic system of SW E-85．The hydraulic pilot control system was replaced by an electro-hydraulic pilot control system．The retrofitted hydraulic system is shown in Fig.2．In this work，because boom，dipper an d bucket are of the same characteristics，the hydraulic system of boom was taken as an example．In the electro-hydraulic pilot control system，the pilot electro—hydraulic proportional valves were derived from adding proportional relief valves on the original SX-l4 main valve，and hydraulic pilot handle was substituted by electrical one．The retrofitted system of excavator was still the LUDV system (Fig.3)of Rexroth with good controllability．In Fig.3，y is the displacement of piston；Q1 and Q2 are the flows in and out to the cylinder respectively；pl，p2，ps and pr are the pressures of head and rod sides of cylinder, system and return oil，respectively；A1 and A2 are the areas of piston in the head and rod sides of cylinder, respectively； xv is the displacement of spool；m is the equivalent mass of load.
Flg.1 Schematic diagtam of excavator’s arm
Flg.2 Schematic diagram of retrofitted electro-hydraulic system of excavator
Flg.3 Schematic diagram of LUDV hydraulic system after retrofitting