Experimental and numerical investigation on surface quality for two-point incremental sheet forming with interpolator

The unsatisfied surface quality seriously impedes the wide application of incremental sheet forming (ISF) in industrial field. As a novel approach, the interpolator method is a promising strategy to enhance the surface quality in ISF. However, the mechanism for the improvement of surface quality and the influence of interpolator properties on surface roughness are not well understood. In this paper, the influences of process variables (i.e. tool diameter, step size and thickness of interpolators) on the forming process (e.g. surface roughness, forming force and geometric error) are investigated through a systematic experimental approach of central composite design (CCD) in two-point incremental sheet forming (TPIF). It is obtained that the increase in thickness of interpolators decreases the surface roughness in direction vertical to the tool path while increases the surface roughness in direction horizontal to the tool path. Nevertheless, the combined influence between thickness of interpolators and process parameters (tool diameter and step size) is limited. Meanwhile, the placement of interpolator has little influence on the effective forming force of blank. In addition, the geometric error enlarges with the increase of step size and thickness of interpolator while decreases firstly and then increase with an increase in tool diameter. Finally, the influencing mechanism of the interpolator method on surface quality can be attributed to the decrease of the contact pressure due to the increase of contact area with the unchanged contact force. Meanwhile, the interpolator method eliminates the sliding friction on the surface of blank due to the stable relative position between the blank and the interpolator.     

全模颤振风洞试验三索悬挂系统多体动力学分析

全模颤振风洞试验需要通过软支撑系统模拟飞行器的自由飞行状态并调整模型姿态达到配平状态。参考NASA双索悬挂方案，提出了一种两电机驱动的三索悬挂系统，利用后方两索的同向/反向联动实现模型俯仰和滚转姿态的调整，利用弹簧刚度以及钢绳张力设计实现支撑频率要求。基于柔性多体动力学方法，建立了包括飞行器刚体模型、柔性索、滑轮、弹簧、气动力模型、伺服电机控制在内的复杂系统动力学模型，其中，利用任意拉格朗日-欧拉（ALE）变长度索单元描述钢绳，利用不约束物质坐标的索结点约束描述钢绳与滑轮相互作用，利用索结点物质输运速度约束描述伺服电机绞盘，利用飞行力学的气动力模型描述吹风下的气动力。基于该模型，通过小扰动响应辨识研究了弹簧刚度、钢绳张力、连接点位置等因素对支撑频率的影响规律，并分析了系统姿态调整能力，俯仰调整范围达到-12.5°~12.5°，滚转调整范围达到-45°~45°。采用滑轮处电位计测量的钢绳相对位移作为反馈信号，基于设计的控制律利用多体动力学求解器与Simulink对风洞吹风下的姿态调整过程进行仿真，模型达到配平状态，获得了吹风下的索拉力和伺服电机功率，为系统设计提供基础。     

Analysis of the dynamic behavior of structures using the high-rate GNSS-PPP method combined with a wavelet-neural model: Numerical simulation and experimental tests

Recently, the high rate global navigation satellite system-precise point positioning (GNSS-PPP) technique has been used to detect the dynamic behavior of structures. This study aimed to increase the accuracy of the extraction oscillation properties of structural movements based on the high-rate (10?Hz) GNSS-PPP monitoring technique. A developmental model based on the combination of wavelet package transformation (WPT) de-noising and neural network prediction (NN) was proposed to improve the dynamic behavior of structures for GNSS-PPP method. A complicated numerical simulation involving highly noisy data and 13 experimental cases with different loads were utilized to confirm the efficiency of the proposed model design and the monitoring technique in detecting the dynamic behavior of structures. The results revealed that, when combined with the proposed model, GNSS-PPP method can be used to accurately detect the dynamic behavior of engineering structures as an alternative to relative GNSS method.     

空天飞行器返回滑翔段在线制导方法

针对空天往返飞行器的返回滑翔段在线制导问题，设计了一种新的滑翔段飞行剖面，实现了滑翔段终端交班高度、位置和倾角约束的自动满足，减少了在线制导算法中需处理的约束数量。推导了滑翔段运动状态、过程约束和性能指标的解析表达式，获得了剩余航程和终端速度间的函数关系。在此基础上，提出了一种双层在线制导方法：内层解析重构飞行剖面，同时通过解析确定路径点来改变剩余航程的变化率，进而对终端交班速度进行控制；外层借助解析表达式，使用粒子群优化算法(PSO)和改进共轭梯度法(CGM)优化飞行剖面，从而满足过程约束和指标要求。最后通过数学仿真验证了方法的正确性。     

Stability of relative equilibria of the full spacecraft dynamics around an asteroid with orbit–attitude coupling

The full dynamics of spacecraft around an asteroid, in which the spacecraft is considered as a rigid body and the gravitational orbit–attitude coupling is taken into account, is of great value and interest in the precise theories of the motion. The spectral stability of the classical relative equilibria of the full spacecraft dynamics around an asteroid is studied with the method of geometric mechanics. The stability conditions are given explicitly based on the characteristic equation of the linear system matrix. It is found that the linearized system decouples into two entirely independent subsystems, which correspond to the motions within and outside the equatorial plane of the asteroid respectively. The system parameters are divided into three groups that describe the traditional stationary orbit stability, the significance of the orbit–attitude coupling and the mass distribution of the spacecraft respectively. The spectral stability of the relative equilibria is investigated numerically with respect to the three groups of system parameters. The relations between the full spacecraft dynamics and the traditional spacecraft dynamics, as well as the effect of the orbit–attitude coupling, are assessed. We find that when the orbit–attitude coupling is strong, the mass distribution of the spacecraft dominates the stability of the relative equilibria; whereas when the orbit–attitude coupling is weak, both the mass distribution and the traditional stationary orbit stability have significant effects on the stability. We also give a criterion to determine whether the orbit–attitude coupling needs to be considered.     

