双月旁转向轨道的修正方法研究

双月旁转向轨道是深空探测中有应用潜力的一类非线性轨道. 在CR3BP模型下得到了双月旁转向轨道, 通过分析轨道的误差传播特性, 制定了双月旁转向轨道的修正原则, 提出了``关键节点'加``显式制导'的修正思路; 给出了考虑太阳引力作用的R4BP模型, 针对初始误差、导航误差、修正执行误差和太阳引力摄动偏差进行了轨道修正仿真, 得到了基于Monte-Carlo方法的轨道修正统计结果. 在此基础上, 采用了``初值速度补偿'与``显式制导'相结合的思路修正初始误差、导航误差、修正执行误差和太阳引力作用, 仿真表明: 这一方法能够较大程度地降低轨道修正所需的冲量. 结果和结论能够为双月旁转向轨道的工程应用提供参考.      

Robust trajectory tracking control for a laboratory helicopter

This paper presents a robust nonlinear control strategy to deal with the trajectory tracking control problem for a laboratory helicopter. The helicopter model is considered as a nominal one with uncertainties such as unmodeled nonlinear dynamics, parametric uncertainties, and external disturbances. The proposed control approach incorporates the feedback linearization technique (FLT) and the signal compensation technique. The FLT is first applied to achieve the linearization of the nominal nonlinear model for reducing the conservation of the robust compensator design. A nominal controller based on the linear quadratic regulation method is designed for the linearized nominal system, whereas a robust compensator is introduced to restrain the influences of the uncertainties. It is shown that the trajectory tracking errors of the closed-loop system are ultimately bounded, and the boundaries can be specified by choosing the controller parameters. Simulation and experimental results on the lab helicopter verify the effectiveness of the proposed method.     

Determination of dynamic three-dimensional soil-tyre contact profile

A technique for measuring the dynamic three-dimensional contact profile between a tyre and deformable soil has been developed. The method involves measuring incremental lateral arc lengths of the profile at discrete locations along the contact length and fitting the coefficients of a model of soil deformation at the soil-tyre interface to the experimental data using a nonlinear constrained optimization algorithm (SUMT). Two representations of the measured contact area were compared: (i) the two-dimensional surface which is the union of all points on the original undeformed soil surface which undergo deformation by the tyre; (ii) the final three-dimensional deformed surface. Contact area measurements were made for two different sized tyres at two levels of inflation pressure, dynamic load and slip in two different soil conditions. The contact width, length and area predicted by the technique were compared with corresponding values for static contact between a tyre and a rigid surface.     

Skip trajectory flight of a ramjet-powered hypersonic vehicle

Possible skip trajectories of a flying vehicle with a periodically actuated ramjet are numerically simulated. An optimal choice of ramjet actuation areas and duration is demonstrated to ensure the maximum flight range with a given amount of the fuel. The main advantage of skip trajectories is found to be a significant (by an order of magnitude) decrease in thermal loads on the flying vehicle.     

Simple analytical model for depth-averaged velocity in meandering compound channels

A simple but applicable analytical model is presented to predict the lateral distribution of the depth-averaged velocity in meandering compound channels. The governing equation with curvilinear coordinates is derived from the momentum equation and the flow continuity equation under the condition of quasi-uniform flow. A series of experiments are conducted in a large-scale meandering compound channel. Based on the experimental data, a magnitude analysis is carried out for the governing equation, and two lower-order shear stress terms are ignored. Four groups of experimental data from different sources are used to verify the predictive capability of this model, and good predictions are obtained. Finally, the determination of the velocity parameter and the limitation of this model are discussed.     

