全文获取类型
收费全文 | 2942篇 |
免费 | 717篇 |
国内免费 | 457篇 |
专业分类
航空 | 2249篇 |
航天技术 | 669篇 |
综合类 | 360篇 |
航天 | 838篇 |
出版年
2024年 | 3篇 |
2023年 | 51篇 |
2022年 | 140篇 |
2021年 | 139篇 |
2020年 | 160篇 |
2019年 | 118篇 |
2018年 | 136篇 |
2017年 | 133篇 |
2016年 | 122篇 |
2015年 | 167篇 |
2014年 | 199篇 |
2013年 | 198篇 |
2012年 | 223篇 |
2011年 | 232篇 |
2010年 | 249篇 |
2009年 | 231篇 |
2008年 | 161篇 |
2007年 | 180篇 |
2006年 | 173篇 |
2005年 | 163篇 |
2004年 | 98篇 |
2003年 | 85篇 |
2002年 | 79篇 |
2001年 | 76篇 |
2000年 | 51篇 |
1999年 | 80篇 |
1998年 | 59篇 |
1997年 | 34篇 |
1996年 | 44篇 |
1995年 | 41篇 |
1994年 | 41篇 |
1993年 | 47篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 22篇 |
1989年 | 31篇 |
1988年 | 27篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有4116条查询结果,搜索用时 843 毫秒
991.
Ming-Tsan Lin Tian-Hua Liu 《IEEE transactions on aerospace and electronic systems》1998,34(4):1149-1164
A systematic controller design for a synchronous reluctance drive system is presented. This controller consists of two parts: a forward-loop H∞ controller to improve the transient response, and a load compensator to reduce the load disturbance. Based on a simplified model of the drive system, a control algorithm has been derived. Detailed analysis of the characteristics of the closed-loop system is presented. The effects of the parameter variations are also studied. A digital signal processor, TMS-320-C30, is used to implement the control algorithm. Both the speed control and the position control of the drive system can be implemented by using the proposed control method. Furthermore, all the control loops are executed by the digital signal processor. The system, as a result, is very flexible. The whole drive system performs well although its hardware is very simple. For speed control, the system can be operated at a speed as low as 1 r/min. For position control, the system can accurately control a one-axis table. In addition, the system also has good position tracking ability. Several experimental waveforms validate the simulated results 相似文献
992.
虚拟量仪——一种新的量仪设计思想 总被引:1,自引:0,他引:1
从虚拟仪器和虚拟现实技术出发,提出了虚拟量仪这一新概念,并对虚拟量仪的定义、特点、优点、结构形式以及虚拟量仪设计的基本思想、基本内容和关键技术作了分析。 相似文献
993.
Mingjun Pang Jinjia Wei Bo Yu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
Since the bubbly flow has extensive applications in the space field, it is very necessary to comprehend the dependence of hydrodynamic characteristics on gravity. In this paper, the dependence of the microbubble distribution and the liquid turbulence modulation by the microbubbles on gravity was investigated in detail with the Euler–Lagrange method. The liquid Navier–Stokes equation was solved using direct numerical simulations (DNS), and the microbubble motion was tracked with Newtonian motion equation considering drag force, shear lift force, added mass force, pressure gradient force, and wall lift force. The coupling between the gas and the liquid phases regarded the interphase force as a momentum source term in the momentum equation of the liquid. The results showed that the phase profile and the turbulence modulation by the microbubbles strongly depend on the magnitude of gravity. When the influence of gravity is relatively weak, the microbubbles approximately uniformly disperse in the wide central region of the channel, and the average statistics of the liquid turbulence are almost not influenced due to the addition of the microbubbles. However, when the influence of gravity is comparatively important, the majority of the microbubbles accumulate near the wall of the channel, and the injection of the microbubbles modifies the profiles of the liquid average statistics. 相似文献
994.
995.
996.
997.
利用全球导航卫星系统反射信号测量技术(GNSS-R)进行土壤湿度反演过程中,实际接收天线方向性会造成GNSS直反信号相关功率测量偏差。针对地基观测场景下天线方向性造成的相关功率的类余弦振荡问题,提出了基于多项式拟合的信号相关功率修正方法。为了验证所提方法的有效性,开展了地基GNSS-R土壤湿度观测实验,结果表明:基于多项式拟合的相关功率修正可以消除信号相关功率的类余弦振荡,提升GNSS-R土壤湿度反演中的观测数据有效性和反演结果准确性。 相似文献
998.
999.
Mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser 总被引:1,自引:3,他引:1
To expand the stable operating range of compressors, understanding the mechanism of flow instability at low flow rates is necessary. In this paper, the mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser is experimentally investigated, where the diffuser blade setting angle can be adjusted. Many dynamic pressure transducers are mounted on the casing surface of the compressor. From the design condition to surge, dynamic pressure data is recorded throughout the gradual process. According to the signal developing status, the typical modes of compressor instability are defined in detail, such as stall, mild surge, and deep surge. A relatively high-frequency stall wave originates in the impeller and propagates to the diffuser, and finally stimulates a deep surge in the compressor. The compressor behavior during surge differs at different diffuser vane angles. When the diffuser vane angle is adjusted, both the unstable form and the core factor affecting the overall machine stability change. A specific indicator is proposed to measure the instability of each component in a compressor, which can be used to determine the best region for stability extension technologies, such as a holed casing treatment, in different compressor applications. 相似文献
1000.