Analytic calculation of magnetic force between two current-carrying coils

Current-carrying coils are basic elements in electromagnetic equipments, for example, in high field magnets from high temperature superconducting wires or tapes. In the assembly of these systems and their current-carrying operation, unavoidable mis-alignment and shift from the original position can be induced by disturbances such as the imbalance of magnetic force due to safety problems. For two current-carrying coils with non-coplanar axes, the analytic expression of the magnetic force between the two coils is presented according to the rule of Ampere circulation and the Biot-Savart law. Based on the expression, the dependence of the magnetic force on the size and the relative position of each other is further investigated, and the variation of the magnetic force is obtained with the above parameters.     

Experimental study on wheel-soil interaction mechanics using in-wheel sensor and particle image velocimetry part II. Analysis and modeling of shear stress of lightweight wheeled vehicle

Wheeled vehicle mobility on loose sand is highly subject to shear deformation of sand around the wheel because the shear stress generates traction force of the wheel. The main contribution of this paper is to improve a shear stress model for a lightweight wheeled vehicle on dry sand. This work exploits two experimental approaches, an in-wheel sensor and a particle image velocimetry that precisely measure the shear stress and shear deformation generated at the interaction boundary. Further, the paper improves a shear stress model. The model proposed in this paper considers a force chain generated inside the granular media, boundary friction between the wheel surface and sand, and velocity dependency of the friction. The proposed model is experimentally validated, and its usefulness is confirmed through numerical simulation of the wheel traction force. The simulation result confirmed that the proposed model calculated the traction force with an accuracy about 70%, whereas the conventional one overestimated the force, and its accuracy was 13% at the best.     

Ram extrusion force for a frictional plastic material: model prediction and application to cement paste

We developed a model to predict the ram extrusion force of frictional plastic materials such as cement-based pastes. The extrusion of cement-based materials has already been studied, but the interaction between shaping force and paste behaviour still have to be understood. Our model is based on the plastic frictional behaviour of cement-based materials and integrates the physical mechanisms that govern material extrusion flow and extrusion force increase. When the process starts, a pressure gradient is created in the extruder due to wall friction of the paste that is submitted to plug flow. It induces a consolidation of the material. As a result, a large increase of extrusion force appears. A Coulomb law is used to model cement-based materials, which is considered as consolidating granular media. Such modelling is compared with experimental results. Tests were carried out on extrudible cement pastes. Modelling and experimental results are in good agreement.Paper presented at the Annual European Rheology Conference, Grenoble, April 2005     

波浪作用下柔性草本植物受力特性研究

草本盐沼植物广泛分布于海岸带形成滨海湿地系统,这些滨海湿地具有生态、旅游、养殖及减灾等多方面价值.草本植物的消浪作用在海岸稳固和沿海防灾减灾等方面发挥了重要作用.已有植物消浪研究通常过度简化,将植物视为刚性圆柱并依赖于调节经验拖曳力系数来提高预测准确度,对柔性植物与波浪的相互作用机理研究还很欠缺.草本植物通常由多片柔性...     

海上风机动冰力模型试验相似律研究

相似律在模型试验中发挥着非常重要的作用,虽然目前静冰力模型试验的研究比较全面,但还没有一个完整适用的动冰力相似律作为动冰力模型试验基础.本文针对冰激结构振动中动冰力模型试验相似律,基于弹性相似与柯西(Cauchy)相似的联合相似准则,提出了两种动冰力相似律.建立并改进了能够反映冰与结构水平相互作用的相互作用系数.以NR...     

Penetration of Cylindrical Projectiles into Saturated Sandy Media

The time and depth of vertical one-dimensional projectile penetration into sandy media in the near shore region are derived. A precise definition for the physical properties and for the behavior of the sandy medium following the projectile impact are evaluated. Three separate time intervals following projectile impact are identified. During the first 3 ms of penetration, the deviatoric friction stress is shown to be negligible and the integrated Mie–Grüneisen equation of state (or, equivalently, the Hugoniot-adiabat) may be applied to compute the normal penetration resistance force from the sand pressure. In order to compute sand pressure as a function of the sand density D by the integrated Mie–Grüneisen equation of state, the Mie–Grüneisen dimensionless constants γ₀ and s and the dimensional speed of sound C ₀ in the sandy medium are required. In order to illustrate the one-dimensional shock wave propagation in both wet and dry sands, Hugoniot data for wet and dry silica sands are evaluated by a three degrees of freedom algorithm to compute these required constants. The numerical results demonstrate that the amplitude of the shock wave pressure in the wet silica sand (41% porosity) is approximately one-third of the shock wave pressure amplitudes in the dry silica sands (22% and 41% porosity). In addition, the shock wave pressure dampens quicker in the wet sand than in the dry sands.     

