Determination of Instantaneous Frequencies of Low Plasma Waves in the Magnetosheath Using Empirical Mode Decomposition (EMD) and Hilbert Transform (HT)

The observations of in-situ spacecraft mission in the magnetosheath and a region of thermalized subsonic plasma behind the bow shock reveal a non-linear behaviour of plasma waves. The study of waves and optics in Physics has given the understanding of the effect of many waves coming together to form a wave field or wave packet. The common aspect of such study shows that two or more waves can superimpose constructively or destructively. The sudden high magnetic field data in the magnetosheath displays such possibility of superposition of waves. In this paper, we use the empirical mode decomposition (EMD) and Hilbert transform (HT) techniques to determine the instantaneous frequencies of low frequency plasma waves in the magnetosheath. Our analysis has shown that the turbulent behavior of magnetic field in the magnetosheath within the selected period is due to superposition of waves.

Efficacy of Hilbert-Huang Transform (HHT) in the Analysis of Instantaneous Low Frequency Waves of Magnetosheath

The flow of supersonic plasma is accompanied by a highly thermalized region called the Magnetoshealth found after the bow shock. Enclosed within this region are different wave modes associated with classes of boundaries which have been determined by different methods. The efficacy of Hilbert-Huang transform (HHT) is based on the conditionality of allowing for the local analysis of frequencies, which presents the physical meaning of the original signal at that instant. The observed data have been taken from Cluster II Fluxgate Magnetometer (FGM) instrument that provides advantage for the analysis in three dimensions. The result compares favourably with instantaneous frequencies computed using simple Hilbert transform (SHT) with electric field measurements of Cluster II mission already carried out in literatures. The result of this study has shown that HHT provides the best applicability in the magnetosheath data analysis than the wavelet and Fast Fourier Transform (FFT). The application of HHT based on its advantages over other methods is viewed to be very critical in the analysis of multi-frequency signals where different frequencies could be determined distinctively at a point.

Efficacy of Hilbert-Huang Transform (HHT) in the Analysis of Instantaneous Low Frequency Waves of Magnetosheath

The flow of supersonic plasma is accompanied by a highly thermalized region called the Magnetoshealth found after the bow shock. Enclosed within this region are different wave modes associated with classes of boundaries which have been determined by different methods. The efficacy of Hilbert-Huang transform (HHT) is based on the conditionality of allowing for the local analysis of frequencies, which presents the physical meaning of the original signal at that instant. The observed data have been taken from Cluster II Fluxgate Magnetometer (FGM) instrument that provides advantage for the analysis in three dimensions. The result compares favourably with instantaneous frequencies computed using simple Hilbert transform (SHT) with electric field measurements of Cluster II mission already carried out in literatures. The result of this study has shown that HHT provides the best applicability in the magnetosheath data analysis than the wavelet and Fast Fourier Transform (FFT). The application of HHT based on its advantages over other methods is viewed to be very critical in the analysis of multi-frequency signals where different frequencies could be determined distinctively at a point.

Instantaneous frequency and wave mode identification in a magnetosheath using few spatial points

Observational data such as those obtained from the magnetosheath in the downstream of Earth's bow shock have waveforms that differ from those of sinusoidal signals. In practice, they are not even aggregates of sinusoidal signals. Therefore, the frequency decomposition for the data requires technique that will account for the time-varying features of the data that will lead to deduction of physical meaning of the observations. The combination of empirical mode decomposition (EMD) and Hilbert transform has been used for extracting the various contributing oscillatory modes (EMDs) and the instantaneous frequency determination (Hilbert transform) of every physically meaningful mode called intrinsic mode function (IMF). The resulting instantaneous frequencies are used to determine instantaneous wave vectors. The combination of the instantaneous frequencies and wave vectors is useful in the identification of wave modes based on the characteristics of the waves. The results show that EMD-Hilbert can be more reliable than simple Hilbert transform alone.

