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   735篇
  免费   224篇
  国内免费   47篇
  2024年   3篇
  2023年   11篇
  2022年   10篇
  2021年   15篇
  2020年   17篇
  2019年   13篇
  2018年   12篇
  2017年   39篇
  2016年   28篇
  2015年   34篇
  2014年   60篇
  2013年   32篇
  2012年   64篇
  2011年   59篇
  2010年   54篇
  2009年   79篇
  2008年   51篇
  2007年   47篇
  2006年   55篇
  2005年   54篇
  2004年   38篇
  2003年   26篇
  2002年   32篇
  2001年   31篇
  2000年   16篇
  1999年   22篇
  1998年   10篇
  1997年   12篇
  1996年   17篇
  1995年   9篇
  1994年   14篇
  1993年   8篇
  1992年   15篇
  1991年   6篇
  1990年   11篇
  1989年   2篇
排序方式: 共有1006条查询结果,搜索用时 46 毫秒
71.
State‐level newborn screening allows for early treatment of genetic disorders, which can substantially improve health outcomes for newborns. As the cost of genetic testing decreases, it is becoming an essential part of newborn screening. A genetic disorder can be caused by many mutation variants; therefore, an important decision is to determine which variants to search for (ie, the panel design), under a testing budget. The frequency of variants that cause a disorder and the incidence of the disorder vary by racial/ethnic group. Consequently, it is important to consider equity issues in panel design, so as to reduce disparities among different groups. We study the panel design problem using cystic fibrosis (CF) as a model disorder, considering the trade‐offs between equity and accuracy, under a limited budget. Most states use a genetic test in their CF screening protocol, but panel designs vary, and, due to cost, no state's panel includes all CF‐causing variants. We develop models that design equitable genetic testing panels, and compare them with panels that maximize sensitivity in the general population. Our case study, based on realistic CF data, highlights the value of equitable panels and provides important insight for newborn screening practices.  相似文献   
72.
高空气球为临近空间中的研究提供了可靠的低成本平台。目前高空气球球形一般直接采用"自然形"或以"自然形"为基础,但通用、简捷地求解"自然形"外形方程一直是个难点。将"自然形"气球外形求解问题转化为最优控制问题,利用最优控制领域成熟的求解工具——高斯伪谱法最优化软件包计算气球外形。此外,通过分析外形数据得到外形变化规律,设计通用算法求解任意球形控制因子、任意高度的"自然形"气球外形。结果显示,该算法无须调参便可计算所需的"自然形"球形,便于科研人员快速获得大量所需  相似文献   
73.
现存的被动式外骨骼存在助力效果不明显等缺陷。基于人体步态特征,提出了将人体行走过程中的重力势能转化为人体抬腿过程中动能的原理,通过弹簧、曲柄滑块机构、棘轮棘爪机构以及链传动机构的组合,设计了一款被动式下肢机械外骨骼,并通过外骨骼动力学分析和生理消耗指数实验测试的方法进行了外骨骼性能评价。动力学分析结果表明,穿戴外骨骼后髋关节峰力矩总量降低了23.43%,髋关节做功总量降低了30.59%,初步验证了外骨骼的有效性。生理消耗指数实验测试结果表明, 穿戴外骨骼对于短距离动作转换未产生显著效果,但在长距离行走和上坡行走中分别降低了8.1%和10.4%的能量消耗。因此,该下肢外骨骼助力性能得到了验证,为相关领域的研究提供了实例化参考。  相似文献   
74.
This work provides a method to predict the three-dimensional equivalent elastic properties of the filament-wound composites based on the multi-scale homogenization principle.In the meso-scale,a representative volume element(RVE)is defined and the bridging model is adopted to establish a theoretical predictive model for its three-dimensional equivalent elastic constants.The results obtained through this method for the previous experimental model are compared with the ones gained respec-tively by experiments and classical laminate theory to verify the reliability of this model.In addition,the effects of some winding parameters,such as winding angle,on the equivalent elastic behavior of the filament-wound composites are analyzed.The rules gained can provide a theoretical reference for the optimum design of filament-wound composites.  相似文献   
75.
This paper presents an actuator used for the trajectory correction fuze, which is subject to high impact loadings during launch. A simulation method is carried out to obtain the peak-peak stress value of each component, from which the ball bearings are possible failures according to the results. Subsequently, three schemes against impact loadings, full-element deep groove ball bearing and integrated raceway, needle roller thrust bearing assembly, and gaskets are utilized for redesigning the actuator to effectively reduce the bearings' stress. However, multi-objectives optimization still needs to be conducted for the gaskets to decrease the stress value further to the yield stress. Four gasket's structure parameters and three bearings' peak-peak stress are served as the four optimization variables and three objectives, respectively. Optimized Latin hypercube design is used for generating sample points, and Kriging model selected according to estimation result can establish the relationship between the variables and objec-tives, representing the simulation which is time-consuming. Accordingly, two optimization algorithms work out the Pareto solutions, from which the best solutions are selected, and verified by the simulation to determine the gaskets optimized structure parameters. It can be concluded that the simulation and optimization method based on these components is effective and efficient.  相似文献   
76.
C4ISR系统体系结构设计是系统综合集成的基础.针对C4ISR体系结构框架2.0,分析了体系结构产品的特点,研究了体系结构产品设计工具的分类以及应该具有的功能,提出体系结构产品设计工具的集成框架和基于体系结构核心数据库的数据管理机制,并实现了体系结构产品设计工具以及体系结构设计集成环境.  相似文献   
77.
本文基于ATDE创新教育理论,以程序设计类课程为教学阵地,对教育技术本科生创新思维培养做出了相关探索与实践。创新思维是高级的问题解决活动。培养学生的创新思维更多体现在对学生解决问题的策略应用方面。对此,本文通过给出的案例,从不同角度详细介绍了程序设计类课程对培养教育技术本科生创新思维的实施对策,以期对提高本专业建设做出有益的探索。  相似文献   
78.
Extending Sastry's result on the uncapacitated two‐commodity network design problem, we completely characterize the optimal solution of the uncapacitated K‐commodity network design problem with zero flow costs for the case when K = 3. By solving a set of shortest‐path problems on related graphs, we show that the optimal solutions can be found in O(n3) time when K = 3, where n is the number of nodes in the network. The algorithm depends on identifying a list of “basic patterns”; the number of basic patterns grows exponentially with K. We also show that the uncapacitated K‐commodity network design problem can be solved in O(n3) time for general K if K is fixed; otherwise, the time for solving the problem is exponential. © 2004 Wiley Periodicals, Inc. Naval Research Logistics, 2004  相似文献   
79.
提出了一种利用陀螺与星敏感器进行组合定姿的方案.为了避免状态及量测向量选取不当而造成的误差,首先采用四元数法对卫星的姿态进行描述;接着分析了陀螺与星敏感器系统的误差源,选取误差四元数及陀螺的系统误差作为组合系统的状态,获得了系统状态方程;同时采用星敏感器的量测作为系统量测,并利用新的数学变换构得到更为准确的量测方程;最...  相似文献   
80.
无人飞艇作为理想的信息平台,在预警、反恐、侦察监视和通信等方面发挥着重要作用。而艇载侦察监视设备CCD摄像机受到飞艇发动机振动的影响,会对成像产生消极影响。为此,分析了影响飞艇摄像机的成像质量的因素,采用被动减振的方案,达到了有效克服振动的目的。  相似文献   
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