电力网络支路参数估计新方法

基于新息图法,提出了一种电力网络中支路参数估计的新方法.根据系统中某支路存在网络参数错误时,该支路及其相连支路的新息值比较大的特点识别网络参数错误.然后利用回路新息相角和,估计存在网络参数错误的支路的参数值.该方法辨识和估计网络参数错误较快,且简单有效.通过IEEE-5节点系统算例的计算,表明此方法能够比较准确地估计网络参数错误,并验证了网络参数错误与坏数据、拓扑错误同时发生时该方法的有效性.     

Quantification of statistical type i and ii errors in correlation analysis of simulated functional magnetic resonance imaging data

The potential of statistical analyses of functional magnetic resonance images using various threshold strategies in combination with correlation analysis was studied by simulating brain activation. Differences in statistical Type I () and II () errors are substantial for the various thresholds. Absolute thresholds and individualized thresholds based on the assumption of a gaussian noise distribution are producing constant -errors and thus do not sufficiently improve discrimination of truly activated pixels even for very high contrast-to-noise ratios (CNR). Only relative threshold strategies related to the maximum correlation coefficient and thus the individual data quality and activation level, i.e., a data-driven approach, can perfectly discriminate true positives, at least for CNR>2.5. To further improve discrimination of activated and non-activated pixel in studies with lower CNR, additional prior knowledge would be necessary. From the data presented, one would also expect that the best performing threshold strategy in this simulation study would perform best underin vivo conditions.Preliminary results presented in abstract form at the 13th Annual ESMRMB Meeting, Prague, Czech Republic, September 12–15, 1996.     

基于等效误差法的直线电机XY平台二阶滑模控制

对于直线电机驱动XY平台,系统动态的非线性、系统不确定性因素以及曲线轨迹的轮廓误差模型相对复杂等问题影响其轮廓加工精度。采用适用于多轴非线性运动系统轮廓控制的等效误差法,建立可用于一般曲线跟踪且容易计算的XY平台等效误差非线性模型。运用二阶滑模控制方法进行轮廓控制器的设计,通过连续控制量使滑模面及其时间导数在有限时间内趋近于零,削弱抖振的同时抑制不确定性因素对系统性能的影响,使直线电机XY平台达到高加工精度要求。理论推导与仿真结果表明,所设计控制系统能有效提高XY平台的轮廓加工精度。     

考虑洪水预报误差的水库防洪控制调度的风险分析

水库动态汛限水位控制调度是水库管理调度的重要方式之一,它可以提高水库的兴利效益.本文通过分析水库动态汛限水位控制调度可能产生的后果,给出了水库动态汛限水位控制调度风险的定义,并建立了考虑洪水预报误差情况下的入库径流的随机模拟方法.通过大清河流域水库实际应用表明:洪水预报误差可能对水库动态汛限水位控制调度的风险产生一定的影响.其影响程度主要都取决于洪水预报的精度情况.在大清河流域,其洪水预报系统的洪水预报误差对各水库动态水位控制调度的影响都是很小的.但各水库汛限水位的提高程度如果超过1m时,对白洋淀的防洪压力会很大.     

折叠腔He-Ne激光器中角锥棱镜直角误差分析

首次研究了应用空心角锥棱镜的折叠腔免调试He-Ne激光器系统.根据矩阵光学理论分析了角锥棱镜的直角误差对其光路反射特性的影响,并应用于折叠腔免调试He-Ne激光器系统,定量给出了返回光线偏角误差与棱镜直角误差的线性关系,计算了免调试激光器系统中角锥棱镜直角误差的临界值.这一结果对控制折叠腔免调试激光器中的角锥棱镜加工误差具有指导意义.     

齿面研磨边界的确定与研齿性能试验

通过对研齿过程的轮齿接触仿真,传动误差与重合度的分析,确定了齿面研磨区域的边界:距齿面中点±0.25～0.3个齿宽,±0.1～0.25个齿高.通过研齿试验,研究了研磨对齿面接触印痕、齿形精度与动态啮合性能的影响.结果表明,研齿能一定程度上消除齿形误差,改善轮齿接触区位置与形状,使热处理后变形的接触区一定程度上得到恢复;研齿对齿轮副动态性能改善明显,振动噪声在转速、载荷较宽的范围内均有明显下降.     

Synthetic bandwidth method and experimental research based on a wideband multi-channel SAR

Synthetic bandwidth technique is used to increase frequency bandwidth of the system and provides an effective way to achieve the ultra high resolution in the field of Synthetic Aperture Radar (SAR). But in the actual SAR system, the synthesis result will be seriously deteriorated by the inner-channel and inter-channel phase error. A frequency domain synthetic bandwidth method based on transfer function extracting technique is proposed in an actual SAR system which consists of single wideband transmit channel and 8-way down-conversion receive sub-channels. The method can accurately get the amplitude and phase characteristics of the sub-channel and compensate the amplitude-phase errors. The final experimental results demonstrate the validity and feasibility of the method and the range resolution down to 0.1 m is obtained.     

Efficient closed-form solution for target localization using TDOA measurements in the presence of sensor position errors

An efficient solution for locating a target was proposed, which by using time difference of arrival (TDOA) measurements in the presence of random sensor position errors to increase the accuracy of estimation. The cause of position estimation errors in two-stage weighted least squares (TSWLS) method is analyzed to develop a simple and effective method for improving the localization performance. Specifically, the reference sensor is selected again and the coordinate system is rotated according to preliminary estimated target position by using TSWLS method, and the final position estimation of the target is obtained by using weighted least squares (WLS). The proposed approach exhibits a closed-form and is as efficient as TSWLS method. Simulation results show that the proposed approach yields low estimation bias and improved robustness with increasing sensor position errors and thus can easily achieve the Cramer-Rao lower bound (CRLB) easily and effectively improve the localization accuracy.     

NTP现网应用的常见问题

NTP是用来使计算机时间同步化的一种协议,在互联网中广泛应用。但在软交换、智能网、BOSS系统等时间敏感型应用中,仍有许多安全问题值得注意。本文从协议自身缺陷与现网设备特性,举例讨论了若干安全问题并提出了相应的防范措施。     

超低副瓣相控阵天线设计中的几个问题

本文论述了超低副瓣相控阵天线的设计方法，它涉及诸多因素，这里主要研究诸如阵列单元互耦，单元与馈电系统之间的传输变换段，阵列激励与位置误差，以及单元通道校准问题。