IMPROVED METHOD FOR HILBERT INSTANTANEOUS FREQUENCY ESTIMATION

In the mechanical fault detection and diagnosis field,it is more and more important to analyze the instantaneous frequency (IF) character of complex vibration signal.The improved IF estimation method is put forward aiming at the shortage of traditional Hilbert transform.It is based on Hilbert transform in wavelet domain.With the help of relationship between the real part and the imaginary part obtained from the complex coefficient of continuous wavelet transform or the analyti- cal signal reconstructed in wavelet packet decomposition,the instantaneous phase function of the subcomponent is extracted.In order to improve the precise of IF estimated out,some means such as Linear regression,adaptive filtering,resampling are applied into the instantaneous phase obtained, then,the central differencing operator is used to get desired IF.Simulation results with synthetic and gearbox fault signals are included to illustrate the proposed method.

CONCENTRATION DISTRIBUTION OF FRACTIONAL ANOMALOUS DIFFUSION CAUSED BY AN INSTANTANEOUS POINT SOURCE

The Fox function expression and the analytic expression for the concentration distribution of fractional anomalous diffusion caused by an instantaneous point source in ndimensional space ( n = 1, 2 or 3 ) are derived by means of the condition of mass conservation, the time-space similarity of the solution, Mellin transform and the properties of the Fox function. And the asymptotic behaviors for the solutions are also given.

Stochastic Resonance of Instantaneous-State in Linear Regime of Single-Mode Lasers Driven by Colored Noises

根据有输入信号的单个模式的激光的激光紧张的一个线性方程，我们在单个模式的激光的 theinstantaneous 状态计算概括 signal-to-noise 比率(GSNR ) ，它被二有颜色的噪音驾驶并且在一个电子 exponential 函数形式相关。我们检测随机的回声(SR ) 也在任何时间发生在即时状态上。当拿不同信号频率时，而且，我们在三种不同形式讨论 GSNR 趋势到稳定的状态。

Application of formant instantaneous characteristics to speech recognition and speaker identification

This paper proposes a new phase feature derived from the formant instantaneous characteristics for speech recognition (SR) and speaker identification (SI) systems. Using Hilbert transform (HT), the formant characteristics can be represented by instantaneous frequency (IF) and instantaneous bandwidth, namely formant instantaneous characteristics (FIC). In order to explore the importance of FIC both in SR and SI, this paper proposes different features from FIC used for SR and SI systems. When combing these new features with conventional parameters, higher identification rate can be achieved than that of using Mel-frequency cepstral coefficients (MFCC) parameters only. The experiment results show that the new features are effective characteristic parameters and can be treated as the compensation of conventional parameters for SR and SI.

考虑频率波动的改进瞬时电压d-q分解算法

电压有效值的准确检测是分析电压暂降、波动和偏差的基础,是电压质量评估与抑制干扰首先要解决的重要问题.以单相电源为参考电压,采用60°延时方法构造虚拟的三相系统,借助d-q坐标变换可以对电压瞬时有效值进行计算.本文在该方法基本原理的基础上,分析了检测信号的频率波动对计算结果的影响,推导了频率波动误差公式,并在Matlab/Simulink平台下对推导结论进行了仿真验证,同时提出了基于u2d+u2q项滤波和局部均值计算的改进算法.通过仿真对比改进算法与传统算法,结果表明,改进算法对频率波动信号电压有效值的计算更准确.

结合SLMSST和DO提取时变结构瞬时频率

为提升局部最大同步挤压变换估算瞬时频率的精度,本文结合2阶局部最大同步挤压变换(Second-order Local Maximum Synchrosqueezing Transform,SLMSST)和动态规划(Dynamic Optimization,DO)方法提出一种识别时变结构瞬时频率的新方法。该方法首先通过引入2阶瞬时振幅与相位得到精度更高的2阶瞬时频率估算位置。其次,搜索频率方向上时频系数的局部最大值所对应的2阶瞬时频率位置并根据这些位置对时频系数进行重排,从而得到2阶局部最大同步挤压变换后的瞬时频带。再次,运用动态规划法在限定频带范围内提取瞬时频率曲线。通过一组数值算例和一个时变拉索试验验证了所提新方法的有效性,研究结果表明:相比既有的局部最大同步挤压变换算法,2阶局部最大同步挤压变换和动态规划的联合算法不仅具有较好的精度,而且具有更好的时频聚集性。

