光速不变原理与一般洛伦兹变换式的推导

洛仑兹变换是狭义相对论的基础,现在使用的大学物理教材和传统文献一般利用线性变换和间隔不变性导出相对论时空坐标变换式,过程复杂学生难以理解。该规律可以直接从光速不变原理出发,用简单的方法推导出洛仑兹变换式,其中蕴含的时空观点变得更加易懂。     

基于PC机控制的交流变频调速实验平台设计

设计了以PC机为控制核心的交流变频调速实验平台,在Visual C++环境下开发了人机交互软件,实现了多个控制算法(SPWM、SVPWM、直接转矩控制、磁场定向控制等),并且可以方便地对各算法进行切换;控制系统设计成以DSP2812为核心的运动控制卡,通过动态链接库实现控制系统的开发;对实验过程中的数据和曲线进行实时显示。该文以磁场定向控制算法为例,对系统的工作状况进行分析。实验结果表明,该实验系统达到了设计要求,有较好的实用价值。     

实时控制的交流变频调速系统实验平台研制

交流变频调速系统实验平台控制系统采用以DSP(digital signal processor)为核心的运动控制卡,该卡以插卡形式嵌入PC机的PCI插槽内,电机的控制算法在PC机上实现.为了满足系统对实时性的要求,提出了一种提高系统实时性的解决方案,即采用硬件中断法保证数据采集的精度和速度,采用多线程技术实现对数据的实时处理.实验结果表明:该方案可行,开发的控制系统性能良好,达到了设计要求,具有实用性.     

基于三次多项式变速算法的中药粉剂下药系统

中药粉剂配方下药系统是一个精度高、反应大滞后的系统,难以使用传统的方法进行下药控制。为有效实现中药粉剂快速、稳定、高精确度的下药问题,本文提出一种利用三次多项式加减速控制算法来实现在线分析医生处方上的中药粉剂配量,并通过Profibus光纤模块控制远程变频电机以计算所得运动曲线进行下药控制。实验结果验证了此方法的有效性。     

基于复合控制的变频调速系统实验平台

矢量控制受电机转子参数影响较大,抗干扰能力较弱,而直接转矩控制(DTC)系统在低速运行时存在转矩和电流脉动大、开关频率不固定、定子电阻变化和磁链估算影响控制精度等问题。该文将两种控制策略相结合,提出了基于空间矢量的直接转矩(SVM-DTC)复合控制策略。该控制策略以智能功率模块(IPM)为硬件平台,以基于PCI总线的DSP运动控制卡为核心,采用Visual C++进行软件编程,具有较强的可读性和开放性,便于在线修改,并且实现了变频调速系统的实时性控制。实验结果表明:该复合控制策略具有较好的动静态性能,系统的性能得到了改善。     

具有转子时间常数辨识的变频调速实验平台研制

研究一种基于模型参考自适应(MRAS)的转子时间常数辨识方法:将转子磁链观测中的电压模型和电流模型分别作为参考模型与可调模型,通过自适应机构得到趋于实际值的转子时间常数辨识值。并设计了具有转子时间常数辨识功能的磁场定向控制变频调速实验平台。该实验平台硬件部分以智能功率模块(IPM)逆变电路和基于PCI总线的DSP运动控制电路为核心。在Visual C++环境下设计人机交互界面,编写优化控制算法。实验结果表明,该方案能减小转子时间常数变化对系统产生的影响,较好地改善系统的动静态性能。     

On maximum power point tracking control strategy for variable speed constant frequency wind power generation

Based on the characteristic of AC-excited variable speed constant frequency （VSCF） wind power generation, the vector control technique was applied in a doubly fed induction generator （DFIG）. Maximum wind energy or maximum output power point can be tracked by decoupling control of active power and reactive power. The research result shows that the net power of generation system delivered to grid in maximum wind energy tracking mode is not the most. We presented a novel maximum power point tracking （MPPT） control strategy by analyzing the DFIG mathematic model and power relations which delivered the maximum power to the grid. The maximum power point could be tracked automatically without measuring wind speed in the control strategy and the control was independent of optimal turbine power curve, which had excellent dynamic and static performances and robustness. Simulation and experimental results testify the accuracy and validity of the control strategy.     

基于空间电压矢量脉宽调制技术的感应电机变频调速系统研究

由于电压空间矢量脉宽调制(SVPWM)技术具有物理意义直观、数学模型简单、便于微处理器实时控制等优点,因此在电机控制中得到了广泛的应用。介绍了一种以数字信号处理器TMS320F2812为核心的感应电动机变频调速系统,采用SVPWM技术实现了对感应电机的变频调速控制。实验结果表明,采用SVPWM技术的感应电动机变频调速系统具有动态特性好、转矩脉动小、噪声低和电压利用率高等特点。     

声速测量实验的理论分析

本文对声速测量———共振干涉法中相关问题进行了理论探讨,使学生对此实验的物理图象,有了更清楚的认识.     

