Machine learning in planetary rovers: A survey of learning versus classical estimation methods in terramechanics for in situ exploration

For the design of space missions in the Moon and planets, analysis of mobility in robots is crucial and poor planning has led to abortion of missions in the past. To mitigate the risk of mission failure, improved algorithms relying intrinsically on fusing visual odometry with other sensory inputs are developed for slip detection and navigation. However, these approaches are significantly expensive computationally and difficult to meet for future space exploration robots. Hence, today the central question in the field is how to develop a novel framework for in situ estimation of rover mobility with available space hardware and low-computational demanding terramechanics predictors. Ranging from pure simulations up to experimentally validated studies, this paper surveys dozens of existing methodologies for detection of vehicle motion performance (wheel forces and torques), surface hazards (slip-sinkage) and other parameters (soil strenght constants) using classical terramechanics maps, and compare them with novel approaches introduced by machine learning, allowing to establish future directions of research towards distributed exteroceptive and proprioceptive sensing for visionless exploration in dynamic environments. To avoid making it challenging to collect all relevant studies expeditiously, we propose a global classification of terramechanics according most common practices in the field, allowing to form an structured framework that condense most works in the domain within three estimator categories (direct/forward or inverse terramechanics, and slip estimators). Likewise, from the experiences collected in previous MER (Mars Exploration Rover) missions, five overlooked problems are documented that will need to be addressed in next generation of planetary vehicles, along three research questions and few hypothesis that will pave the road towards future applications of machine learning-based terramechanics.     

ROSTDyn: Rover simulation based on terramechanics and dynamics

In recent years, rover-based planetary exploration missions have induced some new challenges related to both the speed and the fidelity of rover simulation. This paper introduces ROSTDyn (rover simulation based on terramechanics and dynamics), a good-fidelity (in a linear motion without side forces and related torques), real-time (with an Intel Core2 CPU and ATI Radeon HD 4650 GPU) simulation platform for planetary rovers developed using C++ on the basis of the Vortex physics engine. The inherent trade-off between high fidelity and high speed is overcome by using an improved and simplified terramechanics model and Vortex. This paper presents the key technologies and algorithms constituting ROSTDyn, including the creation of the rover model and terrain model, computation of contact-area parameters, computation of interactive force/torque model, and ROSTDyn’s implementation. Speed tests confirm that ROSTDyn can perform a real-time simulation when the display frequency is less than 45 Hz and the computation frequency is less than 450 Hz. A comparison of the simulation and experiment results for an example involving a six-wheel rover climbing a series of slopes confirms the good fidelity of ROSTDyn.     

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.      

Shape effects of wheel grousers on traction performance on sandy terrain

This paper presents the effects of different wheel grouser shapes on the traction performance of a grouser wheel traveling on sandy terrain. Grouser wheels are locomotion gears that allow small and lightweight exploration rovers to traverse on the loose sand on extraterrestrial surfaces. Although various grouser shapes have been analyzed by some research groups, a more synthetic and direct comparison of possible grousers is required for practical applications. In this study, we developed a single wheel testbed and experimentally investigated the effects of four grouser shapes (parallel, slanted, V-shaped, and offset V-shaped) on the traction performance of linear movement on flat sand. The wheel slip, sinkage, traction and side force acting on the wheel axle, the wheel driving torque, and the efficiency of each wheel were examined. Thereafter, the effects on the lateral slope traversability of a small and lightweight four-wheeled rover with different grouser shapes were also examined. The traversability experiment demonstrated the vehicle mobility performance in order to contribute to the design optimization of rover systems. These experimental results and their comparisons suggested that, of the shapes studies herein, the slanted shape was the optimal grouser design for use in wheeled rovers on lunar and planetary soil.     

On the treatment of soft soil parameter uncertainties in planetary rover mobility simulations

Nowadays soft soil–wheel contact models are widely used for predicting the mobility of rovers in off-road applications. However, most of the contact models used in computer simulations are based on semi-empirical laws for which soil parameters can be assessed only with large uncertainty. This lack of knowledge results in significant uncertainty on the rover position predictions. Applied to a planetary rover model, this paper illustrates probabilistic and non-probabilistic techniques for efficient treatment of soil parameter uncertainty for rover position predictions.     

Predicting the performances of rigid rover wheels on extraterrestrial surfaces based on test results obtained on earth

With a growing number of nations interested in planetary exploration, research and development of extraterrestrial rovers have been intensified. The usual practice is to test the performances of rovers on soil simulants on earth, prior to their deployment to extraterrestrial bodies. It is noted that in the tests the soil simulant is subject to the earth gravity, while the terrain on the extraterrestrial surface is subject to a different gravity. Therefore, it is uncertain whether the rover/rover wheel would exhibit the same performance on the extraterrestrial surface as that obtained from tests conducted on earth. This paper describes a practical methodology that can be employed to predict the performances of rover wheels on extraterrestrial surfaces, based on test results obtained on earth. As rigid wheels are used in many extraterrestrial rovers, this study focuses on examining the effects of gravity on the sinkage and compaction resistance of rigid rover wheels. Predictions obtained using the methodology are shown to correlate reasonably well with test data.     