FAST OPTIMIZATION METHOD OF REENTRY TRAJECTORY CONSIDERING AERODYNAMIC PARAMETER PERTURBATION

针对传统再入轨迹优化方法收敛速度慢、对初值敏感程度高等的局限性,提出了一种基于序列凸优化的再入轨迹快速求解方法.该方法以倾侧角的变化率作为控制量,改进了现有凸化策略,考虑到抑制数值优化过程中由于数值离散方式带来的锯齿化现象,采用 B 样条曲线离散控制量,同时为避免算法在初始猜想值附近出现伪不可行的问题,增加额外虚拟控制量,通过一种"回溯直线"搜索的方法,提高算法的稳定性、快速性和寻优结果的光滑性.为研究飞行器再入过程中的气动参数扰动问题,采用采样点少、易于实现,计算效率高的广义混沌多项式理论研究方法,建立了基于广义混沌多项式和凸优化相结合的再入轨迹鲁棒优化模型,该模型在优化过程中考虑气动参数扰动对寻优结果的影响作用,避免了传统轨迹与制导律的复杂迭代设计环节,可有效降低优化轨迹对气动参数扰动的敏感程度,在气动参数不确定条件的干扰下,依然可以保证飞行器顺利安全的完成飞行任务.最后,以美国某可重复使用飞行器的再入任务为例,验证了基于序列凸优化的再入轨迹优化方法的快速性以及鲁棒优化模型对气动参数扰动的抗干扰性能力,表明了该方法具有一定的工程应用性.     

考虑气动参数扰动的再入轨迹快速优化方法

针对传统再入轨迹优化方法收敛速度慢、对初值敏感程度高等的局限性,提出了一种基于序列凸优化的再入轨迹快速求解方法.该方法以倾侧角的变化率作为控制量,改进了现有凸化策略,考虑到抑制数值优化过程中由于数值离散方式带来的锯齿化现象,采用 B 样条曲线离散控制量,同时为避免算法在初始猜想值附近出现伪不可行的问题,增加额外虚拟控制量,通过一种"回溯直线"搜索的方法,提高算法的稳定性、快速性和寻优结果的光滑性.为研究飞行器再入过程中的气动参数扰动问题,采用采样点少、易于实现,计算效率高的广义混沌多项式理论研究方法,建立了基于广义混沌多项式和凸优化相结合的再入轨迹鲁棒优化模型,该模型在优化过程中考虑气动参数扰动对寻优结果的影响作用,避免了传统轨迹与制导律的复杂迭代设计环节,可有效降低优化轨迹对气动参数扰动的敏感程度,在气动参数不确定条件的干扰下,依然可以保证飞行器顺利安全的完成飞行任务.最后,以美国某可重复使用飞行器的再入任务为例,验证了基于序列凸优化的再入轨迹优化方法的快速性以及鲁棒优化模型对气动参数扰动的抗干扰性能力,表明了该方法具有一定的工程应用性.      

Mathematical model of a brittle crack that takes into account the distribution of cohesive forces between the crack faces and the distance between them

A mathematical model of a circular disk-shaped brittle crack whose faces are acted upon by cohesive forces, whose magnitude depends on the distance between the faces. An algorithm of numerical solution of the singular nonlinear improper integral equation defining the crack profile was developed which can be used for other integral equations of this type. It is shown that taking into account the coupling forces between the crack faces leads to their gradual closing with distance from the center of the crack.     

倾斜轨道电动力绳系卫星回收控制

考虑电动力影响,建立了倾斜轨道绳系卫星系统的动力学模型,研究了子星回收过程的非线性最优控制. 应用Legendre伪谱算法,将连续时间最优控制问题离散化,进而利用非线性规划方法进行求解,通过数值模拟验证了方法的有效性. 结果表明,在满足相关约束的条件下,通过调节系绳张力和电动力,可将子星回收到靠近主星的指定位置.      

条纹投影形貌测量的品质评估位相展开

0Introduction Phaseunwrappingisrelatedtomanyresearchdisciplinesincludingopticalmetrology,synthetic apertureradar,acousticimaging,magneticresonanceimaging(MRI)andmicrowaveinterferometry.AnexcellenttextbookbyGhigliaandPritt[1]presentedthefundamentaltheoryan…     

