基于压缩采样和模态分析的冲击载荷反演研究

近年来,许多学者针对大型飞行器结构的冲击载荷识别进行研究,以期提高这些结构的安全性、降低维护费用、延长使用寿命.在大型结构健康监测中,如何快速、准确地获得冲击力是一个值得深入研究的问题.提出了基于数据的模态分析方法,取代基于有限元分析的模态分析方法,解决传统载荷反演需要依赖结构力学模型的问题;并引入压缩采样理论,基于压缩采样理论提出了一种冲击载荷识别方法,其中包含了稀疏模型,即采样矩阵,还包含重构优化模型;该方法可以解决存储及传输的响应数据过于庞大的问题.最后,用一块大型航空铝板仿真实验来举例说明该冲击力识别方法,结果显示,该方法能够快速、准确地识别出作用在大型结构上的冲击力.     

Arch shape optimization using force approximation methods

The objective of this paper is to provide a method of optimizing areas of the members as well as the shape of both two-hinged and fixed arches. The design process includes satisfaction of combined stress constraints under the assumption that the arch ribs can be approximated by a finite number of straight members.In order to reduce the number of detailed finite element analyses, the Force Approximization Method is used. A finite element analysis of the initial structure is performed and the gradients of the member end forces (axial, bending moment) are calculated with respect to the areas and nodal coordinates. The gradients are used to form an approximate structural analysis based on first order Taylor series expansions of the member end forces. Using move limits, a numerical optimizer minimizes the volume of the arch with information from the approximate structural analysis.Numerical examples are presented to demonstrate the efficiency and reliablity of the proposed method for shape optimization. It is shown that the number of finite element analysis is minimal and the procedure provides a highly efficient method of arch shape optimization.     

六维力／力矩传感器干扰及其标定方法

论述六维力／力矩传感器的典型结构及其特点，并从理论上分析了传感器产生干扰的原因。在建立标定矩阵新概念的基础上，提出了一种传感器的自动标定的方法，其具有简单、快捷、准确，并能完全消除加载力误差影响等特点。     

悬臂梁大变形的向量式有限元分析

为分析悬臂梁的几何非线性行为,用向量式有限元法将结构离散成质点系以及质点间的连接单元.根据牛顿第二定律得到每个质点在内力和外载荷作用下的运动方程以及悬臂梁在每个时刻的变形用该时刻质点系的运动表示.结合刚架元的节点内力和等效质量得出质点位移的迭代计算公式,采用FORTRAN编制计算程序,对悬臂梁分别承受集中载荷和弯矩下的大变形进行算例分析.计算结果与理论解吻合较好,表明该方法能很好地模拟分析悬臂梁的大变形.     

基于SPH 的雨滴打击不规则边界的模拟方法

为实现对雨滴打击树枝等不规则边界过程的模拟,研究了流体粒子在网格表示的 固体边界处的受力情况,提出了一种不需要粒子采样的边界受力模拟方法。采用高斯积分法则 对网格模型的三角面片进行积分,并就此对固液边界的粒子的密度进行修正,以积分的方法对 固体边界处的压力、粘性力等参数进行计算,从而保证边界粒子受力的连续性。同时,还提出 了一种吸引力模型,用来控制粒子在沿着物体表面滑落时的运动。实验结果表明,该方法在模 拟水滴铺展、收缩、沿着边界流动等现象时达到了较为真实的效果。     

采用迭代求解的电动汽车优化充电算法

电动汽车优化充电对配电网的安全经济运行具有十分重要的意义.利用配电网辐射状运行结构采用迭代修正节点电压的方法能够大幅降低优化变量的维数,提高计算速度.该方法已被应用于以网损最小为目标函数、常规负荷模型为恒功率负荷、不计及节点电压、支路功率约束的三相平衡配电网中.该文提出了将其推广至一般情形的方法,目标函数为配电网供电能量最小、运行费用最小或利润最大,不再局限于网损最小,负荷模型不再局限于恒功率负荷.而且计及了配电网的三相不平衡、节点电压、支路功率约束.文中分析了负荷模型对优化计算结果的影响.研究发现,当负荷模型只包含恒功率、恒阻抗负荷时,每次迭代中优化模型仍为线性约束凸二次规划模型;当负荷模型包含恒电流负荷时,每次迭代中优化模型不再为线性约束凸二次规划模型,但仍为线性约束凸规划模型.3个算例的仿真结果表明,所提方法收敛性好、计算精度高、速度快,能够改善电压,提高配电网的经济效益.     

