基于VMD-PNN的砂轮钝化声发射检测

在磨削加工过程中,加工刀具即砂轮会发生钝化现象,砂轮表面磨损影响加工精度和工件质量,需要及时检测并修整。磨粒的塑性变形、破碎、断裂等会产生声发射信号,能够作为精确识别砂轮钝化状态的依据,且不易被噪声干扰,因此提出一种基于变分模态分解（Variational Mode Decomposition,VMD）和概率神经网络（Probabilistic Neural Network,PNN）的砂轮钝化声发射检测方法。VMD可以将原始信号分解为多个本征模态函数（Intrinsic Mode Function,IMF）分量,筛选其中峭度较大的分量重构即得到声发射信号。声发射检测的关键是特征参数的选取,在相关研究基础上本文提出了声发射包络能量占比作为一个重要的特征参数,并选取了共5种特征参数,构建出5维特征向量数据集,输入到PNN中进行训练,经过测试识别准确度达到94.5%。该方法建立了声发射信号特征参数与砂轮不同钝化状态的关系,能够对砂轮严重钝化状态给出准确预警,具有实际应用价值。文章比较了声发射信号不同特征参数用于识别砂轮钝化状态的准确度,对特征参数的选用具有参考意义。     

基于脚步诱发结构振动的人员特征身份识别研究EI北大核心CSCD

身份识别是安防领域一项重要工作,目前生物特征识别方法主要利用静态生理特征,利用脚步振动信号进行身份识别研究相对较少,本文提出利用行走过程中脚步诱发结构振动信号的差异性来识别人员。基于能量阈值法检测脚步事件与非脚步事件,对不同测试人员单一脚步事件在时域、频域方面共16个脚步特征参数进行了对比分析,研究发现可以将不同特征组合下参数差异性作为身份识别的依据。为了验证方法的有效性,采用支持向量机(support vector machine, SVM)作为分类工具,测试人数为10人数据样本500个情况下,选用16个脚步特征参数平均识别率为79.21%;采用皮尔逊相关系数法筛选出彼此不相关的10个脚步特征参数平均识别率为91%,相比于采用16个脚步特征参数平均识别率提高了11.79%;对比了在不同SVM核函数下分类工具对选取的10个脚步特征参数平均识别率的影响,结果采用线性核函数下平均识别率最高达到96%。结果表明,有效的脚步特征参数组合适用于小样本下的身份识别。     

融合时频域特征的舰船识别方法及实验研究

文章提出了一种融合舰船辐射噪声时频域特征的识别方法,将舰船辐射噪声的线谱特征和线性预测倒谱特征作为输入,分别利用反向传播（Back Propagation,BP）神经网络进行训练、降维及初步判别,并采用加权投票方式,引入置信度算法和拒判机制实现决策级融合识别。实验结果表明,对比基于舰船单一特征的识别方法,利用舰船辐射噪声时频域特征的互补性进行融合识别,减小了单一识别方法误判对总识别率的影响,具有较强的鲁棒性,可有效提高对目标的识别率。     

基于脚步诱发结构振动的人员特征身份识别研究

身份识别是安防领域一项重要工作,目前生物特征识别方法主要利用静态生理特征,利用脚步振动信号进行身份识别研究相对较少,本文提出利用行走过程中脚步诱发结构振动信号的差异性来识别人员。基于能量阈值法检测脚步事件与非脚步事件,对不同测试人员单一脚步事件在时域、频域方面共16个脚步特征参数进行了对比分析,研究发现可以将不同特征组合下参数差异性作为身份识别的依据。为了验证方法的有效性,采用支持向量机(support vector machine, SVM)作为分类工具,测试人数为10人数据样本500个情况下,选用16个脚步特征参数平均识别率为79.21%;采用皮尔逊相关系数法筛选出彼此不相关的10个脚步特征参数平均识别率为91%,相比于采用16个脚步特征参数平均识别率提高了11.79%;对比了在不同SVM核函数下分类工具对选取的10个脚步特征参数平均识别率的影响,结果采用线性核函数下平均识别率最高达到96%。结果表明,有效的脚步特征参数组合适用于小样本下的身份识别。     

基于混沌理论和支持向量机的人脸识别方法

针对如何选定主成分分析(PCA)特征维数和如何选定支持向量机(SVM)的参数来进一步提高人脸识别系统性能的问题,提出了一种基于混沌理论和支持向量机的人脸识别方法.首先,在统一的目标函数下,在采用PCA方法对人脸图像进行降维和将得到的特征送入SVM中进行训练期间,使用具有可操作性的改进混沌优化算法同时对PCA图像特征维数和分类器参数进行优化选择,然后用得到的优化人脸特征和最佳参数的分类器对未知图像进行识别.基于该方法,对ORL和Yale人脸库进行实验,其识别率都高达99%以上,仿真结果表明,该方法极大地提高了人脸识别能力.     

