| The first Chinese lunar rover “Jade Rabbit” has successful landed on rainbowbay of the moon surface which is380,000kilometers from the earth in Beijing time at4:35on December15. This indicated that the overall level of Chinese technology inDeep-space Exploration field had officially entered the ranks of the powers.According to the introduce of State Administration of Science, Technology andIndustry for National Defense, as the main task of “Chang’e II program”,“Chang’eIII” will be completed three exploration missions including lunar soft landing, lunarsurface detection, and deep space communications, remote monitoring and controloperations. Among them, the lunar surface detection will depend on the trafficabilityof “Jade Rabbit” lunar rover. Due to great differences between earth’s environmentand lunar surface, such as lunar soil is granular mixtures which are looser than earthsoil, the gravity of lunar surface is only1/6of ground, the research on the influence oflow gravity to pressure properties of lunar regolith and interaction between lunar soilsimulant and lunar rover is significant.This paper based on the “Chang’e Program” lunar exploration project, accordingto the range of parameter values of lunar regolith, with cinder as raw materials, theJLU series lunar soil simulunt for vehicle-terramechanics research in labtory had beenmade. And the basic mechanical properties of JLUs have been tested. The resultsshow that the range of bulk density of JLUs is1.20~1.40g/cm3, specific gravity is2.73g/cm3, range of porosity is50.00~57.00%, range of void ratio is1.00~1.30, rangeof water content is0.20~0.40%. d50of JLU-1is205.9μm, its particle size distributionis close to the lower limit value of the lunar regolith, d50of JLU-2is40.8μm, itsparticle size distribution is close to the upper limit value of the lunar regolith. Themoisture absorption behaviors of JLU-1and JLU-2are basically the same. The largestmoisture absorption of JLU-1is0.54%, of JLU-2is0.78%,both smaller than1.0%.Thismeans in saturated state, JLUs simulant has little water content. Although in along-term storage in natural environment, the absorption of water from environment isvery little. The water in the air has little effect to bulk granular characteristics of lunarsoil simulant. The lunar soil simulant can be storaged directly in the environment.Based on decomposition of forces theory, a kind of low-gravity simulator hasbeen developed called tiltable test-bed, the low gravity along the inclined table can be simulated. Based on Janssen equation, a mathematical model had been built whichcan be used to calculate the internal stress of lunar soil simulant in different tilted state.The axial force only increases with depthof z direction increases, lateral forceincreases with depthof y, z direction and the tilt angle of test-bed platform increases.The additional force eliminate models of JLUs of pressure-sinkage test,shear-stress test, penetration resistance test and impact resistance test have beenestablished. The additional force in impact resistance test can be neglected, theadditional force pain shear-stress test is surface intergral of shear plane, the axialforce pajafter eliminated additional force is volume intergral of cross-sectional area,pressure depth and distribution function of lateral force on the test field.Based on tiltable test-bed, the tests of pressure-sinkage characteristics,shear-stress characteristics, penetration resistance characteristics and impact resistancecharacteristics of JLUs have been done. The results show that low gravity makes thedeformability index n, frictional modulus kφ, cohesion cof JLUs reduced, the cohesivemodulus kc, internal friction angle φ had no obvious changes. In the effect ofequivalent load, low gravity made pressure-sinkage, penetration resistance and impactresistance characteristics of JLUs enhanced; shear-stress characteristics had nosignificant changes. Compared with JLU-1, the mechanical properties of JLU-2isweaker and more sensitive by the affected state dense. Tiny particlesand soft state isnot conducive for rover trafficability. Using two-dimensional discrete element methodto analyze the properties behavior and the two-dimensional and three-dimensionaldiscrete element model of lunar regolith had been established. The trend of lowgravity effect to the mechanical properties of lunar soil simulation of simulationanalysis and soil-bin test are consistend.Using mesh wheel, the static and dynamic sinkage test had been done, the staticsinkage model had been amended and the rut information of wheel through gettingfrom non-contact measurement methods. The results show that the scanning sinkageis smaller than real displacement and he two changes has a linear relationship. Thedifference of scanning and real displacement is affected by subsidence dept, normalload and the state of lunar soil simulant, and can reach above10mm; the averagedeviation rate is23.94%. This research can help to backstepping the real-timemechanical state of lunar soil simulant and guide the work of lunar rover in orbit.Using light lunar rover-soil bin testing system to measure the tractiveperformance of mesh wheel under different vertical load, at the same time, with PFC 2D and PFC3D discrete element software to simulat the wheel-soil relationship underdifferent gravity environment, the results show that the sinkage of wheel increaseswith normal load increases. When wheel slip rate is70%, the sinkage of70N normalload is1.39times of sinkage of30N normal load in soft state, the slip rate has a largerimpact on rutting topography. In this paper, PFC discrete element simulation softwarehad been used to analyze the relationship between soil and wheel under differentgravities. The two-dimensiona simulation results show that, when the wheel load isthe same in different gravities, with the increase of the gravity, wheel DP increased,sinkage reduced, when wheel slip rate is20%, wheel DP in1g environment is1.3times of that in1/6g environment, and wheel sinkage is62.5%of sinkage in1/6g, thetrafficability of lunar rover in ground environment is better than that on lunar surface;when the wheel mass is the same, with the increase of the gravity, wheel DP increased,sinkage had no significant changes, when wheel slip rate is5%, wheel DP in1genvironment is4.03times of that in1/6g environment, wheel torque is4.2times ofthat in1/6g, and wheel sinkage is1.75times of sinkge in1/6g environment, thetrafficability of lunar rover on lunar surface is better than that in ground environment.The three-dimensiona simulation results show that, when the wheel load is the samein different gravities, the influence of low gravity to wheel torque is not obvious,wheel sinkage in1/6g is1.55times of that in1g, wheel DP is about78%of1g wheelDP, all of the traction performance of lunar rover cannot do better over6times of1gperformance. So when the lunar rover-lunar regolith system is in low gravityenvironment, the trafficability of lunar rover on lunar surface is better than that inground environment. The research achievements of this paper can provide basictechnical support to China Deep-Space Exploration. |
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