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.      

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.      

机器人在轨组装结构的耦合动力学与步态优化

机器人在轨移动组装空间结构是建造大型航天器最有潜力的方式之一,但机器人在结构表面作业时两者存在严重的动力学耦合效应,给空间结构的建造带来了新挑战.针对三分支机器人行走在空间柔性结构上形成的耦合动力学问题,提出一种机器人-结构耦合动力学建模与步态优化方法.首先,基于拉格朗日方程和欧拉-伯努利梁模型建立机器人-结构耦合动力学模型,该模型可用于预测机器人在结构表面行走时的耦合动力学响应.然后,基于耦合动力学方程推导出机器人运动与结构振动的关系,以降低结构振动响应为目标开展了机器人行走步态的优化研究.最后,对机器人不同蠕动步态运动方式下的空间结构动力学响应进行了数值仿真,重点分析了机器人以不同步频、不同步长以及不同抬起高度行走移动时对空间结构动力学响应的影响规律.仿真结果表明,空间结构的动力学响应与机器人的运动方式密切相关.因此在设计行走移动组装机器人的运动步态步频时应避免为空间结构固有频率的两倍,同时在保障机器人组装安全稳定的前提下应尽可能减小运动步长和抬起高度.并且通过对机器人运动步态进行优化调整可以有效抑制空间结构的振动.     

燃料大幅晃动等几何分析仿真及航天器耦合动力学研究

现代航天器肩负许多周期长且复杂的航天任务,通常需要携带大量的液体燃料.贮箱中液体燃料大幅晃动会严重影响航天器的姿态稳定性和控制精度,是现代航天器耦合动力学建模和精确控制研究的重要问题.本文提出了一种新的液体大幅晃动数值仿真方法,采用等几何分析方法对贮箱内气体和液体整体进行建模和空间离散,采用压力修正的分步法对控制方程进行时间离散,结合水平集方法划分气体和液体区域并且实时追踪液体晃动自由面.提出了一种质量修正方法以消除水平集函数演化产生的液体质量误差.基于燃料大幅晃动等几何分析仿真方法,对携带太阳能帆板的充液航天器进行动力学建模和耦合运动数值仿真.对于太阳能帆板的振动问题则采用Kirchhoff-Love板理论建模和模态分析法数值求解.通过将数值仿真结果与解析解对比,证明了本文给出方法的正确性.本文还对燃料大幅晃动下的航天器刚-液-柔耦合运动进行了数值仿真,发现液体晃动对航天器的姿态变化和结构振动的幅值和频率具有不可忽视的影响.     

明渠层流近床面颗粒拖曳力研究

泥沙颗粒受到的拖曳力是泥沙运动的主要驱动力,而当前应用于计算流体力学-离散颗粒法(CFD-DPM)耦合模型进行水沙运动模拟的泥沙颗粒拖曳力公式均没有考虑明渠流底床边壁作用的影响。求解不可压缩Navier-Stokes方程,对明渠层流不同雷诺数条件下床面附近不同高度处颗粒所受拖曳力进行了模拟,根据模拟结果变化规律,提出了综合考虑床面和水流惯性对标准拖曳力影响的修正拖曳力计算公式。与常用的单颗粒标准拖曳力公式和考虑遮蔽效应的多颗粒拖曳力公式相比,采用本文修正公式得到的水沙作用力更接近高精度数值解,应用于CFD-DPM输沙模拟获得的输沙结果与输沙率公式结果一致,应用分析表明输沙模拟应当采用粗糙底床边界。     

不规则小行星引力场内的飞行动力学

小行星探测是当前深空探测的主要方向之一,具有重要的科学意义.绝大多数小行星引力场极不规则,探测器在小行星附近运动形态复杂多样.由于同时受到中心引力、快速自旋的不规则形状摄动力、以及光压摄动等作用,探测器容易与小行星发生碰撞或逃逸.概述小行星研究现状和不规则引力场建模方法.重点介绍不规则引力场内动力学特性,包括引力平衡点、局部流形、自然周期轨道和悬停探测轨道等,尝试提出新的研究方向.     

仿青蛙机器人跳跃之力学分析与轨迹优化

目前,相比传统的轮式机器人,离散性运动的跳跃机器人具有更好的移动性能,得到了广泛的应用.本文从仿生学角度出发,选择具有优秀跳跃性能和合理运动结构的青蛙作为研究对象,建立了平面连杆机构仿青蛙机器人跳跃模型.在该模型的基础上,建立了仿青蛙机器人起跳阶段的运动学和动力学模型,得到了起跳阶段机器人各关节力矩与各关节运动轨迹,质心速度,质心加速度间的关系.结合遗传算法,对机器人在起跳阶段的跳跃轨迹进行了优化.在此基础上,从能量角度出发,对机器人在腾空阶段的跳跃轨迹进行了优化,得到了机器人最佳的跳跃轨迹.结合Matlab仿真分析,验证了方案的正确性和可行性.     

