Discrete element modeling of a Mars Exploration Rover wheel in granular material

Three-dimensional discrete element method (DEM) simulations were developed for the Mars Exploration Rover (MER) mission to investigate: (1) rover wheel interactions with martian regolith; and (2) regolith deformation in a geotechnical triaxial strength cell (GTSC). These DEM models were developed to improve interpretations of laboratory and in situ rover data, and can simulate complicated regolith conditions. A DEM simulation was created of a laboratory experiment that involved a MER wheel digging into lunar regolith simulant. Sinkage and torques measured in the experiment were compared with those predicted numerically using simulated particles of increasing shape complexity (spheres, ellipsoids, and poly-ellipsoids). GTSC simulations, using the same model regolith used in the MER simulations, indicate a peak friction angle of approximately 37–38° compared to internal friction angles of 36.5–37.7° determined from the wheel digging experiments. Density of the DEM regolith was 1820 kg/m³ compared to 1660 kg/m³ for the lunar simulant used in the wheel digging experiment indicating that the number of grain contacts and grain contact resistance determined bulk strength in the DEM simulations, not density. An improved correspondence of DEM and actual test regolith densities is needed to simulate the evolution of regolith properties as density changes.     

Discrete element method analysis of single wheel performance for a small lunar rover on sloped terrain

The purpose of this study is to analyze the performance of a lugged wheel for a lunar micro rover on sloped terrain by a 2D discrete element method (DEM), which was initially developed for horizontal terrain. To confirm the applicability of DEM for sloped terrain locomotion, the relationships of slope angle with slip, wheel sinkage and wheel torque obtained by DEM, were compared with experimental results measured using a slope test bed consisting of a soil bin filled with lunar regolith simulant. Among the lug parameters investigated, a lugged wheel with rim diameter of 250 mm, width of 100 mm, lug height of 10 mm, lug thickness of 5 mm, and total lug number of 18 was found, on average, to perform excellently in terms of metrics, such as slope angle for 20% slip, power number for self-propelled point, power number for 15-degree slope and power number for 20% slip. The estimation of wheel performance over sloped lunar terrain showed an increase in wheel slip, and the possibility exists that the selected lugged wheel will not be able to move up a slope steeper than 20°.     

Design and development of a new transformable wheel used in amphibious all-terrain vehicles (A-ATV)

Conventional ground-wheeled vehicles usually have poor trafficability, low efficiency, a large amount of energy consumption and possible failure when driving on soft terrain. To solve this problem, this paper presents a new design of transformable wheels for use in an amphibious all-terrain vehicle. The wheel has two extreme working statuses: unfolded walking-wheel and folded rigid wheel. Furthermore, the kinematic characteristics of the transformable wheel were studied using a kinematic method. When the wheel is unfolded at walking-wheel status, the displacement, velocity and acceleration of the wheel with different slip rates were analyzed. The stress condition is studied by using a classic soil mechanics method when the transformable wheel is driven on soft terrain. The relationship among wheel traction, wheel parameters and soil deformation under the stress were obtained. The results show that both the wheel traction and trafficability can be improved by using the proposed transformable wheel. Finally, a finite element model is established based on the vehicle terramechanics, and the interaction result between the transformable wheel and elastic–plastic soil is simulated when the transformable wheel is driven at different unfold angles. The simulation results are consistent with the theoretical analysis, which verifies the applicability and effectiveness of the transformable wheel developed in this paper.     

