Microstructures and fatigue properties of electron beam welds with beam oscillation for heavy section TC4-DT alloy

With the development of the manufacturing technology, electron beam welding（EBW） is capable of producing titanium alloy large parts in aero fields. To increase the applications and improve the properties, EBW with beam oscillation was investigated on TC4-DT alloy with50 mm thickness. We detected the welding samples by X-ray NDT, observed the microstructures of the welds, and tested the fatigue properties of the joints. The results showed that EBW with beam oscillation improved the weld morphology as well as welding quality, and the microstructure homogeneity of the welds and HAZ along the weld penetration were also improved. The fatigue properties of the joints with beam oscillation were more excellent than those of conventional EBW, even equal to those of the base metal under high stresses. The influences of the processing and the microstructure on the properties with beam oscillation were discussed.     

工件旋转式磨削大直径硅片技术的研究进展

在分析单晶硅片的典型加工工艺基础上,介绍了基于工件旋转式磨削法的大直径硅片超精密加工技术的原理及特点,综述了该项技术的试验研究进展及建立的数学模型,分析了有限元法在工件旋转式磨削硅片研究中的应用,最后提出了工件旋转式磨削大直径硅片技术目前存在的问题和今后的发展趋势。     

中纬电离层暴负相开始时间与磁暴主相开始时间的对应关系

本文假设在磁暴主相期间,由于极光椭圆带处的空气被加热上升,从而使高纬高空出现富含分子的气体。这些气体由于扩散及与中性风的相互作用会向中低纬移动,其所到之处电子消失系数增加,从而导致负相电离层暴的发生。计算给出了全球中纬电离层暴负相的开始时间与磁暴主相开始时间之间的关系,并讨论了负相电离层暴发生的"时间禁区"问题。结果与有关统计结果符合得很好。      

动态测角仪测量误差检测方法研究

检测高速测角仪的测量误差,其检测（方法）装置必须有高的动态响应和高的检测准确度。介绍了动态测角仪测量准确度检测系统的检测原理、测量系统的总体设计及实验结果。     

一种电离层垂直剖面物理模型及其与IRI-90模型的比较

在80-500km范围内考虑了3种中性成份的4种离子,从严格的电子和离子密度连续方程出发,对中性风和扩散效应进行了全面、连续的考虑,由此建立了一种电离层的物理模式;在此模式的基础上针对北京地区分别对太阳活动低年(F_10.7=60)、高年(F_10.7=300)的春(DOY=90)、夏(DOY=180)、秋(DOY=270)、冬(DOY=365)进行计算,并将所得结果与IRI-90进行了比较.结果表明: E层为典型的Chapman层: E-F谷区深度一般为0.2-0.5之间,比IRI要深;F₁缘在太阳活动低年的四季都出现,其中夏天最明显,已形成了一个F₁层,冬天最不明显,仅表现为一个轻微的凸缘,在太阳活动高年只有夏天出现了F₁凸缘,这与现有理论相符合,而IRI-90较少出现明显的F₁缘;F₂层的电子密度是活动高年比低年大,平均冬天比夏天大,这与观测结果也基本符合.      

Study on segmented distribution for reliability evaluation

In practice, the failure rate of most equipment exhibits different tendencies at different stages and even its failure rate curve behaves a multimodal trace during its life cycle. As a result, traditionally evaluating the reliability of equipment with a single model may lead to severer errors. However, if lifetime is divided into several different intervals according to the characteristics of its failure rate, piecewise fitting can more accurately approximate the failure rate of equipment. There-fore, in this paper, failure rate is regarded as a piecewise function, and two kinds of segmented dis-tribution are put forward to evaluate reliability. In order to estimate parameters in the segmented reliability function, Bayesian estimation and maximum likelihood estimation (MLE) of the seg-mented distribution are discussed in this paper. Since traditional information criterion is not suit-able for the segmented distribution, an improved information criterion is proposed to test and evaluate the segmented reliability model in this paper. After a great deal of testing and verification, the segmented reliability model and its estimation methods presented in this paper are proven more efficient and accurate than the traditional non-segmented single model, especially when the change of the failure rate is time-phased or multimodal. The significant performance of the segmented reli-ability model in evaluating reliability of proximity sensors of leading-edge flap in civil aircraft indi-cates that the segmented distribution and its estimation method in this paper could be useful and accurate.     