Quality Guided Phase Unwrapping for Fringe Projection Profilometry

0Introduction Phaseunwrappingisrelatedtomanyresearchdisciplinesincludingopticalmetrology,synthetic apertureradar,acousticimaging,magneticresonanceimaging(MRI)andmicrowaveinterferometry.AnexcellenttextbookbyGhigliaandPritt[1]presentedthefundamentaltheoryandvariousalgorithms ofphaseunwrapping.Thispaperwillfocusonasocalledquality guidedphaseunwrappingapproach.Inthisapproach,aqualitymapgeneratedbyacertainphasequalityevaluationcriterionisincorporated toguidetheunwrappingprocess.Attheprimitivesta…     

Robust Feedback Stabilization of Limit Cycles in PWM DC-DC Converters

Local feedback stabilization of limit cycles in PWM DC-DC converters isconsidered, using recently developed general sampled-data models. Thepaper focuses on converters for which the nominal operating conditionhas lost stability due to off-design operation. The results apply tostabilization of the nominal periodic operating condition. In addition,the same approach can be used to stabilize other limit cycles such asthose embedded in a possible chaotic trajectory. Two feedbackstabilization schemes are proposed and studied in detail. The firststabilization technique uses voltage reference compensation and thesecond uses dynamic ramp compensation. Both employ discrete-time washoutfilters to ensure preservation of the size and shape of the limit cycle.Washout filters ensure that the nominal operating branch is unaffectedby the control, without the need for accurate knowledge of the limitcycle.     

Design of a curved surface constant force mechanism

A new curved surface constant force mechanism which mainly consists of a roller and a curved surface has been proposed. The magnitude and the direction of normal force caused by squeezing between the roller and the curved surface satisfy a certain relationship, thus the decomposed force of the normal force keeps constant in a certain direction all the times. According to the envelope theorem, the trajectory of the roller center and the profile of the curved surface are obtained by ignoring friction. Then, the influence of the friction is discussed in detail. In addition, the simulation is performed to verify the theoretical calculation. The simulation results show that the output force is relatively constant and the friction has little effect on the output force.     

基于ESGA遗传算法的水射流自驱旋转喷头优化设计

超高压水射流自驱旋转型喷头是目前广泛应用于船壁除锈的一种装置,其布局方式直接影响船壁除锈的效率和质量,目前喷头布局多依赖工程经验,缺少准确的理论分析和优化技术支持。针对水射流自驱旋转型喷头的布局优化问题,在传统遗传算法（genetic algorithm,GA）的基础上,提出一种基于“锦标赛选择”的精英策略遗传算法（elitist strategy genetic algorithm,ESGA）,该算法通过采用种群进化过程中精英个体直接保留到下一代的进化策略,从而有效提高算法的全局收敛能力和算法的鲁棒性。结合旋转喷头扫掠冲击性能和轨迹特征,以喷头移动路径垂直打击面上的能量分布均匀度为衡量标准,建立超高压水射流自驱旋转型喷头的螺旋扫掠冲击离散化时间优化模型,并分别利用两种遗传算法对其进行优化改进。对一字形水射流自驱旋转型喷头的布局优化研究发现,经ESGA算法优化的旋转喷头,其扫掠冲击能量分布均匀度较原喷头布局提升了47.2%,其收敛精度也高于GA算法。经对ESGA算法优化后的喷头实验验证发现,ESGA优化方案较原设计方案除锈效率提高了42.0%。改进的ESGA优化算法可行性强,能够在收敛...     

Effects of modifying the blade trailing edge profile on unsteady pressure pulsations and flow structures in a centrifugal pump

In the present work, the effects of modifying the blade pressure side (EPS profile) on unsteady pressure pulsations and flow structures in a low specific speed centrifugal pump are carried out by experimental and numerical methods. Results are compared to the original trailing edge (OTE profile). Unsteady pressure signals are captured at twenty measuring points at flow rate of 0–1.6Q_d. It is observed that the pump head of the EPS profile is improved for all the concerned working conditions. Pressure amplitudes at the blade passing frequency are compared and discussed in detail. It is found that the EPS profile contributes to pressure pulsation reduction obviously. For all the measured flow rates, pressure amplitudes are attenuated evidently at major measuring positions, especially at high flow rates. As for the mean pressure amplitude of twenty measuring points, pressure amplitude is reduced more than 20% at the nominal flow rate using the EPS profile. From relative velocity distribution, it is found that the uniformity of flow field at the blade outlet region would be improved significantly by the EPS profile. Besides, the corresponding vorticity magnitude at the blade outlet would be reduced compared to the OTE profile. The combined effects contribute to the reduction of pressure amplitude using the EPS profile.     