Velocity-based Lateral Stability Control for Four-wheel Independently Actuated Electric Vehicles with Homogeneous Polynomial Approach

This paper presents a control strategy to enhance the lateral dynamics stability and handling performance of the four-wheel independently actuated (FWIA) electric vehicles (EVs). The vehicle longitudinal velocity uncertainty and controller saturation are considered, a double layers control scheme is adopted. In the upper layer, the homogeneous polynomial parameter-dependent approach is introduced to track the uncertainty problem, and a multi-objective controller is designed to obtain the desired external yaw moment. In the lower layer, an optimal force distribution method with considering the distribution error and tire workload is employed to allocate the desired external yaw moment into forces of the four in-wheel motors. Simulation results verify the effectiveness of the proposed control strategy.

     

Motion Control of a 6WD/6WS wheeled platform with in-wheel motors to improve its maneuverability

Multi-axle driving mobile platform that are favored in special environments require high driving performance, steering performance, and stability. Among these, six wheel drive and six wheel steering vehicles hereinafter called 6WD/6WS, gain structural safety by distributing the load and reducing the pitching motion during rapid acceleration and braking. 6WD/6WS mobile platforms are favorable for military use, particularly in off-road operations because of their high maneuverability and mobility on extreme terrains and obstacles. 6WD vehicles that use in-wheel motors can generate independent wheel torque without a need for additional hardware. Conventional vehicles, however, cannot generate an opposite driving force on wheels on both sides. In an independent steering and driving system six-wheel vehicles show better performance than conventional vehicles. This paper discusses the improvement of the cornering performance and maneuverability of 6WD/6WS mobile platform using independent wheel torque and independent steering on each wheel. 6WD/6WS vehicles fundamentally have satisfactory maneuverability under low speed, and sufficient stability at high speed. Consequently, there should be a control strategy for improving their cornering performance using the optimum tire forces that satisfy the driver’s command and minimize energy consumption. From the driver’s commands (i.e., the steering angle and accelerator/brake pedal stroke), the desired yaw moment with virtual steering, desired lateral force, and desired longitudinal force are obtained. These three values are distributed to each wheel as torque and steering angle, based on the optimum tire force distribution method. The optimum tire force distribution method finds the longitudinal/lateral tire forces of each wheel that minimize cost function, which is the sum of the normalized tire forces. This paper describes a 6WS/6WD vehicle with improved cornering performance and the results are validated through TruckSim simulations.

     

Cable installation simulation by using a multibody dynamic model

A major concern when installing the cables into the underground conduit is minimizing the tensile forces exerted on the cables as they are pulled. This knowledge makes it possible to avoid over conservative design practices and to achieve substantial saving during construction. A general computing algorithm of predicting the tensile force of the cable pulled through the underground conduit with an arbitrary configuration is presented in this paper, which is based on multibody system dynamic formulation. The presented multibody dynamic model for this problem consists of the cable, the underground conduit, and the interaction between the cable and the conduit. In this paper, the cable is modeled by the finite cable element based on an absolute nodal coordinate formulation. The interaction between the cable and the underground conduit is described by the Hertz contact theory. Numerical examples are presented to illustrate the effectiveness and efficiency of the proposed method for estimating the cable tension.     

A three-dimensional static torso model for the six human lumbar joints

A simple three-dimensional static torso model for the prediction of the force distributions at the six human lumbar levels in different activities was developed. There are two procedures involved in the model calculations, the intersegmental resultant joint forces and moments of the presumed 26 joints and the internal muscoloskeletal force distributions of the 6 intervertebral disc joints. In formulation of the joint force distribution problem, 6 muscle forces and 3 moment equilibrium equations at one of the six disc joints (e.g., L3/L4) with muscle stress (muscle force divided by physiological cross sectional area - PCSA) upper limits were used. The disc compressive force was minimized as the cost function of the linear programming to solve the force distribution problem. This solution of disc force distribution was further substituted into the next level. Such sequential substitutions of joint level's force equilibrium equations were used to solve for the disc forces at all 6 joint levels.     

