Implementation of a single-phase AC/AC converter based on neutral-point-clamped topology

A single-phase ac/ac converter based on neutral-point-clamped scheme is proposed to perform unity input power factor and to provide a stable ac voltage to the critical loads. The ac/dc rectifier part is controlled to generate a unipolar pulsewidth modulation (PWM) waveform on the ac terminal by using four power switches with voltage stress of half the dc-link voltage. The carrier-based current control scheme is employed in the inner control loop to track the line current command. To regulate the dc bus voltage, a proportional-integral (PI) control is adopted in the outer control loop. The dc/ac inverter part of the system with four power switches is employed to generate a stable and clean sinusoidal output voltage to the critical load. The instantaneous current control scheme is used to track the output voltage command. To verify the effectiveness of the proposed control algorithm, the simulation and experimental results based on a laboratory prototype were implemented and discussed.     

典型R-C系统主光学装置的热光学分析

文章采用热光学集成法对典型R-C成像系统某主光学装置进行热光学分析。     

平均绝对差图象匹配系统的定位精度

本文证明了两个基本定理。在假定图象信号和附加噪声是相互独立的零均值齐次高斯随机场、且二次可微的条件下,解决了平均绝对差图象匹配系统的定位精度的分析和计算问题。用实例说明了精度分析和计算的方法,分析了影响定位精度的因素和参数。为匹配性能的定量分析提供了理论依据和方法。     

Observations of inverted-V electron precipitation

The characteristics of inverted-V electron precipitation fluxes deduced predominantly from observations by the Atmosphere Explorer satellites are reviewed. The energy and pitch angle distributions are presented and shown to be generally in agreement with acceleration by a parallel electrostatic potential. Characteristics of secondary electrons are examined, and effects of beam plasma instabilities on these electrons are discussed. The properties of the monoenergetic component are compared with theoretical models of creating parallel DC electric fields, and found to favor the anomalous resistivity model. The article also discusses relations of inverted-V events with other auroral phenomena including auroras, electrostatic shocks, convective electric field reversals, field-aligned currents and wave emissions. The principal conclusions are: (1) plasma sheet electrons are continuously accelerated to form inverted-V structures in the pre-midnight hemisphere independent of substorm phase, (2) the acceleration processes are probably related to large scale electrostatic wave turbulence observed at altitudes of a few thousand kilometers, (3) narrow bursts of intense electron precipitation fluxes are found to be imbedded within some inverted-V's. It is argued that the narrow bursts of intense electron precipitation have the proper characteristics to cause discrete auroral arcs in the atmosphere. We suggest that these narrow bursts are accelerated by an electrostatic shock at higher altitude and capable of producing discrete auroral arcs below the observing satellite.     

稀薄效应对可倾瓦动压气体轴承性能的影响

以微型三可倾瓦动压气体轴承为研究对象,采用连续模型、一阶滑移模型和WU新滑移模型速度边界条件,建立考虑稀薄效应的滑移修正雷诺方程。结合牛顿迭代法和有限差分法求解修正雷诺方程,计算考虑稀薄效应的三可倾瓦动压气体轴承各瓦块压力分布和承载力。计算结果表明:三可倾瓦动压气体轴承偏心率越大,转速越大,压缩系数越大,轴承承载力越大;考虑稀薄效应后,计算出的可倾瓦动压气体轴承承载力明显下降,且随着克努森数的增大,WU新滑移模型计算得到的承载力明显低于一阶滑移模型。     

波浪形前缘降低后掠叶片宽频噪声实验研究

为了研究波浪形前缘对后掠叶片湍流干涉噪声的影响,通过放置于叶片上游的倾斜圆柱产生尾迹,圆柱尾迹为各向异性的湍流,之后湍流与叶片相互干涉产生干涉噪声。实验采用的叶型后掠角度为30°,截面为NACA0012翼型。在气流来流速度30m/s,40m/,60m/s和70m/s的情况下（基于叶片弦长的雷诺数为300000~700000）,使用31个麦克风线阵列测量了基准后掠叶片与波浪形前缘叶片对应的叶片湍流干涉噪声。采用Clean-SC算法处理数据,得到不同幅值与波长下后掠叶片前缘噪声信息。实验结果表明,波浪形前缘幅值与波长对总声压级降噪量均有影响,使用最大幅值和最小波长的波浪形前缘降噪效果最好;不同气流速度下,采用相同的波浪形前缘,使用斯特劳哈尔数表征的噪声频谱图变化规律相似。     

具结构式不确定性系统的鲁棒镇定

考虑一类与摄动参数呈多项式函数关系的不确定性模型 ,这类模型反应了预知部分不确定性信息的物理意义。给出了含这类不确定性系统鲁棒稳定的充分条件和最大稳定域估计方法。在区间系统情形 ,该条件退化为已有结果 ,这间接表明文中条件的保守性有限。进一步得到了状态和输出反馈鲁棒镇定控制器的设计方法 ,及文中方法意义下具有最大稳定域的镇定控制器设计算法。最后 ,给出了一个飞控系统鲁棒镇定控制器设计的例子 ,说明文中方法的可行性。     

基于HLA架构的雷达对抗训练模拟系统设计

针对雷达对抗部队在构建实战化电磁信号环境、开展模拟训练中存在的问题,分析了训练模拟系统应该满足的技战术需求,深入探讨了基于HLA架构的雷达对抗训练模拟系统的体系结构、系统软件结构和工作流程,为雷达对抗训练模拟系统的研制开发提供了一个较好的设计思路.     

星间网络安全路由选择算法研究

星间网络由于拓扑结构高动态变化、可靠性安全性要求高、节点处理能力有限等特点,使其有别于普通的地面互联网。文章通过分析星间网络特性,对其中安全路由选择的关键技术进行深入研究,提出了一种基于节点信誉度评价与负载感知机制的星间网络安全路由选择算法。该算法采用混合路由配置策略,根据网络实时情况对路由进行动态调整,加入节点信誉度评价策略,减小恶意节点对网络的影响,并且增加节点负载感知机制,根据当前节点负载情况调整路由。最后,文章通过仿真实验验证算法性能,验证了算法在数据包传输成功率方面的优势。     

减少铸造Al-Si合金Sr变质潜伏时间工艺研究

Sr变质潜伏时间延长除了给生产带来很多不便外,还会造成Sr烧损增加,变质效果衰退以及铝合金表面氧化-吸氢加剧,对合金造成污染,其中液态金属温度,液态金属的对流强度,Al-Sr合金的几何尺寸及表面状态是影响Sr变质潜伏时间的重要因素。通过建立物理模型,对Sr扩散过程进行流场模拟计算,并进行了试验验证。通过工艺改进,Sr变质潜伏时间由60分钟,缩短至20分钟,变质后合金性能优良,Al-Sr合金取得了良好的变质效果。