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981.
对相控阵天线扫描时由于相位量化的影响引起的峰值副瓣电平的恶化进行了分析;推导了峰值副瓣电平恶化的估算公式。计算机模拟结果表明,理论分析与仿真结果一致性较好。 相似文献
982.
阵列天线校准是精确控制阵列波束方向和辐射方向图的前提。本文依据测量距离将阵列天线校准方法分为远场校准法和近场校准法两类。随着阵列天线大型化的发展趋势,微波暗室的尺寸越来越难达到阵列天线远场的要求,远场校准法的适用性逐渐减弱,对近场校准法的需求逐渐增加。本文梳理了过往的经典近场校准方法,并对近场校准方法的最新进展进行论述,对不同校准方法的原理做了详细分析和对比,最后给出未来校准研究的发展方向。 相似文献
983.
针对混合推进航天器编队日心悬浮轨道保持控制问题进行了研究.首先推导出在日心悬浮轨道附近的航天器编队相对运动方程,考虑到航天器间距离变化值较小且航天器间距离与航天器到太阳的距离的比值为小量,将其在悬浮轨道附近线性化.基于该线性化方程,设计了一种LQR编队控制方式,该控制方式可通过调节太阳帆的姿态及航天器间库仑力的大小对编队构型进行改变或保持,具有响应速度快和控制简单的特点.最后对控制律进行数值仿真,表明该控制方法能实现编队. 相似文献
984.
针对刚柔耦合的圆形薄膜UltraFlex太阳翼结构动力学建模与分析困难、微重力下薄膜运动复杂和展开精度要求高的问题,搭建了UltraFlex展开动力学数值分析模型,分析了扭簧和绳索驱动下UltraFlex的有序展开动力学特性。基于绝对坐标方法建立包含柔性附件和柔性薄膜的UltraFlex动力学模型,采用两步检测算法处理薄膜间的复杂接触碰撞问题,利用扭簧逐步释放扭矩的方法驱动结构有序展开,通过控制绳索释放速率的方法完成移动箱板的转动规划和限位跟踪,提高展开位置精度。将该展开驱动策略运用到NASA实际样机尺寸的UltraFlex分析模型中,仿真结果表明该展开策略能够使得UltraFlex结构高精度、有序、稳定地展开;绳索始终处于张紧状态,最大拉力为62.5N;薄膜展开过程复杂,重复出现张紧、回弹的现象,最终趋于稳定。 相似文献
985.
986.
《推进技术》2019,40(7):1628-1636
An independent backup control system based on Field Programmable Gate Array (FPGA) is used to ensure air safety when the master control system of aero-engine breaks down. It can realize a smooth transition from the master control system to the backup control system, control the fuel flow and airscrew speed by setting the programmable logic in FPGA. This system can improve the reliability and safety of aero-engine. This paper proposed the general plan, operating principle, control algorithm, switching logic, channel synchronization and Built-In Test (BIT) of the backup control system. The research has been applied in a certain type of aero-engine control system and passed the rig test. Speed control error is less than 0.2% and fuel flow control error is less than 0.35%. During the channel switching, the fluctuation of the fuel flow control is within 1.5%. It turns out that the system works stably and meets the design requirements. 相似文献
987.
988.
针对L型嵌套阵列二维角度估计问题,提出一种三重混合范数块稀疏重构算法。首先,建立一种俯仰角和方位角可分离的二维稀疏测向模型,将两个维度采样点分块,分别计算联合协方差矩阵,二维角度估计问题被转化为联合协方差矩阵稀疏优化问题;为减小计算复杂度,建立三重混合范数块稀疏重构模型,利用交叉迭代的方法得到稀疏解,实现了二维角度估计,并且可以自动配对。仿真试验表明,三重混合范数稀疏重构算法能够有效估计多个辐射源的二维角度,分辨率较高,并且具有一定的鲁棒性。与传统算法相比,在低信噪比和小快拍数的条件下,均优于传统二维角度估计算法。 相似文献
989.
针对燃烧室火焰筒的气动热力条件,在给定的单位壁面面积冷却空气质量流量下,通过数值模拟较为系统地研究了稀疏孔阵层板结构主要参数对冷却性能的影响,冲击孔、扰流柱和气膜孔数之比为1∶10∶1。在本文研究的结构参数范围内,气膜孔和冲击孔直径对于层板的综合冷却效果和压降影响相对较大,增大气膜孔直径有利于改善综合冷却效果并降低冷却气流通过层板的压力损失,增大冲击孔直径虽降低了冷却气流压力损失但同时也导致综合冷却效果有一定的减弱;射流冲击间距和扰流柱直径的改变对压力损失影响甚微,扰流柱直径增加可以提升层板综合冷却效果,小射流冲击间距比的层板综合冷却效果略优。 相似文献
990.
Philippe L. Lamy Imre Toth Björn J. R. Davidsson Olivier Groussin Pedro Gutiérrez Laurent Jorda Mikko Kaasalainen Stephen C. Lowry 《Space Science Reviews》2007,128(1-4):23-66
In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative
to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about
the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as
well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a
baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine
the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional
shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally
different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the
sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions
measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found
to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two
regions for the direction of the rotational axis of the nucleus: RA = 220°+50°
−30° and Dec = −70° ± 10° (retrograde rotation) or RA = 40°+50°
-30° and Dec = +70°± 10° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function,
although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel
1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and
the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction
is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal
effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus
of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared
to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m−3, lead to a mass of ~ 8 × 1012 kg which should ease the landing of Philae and insure the overall success of the Rosetta mission. 相似文献