月球着陆器着陆过程的DEM-FEM耦合分析

实现月球着陆器的安全软着陆是宇航员、搭载设备、仪器安全以及着陆器后续正常工作的重要保证。本文采用离散元-有限元方法建立月壤与着陆器相互作用的耦合模型,其中月壤采用具有粘接作用的球体离散元单元,着陆器采用壳单元和梁单元组合的有限元模型,与缓冲垫连接的支撑腿采用可压缩弹簧模型,以实现着陆器自身的缓冲功能;为分析着陆于不同月表的过程,建立平坦、斜坡月球表面;针对不同坡度的斜坡月表,研究不同着陆模式下着陆器与月壤的相互作用,分析冲击力峰值的大小与作用时间的关系,并从能量方面讨论和解释大坡度下冲击力峰值小的原因。以上研究为月球着陆器的安全着陆分析提供有益的参考。     

Analytical models and laboratory measurements of the soil-tool interaction force to push a narrow tool through JSC-1A lunar simulant and Ottawa sand at different cutting depths

Excavation equipment for developing NASA’s lunar outpost must be carefully designed to reduce launch cost, minimize operation cost, and enhance reliability. Excavation equipment requires knowledge of the stresses and strains in the equipment caused by the forces experienced during excavation. The types of excavation anticipated indicate that blade tools would move the most material. There are several analytical models available to predict forces from blade tools interacting with soil; however, it is not clear which if any, can predict lunar excavation forces precisely enough. Consequently, we measured the forces to push narrow (2.5-cm wide) square and round rods through a control material, Ottawa sand, and JSC-1A lunar mare regolith simulant at different cut depths in a controlled laboratory setting. The measurement results were compared with the forces predicted by eight analytical models. The Zeng, Luth and Wismer, and the Qinsen and Suren models fit the measurements best, considering that our study was limited to pushing stimulant and sand with small rods. The results show that depth of cut has a dramatic effect on the soil-tool interaction forces. Consequently, lunar missions should use a series of shallow cuts to reduce equipment size and power requirements.     

Numerical and experimental studies of gravity effect on the mechanism of lunar excavations

In this paper, the mechanism of soil excavation in partial gravity conditions is investigated by experimental model and numerical study. Experiments were conducted in a parabolic flight, which generated different gravity conditions, focusing on the bearing capacity problem using two soil samples: Toyoura sand and Japanese lunar soil simulant (FJS-1). Corresponding numerical studies were performed by the discrete element method (DEM) for reduced gravity conditions. Herein, the DEM method was modified to include the apparent cohesion that was found in the lunar soil simulant. Two case studies were investigated by the numerical simulations: bearing capacity and soil pushing (as by a bulldozer), and for the former case comparison was made with experiment. Results show that the gravity greatly affects the ultimate bearing capacity of the Toyoura sand; however, such effect becomes insignificant in the lunar soil when the gravity is small or the soil was densely packed. By using the numerical model, this paper suggests that the ultimate bearing capacity observed in the lunar soil simulant was dominated by the apparent cohesive component, rather than gravity or friction. However, gravity causes similar effects on both soil models in the soil pushing problem.     

A modified discrete element method for concave granular materials based on energy-conserving contact model

The development of a general discrete element method for irregularly shaped particles is the core issue of the simulation of the dynamic behavior of granular materials. The general energy-conserving contact theory is used to establish a universal discrete element method suitable for particle contact of arbitrary shape. In this study, three dimentional (3D) modeling and scanning techniques are used to obtain a triangular mesh representation of the true particles containing typical concave particles. The contact volume-based energy-conserving model is used to realize the contact detection between irregularly shaped particles, and the contact force model is refined and modified to describe the contact under real conditions. The inelastic collision processes between the particles and boundaries are simulated to verify the robustness of the modified contact force model and its applicability to the multi-point contact mode. In addition, the packing process and the flow process of a large number of irregular particles are simulated with the modified discrete element method (DEM) to illustrate the applicability of the method of complex problems.     

Reduced testing and modelling of the bearing capacity of rooted soil for wheeled forestry machines

The next generation of forestry machines must be developed to be gentler to soil and to the root mat than present machines, especially in thinning operations. The bearing capacity of the soil is a key property for determining the terrain trafficability and machine mobility. This asks for better and more general terramechanics models that can be used to predict the interaction between different machine concepts and real and complex forest soil.This paper presents results from terramechanics experiments of rooted soil with a new and small-scale testing device. The force–deflection results are analyzed and compared with analytical root reinforcement models found in literature. The presented study indicates that rooted soil properties obtained with the new laboratory test device can be used to create an augmented soil model that can be used to predict the bearing capacity of rooted soil and also to be used in dynamic machine–soil interaction simulations.     