The Effect of Limb Kinematics on the Speed of a Legged Robot on Granular Media

Achieving effective locomotion on diverse terrestrial substrates can require subtle changes of limb kinematics. Biologically inspired legged robots (physical models of organisms) have shown impressive mobility on hard ground but suffer performance loss on unconsolidated granular materials like sand. Because comprehensive limb–ground interaction models are lacking, optimal gaits on complex yielding terrain have been determined empirically. To develop predictive models for legged devices and to provide hypotheses for biological locomotors, we systematically study the performance of SandBot, a small legged robot, on granular media as a function of gait parameters. High performance occurs only in a small region of parameter space. A previously introduced kinematic model of the robot combined with a new anisotropic granular penetration force law predicts the speed. Performance on granular media is maximized when gait parameters utilize solidification features of the granular medium and minimize limb interference.     

鸭式旋翼/机翼飞机悬停及小速度前飞气动干扰实验研究

鸭式旋翼/机翼飞机是一种新概念可垂直起降高速飞行器,为了解该飞机在悬停及小速度前飞时的全机气动干扰特性,在南京航空航天大学开口风洞中进行了飞机全机气动力实验,实验采用多台测力天平分别测量主机翼和机身的气动力.结果表明,悬停时受主机翼高速旋转产生的下洗尾流影响,机身产生了较大的法向力和低头力矩;前飞时下洗尾流对机身的法向力和俯仰力矩有比较严重的干扰,对滚转力矩和偏航力矩干扰较小,对侧向力有一定影响.实验结果为飞机的飞行动力学特性研究以及控制律设计提供了参考.＃     

基于有限时间收敛的双臂空间机器人捕获卫星主动对接力/位姿阻抗控制

针对双臂空间机器人捕获卫星主动对接力/位姿阻抗控制进行了研究.为防止捕获过程中机械臂末端执行器与卫星接触、碰撞时产生的冲击载荷对机器人关节造成冲击破坏,在各关节电机与机械臂之间加入了一种弹簧阻尼缓冲机构.该机构可通过弹簧实现冲击力矩的卸载,阻尼器则用于因弹簧引起的柔性振动的抑制.为解决捕获过程中的非完整动力学约束及捕获后混合体系统的协调控制问题,结合牛顿第三定律、捕获点的速度约束及闭链几何约束,获得捕获后混合体系统的动力学方程,且通过动量守恒关系计算碰撞冲击效应与碰撞冲击力.通过分析对接装置在载体坐标系下的运动学关系,建立对接装置相对载体的运动雅可比矩阵,并基于此建立基于力的二阶线性阻抗模型,实现对接装置输出力的精确控制.考虑到主动对接操作过程要求控制器具有收敛速度快,控制精度高的特点,通过结合终端滑模与超扭滑模的特点,提出一种非奇异快速终端滑模阻抗控制策略.该策略即能实现主动对接操作中位姿与输出力的快速响应,又能有效地抑制滑模的抖振以保证控制精度.通过Lyapunov定理证明系统的稳定性;利用数值模拟验证缓冲装置的抗冲击性能及所提阻抗控制策略的有效性.     

钙质砂的SHPB实验技术及其动态力学性能

开展了11组南海钙质砂和福建石英砂的分离式霍普金森压杆（SHPB）实验,试样相对密实度为90%,厚度分别为10、30和50 mm,得到了冲击荷载下钙质砂和石英砂的应变率时程曲线、应变时程曲线和应力应变关系。实验结果表明：通过严格装样技术可以减小实验设备产生的误差,改变试样厚度、子弹长度、整形器等是实现钙质砂应力平衡和恒应变率的主要手段。在相同的密实度和加载条件下,钙质砂的体积模量和剪切模量约为石英砂的10%,压缩强度和抗剪强度约为石英砂的30%。冲击荷载作用下钙质砂的动态力学性能与石英砂存在较大的差异,因此不能将已有石英砂的研究结果直接用于钙质砂。

     

喷砂冲蚀实验中颗粒轨迹的数值预测

针对冲击磨损实验研究中磨粒群体的运动轨迹难以准确表征的问题,在负压喷射砂粒群冲击Q235钢板的实验中宏观测量了砂粒撞击的速度与位置分布,并使用数值方法模拟了实验砂粒与空气在喷嘴内外的双向耦合过程,以实现负压喷射砂粒群的轨迹预测。计算中提出了非球形粒子在相对马赫数接近1时的曳力模型,以反映空气可压缩引起砂粒表面流动分离的现象,并合理选择Magnus升力模型及壁面反射模型,最终数值预测的砂粒碰撞速度以及撞击位置与实验情况吻合良好。     

石英岩表面分子沉积膜的微观摩擦性能的试验研究

利用原子力显微镜对石英岩表面单层分子沉积膜的微观摩擦特性进行了研究,发现该分子沉积膜具有一定的减摩性.通过对其表面力-位移曲线、表面形貌像、调制力像和摩擦力像的进一步分析表明,石英岩表面分子沉积膜具有减摩作用的原因在于它能够降低表面的粘着力并对表面具有微观修饰作用.     

