六角形阵快速傅立叶变换波束形成算法

在六角形阵阵元数较多时,传统的频域相移求和波束形成方法要求的运算量很大.为此提出一种采用六角形快速傅立叶变换HFFT(Hexagonal Fast Fourier Transform)的波束形成算法.使用六角形傅立叶变换HDFT(Hexagonal Discrete Fourier Transform)完成六角形阵的波束形成,由于HDFT存在快速算法HFFT,因此能够显著降低波束形成的运算量.首先在各个通道上做FFT,将信号变换到频域,然后转角重排,再对各个阵元上相同的频点做HFFT,得到频域常规波束形成输出.理论分析表明,对于窄带信号的六角形阵波束形成,所提出的算法所需的计算量比传统的相移求和方法降低了95%以上.仿真和试验结果表明,提出的算法在不影响阵列处理性能的同时,显著降低了波束形成所需的计算量,易于工程实现.     

基于FFT盲辨识的肌电信号建模及模式识别

针对表面肌电信号(Electromyographic signal,sEMG)产生原理复杂、易受人体自身及外界因素影响的特点,采用基于快速傅里叶变换(Fast Fourier transform,FFT)的盲辨识方法建立肌电信号模型.该方法通过计算即可确定信道阶次,无需人为凭借经验设定,且计算简单、易于实现、运算速度快.其利用输出信道间的相互关系特性,实现信号的频域盲辨识,建立数学模型.此方法适用于小样本信号建模,非常适合易受肌肉疲劳影响的表面肌电信号.将模型系数作为改进的BP神经网络的输入,实现多运动模式识别,与其他盲辨识方法比较,此方法识别效果较好.     

离散S变换的频域算法和时域算法比较

在信号处理领域,S变换具有良好的时频结合特性,应用广泛。在实际系统中,S变换通过离散数值方法实现,有频域和时域两种形式,文献中多使用频域形式。在用S变换对一些信号作时-频分析时,频域和时域两种实现形式得到的时-频谱图有明显的差异,但没有文献对此问题进行相关讨论。离散S变换的频域算法使用快速Fourier 变换（FFT）和其逆变换（IFFT）实现,但由于Fourier变换蕴含周期特性,一些信号的S变换时-频谱图在始末两端会出现虚假频率信息,干扰信号的时-频分析结果。在研究S变换理论的基础上,给出离散S变换的时域实现。离散S变换的时域算法克服了频域算法的缺点,能更准确反映信号的时-频分布情况。通过对比实验,验证了时域算法的有效性。将S变换的离散时域算法用于地震数据拓频处理,结果显示方法能有效增强信号分辨率。     

一种新型分数阶小波变换及其应用

小波变换和分数Fourier变换是应用非常广泛的信号处理工具.但是,小波变换仅局限于时频域分析信号;分数Fourier变换虽突破了时频域局限能够在分数域分析信号,却无法表征信号局部特征.为此,提出了一种新型分数阶小波变换,该变换不但继承了小波变换多分辨分析的优点,而且具有分数Fourier变换分数域表征功能.与现有分数阶小波变换相比,新型分数阶小波变换可以实现对信号在时间-分数频域的多分辨分析.此外,该变换具有物理意义明确和计算复杂度低的优点,更有利于满足实际应用需求.最后,通过仿真实验验证了所提理论的有效性.     

基于模糊模式识别的抽油井示功图判别算法研究

针对有杆抽油井示功图诊断分析问题,研究特定故障情况下示功图特征值算法。把示功图数据通过傅里叶变换由空域转换成频域形式,仅保留较低频率上的系数a0、a1、b1,并将其作为特征值。根据所得到的特征值结合模式识别算法能够实现有杆抽油井示功图快速准确的识别和分类。实际应用表明,利用基准示功图28种类型的特征值的选取,计算出每种示功图的特征矢量,并保存到特征库中。对给定示功图特征值进行模式识别,即可得出该图隶属于哪种模式。     

永磁同步电动机的分数阶时域和频域建模

采用机理与数据相结合的建模方法对永磁同步电动机进行分数阶时域和频域建模. 在分数阶时域建模方法中, 设计伪随机激励信号, 获取实时实验数据并采用输出误差辨识算法来获取分数阶阶次; 在分数阶频域建模方法中, 由实时实验数据绘制出电动机的对数频率特性曲线. 采用分数阶频域建模中经典Levy 辨识算法, 利用加权函数加以改进, 得到永磁同步电动机分数阶模型辨识结果. 最后通过对两种方法得到的结果进行对比表明了所提出模型的可靠性.

