A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metal-lic plate structure.The temporal focusing effect of the time reversal Lamb waves is investigated theoretically.It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation.Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations.The results show that the reconstructed time reversed wave exhibits close similarity to the reversed nar-rowband tone burst signal validating the theoretical model.To enhance the similarity,the cycle number of the excited signal should be increased.Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure.In this work,the time reversal technique is used for the recompression of Lamb wave signals.Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal.It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely,but the cycle number of the excited signal should be chosen reasonably.     

光谱测量在激光加工在线监测上的应用研究进展

随着现代工业的发展,复杂加工环境和对象、大动态范围、高效率和高精度激光加工需求愈加迫切,在线监测并实时优化激光加工参数是一条重要的解决途径。与此同时,激光与物质相互作用时可产生与加工参数、加工过程和目标特性密切相关的光信号和表面光学特性变化,在线测量光信号光谱可分析加工过程和状态,故光谱测量有望成为一种重要的激光加工在线监测手段。实际上,光谱测量已应用于激光焊接、激光切割和钻孔、激光清洗打磨、微纳结构制备和增材制造等几乎所有激光加工工艺,具有分辨率高和光谱信息丰富等特点。分析和总结了用于激光加工在线监测的光谱测量技术,包括等离子体光谱、反射光光谱和非线性光信号光谱等。基于单脉冲和多脉冲激光加工激发等离子体信号的光谱测量,除实现化学成分定性和定量监测外,还可以根据特征谱线相对强度变化实时调焦,根据等离子体温度监测和调控激光加工过程中与热效应相关的物理过程;作为一种无损伤且工作距离较远的监测方法,反射光光谱监测可通过测量特定波段反射光信号光谱积分功率、特征谱线和波段位置和强度来有效监测材料表面清洁度、损伤、色度和成分变化等;而在特定条件下产生的谐波信号、荧光信号和拉曼信号等非线性光信号,尽管应用场景有限,但提供了一种实现成分、焦距和材料损伤等监测的新方法。进而,展望了光谱测量在激光加工在线监测上应用的未来发展趋势,包括多种光信号的光谱协同监测与光、声、温度及图像等多种信号测量的复合监测。同时,人工智能技术与在线监测和激光加工的深入结合将进一步推动激光加工技术的智能化发展。     

A methodology for structural health monitoring with diffuse ultrasonic waves in the presence of temperature variations

Diffuse ultrasonic waves for structural health monitoring offer the advantages of simplicity of signal generation and reception, sensitivity to damage, and large area coverage; however, there are the serious disadvantages of no accepted methodology for analyzing the complex recorded signals and sensitivity to environmental changes such as temperature and surface conditions. Presented here is a methodology for applying diffuse ultrasonic waves to the problem of detecting structural damage in the presence of unmeasured temperature changes. This methodology is based upon the prediction and observation that the first order effect of a temperature change on a diffuse ultrasonic wave is a time dilation or compression. A multi-step procedure is implemented to (1) record a set of baseline waveforms from the undamaged specimen at temperatures spanning the expected operating range, (2) select a waveform from the baseline set whose temperature is the closest to that of a subsequently measured signal, (3) adjust this baseline waveform to best match the signal, and (4) calculate an error parameter between the signal and the adjusted waveform and compare this parameter to a threshold to determine the structural status. This procedure is applied to experimental data from aluminum plate specimens with artificial flaws. Probability of detection and the minimum flaw size detected are presented as a function of the size of the baseline waveform set. It is shown that a probability of detection of over 95% can be achieved with a small number of baseline waveforms.     

非线性调频信号激励下非线性系统的最优共振响应

非线性调频(NLFM)信号在雷达、通信、信号处理中应用广泛,该类信号所激励下的非线性系统响应有着丰富的信息,通过共振来增强NLFM信号具有一定实际意义与价值.本文主要研究了受到不同类型NLFM信号所激励的非线性系统共振现象,提出了实时尺度变换方法来处理高频NLFM信号,克服人为选择造成信号输出响应较差的缺点.同时,提出实时谱放大因子作为共振评价指标,准确评价NLFM信号激励下的系统共振响应,讨论系统参数对系统最优共振响应的影响,参数选择在合理的区间内即可实现最优共振响应,不仅实现信号特征的大幅增强,还保持信号时频特征的连续性.最后,将实时尺度变换方法和固定尺度变换方法进行对比,说明所提方法处理NLFM信号的优越性.     

