汽车转向机构间隙运动副的碰撞接触分析

应用多体动力学分析方法及仿真技术研究了某汽车转向机构间隙运动副在转向过程中的碰撞接触特性.首先应用ADAMS软件创建了该车的仿真模型.由悬架系统、转向系统、行驶系统等系统纰成.其次研究了『H]隙运动副的碰撞接触特性,包括转向横拉杆与转向梯形臂之间的运动副及转向梯形臂与悬架之间的运动副.根据实际工作特征,前者运动副采用空间圆柱铰模型,后者运动副采用平面圆柱铰模型.最后对模型进行了仿真分析.仿真结果表明,转向过程中,两问隙运动副的碰撞接触力与间隙大小成非线性关系,多个运动副间隙共存状念下各间隙运动副的碰撞接触力发生变化.这些研究结果为转向机构零部件设计、动力学特性分析提供了更为准确的实验依据.     

含摩擦滑移铰及驱动约束多刚体系统数值算法

研究了具有驱动约束及非光滑滑移铰多体系统动力学方程的建模与数值计算方法．将驱动约束视为非定常约束，非光滑滑移铰视为双边定常约束，滑移铰的摩擦模型采用库仑摩擦模型；应用第一类Lagrange方程建立系统的动力学方程，应用距离函数建立滑移铰的约束方程；将线性互补方法和Baumgarte约束稳定化方法引入，以解决滑移铰法向约束力的计算以及约束方程违约问题．最后应用曲柄摇杆机构作为算例，说明该方法的有效性．     

基于Karnopp摩擦的柔性滑移铰的建模与仿真

对含Karnopp摩擦的柔性滑移铰系统进行动力学建模和仿真.将滑移铰中的滑块视为柔性体,滑道视为刚性接触面,考虑滑道与滑块之间的间隙.由于柔性滑块与滑道的接触状态和摩擦情况比较复杂,采用有限元方法建立了柔性滑块的力学模型,基于罚函数方法建立含Karnopp摩擦柔性滑移铰接触力模型,通过试算迭代法判断柔性滑块各节点的接触状态,基于KED方法和Newmark方法给出了含该滑移铰机械系统动力学方程的数值算法.最后,以含Karnopp摩擦的柔性滑移铰和驱动摆杆构成的机械系统为例进行动力学仿真,分析了其动力学特性,验证了本文给出的方法的有效性.     

滑动电接触摩擦力的BP与RBF人工神经网络建模

在弓网系统中,受电弓滑板与接触网导线之间的摩擦力受机械、电气以及它们之间的交互作用影响。摩擦力与运行速度、接触电流以及受电弓滑板和接触网导线之间的压力载荷有着密切关系。通过对浸金属碳销（滑板）与铜盘（导线）的载流摩擦实验,得出在不同载流、速度以及载荷条件下的摩擦力特性规律。因摩擦力较难实现数学建模,故采用神经网络中应用最广的BP及RBF算法分别建立了以摩擦力作为输出,以接触压力、滑动速度和接触电流为输入的预测模型,并通过Matlab进行仿真与测试。结果表明：两种算法建立的预测模型均有较高的准确度及良好的泛化能力,为进一步波动载荷下弓网滑动电接触摩擦力预测建模的应用提供参考。     

履带车多刚体建模与仿真分析

首先以车辆地面力学理论为基础,分析了典型的履带与地面相互作用关系及其在RecurDyn/Track(LM)程序中的处理,分别定义了两种典型地面土力学参数,从而建立了基于Bekker理论与Janosi & Hanamoto理论的循环动载荷作用下履带与地面相互作用力学模型;同时分析了履带系统各部件之间的接触力学模型,定义各接触参数及约束关系,从而准确合理地建立了履带车多刚体模型,通过施加运动函数,实现模型在典型地面参数下的行走特性仿真分析,仿真结果与理论分析及实际情况相符合,互为验证,从而为进一步深入研究提供了理论依据与有效分析手段.     