The development of a soil trafficability model for legged vehicles on granular soils

This paper extends previous research in planetary microrover locomotion system analysis at the University of Surrey through the development of a legged microrover mobility model. This model compares various two- and three-dimensional soil cutting models to determine the most applicable model to legged locomotion in deformable soils, and is flexible to use any of these models depending on the leg shape, sinkage and other conditions. This baseline draught force model is used for determining the soil forces available for legged vehicle locomotion, as well as the soil thrust available to the vehicle footprint. Empirical investigations were performed with a robotic arm in planetary soil simulants to validate a legged mobility model through determination of the draft force of a robotic leg pushing through soil at constant and varying sinkage levels. The resulting locomotion performance model will be used to predict the ability of the legged vehicle to traverse a specific soil. An introduction to the planetary soil simulants used in this study (SSC-1 quartz-based sand and SSC-2 garnet-based sand) and the process used to determine their mechanical properties is also briefly presented to provide a baseline for this research.     

双小行星探测轨道动力学研究进展

双小行星系统由在万有引力作用下彼此环绕的两颗小行星组成, 对研究太阳系起源、行星系统演化和行星防御都具有重要的价值, 近年来成为行星科学和航天动力学研究的热门对象, 对双小行星系统的原位探测也即将迎来热潮. 双小行星系统的独特构型和附近的复杂动力学环境为探测器轨道动力学和任务设计带来了全新的挑战, 为应对这些挑战所进行的研究也推动了轨道动力学基础理论的发展. 本文对双小行星探测轨道动力学的研究进展进行综述, 首先介绍了双小行星研究和探测的背景及意义, 简要阐述了双小行星系统形成理论及其附近轨道动力学的研究概况. 其次, 介绍了双小行星系统不规则引力场和相互引力势的建模方法, 进而展示了双星的姿态轨道耦合动力学, 即完全二体问题, 包括双星相对运动的平衡构型和稳定性. 接着, 介绍了描述双星附近探测器轨道运动的限制性完全三体问题的动力学模型, 以及该模型下的平动点、平动点周期轨道、大范围周期轨道、转移轨道和轨道维持等方面的研究进展. 第四部分综述了环绕双小行星系统单颗星的受摄二体问题, 以轨道摄动理论和行星系统中受摄二体问题的研究现状为背景, 介绍了环绕双小行星系统主星的半解析轨道动力学建模与轨道稳定性分析. 之后, 介绍了目前面向探测任务需求和考虑实际约束的轨道动力学研究和轨道设计. 最后, 基于目前研究进展, 分析了面临的若干问题, 对未来双小行星探测轨道动力学及相关技术的发展进行了讨论和展望.      

A method for on-line soil parameters modification to planetary rover simulation

In planetary exploratory rover simulation, the contact model between wheel and terrain inevitably has some differences in contrast with the real one, which can make rover depart the planned track. To eliminate the dynamic errors caused by it, this paper presents a method for on-line soil parameters modification. This paper classifies data errors between virtual rover and real rover as model errors and asynchronous errors. Before modification, data identification is utilized to eliminate asynchronous errors and get a group of effective data with least additional errors. Based on the simplified terramechanics model, the origins of model errors are analyzed in detail from static status and kinetic status; meanwhile, some soil parameters are decoupled from the complicated model, and it makes on-line soil parameters modification feasible. An effective coefficient is also proposed to maintain the stability and convergence of modification. Lastly, through simulations on ROSTDyn (ROver Simulation based on Terramechanics and Dynamics), it is demonstrated that the soil parameters modification method is effective and useful for rover simulation to eliminate dynamic errors of predictive model.     

An equivalent soil mechanics formulation for rigid wheels in deformable terrain, with application to planetary exploration rovers

A simplified, closed-form version of the basic mechanics of a driven rigid wheel on low-cohesion deformable terrain is presented. This approach allows the formulation of an on-line terrain parameter estimation algorithm, which has important applications for planetary exploration rovers. Analytical comparisons of the original and simplified equations are presented, and are shown to closely agree. Experimental results from a single-wheel testbed operating in dry sand shows that the simplified equations can be used for mobility prediction with good accuracy. Methods for incorporating the simplified equations into an on-line terrain parameter algorithm are discussed.     

