Study On Selenographic System Dynamics Of Rigid Wheel Under Light-loading

Lunar exploration has been the important part of space activity, and lunar rover play avital role in lunar exploration. The second stage of China lunar exploration project is―drop‖,that is, lunar rover dorps and carries out field survey on the moon. To satisfy the requirementof science exploration, lunar rover must have high traction ability. The surface of lunar iscoved of soft lunar regolith, the gravity of lunar is only one-sixthes gravity of earth, thecondition is not good for the traction ability of lunar rover. In order to fulfil explorationmission, the relationship between the lunar wheel and lunar regolith should be studied andpredicted by test method, theory method and numerical simulation based onterrainmechanics and real condition of lunar. The practices can provide necessary theorybasis and test method with the study of passing ability of lunar rover.The author is grateful for the financial supported by National Nature ScienceFoundation of China（No.50875107）, National High-tech Research and DevelopmentProgram of China（No.2010AA101401-3） and advanced project of Jilin University andChinese Academy of Space Technology. Firstly, Aimed at the requirement of lunar rover, theresearch developed light-loading soil bin equipment. The research used JLU-2a lunarstimulant which is developed by Jilin University, carried out soil bin test about tractionability for four lunar rover wheels with typical shape, studied the traction ability of the fourdriving wheels. Secondly, the traditional terrainmechnics is summarized, the effect ofmeasure errors of pressure characteristic parameters on sinkage errors is analyzed, thecalculation model of sinkage and rolling resistance are corrected, and the correctness of theformulaes are verified. Thirdly, the change of normal stresses and shear stresses under thesmooth cylinder wheel is analized, and the effect of the position where the velocity of thepoint on the wheel is downward on the forces acting on the wheel is analyzed. Finally, theresearch study the effect of the width and height of the lugs on the traction ability of thedriving wheels by FEM.On the consideration of the actual loading of lunar rover and gravity field of lunar, the reseach develops the light-loading soil bin system. The soil bin system is consistent ofmechanical system and measure and control system. The soil bin is fixed and the wheel canmove in mechanical system, and the measure and control system can fix the position on thesurface of the lunar stimulant, and measure driving torque, horizatla displacement, verticaldisplacement, rotation velocity, slip ratio. Beacause of light-loading, the structure issimplified and lightweight, the friction of mechanisms is reduced to the extent, and smallrange sensors are used. The research selects sinkage, driving torque, drawbar pull, tractionefficiency as test indexes. Based on the acual driving, the vertical loading changes from0to100N, slip ratio changes from0to60%, the velocity of the wheel changes from0to50mm/s.JLU—2a is selected as lunar soil stimulants in the test, the states of the lunar soil stimulantsare natural and soft. The four driving wheels with diffirent shape are used. The appearancecharacteristic has relation with the shape of the driving wheel. The sinkage, driving torqueand slip ratio which are directly gotten from the cylinder wheel with ruts by the soil binsystem are analyzed, these parameters periodically change with the increase of displacementbecause of the lugs.The research analyzes traction ability test of lunar driving wheel. The ruts of smoothwheel have characteristic of water traces or fish scale traces, and the height, width anddegree of damage of edge increase with slip ratio. The depth of the ruts of the wheels withthe lugs increase with slip ratio, width of the lugs decreases with slip ratio, and the lugs stirup more lunar stimulant around the ruts. The sinkage, driving torque, drawbar pull increasewith slip ratio, and tractive efficiency increase firstly, then decrease with slip ratio.Little influence has variation of velocity on the sinkage, driving torque and drawbar pull ofthe four wheels as the velocity changes from15mm to25mm, but lager influence on tractiveefficiency. Drawbar pull of smooth cylinder wheel and smooth drum wheel increase withvelocity, the same as the driving torque of drum wheels, but tractive efficiency of the fourwheels and driving torque of the smooth drum wheel decrease with velocity. As verticalloading changes from30N to90N, the sinkage, drawbar pull, driving torque increase withvertical loading, and tractive efficiency of cylinder wheel with ruts and smooth drum wheeldecrease with vertical loading increases. As slip ratio is smaller than30%,tractive effiencydecrease with vertical loading, the maximum value of tractive efficiency decrease with thevertical loading increases. The sinkage of the four wheels in the state of soft is larger thanthat in the state of nature. Driving torque, drawbar pull and tractive efficiency of smoothcylinder wheel and cylinder wheel with lugs in the state of nature are lager than those of soft. Driving torque of smooth drum wheel and drum wheel with lugs in the state of soft is largerthan that of nature. Drawbar pull of smooth drum wheel and drum wheel with lugs in thestate of nature is larger than that of soft. To the four wheels, the sinkage of the wheels withlungs is smaller than that of smooth wheel, and driving torque, drawbar pull and tractiveeffiency of the wheels with lugs are larger than those of amooth wheel.After summrizing the traditional terrainmechanics formulas, the research determines themechanical models which are fit to be modified. Aimming at sinkage and rolling resistancemodels, the influence of pressure characteristic parameters errors on the erros of sinkage androlling resistance is analyzed. The influence of n on the errors of sinkage and rollingresistance is larger thank candk on those within variaration range which the lunar rovercan work. The relative error of sinkage is16%, and confidence level is90%. The relativeerror of rolling resisitance is8%, and confidence level is96%. On the consideration of slipand the test errors of mechanical parameters, the research modifies the sinkage model androlling resistance model. Furthermore, the research need consider the shape of the wheelsand luggs when the models are modified. Based on ananlysis and the results, the modifiedmodels are given. The errors of sinkage is from1%—19.7%, the errors of rolling resistance is1.5%～19.9%.The research analyzes the relationship between smooth wheels and lunar soil simulants.The normal stresses and shear stresses which exist between the smooth cylinder wheel andlunar soil simulants increase with deformable index, friction modulus and radius of wheels,cohesive modulus and width of wheels have the little influence on normal stresses ans shearstresses, cohesion has little effect on shear stresses, shear sress increase with friction angle.The normal stresses and shear stresses increase when slip ratio decrease when comtact angleis between0and the value where the maximum normal stress. Between the contact valuewhere the maximum normal stress is and the maximum contact angle, the shear stressesincrease with slip ratio, but slip has no influence on shear stresses. The normal stresses andshear stress increase with sinkage and sinkage has larger influence on stresses.There must bea point whose absolute velocity is downward on the brim of the rigid wheel. If the point hascontact with lunar soil simulants, the lunar soil simulants apply bulldozing resistance on thewheel below the point, meanwhile, the lunar soil simulants exert additional traction on thewheel above the wheel. If the point has no contact with the lunar soil simulants, the lunarsoil simulans apply smaller bulldozing resistance on the wheel, meanwhile, a little traction is applied on the wheel. The position of the point has relation with slip ratio, vincreaseswith slip ratio. θ_vmin is increase with velocity and vertical loading decrease. The change ofθ_vmin is not large on the condition of low velocity, θ_vmin occurs when slip ratio changesfrom20%to30%.The research has simulation analysis on traction ability test which is in chapter2andchapter3after modeling on lunar soil stimulant by Druker-Prager constitutive model ofABAQUS and determining corresponding conditions. Simulation ruts and data are similar totest results, it represents that traction ability test of lunar wheels could be availably simulated.The research analyze the effect of lungs with three diffirent dimentions on traction ability oflunar wheels. The change of width of lugs have no influence on pattern, sinkage, drawbarpull and driving torque on the condition of light-loading and low velocity. As the height ofthe lugs increase, the lugs lift seriously rack the soil, sinkage and drawbar pull increase, butdriving torque has little change.