基于曲波系数加权融合的人脸识别

为了提高人脸识别率和缩短识别时间,研究了基于曲波变换的人脸识别技术。考虑到传统曲波变换无法将多尺度多方向的曲波特征进行最优表示且其特征维数过大的缺点,提出了一种基于自适应加权融合的曲波变换和独立分量分析(ICA)的人脸识别算法。该算法通过曲波变换提取原始人脸图像的最优尺度和方向上的曲波系数,并对这些特征系数进行多方向上的融合,根据类别可分离性的判据原则对融合后的系数进行加权,以减少特征数量,提高处理速度;通过独立分量分析降维,将这些特征投影到更具表达力的空间,以获取有效特征,减少冗余信息,便于最近邻分类器进行人脸识别。基于在奥利维帝研究实验室(ORL)人脸库、Yale B人脸库和AR人脸库对该算法进行了测试,结果表明,其识别率分别达到98%、97%和98.57%,单幅图片的识别时间分别为65.43,158.94和20.37ms,从而验证了其实用性。     

基于EMD和神经网络的轮轨故障噪声诊断识别方法研究

针对轮轨故障噪声信号非平稳性特征,提出一种基于经验模式分解(Empirical Mode Decomposition,EMD)与神经网络的轮轨故障诊断方法。该方法首先对轮轨噪声信号进行经验模式分解,信号分解为若干个基本模式分量(Intrinsic Mode Function,IMF)之和,再选取若干个包含主要故障信息的IMF分量,提取各分量的能量与峭度特征,对各分量的峭度特征综合得到多尺度峭度特征,然后将各分量能量特征与多尺度峭度特征作为神经网络的输入来识别轮轨故障的类型。对车轮扁疤、钢轨波浪磨耗和正常状态的分析结果表明,以EMD方法提取特征参数的神经网络诊断方法比以小波包方法提取特征参数的神经网络诊断方法具有更高的故障识别率。该方法能够对轮轨故障类型进行准确、有效地分类识别。     

结合边缘特征的遥感图像融合

曲波(Curvelet)变换是一种更适合于图像处理的多尺度几何分析(MGA)方法,具有很强的方向性.结合 HSI变换将其应用于全色图像和多光谱(MS)图像融合可以更好地表示图像中的有用特征.首先对多光谱图像进行 HSI变换,得到亮度分量 I,对全色图像和 I 分量进行曲波变换得到粗尺度系数和细节尺度系数,对全色图像的粗尺度系数和细节尺度系数进行叠加,计算归一化的全色曲波系数直方图,定义边缘有效因子,利用全色图像的特征信息对融合图像的粗尺度系数进行处理,对细节尺度系数采用函数对弱边缘进行增强,对新的曲波系数设计融合规则进行融合,逆变换后得到新的亮度分量 Inew,用 Inew 替代原亮度分量 I 进行逆 HSI 变换得到最终融合结果,采用统计类指标对融合结果进行评价.实验结果表明,该方法在保持光谱信息和提高空间分辨率上都有较好的效果.     

基于EMD-EDT的行星齿轮箱特征提取及状态识别方法研究

针对行星齿轮箱特征提取及状态识别中存在的难点问题,本文首先总结适用于行星齿轮箱状态识别的36个特征参数并详细说明各参数的来源及提取方法,提出基于EDT的两阶段特征参数选择及加权方法,得到新的组合特征参数。提出基于EMD-EDT的行星齿轮箱状态识别方法;首先提出基于能量比的包含敏感故障信息的固有模态函数（IMF）选择方法,然后选择提取筛选出的各IMF的组合特征参数构成特征矩阵作为EDT的计算输入,最后智能的输出评估结果。利用行星齿轮箱预置故障实验数据验证所提出方法的可行性和有效性及EMD分解、特征选择和加权过程的必要性。     

噪声环境下稳健的说话人识别特征研究

针对噪声环境下说话人识别率较低的问题,提出一种基于正规化线性预测功率谱的说话人识别特征。首先对语音信号线性预测分析和正规化处理求出语音频谱包络,然后通过伽马通滤波器组得到对数子带能量,最后对特征参数进行离散余弦变换,得到了一种说话人识别特征正规化线性预测伽马通滤波器倒谱系数(Regularized Linear Prediction Gammatone Filter Cepstral Coefficient,RLP-GFCC)。仿真结果表明,在噪声环境说话人辨认试验中,相比传统特征美尔频率倒谱系数(Mel Frequency Cepstral Coefficient,MFCC)和伽马通滤波器倒谱系数(Gammatone Filter Cepstral Coefficient,GFCC)的系统识别率得到了明显提高,对噪声环境的鲁棒性得到了增强。     