Investigation of the effects of perturbation forces to buckling in internally pressurized torispherical pressure vessel heads

The object of this paper is to investigate the effects of perturbation forces to buckling in pressure vessel heads. The pressure vessel heads in concern are confined to torispherical geometry with thin walls. Perturbation forces can alter not only the critical load for buckling but the buckled shape as well. In this paper in addition to previously used perturbation model, three more different perturbation force configurations are applied to the knuckle of the vessel. Internally pressurized three-dimensional torispherical pressure vessel head model that is previously used in literature is constructed and finite element program ANSYS Workbench is used for the solutions. First of all eigenvalue solutions are performed for each model. Then nonlinear instability solutions are conducted to obtain more realistic instability pressure values. For the nonlinear analyses not only large deformation static analyses but also large deformation transient analyses are conducted. For nonlinear analyses, perfectly plastic material model is used. It is concluded that instability pressures obtained by transient analyses are closer to plastic pressure values by PWC and ASME TES criteria and perturbation models increase instability pressure and equivalent plastic strain values.     

Modeling and optimal control of a rotary crane using the straight transfer transformation method

This paper presents an open-loop control strategy for sway-free, point-to-point motion of a load mass in the three-dimensional motion of a rotary crane. In order to suppress the sway of the load during transfer and the residual sway after transfer, an optimal control method was applied. It was found that the control effects are best when the load is transferred along a straight line. The straight transfer transformation (STT) model was derived, where the term “STT” simply implies that the load mass is moved along a straight-line trajectory between the initial and final positions by the coordinated motions of the boom and luffing drive of the crane. The minimum time-control problem that considered the change of the rope length was also solved by an optimal technique, such as the DFP (Davidon–Fletcher–Powell) method. The minimum time control was compared to the preshaping control, which effectively controls vibration. The proposed control method using the STT model was demonstrated to be effective in eliminating the influence of centrifugal force through simulation and experiments.     

Muscle force distribution for adaptive control of a humanoid robot arm with redundant bi-articular and mono-articular muscle mechanism

Robot arms driven by bi-articular and mono-articular muscles have numerous advantages. If one muscle is broken, the functionality of the arm is not influenced. In addition, each joint torque is distributed to numerous muscles, and thus the load of each muscle can be relatively small. This paper addresses the problem of muscle control for this kind of robot arm. A relatively mature control method (i.e. sliding mode control) was chosen to get joint torque first and then the joint torque was distributed to muscle forces. The muscle force was computed based on a Jacobian matrix between joint torque space and muscle force space. In addition, internal forces were used to optimize the computed muscle forces in the following manner: not only to make sure that each muscle force is in its force boundary, but also to make the muscles work in the middle of their working range, which is considered best in terms of fatigue. Besides, all the dynamic parameters were updated in real-time. Compared with previous work, a novel method was proposed to use prediction error to accelerate the convergence speed of parameter. We empirically evaluated our method for the case of bending-stretching movements. The results clearly illustrate the effectiveness of our method towards achieving the desired kinetic as well as load distribution characteristics.     

Topology design based on transferred and potential transferred forces

The concepts of transferred force and potential transferred force through certain parts of the structure to its supports are explained. Topology design methods for linearly elastic structures with the combined use of these forces are investigated. Since the methods do not require any formal optimization techniques, they are quite straightforward to use. The methods are applied to a cantilever truss structure and 2-D plates. The problem of dependency of the solution on the finite element mesh is studied for the cantilever plate problem. It is concluded that the methods with the combined use of transferred and potential transferred forces are good for material redistribution problems and they give better solutions than the methods that use only transferred forces.     

Planning for dynamic multiagent planar manipulation with uncertainty: a game theoretic approach

This paper addresses the planning problem for multiagent dynamic manipulation in the plane. The objective of planning is to design the forces exerted on the object by agents with which the object can follow a given trajectory in spite of the uncertainty on pressure distribution. The main novelty of the proposed approach is the integration of noncooperative and cooperative games between agents in an hierarchical manner. Based on a dynamic model of the pushed object, the coordination problem is solved in two levels. In the lower control level, a fictitious force controller is designed by using a minimax technique to achieve the tracking performance. The design procedure is divided into two steps. First, a linear nominal controller is designed via full-state linearization with desired eigenvalues assignment. Next, a minimax control scheme is specified to optimally attenuate the worst-case effect of the uncertainty due to pressure distribution and achieve a minimax tracking performance. In the coordination level, a cooperative game is formulated between agents to distribute the fictitious force, and the objective of the game is to minimize the worst-case interaction force between agents and the object. Simulations are carried out for two-agent and three-agent manipulations, results demonstrate the effectiveness of the planning method.     

