全文获取类型
收费全文 | 256篇 |
免费 | 17篇 |
国内免费 | 12篇 |
专业分类
化学 | 95篇 |
力学 | 8篇 |
综合类 | 1篇 |
数学 | 32篇 |
物理学 | 149篇 |
出版年
2022年 | 5篇 |
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 7篇 |
2017年 | 14篇 |
2016年 | 15篇 |
2015年 | 17篇 |
2014年 | 16篇 |
2013年 | 13篇 |
2012年 | 16篇 |
2011年 | 21篇 |
2010年 | 9篇 |
2009年 | 27篇 |
2008年 | 17篇 |
2007年 | 27篇 |
2006年 | 14篇 |
2005年 | 9篇 |
2004年 | 6篇 |
2003年 | 7篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 6篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1990年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有285条查询结果,搜索用时 15 毫秒
61.
Pointing errors caused by the atmospheric turbulence will degrade the performance of free space optical (FSO) communication systems, especially the bit error rate (BER). In this paper, we innovatively analyze the relationship between BER and pointing errors by the probability density functions (PDFs) and intensity displacement in focal plane under the On-Off Keying (OOK) modulation conditions. The closed-loop experimental system is set up in laboratory, where the fast steering mirror (FSM) is real-time controlled by embedded controller with the parallel processing technology and the atmospheric turbulence is simulated by a turbulence simulation box. The results of repeated experiments show that the method of pointing errors correction we proposed is efficient under the conditions of atmospheric turbulence. By utilizing our method, the BER can decrease from nearly 10−3 to nearly or even below 10−9, thus improving the performance of FSO communication systems significantly. 相似文献
62.
对准光学谐振腔诊断等离子体的传统诊断方法进行了改进,提出一种可用于诊断瞬态等离子体的实时诊断方法。通过实验证明,矢量网络分析仪点频测试(CW Sweep)对信号相位变化的测量速度足够快,可以跟上等离子体鞘套和尾迹的变化速度,因此利用该方法诊断等离子体鞘套和尾迹是可行的。通过对诊断系统性能的评估,表明诊断装置具有较高的实时性,对等离子体电子浓度有较宽的测量范围,测量范围可达10-1~1015m-3。 相似文献
63.
设计了一套嵌入式平台上实现的视频目标跟踪系统.该系统采用CMOS图像传感器获取视频信号,利用Z228多媒体芯片自带的ARM9处理器完成视频信号的控制,并通过MPEG-4硬件编码器实现视频信号的压缩.用Mean Shift算法跟踪运动目标,针对其收敛的局限性设置多个搜索点来提高其跟踪效果.通过减少采样点和标记已计算点来提高代码运行速度,增强了跟踪的实时性.实验结果表明,本系统能以27 fps速率连续稳定地实现视频目标的跟踪. 相似文献
64.
Ruifang Fang Aijun Zeng Linglin Zhu Longhai Liu Huijie Huang 《Optics Communications》2012,285(24):4884-4886
A simultaneous measurement method for the retardation and the fast axis angle of the eighth-wave plate in real time is proposed. The beam emitted from the laser passes through the circular polarizer and the eighth-wave plate to be measured successively, and then is split by the Dammann grating to form three sub-beams. They are analyzed by an analyzer array and detected by a detector array. Three detection signals are obtained simultaneously to calculate the retardation and the fast axis angle in real time. In experiments, a crystal quartz sample is measured at different fast axis angles. The average and standard deviation of its retardation respectively are 40.9°and 0.5°. The maximum measurement deviation of the fast axis angle is 2.1°. The usefulness of the method is verified. 相似文献
65.
差分吸收光谱(DOAS)法是一种有效的监测大气污染气体浓度的光学遥感方法,不仅有好的时间分辨率,而且测量灵敏度也很高。但是由于遥测系统处于复杂的大气环境中,各种干扰因素以及恶劣的气候条件,都会对系统产生影响。针对现有实时、在线监测差分吸收光谱系统中存在的不足,作者提出了一种基于改进Elman网络的实时预测模型。利用逐步回归筛选预测因子,不仅降低了预测网络的复杂程度,而且增强了系统的预测实时性。利用带自适应学习率的动态BP算法对改进的Elman网络进行训练,使预测系统能更好地辨识要预测的差分吸收光谱系统,该模型能较准确地对DOAS系统监测污染物数据进行实时跟踪监控,一定程度上弥补了遥测系统的不足。 相似文献
66.
Parametric nonlinear optimal control problems subject to control and state constraints are studied. Two discretization methods are discussed that transcribe optimal control problems into nonlinear programming problems for which SQP-methods provide efficient solution methods. It is shown that SQP-methods can be used also for a check of second-order sufficient conditions and for a postoptimal calculation of adjoint variables. In addition, SQP-methods lead to a robust computation of sensitivity differentials of optimal solutions with respect to perturbation parameters. Numerical sensitivity analysis is the basis for real-time control approximations of perturbed solutions which are obtained by evaluating a first-order Taylor expansion with respect to the parameter. The proposed numerical methods are illustrated by the optimal control of a low-thrust satellite transfer to geosynchronous orbit and a complex control problem from aquanautics. The examples illustrate the robustness, accuracy and efficiency of the proposed numerical algorithms. 相似文献
67.
基于灰度冗余的红外图像自适应输出窗技术 总被引:6,自引:4,他引:2
根据红外图像的特点,提出了一种基于灰度冗余的红外图像自适应输出窗技术.该技术通过对图像灰度直方图的统计,设定适当的阈值,将灰度像素分布为零的灰度级进行完全压缩,将小于阈值的灰度级映射到阈值灰度级,然后将有效灰度级在整个可显示灰度级范围内作等间距排列.该算法在压缩灰度冗余的同时,实现了对有效图像灰度级的无损均衡,提高了图像质量,并能实现实时处理. 相似文献
68.
69.
A 2.79 COMPETITIVE ON-LINE ALGORITHM FOR TWO PROCESSOR REAL-TIME SYSTEMS WITH UNIFORM VALUE DENSITY 总被引:1,自引:0,他引:1
YANGQIFAN 《高校应用数学学报(英文版)》1997,12(3):333-342
We consider the problem of competitive on-liae scheduling in two processor real-timesystems. In our model all tasks have common value density. Each task has a release time. an exe-cution time and a deadline. The scheduler is given no information about a task until it is released.And the value will be achieved if and only if the task is completed by its deadlirte. Moreover. wesuppose that migratbn is not allowed. The goal of the scheduler is to obtain as much value as pos-sible. In this paper we show that the competitive multipber of Safe-Risky-fixed Algorithm irt [4]is really 3 and presents a modified algorithm (Safe-Risky-unfixed Algorithm) that achieves a com-petitive mukiplier of 2. 79. 相似文献
70.
Among data acquisition techniques in digital photoelasticity, the integrated phase shifting technique (IPST) can real-time analyze the photoelastic parameters at a video rate of the high speed CCD camera. In this paper, fourteen algorithms are described by different configurations of the rotating an analyzer at a constant rate and an output quarter-wave plate at another constant rate. The theoretical comparisons of the algorithms are given by the simulated phase distributions of the isochromatic and isoclinic parameters of the disk under two cases that the load keeps unchangeable or linear increasing in exposure time of the camera. Then a guideline is given to alleviate the influence of the load changing with time on the IPST. 相似文献