首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   331352篇
  免费   46641篇
  国内免费   2019篇
电工技术   24097篇
技术理论   37篇
综合类   34224篇
化学工业   36496篇
金属工艺   14256篇
机械仪表   24102篇
建筑科学   56189篇
矿业工程   16357篇
能源动力   11675篇
轻工业   20565篇
水利工程   14312篇
石油天然气   15589篇
武器工业   4468篇
无线电   21091篇
一般工业技术   27895篇
冶金工业   14538篇
原子能技术   2297篇
自动化技术   41824篇
  2024年   174篇
  2023年   3888篇
  2022年   6919篇
  2021年   9586篇
  2020年   9679篇
  2019年   6969篇
  2018年   6438篇
  2017年   8479篇
  2016年   9709篇
  2015年   10799篇
  2014年   23277篇
  2013年   19344篇
  2012年   24254篇
  2011年   26726篇
  2010年   20837篇
  2009年   21516篇
  2008年   19702篇
  2007年   24144篇
  2006年   21332篇
  2005年   18751篇
  2004年   15610篇
  2003年   14089篇
  2002年   11273篇
  2001年   9351篇
  2000年   7710篇
  1999年   5953篇
  1998年   4390篇
  1997年   3738篇
  1996年   3130篇
  1995年   2623篇
  1994年   2166篇
  1993年   1519篇
  1992年   1207篇
  1991年   914篇
  1990年   703篇
  1989年   653篇
  1988年   425篇
  1987年   318篇
  1986年   233篇
  1985年   283篇
  1984年   251篇
  1983年   188篇
  1982年   152篇
  1981年   106篇
  1980年   115篇
  1979年   65篇
  1978年   33篇
  1977年   33篇
  1975年   23篇
  1959年   35篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Semantic information can help robots understand unknown environments better. In order to obtain semantic information efficiently and link it to a metric map, we present a new robot semantic mapping approach through human activity recognition in a human–robot coexisting environment. An intelligent mobile robot platform called ASCCbot creates a metric map while wearable motion sensors attached to the human body are used to recognize human activities. Combining pre-learned models of activity–furniture correlation and location–furniture correlation, the robot determines the probability distribution of the furniture types through a Bayesian framework and labels them on the metric map. Computer simulations and real experiments demonstrate that the proposed approach is able to create a semantic map of an indoor environment effectively.  相似文献   
2.
ContextIn the last decade, software development has been characterized by two major approaches: agile software development, which aims to achieve increased velocity and flexibility during the development process, and user-centered design, which places the goals and needs of the system’s end-users at the center of software development in order to deliver software with appropriate usability. Hybrid development models, referred to as user-centered agile software development (UCASD) in this article, propose to combine the merits of both approaches in order to design software that is both useful and usable.ObjectiveThis paper aims to capture the current state of the art in UCASD approaches and to derive generic principles from these approaches. More specifically, we investigate the following research question: Which principles constitute a user-centered agile software development approach?MethodWe conduct a systematic review of the literature on UCASD. Identified works are analyzed using a coding scheme that differentiates four levels of UCASD: the process, practices, people/social and technology dimensions. Through subsequent synthesis, we derive generic principles of UCASD.ResultsWe identified and analyzed 83 relevant publications. The analysis resulted in a comprehensive coding system and five principles for UCASD: (1) separate product discovery and product creation, (2) iterative and incremental design and development, (3) parallel interwoven creation tracks, (4) continuous stakeholder involvement, and (5) artifact-mediated communication.ConclusionOur paper contributes to the software development body of knowledge by (1) providing a broad overview of existing works in the area of UCASD, (2) deriving an analysis framework (in form a coding system) for works in this area, going beyond former classifications, and (3) identifying generic principles of UCASD and associating them with specific practices and processes.  相似文献   
3.
ContextMemory safety errors such as buffer overflow vulnerabilities are one of the most serious classes of security threats. Detecting and removing such security errors are important tasks of software testing for improving the quality and reliability of software in practice.ObjectiveThis paper presents a goal-oriented testing approach for effectively and efficiently exploring security vulnerability errors. A goal is a potential safety violation and the testing approach is to automatically generate test inputs to uncover the violation.MethodWe use type inference analysis to diagnose potential safety violations and dynamic symbolic execution to perform test input generation. A major challenge facing dynamic symbolic execution in such application is the combinatorial explosion of the path space. To address this fundamental scalability issue, we employ data dependence analysis to identify a root cause leading to the execution of the goal and propose a path exploration algorithm to guide dynamic symbolic execution for effectively discovering the goal.ResultsTo evaluate the effectiveness of our proposed approach, we conducted experiments against 23 buffer overflow vulnerabilities. We observed a significant improvement of our proposed algorithm over two widely adopted search algorithms. Specifically, our algorithm discovered security vulnerability errors within a matter of a few seconds, whereas the two baseline algorithms failed even after 30 min of testing on a number of test subjects.ConclusionThe experimental results highlight the potential of utilizing data dependence analysis to address the combinatorial path space explosion issue faced by dynamic symbolic execution for effective security testing.  相似文献   
4.
