共查询到19条相似文献,搜索用时 156 毫秒
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本文介绍一种在微型计算机上提高二维快速傅里叶变换速度的算法——子矩阵算法。其关键是在行变换和列变换之间,通过把数据矩阵划分为若干子矩阵的方法,来减少进行矩阵转置的时间,从而达到提高二维快速傅里叶变换速度的目的。 相似文献
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根据傅里叶变换系统中物面平移其频谱面上光强分布不变的特点,提出一种利用二次傅里叶变换实现CCD精密定焦的方法。将CCD和其透镜作为逆变换系统,输入面为两个狭缝,与一个附加透镜组成一个正变换系统。通过镜组移动改变正、负傅里叶变换系统的距离,用软件比较CCD上两个波峰的间隔是否变化来对CCD进行定焦。根据傅里叶变换原理和几何光学成像原理对CCD正焦和离焦时的波峰特点进行了理论分析,证明了离焦时波峰间隔变化量与镜组移动量、离焦量、狭缝间隔成正比,与两透镜的焦距成反比,由此关系即可实现CCD定焦。实验表明,该方法的相对定焦误差为0.2%。 相似文献
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二维谱分析在工程表面的研究中是有效和实用的,在介绍二维快速傅里叶变换基本概念的基础上,推导出了二维功率谱、角谱和半径谱在三维表面形貌中的应用计算方法,并对一些有代表性的精加工表面试件进行了实验研究。结论表明,二维谱分析技术可以表征表面纹理的方向性和不同波长对表面粗糙度高度均方根的影响。 相似文献
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采用有限元方法数值模拟了单脉冲激光热弹激发双层材料中的超声波,得到了材料的上下表面的应力场分布。通过对铝板中的波形进行快速傅里叶变换得到频率信号,计算出波包群速度,判断出材料中兰姆波的模态。结果表明:受基底影响,在双层材料的上层铝板中得到了三种模态的兰姆波。该研究为复合材料激光超声的应用研究提供了新的途径。 相似文献
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本文讨论金属丝网激光在线检测仪系统。该检测仪系统利用了“激光-傅里叶变换法”,并对被采样信号进行实时处理,实现了金属丝网二维分布尺寸的在线检测。 相似文献
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二维傅里叶变换法三维曲面检测 总被引:1,自引:0,他引:1
对投影光栅方法获得的位相调制空间载波图进行二维数字图象处理,实现了散射物体的三维曲面测量,利用二维快速傅里嚅变换将空域信号转变为频域信号,对一级频谱进行傅里叶反变换,逐步解调出其位相值,重建了被测物体表面的形貌。文中给出了对人体模型的实验测量结果。 相似文献
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《振动与冲击》2019,(22)
Lamb波的频散和多模态特性,使得利用Lamb波信号的时域或频域特征实现缺陷的定量检测具有一定的困难。提出了一种在频率-波数内分析激光Lamb波传播特性的方法。基于全光学型激光超声检测系统,脉冲激光在固定位置激励,连续激光一维线扫描接收,获得时间-空间波场信号,可直观地显示激光Lamb波信号的传播特性以及激光Lamb波与缺陷之间的作用规律。采用二维傅里叶变换将波场信号从时间-空间域转换到频率-波数域,可有效识别信号中包含的各模态信息。为了保留空间信息,借鉴短时傅里叶变换的思想,采用短空间二维傅里叶变换得到沿扫描路径上波数的分布,从中可直观看出缺陷的位置。采用带通滤波结合连续小波变换的方法对短空间二维傅里叶变换的结果进行了优化。分别在有、无缺陷铝板中进行了试验研究,试验结果验证了该方法的有效性。 相似文献
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Wilcox PD Lowe MJ Cawley P 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2005,52(12):2370-2383
Planar coil electromagnetic acoustic transducers (EMATs) are investigated for the excitation and detection of Lamb waves in nonferromagnetic metallic wave-guides. Such EMATs are attractive for certain applications due to their omni-directional sensitivity to wave modes with predominantly in-plane surface displacement, such as the So Lamb wave mode. A model is developed that enables the modal content of the radiated Lamb wave field from a transmitting EMAT to be calculated, and the output voltage from a receiving EMAT to be predicted when a Lamb wave mode is incident on it. The predictions from this model are compared with experimental data obtained from 12 different EMATs tested on a 5-mm thick aluminum plate, and good agreement is obtained. The model then is used to analyze the different effects that contribute to the overall Lamb wave modal sensitivity of an EMAT. The relationship between coil geometry and wavelength is examined. 相似文献
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Gachagan A Hayward G McNab A Reynolds P Pierce SG Philp WR Culshaw B 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1999,46(1):72-81
A condition monitoring nondestructive evaluation (NDE) system, combining the generation of ultrasonic Lamb waves in thin composite plates and their subsequent detection using an embedded optical fiber system is described. The acoustic source is of low profile with respect to the composite plate thickness, surface conformable, and able to efficiently launch a known Lamb wave mode, at operating frequencies between 100 and 500 kHz, over typical propagation distances of 100 to 500 mm. It incorporates both piezocomposite technology and interdigital design techniques to generate the fundamental symmetrical Lamb wave mode in both metallic and carbon-fiber composite plates. Linear systems and finite element modeling techniques have been used to evaluate the operation of the transducer structure, and this is supplemented by experimental verification of the simulated data. An optical fiber, either bonded to the surface or embedded across the length of the composite plate samples, is used to detect the propagating ultrasonic Lamb waves. Single mode silica fiber has been used in conjunction with a portable 633 nm Mach-Zehnder interferometer for signal demodulation and subsequent data acquisition. This hybrid system is shown to generate and detect the fundamental symmetrical Lamb wave (s(0)) in both carbon-fiber and glass-fiber reinforced composite plates. Importantly, the system signal-to-noise ratio (SNR) associated with the acoustic source compares favorably with s(0) Lamb wave generation using a conventional transducer and angled perspex wedge arrangement. 相似文献
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J. Pei M. I. Yousuf F. L. Degertekin B. V. Honein B. T. Khuri-Yakub 《Research in Nondestructive Evaluation》1996,8(4):189-197
Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of
bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or
ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point
source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers
is used to excite and detect theA
0 mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe
wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of theA
0 mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application
in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a section
of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate. 相似文献
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针对外界环境噪声等因素造成损伤因子不敏感,导致复合材料损伤识别困难和成像误差大等问题,提出了一种基于经验模态分解(Empirical mode decomposition, EMD)和相关系数的损伤因子。用空气耦合探头采集损伤前后的Lamb波信号进行EMD分解获取多个本征模态分量(Intrinsic mode function, IMF)。根据相关系数获取与信号相关性最大的IMF分量,并定义其能量值的相对变化为损伤因子。在模拟噪声环境前后,分别对玻璃纤维增强聚合物复合材料(GFRP)板中的分层缺陷进行识别和扫查成像,验证了该损伤因子的有效性。结果表明:信号经过EMD分解后,与其相关性最大的IMF分量对损伤最敏感,能够定义为识别损伤的损伤因子。将该损伤因子结合概率成像方法进行空耦Lamb波扫查,不仅能够有效对复合材料中的缺陷进行成像,而且在模拟强噪声环境中具有良好的抗噪性。 相似文献