A methodology for quantitatively assessing vehicular rutting on terrains

This paper presents a quantitative method for assessing the environmental impact of terrain/vehicle interactions during tactical missions. Area wide mobility analyses were conducted using three standard US military tracked and wheeled vehicles over terrain regions representing both fine-grained and course-grained soils. The NATO reference mobility model, Version 2, was used to perform the on- and off-road mobility analysis. Vehicle and terrain characterizations along with different climate scenarios were used as input parameters to predict vehicle rut depth performance for the different vehicles and terrain conditions. The vehicles’ performance was statistically mapped over these terrain regions for percent area traveled and the resulting rut depth created by each vehicle. A selection of tactical scenarios for each vehicle was used to determine rut depth for a range of vehicle missions. A vehicle mission severity rating method, developed at the US Army Engineer Research and Development Center, was used to rate the selected missions and resulting rut depths.     

A survey on orbital dynamics and navigation of asteroid missions

Asteroid exploration is currently one of the most concerned topics among international space agencies. Or- bital dynamics and navigation are obviously crucial for asteroid exploration. This paper aims to give a brief review on the dynamics, control and navigation of asteroid reconnaissance orbits, including the heliocentric transfer orbit and near as- teroid orbit. The developments in optimization techniques of the transfer segment are discussed in detail. We surveyed global researches in this field and made comments on several important progresses. The final section proposed a prospec- tive of future studies with emphasis on the key techniques of these issues in the asteroid exploration missions.     

The ExoMars rover locomotion subsystem

ExoMars is the European Space Agency (ESA) mission to Mars planned for launch in 2018, focusing on exobiology with the primary objective of searching for any traces of extant or extinct carbon-based micro-organisms. The on-surface mission is performed by a near-autonomous mobile robotic vehicle (also referred to as the rover) with a mission design life of 180 sols. The rover has a 6 × 6 × 6 with 6 wheel-walking drive configuration (all 6 wheels are driven, steered and have a ‘walking’ capability) and has flexible wheels providing enhanced traction compared to rigid wheels of the same diameter. The suspension is a passive ‘3-bogie’ system which offers the same 6 wheel contact on uneven ground and mobility performance as the NASA-JPL ‘rocker-bogie’ suspension used on previous Mars rovers, but permits elimination of the differential linkage present in that design. Mars presents several challenges to the rover locomotion subsystem with its rock-strewn surface, sand dunes, rocky outcrops, craters and slopes. The unknown nature of the terrain to be traversed imposes several constraints on the locomotion subsystem design that need to be evaluated and incorporated within the flight model for its successful operation on Mars. In addition, accommodation within the confines of the lander and successful egress from it over deflated airbags places stringent constraints on locomotion subsystem mass, stowage envelope, deployment and wheel design. This paper documents the evolution of the ExoMars rover vehicle locomotion configuration from an early design concept to the current mission baseline design. The discussion involves various tradeoffs supported by mechanical and terramechanical analyses, simulations and testing performed on full-scale locomotion breadboard models at single wheel level and system level.     

Effect of bucket-wheel scale on excavation forces and soil motion

Bucket-wheels enable planetary rovers to perform lightweight digging operations in support of sustained space exploration. Using an excavation tool whose performance scales well for robots of varying sizes builds confidence in a wide range of future digging missions, much as scaled versions of the rocker-bogie suspension have enabled mobility for Mars rovers of vastly different sizes. Bucket-wheel excavation force increases approximately with the cube of excavation dimensions. The excavation forces were measured for bucket-wheels of different scales at proportionate depths and advance speeds, and these results were compared to predictions by excavation models. Analytical and empirical investigated models exhibit force scaling tendency similar to experiment despite their independent backgrounds. Soil particle motion imaging shows that a curved shear interface is prevalent for the conditions tested. This agrees with literature and allows the application of analytical models.     

深空探测自主导航技术综述

深空探测自主导航技术在减少地面测控负担, 提高探测器的生存能力和扩展探测器的特殊应用潜力等方面具有独特的优势, 自主导航在国外的深空探测活动中已经成功验证并逐步开始在实际任务中应用, 未来的自主导航技术将成为深空探测技术发展的一种必然趋势. 由于我国测控资源有限, 在我国的深空探测规划中, 发展自主导航技术将显得更为重要. 在我国即将开展火星和小行星探测计划的背景下, 本文综述了国外深空探测自主导航技术研究状况, 以及在一些探测任务中的试验和应用情况, 并对每个探测活动进行了简要概括;其次, 本文调研了国外自主导航系统中所采用的光学敏感器设备;最后, 结合"深空1号"任务巡航段基于小行星的光学自主导航, 分析提出了深空探测自主导航中需要掌握的关键技术, 并对相应的技术在国内外研究情况进行了总结.     