台风激发的声重力波的可探测性研究

通过对北京大学高频多普勒台站(39.4°N,116.2°E)多年的观测资料及相关台风资料的统计分析,着重研究了高频多普勒探测手段对由台风引起的扰动电离层响应的可探测性问题.通过对台风登陆前后的高频多普勒观测记录的细致分析及与宁静条件下记录的对比研究,表明高频多普勒观测手段可以很好地探测由台风激发的声重力波,尤其是对台风登陆前后所激发的扰动具有较高的可探测性.在所分析的24次台风事件中有明显扰动记录的高达22次(22/24).结合观测事实与Huang等的统计结果(2/12)进行了比较,并进行了可能的原因分析.通过数值模拟的方法对观测到的主要扰动的非线性传播过程进行了模拟再现,结果基本上与观测结果及线性传播理论一致.确认了一般情况下高可探测性这一事实,但同时也表明可探测性和台风激发源与探测位置及中尺度TIDs在电离层中的传播模式有密切关系.      

Robust three-dimensional path-following control for an under-actuated stratospheric airship

This paper proposes a robust three-dimensional (3-D) path-following controller for an under-actuated stratospheric airship in the presence of uncertainties. The resultant control system exhibits an inner-outer loop control structure. In the outer control loop, the path-following error dynamics is constructed in a moving Serret-Frenet frame and a new guidance law with the sideslip and attack angle compensation is designed, which decreases the path-following steady-state error. In the inner control loop, a disturbance observer based backstepping control law is proposed to achieve the desired dynamic behavior on the path. Furthermore, a new velocity tracking control strategy is developed which aligns the resultant velocity tangent to the path. Finally, numerical simulation results are shown to verify the effectiveness of the proposed controller.     

Analysis of BDS-3 distributed autonomous navigation based on BeiDou system simulation platform

Satellite autonomous navigation is an important function of the BeiDou-3 navigation System (BDS-3). Satellite autonomous navigation means that the navigation satellite uses long-term forecast ephemeris and Inter-Satellite Link (ISL) measurements to determinate its own spatial position and time reference without the support of the ground Operation and Control System (OCS) for a long time to ensure that the navigation system can normally maintain the time and space reference. This paper aims to analyze the feasibility of distributed autonomous navigation algorithms. For the first time, a ground parallel autonomous navigation test system (GPANTS) is built. The performance of distributed autonomous navigation is then analyzed using the two-way ISL ranging of BDS-3 satellites. First, the BDS simulation platform and the GPANTS are introduced. Then, the basic principles of distributed satellite autonomous orbit determination and time synchronization based on ISL measurements are summarized. Preliminary evaluation of the performance of the BDS-3 constellation autonomous navigation service under ideal conditions through simulation data. Then the performance of autonomous navigation for 22 BeiDou-3 satellites using ISL measurements is evaluated. The results show that when satellites operate autonomously for 50 days without the support of any ground station, the User Range Error (URE) of autonomous orbit determination is better than 3 m, and the time synchronization accuracy is better than 4 ns.     

Assembly Line Balancing Based on Double Chromosome Genetic Algorithm

Aiming at assembly line balancing problem, a double chromosome genetic algorithm （DCGA） is proposed to avoid trapping in local optimum, which is a disadvantage of standard genetic algorithm （SGA）. In this algorithm, there are two chromosomes of each individual, and the better one, regarded as dominant chromosome, determines the fitness. Dominant chromosome keeps excellent gene segments to speed up the convergence, and re cessive chromosome maintains population diversity to get better global search ability to avoid local optimal solu- tion. When the amounts of chromosomes are equal, the population size of DCGA is half that of SGA, which significantly reduces evolutionary time. Finally, the effectiveness is verified by experiments.