含铰链间隙的刚-柔机械臂动力学模型

根据Hertz接触定律和Coulomb摩擦定律,建立了含间隙平面旋转铰的力学模型;采用几何变形约束法和模态缩聚技术描述柔性机械臂的非线性变形;同时考虑两个旋转铰的间隙特性和柔性臂的弹性变形,最终采用Kane方程建立了含铰链间隙的刚-柔机械臂系统的动力学模型．     

具有积分二次约束属性的网络控制系统H∞容错控制

针对一类具有积分二次约束属性的时滞网络控制系统,研究了系统在不等间隔采样下的H_∞容错控制.首先,分析并处理了网络控制系统中时滞与不等间隔采样之间的关系.以此为基础,建立了不等间隔采样下的不确定时滞网络控制系统的切换模型.通过构造Lyapunov函数,运用H_∞控制方法,以及设计具有积分二次约束属性的控制器,获得了基于LMI描述的系统时滞依赖稳定性判据.然后,建立了网络控制系统的传感器故障模型和传感器故障下的网络控制系统数学模型.通过把结论运用到传感器出现故障的网络控制系统,得到了系统在H_∞容错控制下的稳定性判据.最后,通过与其他文献介绍的方法进行比较的一个例子,说明了该方法的有效性.     

LNG船液舱围护系统安装平台简化分析模型与优化

为实现LNG船液舱围护系统安装平台的最优化设计,分别用MSCPatran和MSC Nastran建立三维模型、进行静力分析;根据力学等效原理将三维模型简化为二维模型,进行变形和支座反力分析计算,依据三维模型的计算结果对二维模型进行修正,使简化的二维模型在不同工况下的变形和支座反力与三维模型的基本一致;最后进行二维模型的灵敏度分析,挑选出对结构质量影响较大的截面尺寸作为设计变量,以节点位移作为约束条件,以结构质量最小化为目标函数,对二雏模型进行结构优化,使结构质量最小．结果表明该方法使安装平台在满足位移约束要求的前提下极大地减轻结构质量,达到优化目的．     

基于恢复系数的碰撞过程模型分析

论述了恢复系数的含义及作用,并在此基础上介绍了几种碰撞过程模型.通过详细推导恢复系数与模型参数之间的关系,使得不同的碰撞过程模型可统一用恢复系数表示能量损失,并用接触刚度表示变形.这也阐明了碰撞过程模型与刚性模型之间的区别和联系,把动态接触理论和古典碰撞理论统一了起来.通过对一个单球碰撞系统进行数值仿真,不仅验证了关系推导的正确性,而且对各种模型从精度、效率、微观接触过程等方面进行了比较.     

基于Kerberos的高兼容性桥接信任模型

分布式网络环境下,信息系统之间安全认证成为关注的热点。各个信息系统利用现有认证技术良好地保证了系统自身的安全,但信任机制和安全策略的不同导致了信息系统之间在协同工作时无法建立信任关系。本文通过对现有成熟的安全信任模型的分析比较提出一种具有较高兼容性和可扩展性的信任模型,以求在不改变原有系统安全体系架构的前提下,有效地在异构系统之间建立信任关系。     

Investigation of joint clearance effects on the dynamic performance of a planar 2-DOF pick-and-place parallel manipulator

In this study, the effects of joint clearance on the dynamic performance of a planar 2-DOF pick-and-place parallel manipulator are investigated. The parallel manipulator is modeled by multi-body system dynamics. The contact effect in revolute joints with clearance is established by using a continuous analysis approach that is combined with a contact force model considering hysteretic damping. The evaluation of the contact force is based on Hertzian contact theory that accounts for the geometrical and material properties of the contacting bodies. Furthermore, the incorporation of the friction effect in clearance joints is performed using a modified Coulomb friction model. By numerical simulation, variations of the clearance joint's eccentric trajectory, the joint reaction force, the input torque, the acceleration, and trajectory of the end-effector are used to illustrate the dynamic behavior of the mechanism when multiple clearance revolute joints are considered. The results indicate that the clearance joints present two obvious separation leaps in a complete pick-and-place working cycle of the parallel manipulator, following a collision. The impact induces system vibration and thus reduces the dynamic stability of the system. The joint clearances affect the amplitudes of the joint reaction force, the input torque, and the end-effector's acceleration, additionally the joint clearances degrade the kinematic and dynamic accuracy of the manipulator's end-effector. Finally, this study proposes related approaches to decrease the effect of joint clearances on the system's dynamic properties for such parallel manipulator and prevent “separation-leap-impact” events in clearance joints.     

Recursive formulations for multibody systems with frictional joints based on the interaction between bodies

In practice, the clearances of joints in a great number of mechanical systems are well under control. In these cases, some of the existing methods become unpractical because of the little differences in the order of magnitude between relative movements and computational errors. Assuming that the effects of impacts are negligible, we proved that both locations and forces of contacts in joints can be fully determined by parts of joint reaction forces. Based on this fact, a method particularly suited for multibody systems possessing frictional joints with tiny clearances is presented. In order to improve the efficiency of computation, recursive formulations are proposed based on the interactions between bodies. The proposed recursive formulations can improve the computation of joint reaction forces. With the methodology presented in this paper, not only the motion of bodies in a multibody system but also the details about the contacts in joints, such as forces of contacts and locations of contact points, can be obtained. Even with the assumption of impact free, the instants of possible impacts can be detected without relying upon any ambiguous parameters, as indicated by numerical examples in this paper.     