混沌直接序列扩频信号盲解调的硬件电路实现

提出了基于现场可编程门阵列 (FPGA) 技术的混沌直接序列扩频信号盲解调的硬件电路实现方法. 设计了混沌直接序列扩频信号发射机与接收机. 发射机可产生10种不同的混沌直接序列扩频信号. 为方便接收机的硬件电路实现, 对无先导卡尔曼滤波混沌拟合盲解调算法进行了简化, 在简化模型的基础上设计了接收机硬件结构. 提出了一种动态调整偏移因子的新方法, 使接收机能实时适应混沌映射的变化. 通过高斯白噪声信道及多径信道条件下的盲解调实验, 验证了盲解调算法硬件实现的抗噪声与抗多径性能, 以及对10种不同的混沌直接序列扩频信号的自适应破译效果. 关键词： FPGA 混沌直接序列扩频通信 盲解调     

一种通用的卫星导航信号码时延估计误差评估方法

卫星导航信号的码时延估计误差是决定系统服务性能的关键因素,迫切需要对多种不同调制与复用方式的导航信号进行全面的码时延估计误差性能评估,从而为后期的系统应用提供重要的选择依据.为此,本文提出了通用的码时延估计误差评估方法.首先,概括了导航接收机的码跟踪环路模型,根据是否匹配接收以及是否相干处理,将目前的导航接收机归纳为四种类型.其次,在假设码时延估计误差非常小的条件下,分别给出了匹配接收下相干处理和非相干处理时的估计误差以及相互之间的关系;推导了非匹配接收下非相干处理时的估计误差,并讨论了与相干处理时的关系.最后,推导了码时延估计误差的齐夫-扎凯界限,解决了估计误差不满足非常小这一假设条件时的评估.本文提出的评估方法均以导航信号的功率谱密度表示,为信号设计和接收机的研制提供了重要的理论指导,同时也给具体信号的评估带来了极大便利.仿真实验中对新一代典型导航信号的码时延估计误差做了有效评估.     

Wavelet filter-based weak signature detection method and its application on rolling element bearing prognostics

De-noising and extraction of the weak signature are crucial to fault prognostics in which case features are often very weak and masked by noise. The wavelet transform has been widely used in signal de-noising due to its extraordinary time-frequency representation capability. In this paper, the performance of wavelet decomposition-based de-noising and wavelet filter-based de-noising methods are compared based on signals from mechanical defects. The comparison result reveals that wavelet filter is more suitable and reliable to detect a weak signature of mechanical impulse-like defect signals, whereas the wavelet decomposition de-noising method can achieve satisfactory results on smooth signal detection. In order to select optimal parameters for the wavelet filter, a two-step optimization process is proposed. Minimal Shannon entropy is used to optimize the Morlet wavelet shape factor. A periodicity detection method based on singular value decomposition (SVD) is used to choose the appropriate scale for the wavelet transform. The signal de-noising results from both simulated signals and experimental data are presented and both support the proposed method.     

智能化计算机安全监控信息网络技术研究

为了保障计算机以及使用者信息的安全性、隐秘性,并且提高计算机的使用生命周期,需要对计算机进行安全智能监控。但采用当前的计算机安全监控技术对计算机安全进行监控时,没有设置具体的安全监控指标,无法计算出计算机安全监控的非安全因素权重,存在计算机网络可能自动泄密,无法智能监控以及监控数据误差大的问题。为此,将信息网络技术应用于计算机安全智能监控中,提出了一种基于LINUX的计算机安全智能监控方法。该方法先将计算机非安全因素进行分类,其中包括计算机网络配置,自带系统和网络病毒。然后利用SAltera EPM7128S芯片对计算机安全智能监控进行硬件构造,采用CPLD结构根据计算机非安全因素分类结果对计算机安全智能监控软件部分进行设计,软件设计中依据计算机安全智能监控失真衰减的抑制方法,实现计算机远程安全智能监控,最后根据Delphi法来建立计算机安全智能监控网络环境总体运行情况的评估指标体系,对大规模无法定量分析的计算机安全监控因素做出概率估计,以概率估计结果为依据对计算机安全智能监控的风险进行评估,从而实现对计算机安全的智能化监控。实验仿真证明,所提方法提高了计算机安全智能监控的全面性和高效性,减少了计算机安全智能监控数据传输的丢包率。     