     

分数阶小波变换

小波变换是对信号时域-频域(Fourier域)的多分辨率分析,也可看作是一种Fourier域伸缩带通滤波.分数阶Fourier变换是对传统Fourier变换的推广,对信号分析处理有更大的灵活性,为了将多分辨率分析理论推广到时域-广义频域(分数阶Fourier域),提出了一种分数阶小波变换,分析了分数阶小波变换在广义频域伸缩带通滤波特性,分析信号时的时域-广义频域平面的多分辨率分析网格划分.分数阶小波变换是传统小波变换的推广,在对原小波变换核作一定改动后增加了小波变换对信号处理的灵活性.可以看到,将分数阶小波变换的变换角度取为π/2,便得到与传统小波变换多分辨率分析理论完全一致的结果.理论分析和计算机仿真表明了所提理论的正确性和有效性.     

基于LabWindows/CVI的振动测试数据分析处理系统设计

电子设备在现代武器装备中起着至关重要的作用,其可靠性对武器装备发挥其作战能力有很大的影响.基于LabWindows/CVI软件开发平台,设计了振动测试数据分析处理系统.通过滤波、快速傅里叶变换(FFT,fast Fourier transform)等数据处理功能,对振动测试数据信号进行时域和频域分析,并且计算出电子设备振动响应传递函数,从而分析评估电子设备在装备工作过程中的振动冲击响应传递特性及影响效应,为电子设备的优化布局及装备的试验定型提供科学的试验数据参考.调试试验表明,该数据分析处理系统基本上实现了设计目的,满足了预期的设计要求及功能需求.     

一种基于小波变换的输油泵故障诊断方法

小波变换突破了传统Fourier变换在信号分析中的局限性,实现了对非平稳信号在时域和频域的局部化处理,提出了一种利用小波变换精确定位突变点的位置和突变类型的故障诊断方法,在输油泵的故障诊断中得到了验证.     

用有限元法计算特征值问题的一种新的动态凝聚方法

有限元法是常用的建模方法,由于所建模型具有较大的自由度,通常需要进行降阶处理.一般来讲,模型前几阶特征值和特征向量可以较精确地得到,利用所得到的特征值和主振型分量(在特征向量中与所给定的主自由度对应的振型分量),本文提出了一种新的动态凝聚方法,该方法是通过迭代方式,利用所得到的特征值和主振型分量对Guyan降阶法所得到的降阶模型进行修正.与同类方法相比,本文方法具有较高的计算精度和很小的计算量,且迭代收敛的稳定性很好.最后本文给出了一个计算实例.     

基于曲率模态振型的刚架结构损伤检测

结构损伤前后动力特性的变化来快速、直接、方便地判定损伤的存在、程度及位置．本文采用曲率模态对刚架结构的损伤检测进行了研究．首先用有限元法计算出结构的位移模态振型,然后用差分法计算出曲率模态振型．数值模拟结果表明：曲率模态振型对结构的损伤敏感,可同时确定结构损伤的存在、程度和位置,并且可以用于结构多位置损伤的检测．实验结果证实了数值模拟结论．     

Dynamics of a circular membrane with an eccentric circular areal constraint: Analysis and accurate simulations

The dynamics of a circular membrane with an eccentric circular areal constraint is studied under arbitrary initial conditions. The symmetric and antisymmetric modes of vibrations are investigated and the results are compared with those in the literature. The accuracy of the eigenfrequencies and mode shapes are studied for different sizes and locations of the constraint and it is shown that proper choice of the number of angular modes is critical to accurate computation of the mode shapes; fewer or more number of angular modes can provide relatively accurate eigenfrequencies but completely inaccurate mode shapes. The orthogonality of distinct modes of constrained membranes is mathematically established in this work. The orthogonality property of the modes is used to compute the modal coefficients for simulation of the dynamics. In comparison to prior work, where the objective has been limited to computing the first few eigenfrequencies, this work presents a method for accurately computing the eigenfrequencies, mode shapes, and modal coefficients needed for dynamics simulation. Numerical simulations and video animations of vibration of constrained membranes are presented for arbitrary initial conditions.     

Characterization of delamination effect on composite laminates using a new generalized layerwise approach

A dynamic analysis method has been developed to investigate and characterize the effect due to presence of discrete single and multiple embedded delaminations on the dynamic response of composite laminated structures with balanced/unbalanced and arbitrary stacking sequences in terms of number, placement, mode shapes and natural frequencies. A new generalized layerwise finite element model is developed to model the presence of multiple finite delamination in laminated composites. The new theory accurately predicts the interlaminar shear stresses while maintaining computational efficiency.     

Numerical simulation of fluid added mass effect on a francis turbine runner

In this paper, a numerical simulation to analyze the influence of the surrounding water in a turbine runner has been carried out using finite element method (FEM). First, the sensitivity of the FEM model on the element shape and mesh density has been analysed. Secondly, with the optimized FEM model, the modal behaviour with the runner vibrating in air and in water has been calculated. The added mass effect by comparing the natural frequencies and mode shapes in both cases has been determined.The numerical results obtained have been compared with experimental results available. The comparison shows a good agreement in the natural frequency values and in the mode shapes. The added mass effect due to the fluid structure interaction has been discussed in detail.Finally, the added mass effect on the submerged runner is quantified using a non-dimensional parameter so that the results can be extrapolated to runners with geometrical similarity.     