Effect of thermal diffusion and electrostatic force on evolution of wind-blown sand flow

A theoretical model is suggested to mathematically describe the effect of thermal diffusion from a sand-bed on evolution of a wind-blown sand flow.An upward wind field is engendered by the thermal diffusion and the coupling interaction among the horizontal and upward wind flow,saltating grains,and a kind of electrostatic force exerted on the grains are considered in this theoretical model.The numerical results show that the effect of the thermal diffusion on the evolution process of wind-blown grain flow is quite obvious and very similar to the effect of the electrostatic force on the evolution.Not only the time for the entire system to reach a steady state(called the duration time),the transport rate of grains,the mass-flux profiles and the trajectory of saltating grains are affected by the thermal diffusion and the electrostatic force exerted on saltating grains, but also the wind profiles and the temperature profiles at the steady state are affected by the wind-blown sand flow.     

钙质砂介质中爆炸波传播规律的试验研究

开展了一系列钙质砂和石英砂的地面爆炸试验，主要对比分析了两种砂土介质中爆炸波的传播规律，包括峰值压力、弹塑性波速及升压时间、爆坑尺寸等。试验结果表明，爆炸波在钙质砂中的传播与在石英砂中存在较大差异：地面爆炸作用下钙质砂爆坑较石英砂爆坑的直径和深度更小，且成坑形状为两阶同心圆；钙质砂中弹性波速为236～300 m/s，石英砂中弹性波速为218～337 m/s，弹性波速和塑性波速均随炸药质量增加而增大；爆炸波在钙质砂中的升压时间随比例距离的增加而增加，而在石英砂中升压时间随比例距离变化不明显，且较钙质砂中升压更迅速；在地面爆炸作用下，低含水率钙质砂的衰减系数为2.86，石英砂为2.79。     

Modeling track-terrain interaction for transient robotic vehicle maneuvers

This article describes integration of a realistic and efficient track-terrain interaction model with a multibody dynamics model of a robotic tracked vehicle. The track-terrain interface continuum is approximated by discretized and parameterizable force elements. Of particular note is a kinematic model used to estimate dynamic shear displacement, taking the form of a partial differential equation. This equation is approximated by a series of ordinary differential equations, making it compatible with multibody dynamics model formulations. Comparisons between simulated results and those obtained from field testing with a remotely-operated unmanned tracked vehicle are made to evaluate the effectiveness of this approach and to validate the use of nominal parameter data from the literature. The test vehicle was subjected to four different types of maneuvers (go-and-stop, j-turn, double lane change, and zero radius turn) on asphalt and dry sand. Simulated results using both the dynamic and steady-state track-terrain interaction models match very well with those obtained from the tests, except for the zero radius turning maneuver in sand. In this case, bulldozing effects must be incorporated to improve prediction of lateral forces.     

双塔联体分幅斜拉桥塔结构模型实验索力测试结果分析

以甬江特大斜拉桥为工程背景,对其整个联塔结构进行室内模型试验研究.针对模型试验中加载斜拉索的布置位置、数量及索力的确定,提出了最少索数的索力优化方法,编制了相应的数值程序.在满足预期目标的前提下,以最少斜拉索数量为原则,确定了试验中斜拉索的优化布置及相应的优化索力及各级加载索力,对优化索力和试验中的测试索力进行比较分析,结果表明索力优化方法可行,两联塔之间索力相互影响较小.     

Compliance characteristic and force control of antagonistic actuation by pneumatic artificial muscles

This paper presents compliance characteristics of antagonistic actuation by the pair of Mckibben pneumatic artificial muscles and force tracking using a sliding control scheme for safe human-robot interaction. The variable stiffness capability of artificial muscles was investigated carefully by resilience tests from a biased initial position and impact tests based on an intended collision between a stationary object and a rotating linkage actuated by pneumatic artificial muscles. Considering the frequency response analysis of a whole pneumatic circuit for artificial muscles, a sliding control system was designed to control contacting force between a linkage actuated by artificial muscles and a rigid environment. Experimental results showed that the proposed force control scheme gave better tracking performance under model uncertainties due to air flow than the conventional PID controller whose feedback gains were well-tuned experimentally.     

Tangential interaction of elastic spherical particles in contact

The elastic interaction of spherical particles is studied. The distribution of the stress, normal to the contact plane, is determined by the rod model suggested recently, which is applicable in the more wide range of deformations as compared with the classical Hertz law. In the rod model context an inner part of compressed particles is regarded as an elastic cylindrical rod, which radius is equal to the contact radius. The rod reaction is added to the normal particle interaction corresponding with the Hertz solution. The resulting normal force passes into the Hertz solution for infinitesimal deformations and gives stronger particle repulsion for finite deformations. Here we solve the Mindlin problem for the rod model, i.e., derive the tangential interaction of initially compressed particles when a relative displacement takes place. The analytical expressions, which determine the total displacement of the sphere’s centers and the corresponding tangential force, are derived. So, the generalization of the classical Mindlin law is obtained for the rod model.     

饱和粉细砂层变形对锚索预应力影响规律研究

土层变形是造成预应力值波动甚至失效的重要原因. 利用载荷位移传递函数,基于弹性理论建立了饱和粉细砂层变形过程中锚索受力模型,通过实例研究了锚索预应力值在施工过程中的变化规律及锚固体在地层变形中的受力特性,结果表明锚固体接触面剪切刚度,锚索倾斜角度以及锚固体直径都是影响锚固体内力及接触面剪切应力的重要因素,为类似的预应力锚索支护工程的设计施工提供经验.