可控停车器控制电路改进方案

针对编组站尾部可控停车器的工程设计，较全面地对现有问题提出了具体的改进方法。改进后电路的可靠性及安全性大大提高。     

金融衍生工具引发的会计问题及对策

金融衍生工具作为一种金融合约 ,对传统财务会计要素定义、会计确认、会计计量以及会计报告的披露带来了全面的挑战 ;文中就此进行了论述并提出了对策。     

校园网的安全问题及对策

指出了校园网安全问题的表现形式,分析了安全问题产生的原因,提出了解决办法.     

同心球壳-球体系感应磁场的解析解

考虑薄壳体和非薄壳体结构的感应磁场可提高舰艇感应磁场数值建模精度,开发相应的磁场数值程序包时,磁场解析解模型是验证程序包数值准确性的有效手段。同心球壳-球体系模型是验证同时考虑薄壳体和非薄壳体感应磁场数值准确性的理想模型。基于分离变量法推导了该模型感应磁场的解析解,以积分方程法和有限元法为参照,解析解误差均在5%以内,表明了导出解析解表达式的准确性。     

基于l1范数稀疏解的水下双层圆柱壳振动声辐射预报影响因素研究

基于模态叠加法理论,采用l1范数稀疏解方法,实现了水下双层圆柱壳由内壳有限测点振速值重构得到内、外壳振速空间分布,进而基于边界元理论对结构水下辐射声场进行预报。通过数值计算,分析了模态数目、测点数目和模态振型误差等因素对振动声辐射预报结果的影响,为指导速度场重构时模态数目、测点数目的选取提供了一定的理论依据;结果表明基于l1范数稀疏解声学预报方法对模态振型误差有一定的鲁棒性。最后开展了水下典型双层圆柱壳结构振动声辐射预报的试验研究,可为工程领域结构的声振预报提供一定的指导思路。     

旧城区停车问题研究及解决思路

虽然近年来停车设施建设取得较快发展,但与日益增长的停车需求相比,停车设施建设速度仍显滞后。分析了现状停车存在的问题,主要为机动车发展与停车设施供给之间的矛盾,且停车管理方面也存在改善空间。针对以上问题提出了相关解决思路,包括停车规划、建设、管理一体化,采用区域差异化的停车供给模式,扩大停车供给与提高停车利用率相结合,以及停车设施建设的机械化和信息化。     

A new functional optimization method applied to the pitch distribution of a marine propeller

Based on Hilbert space theory, an infinite-dimensional optimization method to find the optimal pitch distribution of marine propellers was developed. Thereby, the three-dimensional effect of induced velocity could effectively be controlled in an optimal way; i.e., an investigation was carried out to learn whether the present method is applicable to the design of marine propellers, especially pitch distributions, by maximizing the propeller efficiency functional. In addition, with the help of Hilbert space theory, it was successfully shown that the optimization method developed has a unique mathematical solution. In this work, only the pitch distribution was optimized, while other parameters such as the camber and thickness of the trial propeller remained as in the original. With a numerical study, it is shown that the present optimization method gives an optimal pitch distribution, the open-water characteristic curve being superior to the original curve. Received: August 24, 2000 / Accepted: July 16, 2001     

高速线切割加工的断丝问题及解决方法

高速线切割加工过程中容易出现断丝现象,介绍了常见产生断丝的几个原因,主要有与钼丝结构相关的产生原因、与工件相关的产生原因及工件液相关的原因及工作台移动不匀速引起的原因,并论述了相应的解决办法。     

北京地铁车辆架修资源整合方案研究

针对北京地铁车辆架修资源建设单线保障理念造成部分周期内人员及设备利用率低的情况,提出整合北京地铁局部路网架修资源、实现网络化资源共享的研究思路及原则。按照北京市轨道交通路网三期规划确定的整合后路网规模,测算远期年度车辆架修需求,结合既有车辆基地建设基础以及整合原则形成3种整合方案。从改造规模、过轨距离、投资估算等方面对3种方案进行比选,最终确定推荐方案及整合后车辆基地改造思路。     

Critical assessment of five methods to correct for endogeneity in discrete-choice models

Endogeneity often arises in discrete-choice models, precluding the consistent estimation of the model parameters, but it is habitually neglected in practical applications. The purpose of this article is to contribute in closing that gap by assessing five methods to address endogeneity in this context: the use of Proxys (PR); the two steps Control-Function (CF) method; the simultaneous estimation of the CF method via Maximum-Likelihood (ML); the Multiple Indicator Solution (MIS); and the integration of Latent-Variables (LV). The assessment is first made qualitatively, in terms of the formulation, normalization and data needs of each method. Then, the evaluation is made quantitatively, by means of a Monte Carlo experiment to study the finite sample properties under a unified data generation process, and to analyze the impact of common flaws. The methods studied differ notably in the range of problems that they can address; their underlying assumptions; the difficulty of gathering proper auxiliary variables needed to apply them; and their practicality, both in terms of the need for coding and their computational burden. The analysis developed in this article shows that PR is formally inappropriate for many cases, but it is easy to apply, and often corrects in the right direction. CF is also easy to apply with canned software, but requires instrumental variables which may be hard to collect in various contexts. Since CF is estimated in two stages, it may also compromise efficiency and difficult the estimation of standard errors. ML guarantees efficiency and direct estimation of the standard errors, but at the cost of larger computational burden required for the estimation of a multifold integral, with potential difficulties in identification, and retaining the difficulty of gathering proper instrumental variables. The MIS method appears relatively easy to apply and requiring indicators that may be easier to obtain in various cases. Finally, the LV approach appears as the more versatile method, but at a high cost in computational burden, problems of identification and limitations in the capability of writing proper structural equations for the latent variable.