Design and characterization of GRC-1: A soil for lunar terramechanics testing in Earth-ambient conditions

Earth experiments must be carried out on terrain that deforms similarly to the lunar terrain to assess the tractive performances of lunar vehicles. Most notably, terrain compaction and shear response underneath the lunar vehicle wheels must represent that of the Moon. This paper discusses the development of a new lunar soil simulant, Glenn Research Center lunar soil simulant #1 (GRC-1), which meets this need. A semi-empirical design approach was followed in which the soil was created by mixing readily available manufactured sands to a particle size distribution similar to the coarse fraction of lunar soil. By varying terrain density, a broad range of in situ cone penetration measurements collected by the Apollo mission astronauts can be replicated. An extensive set of characterization data is provided in this article to facilitate the use of this material. For reference, the index and geotechnical properties of GRC-1 are compared to the lunar soil and existing lunar soil simulants.     

Digging and pushing lunar regolith: Classical soil mechanics and the forces needed for excavation and traction

There are many notional systems for excavating lunar regolith in NASA’s Exploration Vision. Quantitative system performance comparisons are scarce in the literature. This paper focuses on the required forces for excavation and traction as quantitative predictors of system feasibility. The rich history of terrestrial soil mechanics is adapted to extant lunar regolith parameters to calculate the forces. The soil mechanics literature often acknowledges the approximate results from the numerous excavation force models in use. An intent of this paper is to examine their variations in the lunar context. Six excavation models and one traction model are presented. The effects of soil properties are explored for each excavation model, for example, soil cohesion and friction, tool–soil adhesion, and soil density. Excavation operational parameters like digging depth, rake angle, gravity, and surcharge are examined. For the traction model, soil, operational, and machine design parameters are varied to probe choices. Mathematical anomalies are noted for several models. One conclusion is that the excavation models yield such disparate results that lunar-field testing is prudent. All the equations and graphs presented have been programmed for design use. Parameter ranges and units are included.     

地面力学及其在行星探测研究中的应用

地面力学是研究越野行驶中机器与地面相互作用的一门力学学科,包括对机器通过性的预测和评价,行走机构的优化设计以及对地面可行驶性的预测判断等几个方面.首先简介地面力学的研究方法、试验仪器和设备以及主要的成果和结论,其中包括在这些方面的最新研究进展情况.之后重点介绍行星探测领域中所开展的地面力学研究,主要从行星探测器设计阶段对地面力学理论和方法的应用、行星模拟土壤研制和力学特性研究、行星就位土壤力学参数测量等几个方面进行了综述.最后对这一领域今后的研究方向进行了探讨.      

On the degradation of granular materials due to internal erosion

A new state-based elasto-plastic constitutive relationship along with the discrete element model is established to estimate the degradation of granular materials due to internal erosion.Four essential effects of internal erosion such as the force network damage and relaxation are proposed and then incorporated into the constitutive relationship to formulate internal erosion impacts on the mechanical behavior of granular materials.Most manifestations in the degradation of granular materials,such as reduction of peak strength and dilatancy are predicted by the modified constitutive relationship in good agreement with the discrete element method(DEM)simulation.In particular,the sudden reduction of stress for conspicuous mass erosion in a high stress state is captured by force network damage and the relaxation mechanism.It is concluded that the new modified constitutive relationship is a potential theory to describe the degradation of granular materials due to internal erosion and would be very useful,for instance,in the prediction and assessment of piping disaster risk during the flood season.     