Frictional contact analysis of spatial prismatic joints in multibody systems

The contact analysis of spatial prismatic joints remains a hard problem due to its complex nature. In this paper, a methodology for the frictional contact analysis of rigid multibody systems with spatial prismatic joints is presented, which is free of calculating the relative motion between the slider and guide, and is particularly suitable to the case of clearances being tiny. Under the assumption of the slider and guide being rigid, we prove that all types of contacts in the joint can be converted to point-to-point contacts. At each of the candidate points, two gap functions are introduced. However, in the proposed method, not the values of these gap functions but the relations between them are essential. In view of the non-colliding contacts being predominant when clearances of joints are tiny, we formulate the contact forces in terms of resultant frictional forces in the joint, resulting in a linear complementarity problem. By the proposed method, details about the contacts including the impact instants can be obtained, although impacts are not taken into consideration explicitly, as indicated by the numerical examples in this paper.     

Dynamic Analysis for Planar Multibody Mechanical Systems with Lubricated Joints

The dynamic analysis of planar multibody systems with revolute clearance joints, including dry contact and lubrication effects is presented here. The clearances are always present in the kinematic joints. They are known to be the sources for impact forces, which ultimately result in wear and tear of the joints. A joint with clearance is included in the multibody system much like a revolute joint. If there is no lubricant in the joint, impacts occur in the system and the corresponding impulsive forces are transmitted throughout the multibody system. These impacts and the eventual continuous contact are described here by a force model that accounts for the geometric and material characteristics of the journal and bearing. In most of the machines and mechanisms, the joints are designed to operate with some lubricant fluid. The high pressures generated in the lubricant fluid act to keep the journal and the bearing surfaces apart. Moreover, the lubricant provides protection against wear and tear. The equations governing the dynamical behavior of the general mechanical systems incorporate the impact force due to the joint clearance without lubricant, as well as the hydrodynamic forces owing to the lubrication effect. A continuous contact model provides the intra-joint impact forces. The friction effects due to the contact in the joints are also represented. In addition, a general methodology for modeling lubricated revolute joints in multibody mechanical systems is also presented. Results for a slider-crank mechanism with a revolute clearance joint between the connecting rod and the slider are presented and used to discuss the assumptions and procedures adopted.     

Analysis of Joint Clearances in Multibody Systems

A methodology for the study of typical smooth joint clearances in multibody systems is presented. The proposed approach takes advantage of the analytical definition of the material surfaces defining the clearance, resulting in a formulation where the gap does not play a central role, as it happens in standard contact models. The contact forces are formulated in conserving form, such that the balance of total energy during the intermittent contact is exactly established in the discrete time integration scheme. Some numerical applications are presented, showing that the proposed methodology is very stable in long-term simulations with relatively large time step sizes. Therefore, it appears to be promising in terms of efficiency and robustness for the numerical analysis of real joints with clearances.     

Computed torque control of fully-actuated nondeterministic multibody systems

In this paper we are interested in the dynamic behavior of a slider-crank mechanism with single and two revolute clearance joints. Due to the clearance existence in the revolute joints, it is important to choose an appropriate contact force model in analyzing the dynamic response of a slider-crank mechanism with clearances. The dynamic equations are established by combining the Newton–Euler equations with modified contact force model and improved Coulomb friction force model, and the Baumgarte stabilization approach is used to improve the numerical stability. According to numerical and experimental results, the method of continuous contact can be verified to be reasonable. Comparing dynamic the response between one clearance joint and two clearance joints in a crank-slider mechanism, it is easy to find a significant mutual coupling region due to the presence of two clearance joints by simply contact figures. The dynamic response in a mechanism with two clearance joints is not a simple superposition of that in mechanism with one clearance joint. Therefore, all the joints in a multibody system should be modeled as clearance joints.     

