全文获取类型
收费全文 | 6499篇 |
免费 | 624篇 |
国内免费 | 918篇 |
专业分类
航空 | 3625篇 |
航天技术 | 2042篇 |
综合类 | 804篇 |
航天 | 1570篇 |
出版年
2022年 | 43篇 |
2021年 | 59篇 |
2020年 | 111篇 |
2019年 | 41篇 |
2018年 | 65篇 |
2017年 | 60篇 |
2016年 | 84篇 |
2015年 | 138篇 |
2014年 | 292篇 |
2013年 | 304篇 |
2012年 | 315篇 |
2011年 | 340篇 |
2010年 | 465篇 |
2009年 | 386篇 |
2008年 | 457篇 |
2007年 | 352篇 |
2006年 | 267篇 |
2005年 | 254篇 |
2004年 | 204篇 |
2003年 | 212篇 |
2002年 | 139篇 |
2001年 | 242篇 |
2000年 | 234篇 |
1999年 | 243篇 |
1998年 | 282篇 |
1997年 | 223篇 |
1996年 | 220篇 |
1995年 | 190篇 |
1994年 | 193篇 |
1993年 | 171篇 |
1992年 | 159篇 |
1991年 | 121篇 |
1990年 | 101篇 |
1989年 | 146篇 |
1988年 | 71篇 |
1987年 | 58篇 |
1986年 | 66篇 |
1985年 | 96篇 |
1984年 | 57篇 |
1983年 | 64篇 |
1982年 | 62篇 |
1981年 | 71篇 |
1980年 | 34篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1975年 | 26篇 |
1974年 | 25篇 |
1973年 | 24篇 |
1971年 | 31篇 |
1969年 | 26篇 |
排序方式: 共有8041条查询结果,搜索用时 62 毫秒
991.
This paper presents a two-level geometric calibration method for the permanent magnet (PM) spherical actuator to improve its motion control accuracy. The proposed actuator is com- posed of a stator with circumferential coils and a rotor with multiple PM poles. Due to the assembly and fabrication errors, the real geometric parameters of the actuator will deviate from their design values. Hence, the identification of such errors is critical for the motion control tasks. A two-level geometric calibration approach is proposed to identify such errors. In the first level, the calibration model is formulated based on the differential form of the kinematic equation, which is to identify the geometric errors in the spherical joint. In the second level, the calibration model is formulated based on the differential form of torque formula, which is to calibrate the geometric parameters of the magnetization axes of PM poles and coils axes. To demonstrate the robustness and availability of the calibration algorithm, simulations are conducted. The results have shown that the proposed two-level calibration method can effectively compensate the geometric parameter errors and improve the positioning accuracy of the spherical actuator. 相似文献
992.
The equivalent linearization method (ELM) is modified to investigate the nonlinear flut- ter system of an airfoil with a cubic damping. After obtaining the linearization quantity of the cubic nonlinearity by the ELM, an equivalent system can be deduced and then investigated by linear flut- ter analysis methods. Different from the routine procedures of the ELM, the frequency rather than the amplitude of limit cycle oscillation (LCO) is chosen as an active increment to produce bifurca- tion charts. Numerical examples show that this modification makes the ELM much more efficient. Meanwhile, the LCOs obtained by the ELM are in good agreement with numerical solutions. The nonlinear damping can delay the occurrence of secondary bifurcation. On the other hand, it has marginal influence on bifurcation characteristics or LCOs. 相似文献
993.
It is urgent to carry out detailed research on storage performance of rubber sealing ring to get the criterion for its storage life. This paper acquires material ageing regularity by theoretical analysis and experimental confirmation. On this condition, failure mode and failure criterion of typical sealing structure is studied, and the failure mechanism is found. Thus by analyzing the stress distribution, the relationship between ageing state and sealing condition is established. Rationalization proposal is put forward and storage life of sealing ring is evaluated. The research mentionedabove has special reference to the design of sealing structures and can provide reference for prolonging their service life. 相似文献
994.
995.
周烨斐刘艳涛 《民用飞机设计与研究》2014,(3):88-91
对比航电传统系统架构,分析了综合模块化航电系统(Integrated Modular Avionics,简称IMA)架构的优点;针对IMA系统功能的高集成度带来的资源分配难度和安全性要求,提出了IMA系统分区和资源分配策略,并给出了具体方法,对民用飞机IMA系统设计具有一定的指导意义和实用价值。 相似文献
996.
997.
Harry Y. McSween Jr. David W. Mittlefehldt Andrew W. Beck Rhiannon G. Mayne Timothy J. McCoy 《Space Science Reviews》2011,163(1-4):141-174
Howardite-eucrite-diogenite (HED) meteorites, thought to be derived from 4 Vesta, provide the best sampling available for any differentiated asteroid. However, deviations in oxygen isotopic composition from a common mass-fractionation line suggest that a few eucrite-like meteorites are from other bodies, or that Vesta was not completely homogenized during differentiation. The petrology and geochemistry of HEDs provide insights into igneous processes that produced a crust composed of basalts, gabbros, and ultramafic cumulate rocks. Although most HED magmas were fractionated, it is unresolved whether some eucrites may have been primary melts. The geochemistry of HEDs indicates that bulk Vesta is depleted in volatile elements and is relatively reduced, but has chondritic refractory element abundances. The compositions of HEDs may favor a magma ocean model, but inconsistencies remain. Geochronology indicates that Vesta accreted and differentiated within the first several million years of solar system history, that magmatism continued over a span of ??10 Myr, and that its thermal history extended for perhaps 100 Myr. The protracted cooling history is probably responsible for thermal metamorphism of most HEDs. Impact chronology indicates that Vesta experienced many significant collisions, including during the late heavy bombardment. The age of the huge south pole crater is controversial, but it probably ejected Vestoids and many HEDs. Continued impacts produced a regolith composed of eucrite and diogenite fragments containing only minor exotic materials. HED meteorites serve as ground truth for orbital spectroscopic and chemical analyses by the Dawn spacecraft, and their properties are critical for instrument calibration and interpretation of Vesta??s geologic history. 相似文献
998.
基于FDUC模型和Ⅲ-Ⅴ族元素共价半径不变的假设,本文出了Ⅲ-Ⅴ族固溶体中原子近邻和次近邻的键长表达式。这套键长表达式除依赖于四个Ⅲ-Ⅴ族元素的含量(x,y)外,只与四个纯化合物的晶胞参数或这四个Ⅲ-Ⅴ族元素的共价半径有关。而且也含盖了相应的伪二元固溶体的情况。本文还对其他的理论模型作了进一步的比较和讨论。 相似文献
999.
1000.
Robbins Earl R. Moody Douglas E. Happ William W. 《IEEE transactions on aerospace and electronic systems》1966,(4):370-375
Flowgraph techniques are extended to systems with piecewise-linear characteristics by developing criteria for construction of an optimum model from related subregions in which linearity holds. This requires the synthesis of several known techniques and results in a wide range of useful applications including: 1) devices with nonlinear characteristics which may be considered as linear over certain subregions; 2) networks whose response to changes in applied signal frequency or magnitude may be approximated by piecewise-linear asymptotes; 3) systems processing two or more signals simultaneously with different transfer or immitance characteristics for each signal; 4) circuits approximated by different equivalent circuits depending on the numerical values of critical parameters. Representative examples will illustrate these and similar applications. Procedures are presented to provide a logical, orderly, and effective approach to construct a model, to determine figures of merit, and to optimize the model for a prescribed region of operation or for a desired range of parameters. 相似文献