Discrete element method-based studies on dynamic interactions of a lugged wheel with granular media

Terramechanics plays an important role in determining the design and control of autonomous robots and other vehicles that move on granular surfaces. Traction capabilities, slippage, and sinkage of a robot are governed by the interaction of a robot’s appendage with the operating terrain. It is important to understand how the terrain flows under this appendage during such an interaction. In this work, dynamics of soil performance and locomotion performance of a lugged wheel travelling on soft soil are numerically investigated. Studies are conducted with a two-dimensional model by using the discrete element method to analyze the interactions between a lugged wheel and the soil. The soil performance is studied by examining the force distribution and evolution of force networks during the course of the wheel travel. For two different control modes, namely, slip-based wheel control and angular velocity-based wheel control, the performance parameters such as, sinkage, traction, traction efficiency, and power consumption of the wheel are compared for various wheel configurations. The findings of this work are expected to be useful for optimal design and control of the lugged wheel travelling on deformable surfaces.     

Design of manned lunar rover wheels and improvement in soil mechanics formulas for elastic wheels in consideration of deformation

Most of the current lunar rover vehicle wheels are inconvenient for changing broken wheels and have poor shock absorbing in driving, so they cannot be used to carry people on the moon. To meet the demands for manned lunar transportation, a new wheel possessing a woven metal wire mesh tire and using hub-rim combination slide mechanism is designed in this article. The characteristics of the new wheel is analyzed by comparing with the same-size conventional rover wheels after demonstrating the validity of FEM simulation. The new wheel possesses lighter structure and superior shock absorbing. It also provides stronger traction because the deformation of the designed wheel increases the contact area between the tire and lunar terrain. In order to establish an on-line soil parameter estimation algorithm for low cohesion soil, the stress distribution along a driven deformable wheel on off-road terrain is simplified. The basic mechanics equations of the interaction between the wheel and the lunar soil can be used for analytical analysis. Simulation results show that the soil estimation algorithm can accurately and efficiently identify key soil parameters for loose sand.     

Numerical simulation and testing verification of the interaction between track and sandy ground based on discrete element method

Tractive effort of tracked vehicles plays an important role in military and agricultural fields. In order to solve the problem of low precision in numerical simulation of the interaction between track and sandy ground, a systematic and accurate discrete element modeling method for sandy road was proposed. The sandy ground was modeled according to the mechanical parameters measured by soil mechanics tests. The interaction coefficients of sandy soil were measured by the repose angle test and triaxial compression test combined with the corresponding simulation. On this basis, a discrete element interaction model of track-sandy ground was established, which can be used to test the tractive effort of track. Numerical simulation calculation of track model at different speeds was carried out, and the simulation results were compared with the results of indoor soil bin test for verification. The verification results show that the interaction between track and sandy ground based on DEM simulation is consistent with the actual soil bin test. The discrete element modeling method in this paper can be used to model the track and sandy ground accurately, and the simulation model can be used to test the tractive effort of tracked vehicle.     

Equivalent boundary model of lunar soil drilling simulation by DEM

Aiming to solve the computational cost problem in the discrete element simulation for lunar soil drilling sampling, an equivalent boundary method was proposed. A high-accuracy DEM model of lunar soil was established firstly. As the novel alterable constitutive law, the accuracy of the model was verified to meet the performance of real lunar soil very much both in shear strength indices and elastic–plastic behavior. A common drill bit in the geological exploration field for sampling soil was chosen as the simulation object. In preanalysis, it was known that with the increase of drilling depth, the stress concentration area was always near the drill bit, while the affected area of the lunar soil was a cylindrical area around the drill pipe, which extended towards the drilling direction instead of extending around it. Then a big boundary drilling simulation scene was established to investigate the flow direction of lunar soil particles. The motion law of particles and the velocity field information were obtained, and a U-shape chain was described around the drill bit. Finally an equivalent boundary was set near the U-shaped chain, and the size was determined by comparing the soil stress in the fierce collision zone and around the reference boundary. This method could be a reference for other lunar soil drilling researches with other drills of different sizes.     