金属橡胶热物理性能理论与试验研究

针对金属橡胶材料在高温环境下的热稳定性能和热传导性能, 基于金属橡胶的内部基本组分以及特殊的编织制作工艺, 构建了两种典型排列微元体结构, 并以此描述微元体在三种接触状态下的热膨胀特性, 提出了金属橡胶热膨胀Schapery分析模型, 从理论上解释了金属橡胶热膨胀的产生机理.根据金属橡胶基本组成单元的传热模式, 研究了金属橡胶的传热过程.利用热电比拟法和有限元法, 分析微元体的导热性能, 结合微元体的分布形式, 提出了金属橡胶的导热分析模型.试验测试了不同相对密度金属橡胶的热膨胀和热传导性能, 验证了金属橡胶热膨胀和导热特性分析理论模型的适用性.所得到的金属橡胶的热膨胀和热传导性能理论模型和试验结果, 为金属橡胶材料在高温环境下的热物理特性分析提供了理论基础和计算分析依据. 关键词： 金属橡胶 热膨胀 热传导 金属丝螺旋卷     

海洋赤潮温度盐度双参数检测技术研究

针对海洋赤潮的实时监测和预报,提出了一种通过检测海水中布里渊散射回波信号,获取海洋赤潮水体温度和盐度分布信息的双参数检测技术。通过分析布里渊散射理论,建立基于机载蓝绿激光雷达的布里渊散射信号频移量和布里渊散射信号能量与海水温度和盐度的关系模型。通过仿真计算,可由海水中的蓝绿激光布里渊散射回波信号的频移量和能量,分别得出海表及海水50 m深度以内温度和盐度的分布情况,从而可以实现海水温度和盐度的双参数实时获取,为实时判断海洋赤潮的消长过程提供理论依据。     

基于多模态信号的金属材料缺陷无损检测方法

金属材料缺陷检测对于经济发展具有重要意义.针对现有无损检测技术信号模态单一、检测范围有限等不足,提出了一种基于多模态信号的金属材料缺陷无损检测方法.该方法以光声无损检测方法为主体,首先利用有限元方法分析了缺陷对激光能量吸收量和光声表面波传播的影响,提出了基于激光吸收量和光声表面波的缺陷检测方法;然后搭建了多模态信号检测平台,采集了缺陷的光学、光声和超声三种模态的信号,检测出了裂纹的宽度和分布信息,以及深度和在内部的延伸状况.研究结果表明,本文提出的基于金属材料多模态信号的无损检测方法能够准确、全面地检测出金属材料的杂质和裂纹的尺寸信息,弥补了现有无损检测方法在检测范围上的不足,为缺陷定量检测和全面诊断提供了一个新的思路.     

小波包节点分段阈值降噪在水声监听中的应用*

水声目标信号在发送、传播过程中,易受到环境噪声、系统自噪声等影响,因此水声监听过程中目标信号会掺杂大量噪声信息。为提高获取目标信号的准确性和可靠性,降低噪声,在已有小波分析基础上,提出小波包节点相对能量判断最优分解层,最优分解层节点系数分段阈值处理重构方法,实现水声监听信号分频段去噪。将0.1 kHz~8.4 k Hz实验数据按节点频率排序划分为5个强弱不同的频段信号实现消噪提取,结果表明该方法可将噪声信号与目标信号有效分离,与全局单一阈值相比,具有较好降噪能力。该方法打破了小波阈值去噪高频处理的局限性,提高了识别精度,改善了全局单一阈值去噪存在的短板,在鱼类分析识别、舰船监听、深海探测等方面具有一定的推广和应用价值。     