We present two dual control approaches to the model maintenance problem based on adaptive model predictive control (mpc). The controllers employ systematic self-excitation and design experiments that are performed under normal operation, resulting in improved control performance with smaller output variance and less control effort. Our control formulations offer a novel approach to the question of how to excite the plant input to generate informative data within the context of mpc and adaptive control. One controller actively tries to reduce the parameter-estimate error covariances; the other controller maximizes the information in the signals for enhanced learning. Our approach differs from existing ones in that we let our controllers converge to standard certainty equivalence (ce) mpc when the parameter uncertainty decreases or more information is generated, and as a result we avoid plant excitation when the uncertainty is low or enough information has been generated. We demonstrate that the controllers work well with a large number of tuning configurations and also address the issue of models that are not admissible for control design.  相似文献   
5.
The present work concerns model predictive control (MPC) of centrifugal gas compressors and describes the development of an MPC application for the tasks of anti-surge and process control. More specifically, the MPC formulation focuses on the question of how the transient manipulation of driver torque can be used to improve the performance of anti-surge and process control. For the purpose of testing and validating the proposed control algorithm, an experimental compressor test rig is presented, which is designed to mimic a typical centrifugal compressor application in the oil and gas industry. Modeling and parameter identification of the experimental setup is followed by the realization of the MPC solution on an embedded system to comply with the stringent real-time requirements for anti-surge control. Testing is performed with experiments using suction and discharge side disturbances, which are created by rapid valve closures. For comparison the same tests are repeated with conventional control approaches. The test results indicate improvements in maintaining the distance to surge by up to 11%, while at the same time reducing the process control settling time by up to 50%.  相似文献   
6.
The relative sliding motion of two elastic bodies in contact induces wear process and contact shape evolution. In the case of a punch sliding on a substrate the transient process tends to a steady state for which the fixed contact stress and strain distribution develops in the contact zone. This state usually corresponds to a minimum of the wear dissipation power. The optimality conditions of the wear dissipation functional provide the contact stress distribution and the wear rate compatible with the rigid body punch motion. The present paper is aimed to extend the previous analyses [1], [2], [3], [4], [5] of steady state conditions to cases of periodic sliding of punch, assuming cyclic steady state conditions for both mechanical and thermal fields.  相似文献   
7.
The paper presents an improved sectional discretization method for evaluating the response of reinforced concrete sections. The section is subdivided into parametric subdomains that allow the modelization of any complex geometry while taking advantage of the Gauss quadrature techniques. In particular, curved boundaries are dealt with two nested parametric transformations, reducing the modeling approximation. It is shown how the so-called fiber approach is simply a particular case of the present more general method. Many benchmarks are presented in order to assess the accuracy of the results. The influence of the discretization into subdomains and of the quadrature rules, chosen for integration, is discussed. The numerical tests highlight also the effects of spurious stress distributions in the tensile concrete zone, due the interpolation functions adopted for the Gauss integration. It is shown how balancing the number of subdomains and the number of sampling points such spurious effects vanish. The method shows to be accurate, very flexible in the discretization process and robust in analyzing any sectional state. Moreover, it converges faster than the fiber method, reducing the computational demand. All these properties are of great importance when the computations are iteratively repeated, as for the case of the sectional analysis within a computational procedure for a R.C. frame analysis.  相似文献   
8.
Installing pedestrian ramps is a common improvement towards a barrier-free environment. This paper introduces a graph-theoretical method of retrofitting of a single-branch Truss-Z (TZ) ramp in a constrained environment. The results produced by this exhaustive search method are usually ideal and better than those produced previously with meta-heuristic methods.A large case study of linking two sections of the Hongo Campus of Tokyo University using an overpass in an extremely constrained environment is presented. TZ modules with 1:12 (8.3%) slope are used, which is allowable in most countries for ramps for self-powered wheelchairs.The results presented here are highly satisfactory both in terms of structural optimization and aesthetics.Visualizations of the TZ ramp system, composed of 124 units, are presented.  相似文献   
9.
Model-based control design requires a careful specification of performance and robustness requirements. In typical norm-based control designs, performance and robustness requirements are specified in a scalar optimization criterion, even for complex multivariable systems. This paper aims to develop a novel approach for the formulation of this optimization criterion for multivariable motion systems that exhibit spatio-temporal deformations. To achieve this, characteristics of the underlying system are exploited to design multivariable weighting functions. In contrast to pre-existing approaches, which typically lead to diagonal weighting functions, the proposed approach enables the design of non-diagonal weighting functions. Extensive experimental results confirm that the proposed procedure can significantly improve the performance of an industrial motion system compared to earlier approaches.  相似文献   
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号