楔环连接结构动态特性的试验与分析

在动力学环境下,楔环连接结构中存在接触、空隙、磨损等多种非线性因素,其动力学特性很难用理论方法精确描述。文中首先用试验模态分析的方法对楔环连接结构在不同大小激振力作用下的模态参数进行辨识。结果表明,结构的模态参数随激振力的变化呈现规律性的变化;对结构上不同部位在不同大小、不同类型的激振力作用下的响应情况进行试验研究,发现结构上的响应与激振力呈现非线性关系,且不同部位测点表现出的非线性程度也不同。     

600MW发电机组凝结水系统变频改造及经济性分析

阐述了华电包头电厂凝结水泵变频改造的方案,通过对凝结水泵节能技术改造的实施,对改造后运行方式及经济性做了详细的分析,为同类型机组实施凝结水节能技术改造提供了科学依据和宝贵经验。     

风机盘管机组噪声分析

描述我国风机盘管机组噪声问题的现状,分析导致风机盘管机组噪声的3个主要因素,并着重阐述低频噪声的判断方法以及施工规范对预防风机盘管机组噪声的重要性。     

基于EMG频域特征的假肢机电信号识别研究

采集人体表面肌电信号并进行有效模式识别,是多功能假肢研究的关键技术之一.本文研究肌电信号(EMG)的频域信息,以提取有效特征识别不同的手腕部动作.实验采集了8种不同的手腕动作的EMG信号.分别提取了EMG信号的时间序列模型(AR)、时域统计量(DT)、功率谱估计(PSD)、短时傅里叶变换(STFT)和互功率谱(CPSD...     

Enhanced EM-based channel estimation for MIMO-OFDM in highly mobile channels

An enhanced expectation-maximization (EM)-based iterative channel estimator for coping with channel time variation is proposed for mobile multiple-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems.In the proposed scheme,the recursive least squares (RLS) algorithm is applied to track the time-varying channel impulse response (CIR) within several symbols.By using the tracked time-varying CIR,the ICI are constructed and then cancelled from the received signal,thus reducing their impactions on the channel estimation.Moreover,based on an ovet-sampled complex exponential basis expansion model (OCE-BEM),an improved channel predictor is derived in order to improve the initial channel estimates accuracy of the iterative estimator.Simulation results show that the proposed scheme outperforms the classic counterpart in time-varying scenarios with a smaller cost of complexity.     

Seeing a phrase “time and again” matters: The role of phrasal frequency in the processing of multiword sequences.

Are speakers sensitive to the frequency with which phrases occur in language? The authors report an eye-tracking study that investigates this by examining the processing of multiword sequences that differ in phrasal frequency by native and proficient nonnative English speakers. Participants read sentences containing 3-word binomial phrases (bride and groom) and their reversed forms (groom and bride), which are identical in syntax and meaning but that differ in phrasal frequency. Mixed-effects modeling revealed that native speakers and nonnative speakers, across a range of proficiencies, are sensitive to the frequency with which phrases occur in English. Results also indicate that native speakers and higher proficiency nonnatives are sensitive to whether a phrase occurs in a particular configuration (binomial vs. reversed) in English, highlighting the contribution of entrenchment of a particular phrase in memory. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Perceptual grouping affects pitch judgments across time and frequency.

Pitch, the perceptual correlate of fundamental frequency (F0), plays an important role in speech, music, and animal vocalizations. Changes in F0 over time help define musical melodies and speech prosody, while comparisons of simultaneous F0 are important for musical harmony, and for segregating competing sound sources. This study compared listeners' ability to detect differences in F0 between pairs of sequential or simultaneous tones that were filtered into separate, nonoverlapping spectral regions. The timbre differences induced by filtering led to poor F0 discrimination in the sequential, but not the simultaneous, conditions. Temporal overlap of the two tones was not sufficient to produce good performance; instead performance appeared to depend on the two tones being integrated into the same perceptual object. The results confirm the difficulty of comparing the pitches of sequential sounds with different timbres and suggest that, for simultaneous sounds, pitch differences may be detected through a decrease in perceptual fusion rather than an explicit coding and comparison of the underlying F0s. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

北极涛动对华北沙尘暴频次的影响

This study has investigated the influence of Arctic Oscillation (AO) on dust storm frequency in North China in spring seasons during 1961-2007. There is a significant linkagebetween dust storm frequency and AO; a negative (positive) AO phase is related to an in-creased (decreased) dust storm frequency in North China. This relationship is closely related to changes in the cold air activity in Mongolia. The cold air activity exerts large impacts on the dust storm frequency; the frequency of cold air activity over Mongolia not only positively cor-relates with the dust storm frequency in North China, but also shows a long-term decreasing trend that is an important reason for the long-term decreasing of dust storm frequency in North China. The AO has large influence on the frequency of cold air activity over Mongolia; a negative (positive) AO phase is highly related to an increased (decreased) frequency of cold air activity over Mongolia, which results in an increased (decreased) dust storm frequency in North China.     

Timedelay based all-frequency correction method for discrete spectrum

Many spectrum correction methods have been developed, but their performance degrades signif- icantly when they are applied to the correction of low frequency component ( LFC ). It owns to that the model underlying the conventional approaches neglects the interference of the negative frequency in the real signal. A new approach for the correction of the LFC is proposed, which suits all kinds of symmetrical windows. It divides a signal into three sections and makes use of the first spectrum line of each section. Then this approach is modified so that it is also applicable to the correction of the high frequency component. Thus a timedelay-based all-frequency correction method is proposed. The simulation results show that this method is simple and feasible. By this method, the accurate frequency, amplitude and phase of the spectral line can be obtained whether it is close to or far from OHz.     

Theoretical performance analysis of radio frequency fingerprinting under receiver distortions

Radio frequency fingerprinting (RFF) is used to uniquely identify individual emitters by exploiting the radio frequency characteristics, which are originated from transmitter imperfections. The theoretical performance is analyzed to evaluate the feasibility of RFF with channel noise and receiver imperfections. The distortions from oscillators and analog‐to‐digital converter (ADC) are considered, and a RFF signal model describing the discrepancies of transmitter imperfections, channel noise, and receiver distortions are constructed. A likelihood ratio test algorithm for RFF is proposed to analyze the theoretical performance. The upper and lower bound of theoretic performance is derived through the analysis of the likelihood ratio test statistic, which enlightens how to design a “just enough” receiver to meet the demands of RFF. The simulation results are in agreement with the theoretical evaluation. Copyright © 2013 John Wiley & Sons, Ltd.