Characterisation of martian soil simulants for the ExoMars rover testbed

The European Space Agency (ESA) ExoMars mission involves landing a rover on the surface of Mars on an exobiology mission to extend the search for life. The locomotion capabilities of the ExoMars rover will enable it to use its scientific instruments in a wide variety of locations. Before it is sent to Mars, this locomotion system must be tested and its performance limitations understood. To test the locomotion performance of the ExoMars rover, three martian regolith simulants were selected: a fine dust analogue, a fine Aeolian sand analogue, and a coarse sand analogue. To predict the performance of the ExoMars rover locomotion system in these three regolith simulants, it is necessary to measure some fundamental macroscopic properties of the materials: cohesion, friction angle, and various bearing capacity constants. This paper presents the tests conducted to determine these properties. During these tests, emphasis was placed on preparing the regolith simulants at different levels of density in order to evaluate its impact on the value of the parameters in particular. It was shown that compaction can influence the Bekker coefficients of pressure-sinkage. The shear properties are consistent with the critical state model at normal stresses similar to those of the ExoMars rover in all but one of the simulants, which showed behaviour more consistent with transitional soil behaviour. It is necessary to give due consideration to these variations to ensure a robust test regime is developed when testing the tractive ability of the ExoMars mobility system.     

Advanced mobility in difficult terrain

The development and success of the Swedish Combat Vehicle CV90 has demonstrated the abilities of the author in the field of terramechanics related to tracked military vehicles. The honour of the Bekker–Reece–Radforth Award 2002 has been granted in recognition of these achievements made during the author's employment at Hägglunds Vehicle AB since 1975. Hägglunds Vehicle AB has been a producer of military vehicles since the late 1950s, although the first years concentrated on production only. From the early 1960s, Hägglunds developed a number of its own tracked vehicles, all of which were influenced by the mobility demands dictated by their intended use in severe terrain conditions, such as those found in Northern Scandinavia. This paper presents a brief history of the advancement of tracked vehicle technology at Hägglunds Vehicle AB. The concepts discussed include: ground pressure, the number of road-wheels, articulated steering, track tension, track attack angle, sinkage, belly effects, and the use of terramechanic simulation. The success of the CV90 demonstrates that the combination of practical experience, terrain knowledge, and terramechanic simulations can effect substantial improvements in vehicle mobility. Evaluation of the CV90 versus other modern combat vehicles of the same class has shown that the CV90 possesses considerably higher mobility and speed under severe terrain conditions. These two attributes provide CV90 with the ability to access terrain that similar vehicles cannot, thus giving the military user greater mobility options.     

Bevameter testing on simulant Fillite for planetary rover mobility applications

This paper examines pressure-sinkage and shearing behavior via bevameter testing of a light-weight, granular simulant called Fillite in support of laboratory modeling of rover mobility in high-sinkage, high-slip environments typically found on Mars, the Moon, and other planetary bodies. Normal bevameter test results helped to determine parameters for the Bekker model, the New Model of Mobility (N2M) sinkage model, and the Bekker-Wong model. A case study used the Bekker-Wong model parameters to predict the possible sinkage of 84% into Fillite of a wheel on the Mars Spirit rover, a value within the observed sinkage of 50–90% of the wheel diameter of the Spirit rover on Mars. Shear bevameter testing of Fillite provided a second set of parameters to assess shear behavior, this time simulating the stresses and shear deformations imparted by rotating wheels. The results compared well to the estimated shear stresses and deformations of Martian soil caused by the wheels of the Spirit rover. When compared to other simulants (e.g. GRC-1), the results confirm that Fillite is possibly more suitable for high-sinkage and high-slip rover studies than other typical simulants derived from natural terrestrial soils and rocks.     

行星探测中的大气制动技术研究进展

大气制动技术正成为一种有效的行星探测技术, 该技术利用目的行星的大气阻力作用来减缓探测器的速度以达到预定轨道, 不需要或很少使用携带的燃料进行反推减速入轨, 能够降低行星探测器的发射成本, 增加探测器的入轨有效载荷. 本文描述了大气制动的特点, 以及NASA成功应用大气制动技术的4次行星探测任务, 重点评述了大气制动期间气体动力学特性和轨道模拟的研究情况, 并对大气制动技术的风险性和成本分析进行了总结. 最后, 指出了今后大气制动技术研究和发展的重点.      

The present status and prospects in the research of orbital dynamics and control near small celestial bodies

Small celestial body exploration is of great significance to deep space activities. The dynamics and control of orbits around small celestial bodies is of top priority in the exploration research. It includes the modeling of dynamics environment and the orbital dynamics mechanism. This paper introduced state-of-the-art researches, major challenges, and future trends in this field. Three topics are mainly discussed: the gravitational field modeling of irregular-shaped small celestial bodies, natural orbital dynamics and control, and controlled orbital dynamics. Finally, constructive suggestions are made for China's future space exploration missions.