Bias in modal parameters due to aliasing in the time domain

This paper deals with some aliasing effects in the time domain that can lead to unacceptable misestimations of modal parameters. When a frequency response function of a vibrating system is sampled and inverse Fourier transformed, the resulting impulse response is given by an infinite geometric series, the single term of which is the impulse response itself shifted in time. For this reason, some modal parameters, if estimated in the time domain, are biased; in particular, while the damping factor and the natural frequency are not influenced by the aliasing phenomenon, the magnitude and phase of the residue can be highly biased. Corrective terms are theoretically evaluated and their efficiency is shown in numerical simulations.

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Sommario In questo articolo e' presentato un metodo per correggere gli errori che si compiono nella stima di alcuni parametri modali, quando essi vengono ricavati nel dominio del tempo. Infatti se la funzione di risposta in frequenza e' ottenuta con eccitazioni particolari—quali ad esempio lo pseudo-random, lo stepped-sine a passo costante o lo sweep in frequenza-la risposta impulsiva, ottenuta per mezzo dell'antitrasformata discreta di Fourier, risulta periodica, con periodo pari all'inverso della spaziatura tra le righe spettrali. Cio' comporta un errore nella stima dell'ampiezza e della fase del residuo, mentre nessuna conseguenza si ha sulla frequenza naturale e sul fattore di smorzamento. L'errore sulle stime risulta tanto piu' grande, quanto meno smorzata e' la risposta impulsiva all'interno dell'intervallo di osservazione.Partendo dall'inviluppo e dalla fase istantanea dei segnali complessi, ottenibili per mezzo della trasformata di Hilbert, si sono ricavati i termini correttivi sia per il modulo, che per la fase del residuo. La validita' delle correzioni e' mostrata con esempi numerici.

     

Correlation investigation between contact approach speed of handheld metal rod and discharge parameters from charged human body

Characteristic measurement of contact discharge currents are made through a hand-held metal rod from charged human body. Correlation coefficients are obtained, through Statistic Package for Social Science （SPSS）, for various charge voltages, which is based on the effect test of electrode contact approach speeds on discharge current parameters of current peaks, maximum rising slope and spark lengths. Discharge parameters at charge voltage 300V are independent on approach speed. For charge voltages equal to and higher than 500V, the contact approach speed has strong positive correlation with discharge parameters of the peak current and the maximum rising slope, whereas has strong negative correlation with the spark length.     

Rebuilding Babel: A Constitutive Approach to Tongues‐in‐use

In this paper, we suggest that the Montreal School (TMS) tradition of organizational communication offers a fruitful analytical framework that allows us to better take into account the way people practically deal with plurilingual situations as they go on with their daily activities and contribute to shaping their organizations. We identify six core features of TMS and show their analytical power in studying plurilingual interactions. TMS, we argue, is conceptually well equipped to reveal the ways in which multiple tongues are dealt with in everyday organizational settings and to uncover the constitutive nature of tongue‐in‐use.     

多尺度分析用于图像边缘的精确检测

图像中边缘往往存在于一个较宽的尺度范围内，边缘的定位精确性是衡量一个边缘检测算法性能的重要依据。众多算法中，引起较大关注的是M—H的二阶导数过零点检测和Canny的一阶导数最大值检测，但都存在各自的不足。本文提出将线性权重函数(LWF)作为平滑函数，并利用小波理论的多尺度分析方法，对图像边缘进行检测，其在定位精确性、噪声抑制能力方面均有较好性能。     

基于模式间混淆关系自适应构建层次化分类器

为了解决层次化分类器的设计难点——子分类器的层属关系及其内部组成的确定,本文首先定义了模式间混淆关系,用于描述不同模式在判决域中的相互作用;进而提出了基于混淆矩阵度量模式间混淆关系的方法。设计并实现了多模式混淆关系分析机MPCRAM,将有师指派和无师自组两种常用的模式重组方法有机结合,遵循Fisher准则,自适应地产生层次化分类器结构。大量综合测试证实了该方法有效、实用,可显著提高分类器的识别性能和稳健性。     

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。     

真空炉加热区的热场分析

加热区是真空炉的核心部分,加热区设计是否合理直接影响着真空炉的使用性能及其运营成本。运用有限元仿真对典型真空炉加热区的热场进行分析,并通过实验数据,进一步验证真空炉加热区的设计合理性,掌握了其温场的实际温度分布情况,为今后设计类似炉型的真空炉加热区提供重要参考依据。     

基于主元分析和Fuzzy ART模型的人脸识别算法

本文从子空间的理论出发,先用一写数量的人脸榇一构造优化的“人脸空间”,“人脸空间”能更好的描述人脸图象矢量的分布。把人脸图象在子空间进行投影得到人脸的特征向量。然后在Ｆｕｚｚｙ ＡＲＴ模型的基础上设计发类器。用４０组共计４００张人脸图象结系统进行测试,实验结果表明识别率在９５％以上,系统具有良好的识别能力和鲁棒性。     

浅析220 kV线路跳闸故障及应对措施

随着经济的发展,城市配电网络以及山区高压电缆架设工程进展迅速。但是由于部分地区的自然灾害、人为事故频繁等原因,电力线路经常出现故障。比如雷电活动导致的跳闸现象。对此,本文作者通过对220kV某线雷击跳闸故障进行实例分析,寻找跳闸事故的原因。并结合多年工作经验对事故之后的改进措施提出了自己的看法。