Discrete element modelling of pebble beds: With application to uniaxial compression tests of ceramic breeder pebble beds

In this paper, a discrete element simulation scheme for pebble beds in fusion blankets is presented. Each individual pebble is considered as one element obeying equilibrium conditions under contact forces. We study not only the rearrangement of particles but also the overall behaviour of an assembly under the action of macroscopic compressive stresses. Using random close packing as initial configurations, the discrete element simulation of the uniaxial compression test has been quantitatively compared to experiments. This method yields the distribution of the inter-particle contact forces. Moreover, the micro-macro relations have been investigated to relate the microscopic information, such as the maximum contact force and the coordination number inside the assembly, to the macroscopic stress variables.     

Analysis of the movement behaviour of soil between subsoilers based on the discrete element method

The identification of the movement behaviour of soil in the area under the combined effect of two subsoilers (i.e., the area between two subsoilers) is one of the key issues in determining a reasonable inter-subsoiling shovel distance. The present study established a working model of subsoiling using the discrete element method (DEM). Based on this model and an indoor soil-bin experiment, the present study focused on investigating the micro-movement and macro-disturbed behaviour of soil in the area under the combined effect of two subsoilers. The results show the following. (1) The range of transverse and longitudinal disturbance of soil decreased with increasing distance between the soil and subsoiler. The range of disturbance of the soil in the shallow layer was the widest, followed by the range of disturbance of the soil in the middle layer and the range of disturbance of the soil in the deep layer. The simulated and experimental values of the mean displacement of the soil in the shallow layer (tracer blocks with a side length of 10 mm) were 34.81 mm and 34.55 mm, respectively (error: 0.75%). (2) The force on the soil particles in the deep layer was the greatest, followed by the force on the soil particles in the middle layer and the force on the soil particles in the shallow layer. The force on and the velocity of movement of the soil particles at different locations decreased with increasing distance between the soil and subsoiler. (3) The discrete element simulation could accurately simulate the disturbance process of the soil subjected to subsoiling. The sectional profiles of the disturbed soil obtained from the simulation and experiment were consistent with each other. The relative error between the simulated and experimental values of the soil looseness and the soil disturbance coefficient was 14.45% and 12.06%, respectively. Based on the DEM combined with an indoor soil-bin experiment, the present study determined the movement behaviour of the soil in the area under the combined effect of two subsoilers. The results of the present study can facilitate in-depth investigations of the subsoiling shovel–soil interaction and provide a basis for making decisions to optimise subsoiler arrangements.     

Discrete element modelling of large soil deformations under heavy vehicles

This paper addresses the challenges of creating realistic models of soil for simulations of heavy vehicles on weak terrain. We modelled dense soils using the discrete element method with variable parameters for surface friction, normal cohesion, and rolling resistance. To find out what type of soils can be represented, we measured the internal friction and bulk cohesion of over 100 different virtual samples. To test the model, we simulated rut formation from a heavy vehicle with different loads and soil strengths. We conclude that the relevant space of dense frictional and frictional-cohesive soils can be represented and that the model is applicable for simulation of large deformations induced by heavy vehicles on weak terrain.     

Simulation of a soil loosening process by means of the modified distinct element method

We apply the Distinct Element Method (DEM) to analyze the dynamic behavior of soil. However, the conventional DEM model for calculation of contact forces between elements has some problems; for example, the movement of elements is too discrete to simulate real soil particle movement. Therefore, we modify the model to solve the difficulties. To investigate the validity of the modified model, we conduct an experiment in which soil is cut with a pendulum-typeblade, and simulate the soil loosening process with the modified DEM model. This paper presents details of the experimental apparatus and the comparison of soil behavior and energy absorption between the simulation and the experiment. Some characteristic phenomena of the experiment are reproduced in the simulation giving us confidence that the modified model is better than the conventional model for the simulation of soil behavior.