Adaptive noise cancelling and condition monitoring

When mechanical signature analysis methods are applied to the detection of faults within a complex machine, one is often confronted with a situation in which the diagnostic signal is embedded in a background noise. Coherent filtering techniques are of help in improving the signal to noise ratio (SNR) only when a synchronizing signal is available; on the other hand, the adaptive noise cancelling (ANC) technique can be successfully applied to increase the signal to noise ratio even in those situations where a synchronization signal is not available. Adaptive noise cancelling is a form of optimal filtering in which use is made of an auxiliary or a reference signal. In the work reported here it has been shown that the statistical and spectral analyses techniques which fail to detect and diagnose faults because of a poor signal to noise ratio can be made effective by using an adaptive noise cancelling technique. The expression for the signal to noise density ratio at the output of the noise canceller is derived for a simplified model of a machine.     

基于可调谐半导体激光吸收光谱技术的气体浓度测量温度影响修正方法研究

可调谐二极管激光吸收光谱技术测量气体浓度时,由于测量氛围温度变化的影响引起解调的二次谐波信号发生变化,最终导致浓度测量的较大误差. 为了修正温度变化对浓度反演结果的影响,适应工业测量、燃烧诊断的需要,采用通过实验所得温度关系的数值拟合修正方法即经验公式修正和根据HITRAN数据库参数的理论关系即理论公式修正两种方法进行分析与讨论. 实验中采用在50 cm长的高温管式炉中通入高温安全的21%浓度的 氧气为目标测定气体,选定760.77 nm的中心吸收波长,测量了温度变化范围为300–900 K,间隔50 K的情况下所得到的谐波信号,并利用一次谐波比值消元法消除光强波动影响后的结果,得出了不同温度下未修正的原始浓度值和通过修正方法后的修正值. 实验结果表明所述的经验公式和理论公式两种修正方法对温度影响都有一定的抑制作用,可以应用到温度变化引起的气体浓度误差修正监测中,为下一步开展燃烧诊断实时在线监测提供了依据. 关键词： 可调谐二极管激光吸收光谱 (TDLAS) 温度修正 经验公式 理论公式     

核磁共振陀螺中原子气室温度场的研究

为研究核磁共振陀螺中加热机构对原子气室性能的影响,设计了5种典型加热方式。利用有限元分析软件ANSYS建立了原子气室的温度场模型,给出了原子气室表面的稳态温度场分布情况。同时设计了探测精度为0.01℃的测温电路,对原子气室表面不同位置的温度进行监控,获得了不同加热方式下原子气室表面的温度变化情况。将仿真和实验结果进行比较,发现误差在5%之内,验证了仿真模型的正确性。综合仿真和实验结果比较了不同加热方式下原子气室表面温度分布情况,获得了能够使原子气室表面温度分布最均匀的加热方式。     

Internal tissue displacement measurement based on ultrasonic wave Doppler signal digital detection and its application to fetal movement monitoring

We have developed a fetal movement monitoring system based on small displacement measurement of internal tissues. When ultrasonic pulses are transmitted to the fetus, the reflected ultrasonic waves which have a Doppler frequency shift due to the fetal movements are detected by using an ultrasonic pulsed Doppler technique. In this paper, we propose a displacement measurement method for internal tissues which is based on the Doppler signal digital detection technique. In the method, the received ultrasonic RF signals are sampled with a sampling frequency of four times higher than the centre frequency of the ultrasonic waves; the Doppler frequency shift signals are derived using digital signal processing. From the detected signals, the internal displacements are estimated using the arc-tangent method. The basic algorithm of the detection method has already been used in the area of blood flow sensing, however, we apply the algorithm to the displacement measurement of internal tissues. The comparison between the proposed method and the conventional method is presented. The fetal movement quantitative monitoring system based on the method which has been constructed is shown.     

