全文获取类型
收费全文 | 1972篇 |
免费 | 288篇 |
国内免费 | 553篇 |
专业分类
航空 | 1741篇 |
航天技术 | 368篇 |
综合类 | 370篇 |
航天 | 334篇 |
出版年
2024年 | 10篇 |
2023年 | 28篇 |
2022年 | 44篇 |
2021年 | 71篇 |
2020年 | 77篇 |
2019年 | 105篇 |
2018年 | 109篇 |
2017年 | 109篇 |
2016年 | 114篇 |
2015年 | 93篇 |
2014年 | 132篇 |
2013年 | 99篇 |
2012年 | 128篇 |
2011年 | 161篇 |
2010年 | 134篇 |
2009年 | 142篇 |
2008年 | 134篇 |
2007年 | 130篇 |
2006年 | 124篇 |
2005年 | 89篇 |
2004年 | 97篇 |
2003年 | 73篇 |
2002年 | 61篇 |
2001年 | 67篇 |
2000年 | 47篇 |
1999年 | 44篇 |
1998年 | 53篇 |
1997年 | 31篇 |
1996年 | 49篇 |
1995年 | 25篇 |
1994年 | 40篇 |
1993年 | 43篇 |
1992年 | 32篇 |
1991年 | 29篇 |
1990年 | 25篇 |
1989年 | 35篇 |
1988年 | 17篇 |
1987年 | 6篇 |
1986年 | 6篇 |
排序方式: 共有2813条查询结果,搜索用时 46 毫秒
51.
Air route network optimization,one of the essential parts of the airspace planning,is an effective way to optimize airspace resources,increase airspace capacity,and alleviate air traffic con gestion.However,little has been done on the optimization of air route network in the fragmented airspace caused by prohibited,restricted,and dangerous areas (PRDs).In this paper,an air route network optimization model is developed with the total operational cost as the objective function while airspace restriction,air route network capacity,and non-straight-line factors (NSLF) are taken as major constraints.A square grid cellular space,Moore neighbors,a fixed boundary,together with a set of rules for solving the route network optimization model are designed based on cellular automata.The empirical traffic of airports with the largest traffic volume in each of the 9 flight information regions in mainland China is collected as the origin-destination (OD) air port pair demands.Based on traffic patterns,the model generates 35 air routes which successfully avoids 144 PRDs.Compared with the current air route network structure,the number of nodes decreases by 41.67%,while the total length of flight segments and air routes drop by 32.03% and 5.82% respectively.The NSLF decreases by 5.82% with changes in the total length of the air route network.More importantly,the total operational cost of the whole network decreases by 6.22%.The computational results show the potential benefits of the model and the advantage of the algorithm.Optimization of air route network can significantly reduce operational cost while ensuring operation safety. 相似文献
52.
53.
《中国航空学报》2016,(5):1397-1404
Due to the lack of information of subsurface lunar regolith stratification which varies along depth, the drilling device may encounter lunar soil and lunar rock randomly in the drilling process. To meet the load safety requirements of unmanned sampling mission under limited orbital resources, the control strategy of autonomous drilling should adapt to the indeterminable lunar environments. Based on the analysis of two types of typical drilling media (i.e., lunar soil and lunar rock), this paper proposes a multi-state control strategy for autonomous lunar drilling. To represent the working circumstances in the lunar subsurface and reduce the complexity of the control algo-rithm, lunar drilling process was categorized into three drilling states:the interface detection, initi-ation of drilling parameters for recognition and drilling medium recognition. Support vector machine (SVM) and continuous wavelet transform were employed for the online recognition of dril-ling media and interface, respectively. Finite state machine was utilized to control the transition among different drilling states. To verify the effectiveness of the multi-state control strategy, drilling experiments were implemented with multi-layered drilling media constructed by lunar soil simulant and lunar rock simulant. The results reveal that the multi-state control method is capable of detect-ing drilling state variation and adjusting drilling parameters timely under vibration interferences. The multi-state control method provides a feasible reference for the control of extraterrestrial autonomous drilling. 相似文献
54.
为解决框架类零件的高位置度孔的精密镗削加工问题,研发了ZYTC600专用镗削机床.其精度指标优于现有的通用镗床(包括进口镗床),尺寸在500mm范围内,同轴度优于2μm,垂直度优于5μm,其中的关键部件双回转轴系分度精密转台采用了一套精密调整机构,可以实现0.2s”的精密调整,通过有限元分析得到其驱动刚度可达到800N/μm,可以满足加工需求. 相似文献
55.
在设计控制系统模型时,首先引入了有限差分法(FDM)离散化后的波动方程,然后基于H∞最优控制原理,检查闭环控制系统的稳定性和可探测性,对控制系统传感器和作动器的位置进行优化配置.通过计算闭环反馈系统的范数,最终得到了控制系统的最优控制算法.仿真结果表明,利用波动方程来优化控制系统传感器和作动器位置的算法是可行的,可广泛应用于飞控系统的设计中. 相似文献
56.
R.V. Semenyshyn A.N. VeklichI.L. Babich V.F. Boretskij 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Plasma of the free burning electric arc between Ag–SnO2–ZnO composite electrodes as well as brass electrodes were investigated. The plasma temperature distributions were obtained by Boltzmann plot method involving Cu I, Ag I or Zn I spectral line emissions. The electron density distributions were obtained from the width and from absolute intensity of spectral lines. The laser absorption spectroscopy was used for measurement of copper atom concentration in plasma. Plasma equilibrium composition was calculated using two independent groups of experimental values (temperature and copper atom concentration, temperature and electron density). It was found that plasma of the free burning electric arc between brass electrodes is in local thermodynamical equilibrium. The experimental verification of the spectroscopic data of Zn I spectral lines was carried out. 相似文献
57.
针对复合壳阻尼结构的拓扑减振优化问题,以约束阻尼层的有限单元为设计变量,采用体积比、模态频率和振型为优化约束条件,构建以多模态权重系数的结构模态损耗因子数值关系为优化目标函数的拓扑减振优化模型。为了拓展优化目标灵敏度具有不局限于某一变密度法插值模型的形式,推导了数值表达式的一般函数式。动力学优化中优化目标灵敏度正、负数集共存,使得非凸性的目标函数设计变量出现负值或优化函数寻优于局部极值点。为此,推导出复合壳阻尼结构的全域灵敏度改进优化准则法迭代格,以确保每次迭代域均为全域设计变量集。结合有限单元法编程实现了复合壳阻尼结构改进准则法,并对复合壳结构进行拓扑减振优化分析。结果表明:在敷设体积减为全覆盖的50%时,复合壳结构的模态损耗因子增减偏差为10%,具有提升减振的轻量化设计目的;各阶目标函数和拓扑构型所需的迭代次数少,中间密度区域较小,多阶优于单阶模态优化函数,易于获得全域寻优的有效减振。 相似文献
58.
59.
针对近地低轨三轴稳定卫星在轨管理后期,除磁强计外其他姿态敏感器都无效情况下的姿态异常问题,分析了三轴稳定卫星失去姿态基准后,星体自旋状态下三轴磁强计测量数据的特点,提出了使用磁强计测量数据的矢量信息,以找到能够获取卫星状态的方法,从而建立了磁强计测量矢量与卫星自旋轴的几何关系,给出了处于自旋状态下的卫星自旋轴确定方法。通过此种辨识方法,获得了某气象卫星姿态异常翻转状态下的自旋矢量方向和自旋角速度,从而证明了该辨识方法快速、有效,可以作为姿态异常卫星自旋状态的辨识手段,为恢复卫星姿态提供了重要信息,具有一定的工程应用价值。 相似文献
60.