Optimization of transmission angle for slider-crank mechanism with joint clearances

In this study, kinematic analysis of a planar slider-crank mechanism having revolute joints with clearances was presented. Joint clearance was modelled as a massless virtual link, and Multi-Layered Neural Network (MLNN) structure was used for approximating the motion of this link with respect to the position of input link. Training and testing data sets for the neural network were obtained from mechanism simulation using the ADAMS software. A genetic algorithm was also used to optimize the design parameters for minimizing the deviations due to clearances. When two joint clearances at crank-pin and piston-pin centers were considered, the effects of these clearances on the kinematic characteristics and transmission quality of the mechanism were investigated using continuous contact model between the journal and bearing at a joint.     

Multibody systems with 3D revolute joints with clearances: an industrial case study with an experimental validation

This article is devoted to the analysis of the influence of the joint clearances in a mechanism of a circuit breaker, which is a 42 degree-of-freedom mechanism made of seven links, seven revolute joints, and four unilateral contacts with friction. Spatial (3D) revolute joints are modeled with both radial and axial clearances taking into account contact with flanges. Unilateral contact, Coulomb’s friction and Newton impact laws are modeled within the framework of nonsmooth mechanics without resorting to some regularizations or compliance/damping at contact. The nonsmooth contact dynamics method based on an event-capturing time-stepping scheme with a second order cone complementarity solver is used to perform the numerical integration. Furthermore, the stabilization of the constraints at the position level is made thanks to the stabilized combined projected Moreau–Jean scheme. The nonsmooth modeling approach together with an event–capturing time-stepping scheme allows us to simulate, in an efficient and robust way, the contact and impacts phenomena that occur in joints with clearances. In particular, comparing with the event-detecting time-stepping schemes, the event-capturing scheme enables us to perform the time-integration with a large number of events (impacts, sliding/sticking transitions, changes in the direction of sliding) and possibly with finite-time accumulations with a reasonable time-step length. Comparing with compliant contact models, we avoid stiff problems related with high stiffnesses at contact which generate some issues in contact stabilization and spurious oscillations during persistent contact periods. In the studied mechanisms of the circuit breakers, the numerical method deals with more than 70 contact points without any problems. Furthermore, the number of contact parameters is small—one coefficient of restitution and one coefficient of friction. Though they are sometimes difficult to measure accurately, the sensitivity of the simulation result with respect to contact parameters is low in the mechanism of the circuit breaker. It is demonstrated that this method, thanks to its robustness and efficiency, allows us to perform a sensitivity analysis using a Monte Carlo method. The numerical results are also validated by careful comparisons with experimental data, showing a very good correlation.     

A unified formulation for mechanical joints with and without clearances/bushings and/or stops in the framework of multibody systems

Virtually all machines and mechanisms use mechanical joints that are not perfect from the kinematic point of view and for which tolerances, in the fitting of their components, are specified. Together with such controlled clearances, mechanical joints may require the use of bushing elements, such as those used in vehicle suspensions. Furthermore, in many situations the joints exhibit limits (stops) in their translational or rotational motion that have to be taken into account when modeling them. The dynamic response of the mechanical systems that use such realistic mechanical joints is largely dependent on their characteristic dimensions and material properties of the compliant elements, implying that correct models of these systems must include realistic models of the bushing/clearance joints and of the joint stops. Several works addressed the modeling of imperfect joints to account for the existence of clearances and bushings, generally independently of the formulation of the perfect kinematic joints. This work proposes a formulation in which both perfect and clearance/bushing joints share the same kinematic information making their modeling data similar and enabling their easy permutation in the context of multibody systems modeling. The proposed methodology is suitable for the most common mechanical joints and easily extended to many other joint types benefiting the exploration of a wide number of modeling applications, including the representation of cut-joints required for some formulations in multibody dynamics. The formulation presented in this work is applied to several demonstrative examples of spatial mechanisms to show the need to consider the type of imperfect joints and/or joints with stops modeling in practical applications.     

Experimental investigation of joint clearance effects on the dynamics of a slider-crank mechanism

Clearance is inevitable for assembly and mobility in the kinematic joints of mechanisms. Excessive value of this clearance leads to poor operational characteristics, and these result in losses in kinematic and dynamic performances of mechanism. In this study, effects of joint clearances on vibration and noise characteristics of mechanism are investigated. An experimental test rig has been set up, and a planar slider-crank mechanism having two joints with clearance has been used as a model mechanism. Joint clearance is modeled as a massless virtual link and continuous contact mode between journal and bearing in joint connection is considered in theoretical analyses. Three accelerometers and two microphones have been located at different points to measure the vibrations and noises on system during the mechanism motion. The results obtained for the cases with and without joint clearance are evaluated for vibration and noise characteristics of mechanism.