不规则小行星引力场内的飞行动力学

小行星探测是当前深空探测的主要方向之一,具有重要的科学意义.绝大多数小行星引力场极不规则,探测器在小行星附近运动形态复杂多样.由于同时受到中心引力、快速自旋的不规则形状摄动力、以及光压摄动等作用,探测器容易与小行星发生碰撞或逃逸.概述小行星研究现状和不规则引力场建模方法.重点介绍不规则引力场内动力学特性,包括引力平衡点、局部流形、自然周期轨道和悬停探测轨道等,尝试提出新的研究方向.     

Flight dynamics over irregular asteroids

小行星探测是当前深空探测的主要方向之一,具有重要的科学意义.绝大多数小行星引力场极不规则,探测器在小行星附近运动形态复杂多样.由于同时受到中心引力、快速自旋的不规则形状摄动力、以及光压摄动等作用,探测器容易与小行星发生碰撞或逃逸.概述小行星研究现状和不规则引力场建模方法.重点介绍不规则引力场内动力学特性,包括引力平衡点、局部流形、自然周期轨道和悬停探测轨道等,尝试提出新的研究方向.     

考虑道砟侵切土质路基的有砟轨道离散元-有限元耦合方法

论文考虑有砟轨道道床结构中底层道砟颗粒会逐渐侵切土质路基上表层的实际情况,引入基于双曲线函数的土阻力公式,发展了离散元颗粒与有限元网格体内耦合的接触力算法,建立了考虑道砟颗粒侵切路基表层的有砟轨道离散元-有限元耦合模型.其中道床中的非规则形状道砟采用离散元镶嵌单元构造,路基采用有限元20节点六面体实体单元构造.分析了道砟侵切路基下列车荷载及路基软硬程度对道床整体沉降的影响,绘制了有砟道床-路基的横截面应力及位移云图.研究表明提出的耦合模型不仅能反映道床自身的弹性变形和累积塑性变形,还能真实反映底层道砟侵切软土路基表层所导致的侵切沉降,为深入了解有砟道床的沉降劣化机理提供重要借鉴作用.     

Research in mechanical model of bionic foot intruding into sands with different physical characteristics

In this study, a bionic foot with sand fixation and fluidization limitation functions was designed. Also a rectangular foot with the same sizes, named the common foot, was designed for comparison. Three kinds of quartz sands were selected to study how particle size, shape and compactness affected the intrusion performances of mechanical feet. The intrusion resistive forces and pressures of the bionic foot on these three kinds of quartz sands were all smaller compared with the common foot. Discrete element simulations showed particle disturbance areas were smaller and particle motion trends were more consistent under the bionic foot versus the common foot. The intrusion resistive forces of these two kinds of mechanical feet firstly increased and then decreased with the increasing particle sizes of quartz sands. Moreover, the intrusion resistive force on spherical particles was less than that of irregular particles for both the bionic foot and the common foot. The corresponding resistive forces of mechanical feet were characterized based on quartz sand compactness. The classic pressure-sinkage model was modified based on the intrusion tests, and the relationships between intrusion resistive force and mechanical foot depth were obtained.     

Three-dimensional dynamic imaging of sand particles under wheel via gamma-ray camera system

In recent years, attempts have been made to deploy robots for use in various activities such as planetary exploration, post-tsunami seashore reconnaissance, and volcano investigations. These robots may have to move on soft terrain. The movement of sand or soil particles under the wheels or tracks greatly affects the robot’s ability to maneuver. There is a simple but difficult problem with measuring particle movement: the sand and soil particles beneath the surface are not visible. Only 2D visualization techniques that take a surface picture of the ground or use transparent boards are available. A nuclear 3D imaging technique called positron emission particle tracking (PEPT) was developed at the University of Birmingham for this purpose. PEPT detects pairs of gamma rays emitted by a positron-emitting radionuclide of a tracer particle, which produces an image of the tracer. Thus, the overarching goal of this study was to explore the 3D terramechanics between terrain particles and a wheel or track using PEPT. As an initial step, this paper introduces an imaging technique for standard sand under a rotating wheel using PEPT and presents some images of sand particles under various conditions. Absolute displacements along the longitudinal, vertical, and lateral axes are presented.     