最大系数多重同步挤压变换识别结构瞬时频率

针对多重同步挤压变换及其改进算法存在的未重排点问题,提出了一种基于最大系数的多重同步挤压变换(maximum coefficient based multi-synchrosqueezing transform,简称MCMSST)方法来识别时变结构非平稳响应信号的瞬时频率(instantaneous frequency,简称IF)。首先,引入傅里叶频谱来辅助多分量响应信号选取截止频率;其次,对响应信号进行短时傅里叶变换(short time fourier transform,简称STFT),针对短时傅里叶变换系数求取针对时间的偏导,从而获得估算的瞬时频率;然后,在对瞬时频率的估算值进行多次迭代的基础上,仅保留时频系数模值最大处所对应的估算瞬时频率,并将其余位置的瞬时频率值归零;最后,对时频系数模值最大处所对应的瞬时频率进行时频重排即可得到细化后的瞬时频带。由于基于MCMSST方法提取的是瞬时频带,故采用时频系数模极大值法在限定的频带范围内提取瞬时频率曲线。通过2组数值算例和1个铝合金悬臂梁质量突变试验,验证了所提方法的有效性。研究结果表明,MCMSST方法不仅彻底解决了未重排点问题,而且提高了瞬时频率的识别精度和抗噪能力。

基于ILMSST识别时变结构非平稳响应信号瞬时频率

为解决局部最大同步挤压变换算法识别的频率精度不足及频带能量发散的问题,提出一种改进算法并将之命名为改进局部最大同步挤压变换方法(improved local maximum synchrosqueezing transform,ILMSST)。该方法首先对瞬时频率(instantaneous freguency,IF)位置进行多次迭代,从而获得更高精度的瞬时频率位置。其次,搜索短时傅里叶系数模极大值的位置并上下平移该位置,得到初步估算的频带并将频带外的短时傅里叶系数归零。最后,搜索频率方向上短时傅里叶系数的局部最大值所对应的瞬时频率位置,根据这些位置对时频系数进行重排,进而得到细化的瞬时频带。通过2组数值算例、1个7层钢筋混凝土剪力墙振动台试验和1个时变拉索试验验证了所提新方法的有效性,研究结果表明:相比现有的局部最大同步挤压变换方法,改进算法不仅提高了瞬时频率的估算精度,而且改善了响应信号瞬时频带的时频聚集性。

冻土区天然气水合物正演数值模拟与瞬时地震属性研究

天然气水合物是一种具有巨大潜能的新型能源,研究冻土区天然气水合物的地震响应特征,对我国陆域天然气水合物的勘探和开发具有重要意义。本文运用Kelvin粘弹性介质模型,基于祁连山冻土区的实际地质地层条件,建立理论地质-地球物理模型;采用交错网格有限差分法进行正演数值模拟,并对自激自收地震记录进行波场特征分析和提取瞬时地震属性。研究结果表明:地震波通过天然气水合物地层时,反射振幅能量较弱;在瞬时频率属性剖面可分辨层厚的范围内,瞬时频率随着层厚增加,频率在小幅度衰减;地震波通过含天然气地层时,反射波表现为强反射特征,瞬时频率能量明显增大;瞬时地震属性对波阻抗界面有更好的分辨能力,特别是瞬时相位属性剖面,作用明显。因此,综合分析波场特征与瞬时属性特征可以为陆域天然气水合物的识别、预测提供依据。

考虑电流瞬时频率特性的水上浮吊供电系统定时限过流保护方法

水上浮吊供电系统中,通常含有多台大功率异步电动机,多个电动机在并网启动时会产生较大的启动电流,当各电机启动电流叠加,可能导致变压器电流激增,触发过电流保护误动,给工业生产带来巨大损失。本文首先基于异步电动机等效模型,揭示了冲击电流的产生机理和箱式变压器保护误动原因;接着针对水上浮吊电机断续周期工作场景,提出了随机运行场景生成方法,基于希尔伯特黄变换技术,对系统的故障短路和电机启动状态下变压器的电流信号进行瞬时频率特性分析,揭示不同状态下箱式变压器电流的瞬时频率特性差异,在此基础上构建定时限过电流主动闭锁模块,确保浮吊供电系统正常工作时的可靠供电;最后通过湖南省某水上浮吊系统的仿真模型对所提出的保护方案进行了验证。

边界附近匀加速Unruh-DeWitt粒子探测器的响应

本文采用含时微扰论计算了真空中平行于一块全反射导体板匀加速运动的Unruh-DeWitt粒子探测器的跃迁速率.通过将结果分别与无边界真空中静态和匀加速粒子探测器的跃迁速率进行对比,发现匀加速运动将引起边界附近基态粒子探测器自发激发、边界可能引起探测器跃迁速率的重要修正.