System identification of torsionally coupled buildings

In this study, an extended random decrement method, which considers the correlation among measurements, was employed to reduce the measured dynamic responses of general torsionally coupled multi-story building under random excitations. The Ibrahim time domain technique was then applied to calculate the modal frequencies and damping ratios based on only a few floor response measurements. To obtain the complete mode shapes, an interpolation method was developed to estimate the mode shape values for the locations without measurements. The seismic responses at floors with and without measurements were also calculated. Numerical results through a seven-story torsionally coupled building under ambient random excitations demonstrated that the proposed method is able to identify structural dominant modal parameters accurately even with highly coupled modes and noise contamination. A small number of response measurements, no requirement for input excitation measurements and simple on-line calculations make the proposed method favorable for implementation.     

Damage detection of truss bridge joints using Artificial Neural Networks

Recent developments in Artificial Neural Networks (ANNs) have opened up new possibilities in the domain of inverse problems. For inverse problems like structural identification of large structures (such as bridges) where in situ measured data are expected to be imprecise and often incomplete, ANNs may hold greater promise. This study presents a method for estimating the damage intensities of joints for truss bridge structures using a back-propagation based neural network. The technique that was employed to overcome the issues associated with many unknown parameters in a large structural system is the substructural identification. The natural frequencies and mode shapes were used as input parameters to the neural network for damage identification, particularly for the case with incomplete measurements of the mode shapes. Numerical example analyses on truss bridges are presented to demonstrate the accuracy and efficiency of the proposed method.     

Shape estimation of transparent objects by using inverse polarization ray tracing

Few methods have been proposed to measure three-dimensional shapes of transparent objects such as those made of glass and acrylic. In this paper, we propose a novel method for estimating the surface shapes of transparent objects by analyzing the polarization state of the light. Existing methods do not fully consider the reflection, refraction, and transmission of the light occurring inside a transparent object. We employ a polarization raytracing method to compute both the path of the light and its polarization state. Polarization raytracing is a combination of conventional raytracing, which calculates the trajectory of light rays, and Mueller calculus, which calculates the polarization state of the light. First, we set an initial value of the shape of the transparent object. Then, by changing the shape, the method minimizes the difference between the input polarization data and the rendered polarization data calculated by polarization raytracing. Finally, after the iterative computation is converged, the shape of the object is obtained. We also evaluate the method by measuring some real transparent objects.     

The approximate reanalysis method for topology optimization under harmonic force excitations with multiple frequencies

In mode acceleration method for topology optimization related harmonic response with multiple frequencies, most of the computation effort is invested in the solution of the eigen-problem. This paper is focused on reduction of the computational effort in repeated solution of the eigen-problem involved in mode acceleration method. The block combined approximation with shifting method is adopted for eigen-problem reanalysis, which simultaneously calculates some eigenpairs of modified structures. The triangular factorizations of shifted stiffness matrices generated within a certain number of design iterations are utilized to calculate the modes. For improving computational efficiency, Basic Linear Algebra Subprograms (BLAS) are utilized. The reanalysis method is based on matrix-matrix operations with Level-3 BLAS and can provide very fast development of approximate solutions of high quality for frequencies and associated mode shapes of the modified structure. Numerical examples are given to demonstrate the efficiency of the proposed topology optimization procedure and the accuracy of the approximate solutions.     

Method for finding multiple roots of polynomials

Conventional numerical methods for finding multiple roots of polynomials are inaccurate. The accuracy is unsatisfactory because the derivatives of the polynomial in the intermediate steps of the associated root-finding procedures are eliminated. Engineering applications require that this problem be solved. This work presents an easy-to-implement method that theoretically completely resolves the multiple-root issue. The proposed method adopts the Euclidean algorithm to obtain the greatest common divisor (GCD) of a polynomial and its fast derivative. The GCD may be approximate because of computational inaccuracy. The multiple roots are then deflated into simple ones and then determined by conventional root-finding methods. The multiplicities of the roots are accordingly calculated. A detailed derivation and test examples are provided to demonstrate the efficiency of this method.     

Unsupervised learning of an atlas from unlabeled point-sets

One of the key challenges in deformable shape modeling is the problem of estimating a meaningful average or mean shape from a set of unlabeled shapes. We present a new joint clustering and matching algorithm that is capable of computing such a mean shape from multiple shape samples which are represented by unlabeled point-sets. An iterative bootstrap process is used wherein multiple shape sample point-sets are nonrigidly deformed to the emerging mean shape, with subsequent estimation of the mean shape based on these nonrigid alignments. The process is entirely symmetric with no bias toward any of the original shape sample point-sets. We believe that this method can be especially useful for creating atlases of various shapes present in medical images. We have applied the method to create mean shapes from nine hand-segmented 2D corpus callosum data sets and 10 hippocampal 3D point-sets.