基于多天线中继的卫星-地面混合无线通信系统上行链路性能分析

研究了Rayleigh Rician非对称衰落信道下,以基站作为固定增益放大转发(AF)中继的卫星-地面混合无线通信系统上行链路性能。首先利用信道状态信息(CSI)得到中继配置多天线时系统的最大输出等效信噪比(SNR)。基于特殊函数推导出系统中断概率(OP)和概率密度函数(PDF)的解析表达式,并进一步得到各种调制方式下系统平均误符号率(ASER)的理论计算公式。最后通过无穷级数展开推导出高信噪比时平均误符号率的渐近闭合表达式,分析系统能获得的分集增益和阵列增益。计算机仿真表明所提出的性能分析方法的正确性,分析了天线数量和信道参数对卫星-地面混合无线通信系统性能的影响,为实际系统设计提供了参考和依据。     

电子式互感器校验方法的设计与实现

对电子式互感器进行校验是确保其在电力系统中成功应用的前提.针对目前电子式互感器校验过程中存在的问题,提出了一种新的校验方法.利用有限次迭代和最小二乘法相结合的组合测量原理实现频率的快速准确测量;引入准同步算法并对其实现过程中几个关键环节的设计及参数选取问题进行讨论,有效抑制了因电网频率波动而导致的非同步采样对校验结果的影响;最后对校验方法的实现过程进行了仿真与实测,验证了方法的性能及效率.为便携式智能化校验装置的研制提供了强有力的理论支持.      

PWM型半桥式开关稳压电源的设计

介绍了一款基于PWM技术的半桥式开关稳压电源.给出了高频变压器、PWM控制及驱动电路的详细设计方法.用该方法设计了一台功率为150 W、输出电压为 15 V的半桥式开关稳压电源,并给出了实验波形.     

铜颗粒污染物对变压器油运动黏度的影响

变压器油中铜颗粒污染物的存在极易影响油液的理化性能,甚至导致重大生产事故,因而根据ISO4406:1999标准,配制了24组不同污染度的含铜颗粒油样并通过实验测试了变压器油样的运动黏度性能（40℃）,获得了实验范围内铜颗粒污染度对油液运动黏度的影响规律;然后根据测试数据采用偏最小二乘（PLS）法和支持向量机（SVM）法建立了油样中不同粒径、含量的铜颗粒与油液运动黏度的数学模型,探讨了不同粒径、含量的铜颗粒污染物对油液运动黏度的影响规律。结果表明:随着铜颗粒污染度的增加,油液的运动黏度减小;铜颗粒污染物粒径在15～25 μm范围内对油液运动黏度的影响较大,且粒径越大,油液运动黏度也越大;所建立SVM模型对验证集进行预测的相关系数和均方根误差（RMSE）分别是0.962 6、4.597×10-5。为消除实验误差,减少人为因素的影响比较准确地掌握铜颗粒污染物对油液运动黏度的影响提供了新的途径。      

浅议变压器的经济负荷率

结合按电度表计费的电度电价、按投运变压器容量计费的基本电价、变压器市场价格、银行利率等因素,以S9系列10KV/0.4KV变压器为例,推导出变压器的经济负荷率。并注意到,对同一设计系列变压器而言,随着其容量的增长,铜损、铁损和价格都在增加,而这些数据又都是离散的,为便于分析,文中分别把铜损、铁损和价格表示成变压器容量的函数,给变压器的运行成本分析带来了方便。     

单相四开关Z源AC-AC变换器

提出了一种单相四开关Z源AC-AC变换器,它包括电压型与电流型两类。与传统脉宽调制AC-AC变换器相比,它具有如下特点:输出电压范围宽且可升可降,输出电压与输入电压可同相或反相,冲击和谐波电流小,可靠性高。针对AC-AC变换器中固有的开关管换流问题,提出了一种适用于单相四开关Z源AC-AC变换器的换流方案,无需外加缓冲吸收电路即可实现开关管的安全换流。以电压型单相四开关Z源AC-AC变换器为例,对其进行了理论分析、仿真和原理试验研究。实验结果证实了Z源变换器的特性及换流方案的可行性。     