The textbook as classroom context variable

To explore the role of the textbook as a context variable in process—product relationships, data on teaching practices and learning outcomes from the IEA Classroom Environment Study in The Netherlands were used. The sample consisted of 50 secondary school mathematics classes and their teachers. Three textbooks were represented in the sample. Data collection included systematic observation of lessons and administration of tests and questionnaires to students and teachers. Nine teaching practices analogous to the distinguishing features of the textbooks were identified, and data relating to these practices were analysed. Four of the teaching practices occurred to different degrees in the three groups of textbook users. The correlations with two learning outcomes in different groups of textbook users were significantly different for 5 of the 18 cases. These results suggest that the textbook is an important context variable.     

The motion variable in procedural learning

Conclusions Differences in performance of a procedural task, attributable to motion, are identified with the putting together of a new motor response. No reliable difference was shown between motion and still sequences in the overall procedural sequence of steps. The use of animation arrows to direct attention and to show the direction of motion did not help learning. Motion does not appear to function as a cue focusing attention upon the direction of movement or critical elements of the display. The results of the present studies support the recommendations of Allen and Weintraub (1968). They suggest that the use of motion in a display is definitely indicated when the particular content to be learned consists of the movement itself and its characteristics, or where the content is enhanced and differentiated by the cues provided in the action of the movement. Until the bases for differential individual needs for motion can be determined, displays that provide motion are best used wherever procedural tasks requiring motions new to the student are taught. The research reported in this paper was performed at HumRRO Division No. 2, Fort Knox, Kentucky, under Department of the Army Contract with HumRRO. The contents of this paper do not necessarily reflect official opinions or policies of the Department of the Army.     

The interference of polarised light

This article brings out a remarkable contribution to the understanding of light, made by S (for Sivaramakrishnan) Pancharatnam, when he was a doctoral student working on the optics of crystals with C V Raman at the Raman Research Institute (RRI) in Bangalore in the 1950s. We will first illustrate this contribution by means of specific examples, which already calls for considerable background, and then go on to place the work in a wider context.     

ADSL高速接入技术的新发展

本文介绍了ADSL(Asymmetric Digital SubscriberLoop)接入技术的原理、技术优势,并分析了三种调制方式的特点及ADSL技术当前的新发展。     

光速可变理论中局域Lorentz变换的错误,超光速和泛狭义相对论

证明由不同系统的局域Lorentz变换不能得到光速可变.基于狭义相对性原理必然得到一个不变速度ch.因此,狭义相对论基本原理应该严格地重新定义为：Ⅰ.狭义相对性原理,其必然导出一个不变速度ch.Ⅱ.假设理论中的这个不变速度ch是真空中的光速c.如果第二个原理不成立,例如存在超光速运动,则理论仍是形式相同,仅仅c→ch的泛狭义相对论.这是目前许多问题的一个最简单的修改.如果ch是各种不同的不变速度,则多时空体系相应于多世界.结合超光速等,发展理论的其他方法也被探索.光速可变理论应该联系于广义相对论.     

学习的进度就是赚钱的速度

每一次技术的重大创新和商业思想的革命，都深刻地改变经济的格局，扭转企业的命运。     

加强心理护理对促进分娩的意义

目的:通过对110例心理护理分娩产妇的临床观察，探讨心理护理对分娩的影响。方法：正常足月妊娠，无产科手术指症及妊娠合并症并发症的初产妇220例，随机分为心理护理且和一般护理组，每组各110例，结果：心理护理的产妇、产程，剖宫产率及新生儿窒息率明显低于对照组。结论：适时恰当的心理护理能够缩短产程，促进顺利分娩，减少剖宫产及新生儿窒息率。     

浅谈光折射的起因和规律

长期以来,高中物理教材对光的折射一直没有给出详细的本质性解释.新版的高中物理教材也只是给出了结论:"研究表明,光在不同介质中的速度不同是光发生折射的原因",并没有作深入的分析.教学实践表明,一些同学不愿停留在对光的折射的起因和规律的识记水平,本文结合教学实践,对"光在不同介质中的速度不同是光发生折射的原因"进行剖析,以期对光的折射加深理解,并由此推导出"入射角的正弦跟折射角的正弦成正经"的折射规律.     

实变函数论课程建设改革的几点浅议

本探讨了当今教育喀革的进展现状中存在的问题，就实变函数论的定位讨论其课程建设改革中的一些问题．     

The effect of print size on reading speed in dyslexia

This article details a study which predicted that across a wide range of print sizes dyslexic reading would follow the same curve shape as skilled reading, with constant reading rates across large print sizes and a sharp decline in reading rates below a critical print size. It also predicted that dyslexic readers would require larger critical print sizes to attain their maximum reading speeds, following the letter position coding deficit hypothesis. Reading speed was measured across twelve print sizes ranging from Snellen equivalents of 20/12 to 20/200 letter sizes for a group of dyslexic readers in Grades 2 to 4 (aged 7 to 10 years), and for non-dyslexic readers in Grades 1 to 3 (aged 6 to 8 years). The groups were equated for word reading ability. Results confirmed that reading rate-by-print size curves followed the same two-limbed shape for dyslexic and non-dyslexic readers. Dyslexic reading curves showed higher critical print sizes and shallower reading rate-by-print size slopes below the critical print size, consistent with the hypothesis of a letter-position coding deficit. Non-dyslexic reading curves also showed a decrease of critical print size with age. A developmental lag model of dyslexic reading does not account for the results, since the regression of critical print size on maximum reading rate differed between groups.