Sand particle dislodgement in windblown sand

The incipient motion of sand particle from sand bed plays a very important role in the prediction of windblown sand.In this paper,we proposed a new method for predicting the incipient motion of sand particle based on wind speed fluctuation as follows,when the wind speed is larger than the critical wind speed,if the total impulse on sand particle is larger than the critical impulse,incipient motion of sand particle would take place,otherwise if not.Furthermore,from the analysis of entrainment in the rolling and lifting modes,we come to the following conclusion.When the average wind speed is smaller than the critical wind speed,if the average wind speed is used to judge the incipient motion of sand particle,one will underestimate the number of sand particles jumping from the bed,if the instantaneous wind speed is used to judge incipient motion of sand particle,one will overestimate the number of sand particles jumping from the bed;When the average wind speed is larger than the critical wind speed,either the average or the instantaneous wind speeds is used to judge the incipientmotion of sand particles,one will overestimate the number of sand particles jumping from the bed.     

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.     

沉积方向和粒径组成对砂土力学特性的影响

运用砂土毛细效应所形成的假凝聚力制备具有不同沉积方向的试样,开展三轴固结排水剪切试验,研究粒径组成对初始各向异性砂土力学特性的影响,分析了粒径组成、沉积方向、强度特性和体积变化之间的相互关系.研究结论表明,试样剪切带的形成受到颗粒级配的影响,它是应力应变关系中陡降阶段的产生原因.沉积方向对粗砂和细砂的偏应力峰值强度均有...     

Biomechanical and dynamic mechanism of locust take-off

The biomimetic locust robot hopping vehicle has promising applications in planet exploration and reconnaissance. This paper explores the bionic dynamics model of locust jumping by using high-speed video and force analysis. This paper applies hybrid rigid-flexible mechanisms to bionic locust hopping and studies its dynamics with emphasis laid on the relationship between force and jumping performance. The hybrid rigid-flexible model is introduced in the analysis of locust mechanism to address the principles of dynamics that govern locust joints and mechanisms during energy storage and take-off. The dynamic response of the biomimetic mechanism is studied by considering the flexi- bility according to the locust jumping dynamics mechanism. A multi-rigid-body dynamics model of locust jumping is established and analyzed based on Lagrange method; elastic knee and tarsus mechanisms that were proposed in previous works are analyzed alongside the original bionic joint configurations and their machinery principles. This work offers primary theories for take-off dynamics and establishes a theoretical basis for future studies and engineering applications.     

Studies of dynamics of a lightweight wheeled mobile robot during longitudinal motion on soft ground

The paper is concerned with the study of longitudinal motion of a lightweight wheeled mobile robot on soft ground. The study is focused on the influence of the desired longitudinal velocity of a robot on both the longitudinal slip of the wheels and the ratio of wheel-terrain contact angles. Design of the four-wheeled skid-steered robot and research environment are described. Experimental investigations were conducted on a dedicated test stand with dry sand. A dynamics model of the robot-ground system taking into account properties of soft ground is presented. The classical terramechanics models of Bekker and Janosi-Hanamoto are used. Results of simulation research of robot motion and of the analogous experimental investigations are presented. Actual motion parameters of the robot and the values of longitudinal slip ratio of the wheels are determined. The results of simulation and experimental investigations are compared and discussed. A formula to describe front-to-back wheel-terrain contact angle ratio dependency on the desired velocity is proposed.     

饱和砂土的三剪弹塑性边界面模型研究(一)——模型理论

基于边界面模型方法研究了不同密实度和含泥量的饱和砂土在静、动荷载作用下的力学和变形特性。其中,初始边界面方程是通过改进修正剑桥模型椭圆形屈服方程得到的,后继边界面方程则是在前次边界面方程的基础上采用中心点映射法获得。针对原模型破坏应力比为定值,不能反映全应力状态和黏聚力在土体受力过程中的作用这一问题,基于三剪统一强度准则,通过等量代换法、坐标平移法对破坏应力比、相变应力比进行了修正,使修正后的椭圆形边界面适用于不同密实度和含泥量的饱和砂土。在此基础上应用边界面理论,研究了饱和砂土的三剪弹塑性边界面本构模型及有限元算式。该模型能够描述密实砂土在受力时表现的剪胀特性,同时可以反映中间主应力效应、应力区间效应、拉压差效应和黏聚力对应力-应变特性的影响。     

Determination of soil parameters using plate test

Previous plate-sinkage relations used in different fields of applications are reviewed. A modified formula which takes into consideration the effect of both plate size and shape is developed. Sensitivity of the new formula to variations in soil shear strength parameters and, consequently, to the soil response is examined. Application to sandy and clayley sand soils, together with the manner in which the plate-predicted soil parameters reflect the behaviour of the soil types and conditions, are carefully studied. Comparisons are made between these soil parameters and the corresponding ones associated with other similar formulae. Predicted soil responses compared favorably with the measured results.     

饱和砂土动本构理论研究进展

饱和砂土由于它对动载荷作用的特殊敏感性在土动力学和土工抗震问题研究中占据了重要的地位,本文介绍了饱和砂土的基本动力学特性,并对当前饱和砂土动本构理论的研究进展作了较为系统的评述,以期对进一步的研究有所稗益．