An experimental investigation on static directional stability

A generic aircraft usually loses its static directional stability at moderate angle of attack (typically 20–30?). In this research, wind tunnel studies were performed using an aircraft model with moderate swept wing and a conventional vertical tail. The purpose of this study was to investigate flow mechanisms responsible for static directional stability. Measurements of force, surface pressure and spatial flow field were carried out for angles of attack from 0? to 46? and sideslip angles from ?8? to 8?. Results of the wind tunnel experiments show that the vertical tail is the main contributor to static directional stability, while the fuselage is the main contributor to static directional instabil-ity of the model. In the sideslip attitude for moderate angles of attack, the fuselage vortex and the wing vortex merged together and changed asymmetrically as angle of attack increased on the wind-ward side and leeward side of the vertical tail. The separated asymmetrical vortex flow around the vertical tail is the main reason for reduction in the static directional stability. Compared with the wing vortices, the fuselage vortices are more concentrated and closer to the vertical tail, so the yaw-ing moment of vertical tail is more unstable than that when the wings are absent. On the other hand, the attached asymmetrical flow over the fuselage in sideslip leads to the static directional instability of the fuselage being exacerbated. It is mainly due to the predominant model contour blockage effect on the windward side flow over the model in sideslip, which is strongly affected by angle of attack.     

400 Hz电流互感器检定及量值溯源系统

介绍了400Hz电流互感器检定及量值溯源系统的组成、工作原理、不确定度分析及量值溯源问题。该系统可检定0.02级及以下等级的400Hz电流互感器。     

Direct thrust test and asymmetric performance of porous ionic liquid electrospray thruster

In order to meet the demand of CubeSats for low power and high-performance micro-propulsion system, a porous ionic liquid electrospray thruster prototype is developed in this study. 10 × 10 conical emitter arrays are fabricated on an area of 3.24 cm² by computer numerical control machining technology. The propellant is 1-ethyl-3-methylimidazolium tetrafluoroborate. The overall dimension of the assembled prototype is 3 cm × 3 cm × 1 cm, with a total weight of about 15 g (with propellant). The performance of this prototype is tested under vacuum. The results show that it can work in the voltage range of ±2.0 kV to ±3.0 kV, and the maximum emission current and input power are about 355 μA and 1.12 W. Time of Flight (TOF) mass spectrometry results show that cationic monomers and dimers dominate the beam in positive mode, while a higher proportion of higher-order solvated ion clusters in negative mode. The maximum specific impulse is 2992 s in positive mode and 849 s in negative mode. The thrust is measured in two methods: one is calculated by TOF results and the other is directly measured by high-precision torsional thrust stand. The thrust (T) obtained by these two methods conforms to a certain scaling law with respect to the emission current (I_em) and the applied voltage (V_app), following the scale of T ∼ I_emV_app^0.5, and the thrust range is from 2.1 μN to 42.6 μN. Many thruster performance parameters are significantly different in positive and negative modes. We speculate that due to the higher solvation energy of the anion, more solvated ion clusters are formed rather than pure ions under the same electric field. It may help to improve thruster performance if porous materials with smaller pore sizes are used as reservoirs. Although there are still many problems, most of the performance parameters of ILET-3 are good, which can theoretically meet the requirements of CubeSats for micro-propulsion system.     

非对称安装布局下大型高超声速风洞引射增强型二次喉道数值优化

增强二次喉道扩压能力,降低接力引射器的引射流量是提高大型高超声速风洞运行经济性的重要技术措施。引射增压型二次喉道通过在二次喉道侧壁引射低总压介质增强其扩压能力,是一种非常有潜力的实用方案。本文通过数值模拟研究了其在长模型大攻角等非对称安装布局条件下的运行流场,并进一步优化了压力、介质类型等引射条件。结果表明,优化方案在高超声速风洞复杂运行条件下仍能保持较高的二次喉道扩压能力,较大幅度降低接力引射器的引射流量,具有可行性和实用性。