Atmospheric pressure variations at extratropical latitudes associated with Forbush decreases of galactic cosmic rays

Changes of troposphere pressure associated with short-time variations of galactic cosmic rays (GCRs) taking place in the Northern hemisphere’s cold months (October–March) were analyzed for the period 1980–2006, NCEP/NCAR reanalysis data being used. Noticeable pressure variations during Forbush decreases of GCRs were revealed at extratropical latitudes of both hemispheres. The maxima of pressure increase were observed on the 3rd–4th days after the event onsets over Northern Europe and the European part of Russia in the Northern hemisphere, as well as on the 4th–5th days over the eastern part of the South Atlantic opposite Queen Maud Land and over the d’Urville Sea in the Southern Ocean. According to the weather chart analysis, the observed pressure growth, as a rule, results from the weakening of cyclones and intensification of anticyclone development in these areas. The presented results suggest that cosmic ray variations may influence the evolution of extratropical baric systems and play an important role in solar-terrestrial relationships.     

Thrusting maneuver control of a small spacecraft via only gimbaled-thruster scheme

The thrust vector control (TVC) scheme is a powerful method in spacecraft attitude control. Since the control of a small spacecraft is being studied here, a solid rocket motor (SRM) should be used instead of a liquid propellant motor. Among the TVC methods, gimbaled-TVC as an efficient method is employed in this paper. The spacecraft structure is composed of a body and a gimbaled-SRM where common attitude control systems such as reaction control system (RCS) and spin-stabilization are not presented. A nonlinear two-body model is considered for the characterization of the gimbaled-thruster spacecraft where, the only control input is provided by a gimbal actuator. The attitude of the spacecraft is affected by a large exogenous disturbance torque which is generated by a thrust vector misalignment from the center of mass (C.M). A linear control law is designed to stabilize the spacecraft attitude while rejecting the mentioned disturbance torque. A semi-analytical formulation of the region of attraction (RoA) is developed to ensure the local stability and fast convergence of the nonlinear closed-loop system. Simulation results of the 3D maneuvers are included to show the applicability of this method for use in a small spacecraft.     

反推力装置结构优化设计

根据叶栅式反推力装置结构及其工作原理,建立了反推力装置运动学与动力学数学模型,以此为基础,取反推力装置对作动系统的最大负载力极小化为目标函数,装置各运动机构满足几何关系为约束条件,建立反推力装置结构优化模型。通过反推力装置运动学及动力学仿真,并分析在不同结构参数下反推力装置运动学及动力学特性,验证了所建立模型的合理性与正确性。在Matlab环境下采用惩罚函数法对反推力装置进行结构优化设计,结果表明：优化后的反推力装置正向最大负载力下降了24.5%,负向最大负载力下降了16.3%,且各个机构运动不出现干涉,整个反推力装置可正常工作。论文建模方法与优化结果可为反推力装置结构设计提供参考。     

Structurally coupled characteristics of rotor blade using new rigid-flexible dynamic model based on geometrically exact formulation

In rotor dynamics, blades are normally modelled as a slender beam, in which elastic deformations are coupled with each other. To identify these coupling effects, new rigid-flexible structural model for helicopter rotor system is proposed in this paper. Finite rotations of the whole blade(on flapwise, lagwise, and torsional) are described as three global rigid degrees of freedom.The nonlinear deformation geometrics of the beam is built on geometrically exact beam theory.New expressions for blade ...     

滚珠自转对高速球轴承环下润滑内部流动的影响

高速球轴承正常工作时,处于高速旋转状态,此时油相和气相因重力和离心力的作用在轴承环间剧烈运动。为了更准确地分析轴承环间的两相流动,采取VOF模型进行内部流场的模拟,采用多重旋转坐标系描述部件运动。建立球轴承环下润滑计算模型,分析了考虑滚珠自转因素下轴承内部的流动,并在此基础上探究了转速及供油量对轴承工作状态的影响。结果表明,对比整体模型与滚珠自转模型,发现滚珠自转使得轴承内部油体积分数增大,同时也使得滑油穿透间隙达到外环的能力增加;在考虑滚珠自转情况下,转速的不断增大,使得轴承内部的油相体积分数不断减小,在低转速情况下滚珠自转对流体运动影响较为明显,在高转速情况下公转速度对流体运动起到主导作用,滚珠自转对流体运动影响减弱;供油量不断的增大,使得滚珠自转模型内部的油相体积分数也在不断增大,而且滚珠自转运动会加强滑油在轴承内部的分布。