复杂曲面零件五轴加工刀轴整体优化方法

 针对复杂曲面零件五轴加工中刀轴矢量变化剧烈、严重影响工件表面加工质量的问题,提出一种基于临界约束的五轴刀轴矢量整体优化方法。首先,构造了给定切触点处所有可行摆刀平面,并在摆刀平面内根据临界约束计算出临界刀轴矢量,在获得临界刀轴矢量的基础上,对其进行平面映射,建立了刀轴摆动的初始可行域;其次,通过对初始可行域进行均匀离散,根据离散点之间相对位置关系构造邻接矩阵,并结合最短路径搜索算法获得了初始参考刀轴,从而构造了新的刀轴摆动可行域;最后,建立当前切削行内无干涉且相邻刀轴变化最小的刀轴矢量优化模型,实现自由曲面五轴加工无干涉刀轴矢量的光滑控制。两种自由曲面叶轮的加工算例分析表明,采用本文方法获得的刀轴矢量可以明显改善机床的运动性能,避免了刀具干涉的产生,可提高复杂曲面零件的加工质量与效率。     

The yaw stability analysis for stratospheric airships in float situation

When a stratospheric airship free floats at pressure altitude, the sideslip angle of the airship is neither random nor against the wind, but is stable on certain values. According to classical potential flow theory, a simplified two-dimensional ellipse and three-dimensional ellipsoid are firstly analyzed respectively, which implied that the airship could present crosswind orientation. The numerical investigations (CFD) on the yaw stability based on a bare hull and a finned airship are employed for verifying the theory conclusion. It is found that the finned airships can remain stable when its sideslip Angle is 55–70°, which is less than 90° of the stable angle of the ellipsoid and bare hull, but statically unstable at low sideslip angles, its static instability is similar to that of dynamic flight. Then the fight data of three stratospheric airships is analyzed. The yaw stability in flight data generally agrees with expectations drawn of theoretical and numerical simulation. These investigations serve to provide references for yaw control and configuration design of airships.     

Primary Solidification Phase and Lamellar Orientation in Directionally Solidified Ti-45Al-7Nb Alloy

Primary solidification phase and lamellar orientation are investigated in Ti-45Al-7Nb alloy at very high ratio of temperature gradient to growth rate (G/v) by a liquid-metal-cooled directionally solidified method. It shows that Ti-45Al-7Nb alloy solidifies with primary α phase. Longitudinal (parallel to growth direction) microstructure shows that α dendrites in solid-liquid mushy zone are discontinuous and transverse microstructure of α dendrites is worm-like feature. Growth direction of α phase is about 80° away from〈0001〉α direction, and close to〈1120〉α direction. The corresponding lamellar orientation is aligned at the angle of about 10° to growth direction, which is consistent with α-dendrite growth direction according to Blackburn orientation relationship. Therefore, due to the altered growth direction of α phase, the lamellar orientation in Ti-45Al-7Nb alloy is controlled at the G/v ratio of 5×109 K·sm-2.     

显式制导下定位定向误差的影响分析

建立了定位定向误差对显式制导精度影响的解析模型,提出了利用标准弹道参数估算定位定向偏差对制导精度影响的近似解析方法.结果表明,该方法计算精度高、速度快,可以满足显式制导下定位定向误差影响分析的需要.     

Three dimensional spatial memory and learning in real and virtual environments

Human orientation and spatial cognition partlydepends on our ability to remember sets ofvisual landmarks and imagine their relationshipto us from a different viewpoint. We normallymake large body rotations only about a singleaxis which is aligned with gravity. However,astronauts who try to recognize environmentsrotated in 3 dimensions report that theirterrestrial ability to imagine the relativeorientation of remembered landmarks does noteasily generalize. The ability of humansubjects to learn to mentally rotate a simplearray of six objects around them was studied in1-G laboratory experiments. Subjects weretested in a cubic chamber (n = 73) and aequivalent virtual environment (n = 24),analogous to the interior of a space stationnode module. A picture of an object waspresented at the center of each wall. Subjectshad to memorize the spatial relationships amongthe six objects and learn to predict thedirection to a specific object if their bodywere in a specified 3D orientation. Percentcorrect learning curves and response times weremeasured. Most subjects achieved high accuracyfrom a given viewpoint within 20 trials,regardless of roll orientation, and learned asecond view direction with equal or greaterease. Performance of the subject group thatused a head mounted display/head tracker wasqualitatively similar to that of the secondgroup tested in a physical node simulator. Body position with respect to gravity had asignificant but minor effect on performance ofeach group, suggesting that results may alsoapply to weightless situations. A correlationwas found between task performance measures andconventional paper-and-pencil tests of fieldindependence and 2&3 dimensional figurerotation ability.     

基于双圆特征的无人机着陆位置姿态视觉测量方法

提出了一种无人机自主着陆位置姿态的单目视觉测量方法,建立了机载摄像机的运动和投影模型。设计了新型双圆图案着陆平面靶标,采用双圆的8个公切点,产生21个具有透视投影不变性的特征点,并提出了在复杂背景中全自动双圆特征的图像提取新方法及标记特征点的方案,实验表明,768×576像素大小的图像,特征提取及标记耗时小于9ms。仿真试验表明,摄像机距离靶标10m左右,噪声偏差达到1.5像素时,单轴位置RMS误差小于6cm,单轴姿态RMS误差小于0.7°,所提出的算法具有很强的抗噪声能力,能够满足无人机自主着陆位置姿态实时测量的要求。     

出流方式对阵列冲击换热的影响

采用瞬态液晶技术测量了阵列冲击靶面的局部传热系数.获得了冲击雷诺数为5 000,10 000,15 000,20 000,25 000时三种不同出流方式时的靶面传热系数分布规律.结果表明:双侧出流时,靶面的平均传热系数比单侧出流要高;不同方式的单侧出流,由于产生的横流强弱作用不同,所以靶面的平均传热系数也有所不同.不同出流方式时,靶面的局部传热系数均随冲击雷诺数的升高而升高.      

次孔方位角对单入口-双出口孔射流气膜冷却效率影响

为了探讨次孔方位角对单入口-双出口孔射流气膜冷却效率的影响,利用商业软件提供的有限体积法求解Navier-Stokes方程,对次孔方位角分别为30°,45°,60°和90°的单入口-双出口孔射流冷却效率进行数值模拟.吹风比变化范围为0.5~2.0.研究了流场、气膜冷却效率和径向平均气膜冷却效率的变化规律.结果表明,相对于圆柱孔,单入口-双出口孔射流气膜冷却效率明显改进.基于气膜冷却效率的最佳次孔方位角度为45°,高于圆柱孔射流气膜冷却效率300%.吹风比越大,次孔方位角对气膜冷却效率的影响越明显.      

单星修正惯导系统初始定位定向误差原理

本文对利用单星信息修正惯导系统初始定位定向误差的方法进行分析,从几何角度说明其基本工作原理。并指出应用这种方法的特定情况和所需条件。     

潜射弹道导弹定位定向误差传播模型

定位定向误差是影响潜射弹道导弹命中精度的重要因素。同时考虑几何项、初值项和引力项,得到了定位定向误差影响分析的完整解析解,并利用主动段状态空间摄动方程推导了定位定向误差的线性化传播模型。计算结果表明:所提方法计算速度快,速度偏差估计误差小于10%,位置偏差估计误差小于1%,可以满足精度分析的需要。