An effective method of determining the residual stress gradients in a micro-cantilever

A method of determining the micro-cantilever residual stress gradients by studying its deflection and curvature is presented. The stress gradients contribute to both axial load and bending moment, which, in prebuckling regime, cause the structural stiffness change and curving up/down, respectively. As the axial load corresponds to the even polynomial terms of stress gradients and bending moment corresponds to the the odd polynomial terms, the deflection itself is not enough to determine the axial load and bending moment. Curvature together with the deflection can uniquely determine these two parameters. Both linear analysis and nonlinear analysis of micro-cantilever deflection under axial load and bending moment are presented. Because of the stiffening effect due to the nonlinearity of (large) deformation, the difference between linear and nonlinear analyses enlarges as the micro-cantilever deflection increases. The model developed in this paper determines the resultant axial load and bending moment due to the stress gradients. Under proper assumptions, the stress gradients profile is obtained through the resultant axial load and bending moment.     

Investigation of plane elastic contact allowing for friction

This paper describes an analysis of plane elastic contact using the finite element method. Solids in contact are subject to small strains, and friction is present between them. Triangular elements are used, in plane strain or stress, with, in each node, six parameters which are the displacements and their partial derivatives combined to obtain strains or stresses according to the continuities to be satisfied through the contact. Opposite nodal points are defined on the two surfaces, in the area which is expected to be in contact under load. Stiffness matrices are first calculated for each solid, respectively and, after that, coupled relatively to the degrees of freedom of all the previous common nodal points. Loads are applied progressively and the equilibrium state is sought, for each increment, by means of an iterative procedure: (1) the contact area is defined; if the calculated normal stress is tension at some opposite nodal points, these are uncoupled; (2) according to the friction law used, the coupled nodal points are labelled “adhering” or “sliding” and, in this case, the tangential stress is made equal to the maximum allowable value. The method is checked for the case with zero friction, the well known Hertz problem. Results are in very good agreement with the analytical solution, for both the contact area dimensions and the pressure distribution. The case of a connecting rod with a loose-fitting bush in one joint is investigated. The calculated normal pressure distribution is shown, at two load levels, for several values of the clearance. As verification, the external circumferential stress distribution is compared with strain-gage measurements.     

Sensitivity analysis of structural response to position of external applied load: in plane stress condition

Procedures for sensitivity analysis of the structural responses, i.e., nodal displacement, mean compliance and local stresses within an element, with respect to the location of an external applied load are developed. This is mainly because the external loads are often of some freedom to change their application positions in the structural preliminary design. Apart from the structural response evaluation, the finite element method is employed in this work for the sensitivity analysis implementation of a plane stress continuum structure. First, an external load is transformed into the equivalent nodal forces such that the influence of an external load shift is represented completely by the magnitude variation of the associated nodal forces, upon which the first- and second-order derivatives of an external load to its location change are performed respectively in a closed form by the aid of the features of the element shape functions. Then, the relevant sensitivities of the structural responses aforementioned are formulated readily with the discrete method. Finally, two typical examples are provided to demonstrate the validity of the sensitivity formulations presented, and the numerical results show a perfect accuracy of calculation of the response sensitivity.     

Synthesis of elastic structures with controlled forces

An analytical model is presented for synthesis of elastic structural systems under external loads and desired “controlled forces”. In the first stage, the optimal force distribution and the final cross sectional dimensions are chosen, solving an optimization problem with elastic compatibility temporarily excluded. In the second stage, the force due to the external loads is determined for the optimal structure by elastic analysis, and the desired controlled force needed for the optimal cross sections and force distribution is obtained by subtracting the above result from the optimal force of the first stage. The final design which has been determined in the first stage, satisfies both conditions of equilibrium and compatibility. It is a lower-bound solution for a similar incontrolled elastic system. In some cases the first-stage optimization problem may be cast in linear programming form. Numerical examples of continuous beam and indeterminate truss demonstrate application of the proposed method.