圆迹合成孔径雷达星载实现的可行性分析

圆迹合成孔径雷达(circular synthetic aperture radar,CSAR)是一种通过传感器平台作360°圆周运动来获得全方位高空间分辨率的聚束合成孔径雷达,本文通过对其星载平台进行动力学分析来研究其星载的可行性.首先以CSAR的工作条件作为约束,建立了星载平台轨道模型.然后以圆迹中心为基点,分析了星载平台的运动状态及受力情况,最终得到形成圆迹平台所需要的主动控制力.研究结果表明:形成圆迹的星载平台,作用在卫星平台上的主动控制力大小、方向是不断改变的,使得CSAR对任意纬度区域的观测很难实现.但对于两极点区域,当星载平台沿圆迹以一定的角速度运行时,主动控制力的大小、方向不变,易于实现控制,有星载实现的可能性.     

Modeling of the free-surface shape in laser cutting of metals. 2. M odel of multiple reflection and absorption of radiation

A physical model of multiple reflection of focused laser radiation propagating in narrow channels and slots is proposed to be applied in laser treatment of metals. Based on the laws of geometric optics, an algorithm for calculating the shape of the surface formed by interaction of laser radiation and metal with the use of the trajectory technique is proposed. The method used allows one to describe energy transfer inward the channel in simulation of laser-treatment processes (drilling, cutting, and welding). It is shown that the efficiency of multiple reflection is most significant in cutting thick materials with a large ratio of the plate thickness to the Gaussian beam width.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 16–20, January–February, 2005     

Finite Spherical Cavity Expansion Method for Layering Effect

A decay function for the layering effect during the projectile penetrating into layered targets is constructed, which is obtained via the theoretical solution of a dynamically expanding layered spherical cavity with finite radius in the layered targets that are assumed to be incompressible Mohr-Coulomb materials. By multiplying the decay function with the semi-empirical forcing functions that account for all the constitutive behavior of the targets, the forcing functions for the layered targets are obtained. Then, the forcing functions are used to represent the targets and are applied on the projectile surface as the pressure boundary condition where the projectile is modeled by an explicit transient dynamic finite element code. This methodology is implemented into ABAQUS explicit solver via the user subroutine VDLOAD, which eliminates the need for discretizing the targets and the need for the complex contact algorithm. In order to verify the proposed layering effect model, depth-of-penetration experiments of the 37 mm hard core projectile penetrating into three sets of fiber concrete and soil layered targets are conducted. The predicted depths of penetration show good agreement with the experimental data. Furthermore,the influence of layering effect on projectile trajectory during earth penetration is investigated. It is found that the layering effect should be taken into account if the final position and trajectory of the projectile are the main concern.     

Experimentally separating fluid and matrix contributions to polymeric foam behavior

A new experimental procedure to determine the loads carried by the fluid (air) and matrix components of a polymeric foam is presented. Testing is carried out in a sealed chamber equipped with a differential pressure transducer to measure changes in the chamber air pressure and a load cell to measure the load applied to the specimen. Multiexposure photographs are used to determine lateral specimen expansion at various degrees of compression. From these data the amount of air trapped and compressed within the foam can be determined. Theoretical analyses suggest and tests confirm that for the strain rates used here the trapped air undergoes isothermal compression. By treating compression of the air trapped in the specimen as an isothermal process, an equivalent volume-average pore pressure can be determined, and the load carried by the fluid phase calculated. The load carried by the polymer matrix component is the difference between the total response and the fluid component. The energy input into each phase during compression can then be calculated.The effectiveness of the procedure is demonstrated by displacement-controlled compression tests of 50×100×100-mm semi-rigid, polyurethane foam specimens. Two types of foam were compressed to 75-percent strain at nominal strain rates of 1.4/s and 14/s. Calculated values show a high degree of repeatability.