机载激光测深系统的最小可探测深度研究

针对回波特性进行理论分析和数值模拟，讨论影响机载激光测深系统的最小可探测深度的各种因素和解决的技术途径。采用窄激光脉冲，高速探测器，小接收视场角，可以改善海表和海底反射信号的叠加。分析海表和海底反射脉冲的特征，提出采用双高斯脉冲拟合方法，从叠加的回波信号中分离海表、海底反射信号，降低机载激光测深系统的最小可探测深度，满足海岸带测绘应用。利用实验室模拟装置进行测量实验，利用提出的方法分离表面和底部反射信号，得到0．4m的最小可探测深度。同时比较了雪崩二极管和光电倍增管的测量结果，两者的测量精度相当。     

磁性应力监测中力磁耦合特征及关键影响因素分析

针对磁性应力监测研究中力磁耦合关系有多种变化趋势且一直没有合理理论解释这一问题, 从磁性材料的微观磁畴运动出发, 根据磁致磁化过程, 详细分析了两种不同磁化状态下力致磁化的变化规律, 揭示出力磁耦合关系的变化特征, 进而提出磁化状态是导致力磁耦合关系差异的本质因素; 对工程上实际缆索用镀锌钢丝在不同磁化状态下进行了力磁耦合试验, 结果与理论一致: 力磁耦合关系随磁化状态的不同而不同, 本质上有效地解释了已有研究中力磁耦合关系的矛盾多样性. 关键词： 应力监测 磁化状态 力磁耦合 磁畴运动     

Estimation of ultrasound attenuation and dispersion using short time Fourier transform

Determination of the acoustic attenuation and dispersion has important applications in ultrasound tissue characterization and non-destructive material testing. Current signal processing methods Fourier transform of ultrasound signals to get the spectra of amplitude and phase to estimate respectively the attenuation and dispersion of a given medium. These methods are frequency domain method and obsessed with ambiguity issue in the phase unwrapping calculation. Conventional ultrasound velocity measuring method detects the time of arrival of a pulse (or echo) signal, which is a time domain method to compute group velocity (not phase velocity). This paper presents a novel approach based on the short time Fourier transform (STFT)--a time-frequency analysis, to estimate the ultrasonic dispersion and attenuation. Only the amplitude information of the pulse-signal spectra is used. Based on the time-frequency presentation, the attenuation coefficient of the signal is obtained by computing the amplitude decay of pulse spectrum in time domain, while phase velocities are obtained based on the "time-of-flight" (TOF) of the mono frequency component of the pulse signals. As a result, we eliminate the ambiguity issue in phase angle calculation. Furthermore, the proposed method makes the phase velocity pedagogically intuitive for novice users. The paper presents experiments to evaluate demonstrate the performance of the proposed method.     

基于电阻抗层析成像的高强度聚焦超声温度监测技术

作为一种对正常组织无损伤且不易引起癌细胞转移的非入侵肿瘤治疗手段,高强度聚焦超声(HIFU)治疗过程中焦域的温度监测是实现剂量精准控制的关键.本文基于生物组织的温度-电阻抗的关系,将电阻抗层析成像(EIT)和HIFU治疗相结合,提出了一种利用组织焦平面的表面电压实现电阻抗重构的检测技术.建立了HIFU治疗和EIT综合系统模型,在考虑组织的声吸收条件下,对三维Helmholtz方程在柱坐标下的声场计算进行了二维简化,并引入Pennes生物热传导方程来计算HIFU焦域的声压和温升分布特性;引入生物组织的温度-电阻抗关系,基于麦克斯韦电磁场理论,建立了具有温度分布HIFU焦域的电流和电压计算模型,利用恒流注入的边界条件实现电场计算,获得焦平面的表面电压分布.在数值计算中,利用实验聚焦换能器参数,模拟了在固定声功率下组织焦域的声场和温度场分布,以及中心和偏心聚焦条件下不同治疗时刻的电导率分布;然后通过对称电极的循环电流注入,计算了组织模型焦平面内的电流密度和电势分布,获得了焦平面圆周分布的表面电极电压;进一步采用修正的牛顿-拉夫逊算法,利用32×32的表面电极电压实现了焦平面内电导率分布的重建.结果表明,基于温度-电阻抗关系的EIT电导率重建技术不但能准确定位HIFU焦域中心,还能恢复HIFU治疗中焦域的温度分布,证明了EIT用于HIFU治疗中温度监测的可行性,为其疗效评估和剂量控制提供了一种无创电阻抗测量和成像新方法.