Enhancing the resolution of seismic data based on the generalized S-transform

In this paper, we analyze the seismic signal in the time-frequency domain using the generalized S-transform combined with spectrum modeling. Without assuming that the reflection coeffi cients are random white noise as in the conventional resolution-enhanced techniques, the wavelet which changes with time and frequency was simulated and eliminated. After using the inverse S-transform for the processed instantaneous spectrum, the signal in the time domain was obtained again with a more balanced spectrum and broader frequency band. The quality of seismic data was improved without additional noise.

Effect of plunger number on hydraulic transformer's flow pulsation rate

In order to supply theoretical guidance to hydraulic transformer's design and application,the effect of the number of plungers in hydraulic transformer on its flow characteristic is analyzed,theoretical analysis and simulation are done on hydraulic transformer's flow characteristic when the number of plungers is different.Based on the working principle of swash plate piston hydraulic components,mathematical models of instantaneous flow and flow pulsation rate are built,and simulation study is done with MATLAB.As a result,the effect is found,and some conclusions worth referring to are obtained.

Seismic signal analysis based on the dual-tree complex wavelet packet transform

We tried to apply the dual-tree complex wavelet packet transform in seismic signal analysis. The complex wavelet packet transform (CWPT) combine the merits of real wavelet packet transform with that of complex continuous wavelet transform (CCWT). It can not only pick up the phase information of signal, but also produce better ″focal- izing″ function if it matches the phase spectrum of signals analyzed. We here described the dual-tree CWPT algo- rithm, and gave the examples of simulation and actual seismic signals analysis. As shown by our results, the dual-tree CWPT is a very effective method in analyzing seismic signals with non-linear phase.

Flow characteristics of variable hydraulic transformer

A new kind of hydraulic transformer, called variable hydraulic transformer(VHT), is proposed to control its load flow rate. The hydraulic transformer evolves from a pressure transducer to a power transducer. The flow characteristics of VHT, such as its instantaneous flow rates, average flow rates, and flow pulsations in the ports, are investigated. Matlab software is used to simulate and calculate. There are five controlled angles of the port plate that can help to define the flow characteristics of VHT. The relationships between the flow characteristics and the structure in VHT are shown. Also, the plus-minus change of the average flow rates and the continuity of the instantaneous flow rates in the ports are presented. The results demonstrate the performance laws of VHT when the controlled angles of the port plate and of the swash plate change. The results also reveal that the special principle of the flow pulsation in the ports and the jump points of the instantaneous curves are the two basic causes of its loud noise, and that the control angles of the port plate and the swash plate and the pressures in the ports are the three key factors of the noise.

Chandler Wobble Period and Q Derived by Wavelet Transform

We apply complex Morlet wavelet transform to three polar motion data series,and derive quasi-instantaneous periods of the Chandler and annual wobble by differencing the wavelet transform results versus the scale factor,and then find their zero points.The results show that the mean periods of the Chandler(annual)wobble are 430.71±1.07(365.24±0.11)and 432.71±0.42(365.23±0.18)mean solar days for the data sets of 1900-2001 and 1940-2001,respectively.The maximum relative variation of the quasi-instantaneous period to the mean of the Chandler wobble is less than 1.5% during 1900-2001(3%-5% during 1920-1940),and that of the annual wobble is less than 1.6% during 1900-2001.Quasi-instantaneous and mean values of Q are also derived by using the energy density-period profile of the Chandler wobble.An asymptotic value of Q = 36.7 is obtained by fitting polynomial of exponential of σ-2 to the relationship between Q and σ during 1940-2001.