Real gas choked flow conditions at low reduced-temperatures

Real gas choked mass flux is calculated for a frictionless stream expanding isentropically until it reaches the speed of sound and without phase changes. The other parameters associated with the choked state are the pressure, density, temperature ratios, and the speed of sound. Departure of the choked mass flux from the ideal gas model is discussed first in absolute terms and then in relative terms, using the Principle of Corresponding States, for gases with boiling points in the low temperature range. Reduced-stagnation pressures are examined up to values of 30 for hydrogen, neon, nitrogen, argon, methane, krypton, xenon, and R-14 and up to 100 for ⁴He. The corresponding reduced-stagnation temperatures go down to 1.4 and in some cases down to 1.2 for nitrogen and argon. Also discussed are the limiting values of stagnation parameters for which no phase change occurs in the choked state. Compared to the ideal gas, the mass flux may almost double and the critical pressure ratio may decrease by an order of magnitude. The relevance of results is discussed qualitatively and quantitatively for Joule-Thomson cryocooling.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Degradation behaviour of sputtered Co–Al coatings on superalloy

Degradation behaviour of sputtered Co–Al coatings on Superni-718 substrate has been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 900 °C for up to 100 thermal cycles between the peak and room temperatures. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS, AFM, and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al₂O₃ and spinel phases such as CoCr₂O₄ and CoAl₂O₄ in scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compare to bare substrate in air at 900 °C temperature up to 100 cycles.     

Capacitance based mass metering for cryogenic fluids

This paper describes a method for measuring the mass of cryogenic fluids in on-board rocket propellant tanks or ground storage tanks. Linear approximations to the Clausius-Mossotti relationship serve as the foundation for a capacitance based mass sensor, regardless of fluid density stratification or tank shape. Sensor design considerations are presented along with the experimental results for a capacitance based mass gage tested in liquid nitrogen. This test data is shown to be consistent with theory resulting in a demonstrated mass measurement accuracy of ±0.75% and a deviation from linearity of less than ±0.30% of full scale mass. Theoretical accuracies are also shown to be ±0.73% for hydrogen and ±1.39% for oxygen for a select range of pressures and temperatures.     

Equipment using a “static-analytic” method for solubility measurements in potentially hazardous binary mixtures under cryogenic temperatures

A new apparatus designed to study, at cryogenic temperatures, thermodynamic equilibria of potentially explosive binary systems such as hydrocarbon-oxygen mixtures is described herein. This equipment has an equilibrium cell which was especially designed to minimize hazards while allowing accurate phase equilibrium measurements. Reliability of results, obtained with this equipment has been verified by working on the nitrogen-propane system, for which data are already available in literature, over a large range of compositions and at various temperatures. Four isothermal curves describing liquid phase compositions at 109.98, 113.77, 119.75 and 125.63 K have been determined. Our experimental data are represented within 2% in compositions and in pressures through the Peng-Robinson equation of state implying Mathias-Copeman alpha function and Huron-Vidal mixing rule. Comparisons to literature allow pointing out: good agreement is observed with Kremer and Knapp data (1983) while the three sets of Poon and Lu data (1974) presenting systematic positive deviation are consequently judged as suspicious.     

Thermal Stability of Amorphous Hydride Mg_(50)Ni_(50)H_(54) and Mg_(30)Ni_(70)H_(45)

The thermal stability of amorphous ternary hydrides Mg_(50)Ni_(50)H_(54) and Mg_(30)Ni_(70)H_(45) and their corre-sponding amorphous binary alloys Mg_(50)Ni_(50) and Mg_(30)Ni_(70) were studied with X-ray diffraction(XRD) and differential scanning calorimetry(DSC). Samples of the amorphous alloys were preparedby mechanical alloying and the amorphous hydrides were obtained by charging the alloys with gas-eous hydrogen at 3.0 MPa and 423 K. It was found that the amorphous hydrides released most oftheir hydrogen before the crystallization of the essentially hydrogen depleted amorphous alloy. Thecrystallization temperature of amorphous Mg_(50)Ni_(50)H_(54) elevated and that of amorphousMg_(30)Ni_(70)H_(45) did not change in relation to the original binary amorphous alloy. This is very excep-tional for amorphous hydrides. The reason for the effects of hydrogen absorption/desorption on thecrystallization of amorphous alloys was discussed.     

Percolation and the resistive transition of the critical temperature Tc of Nb3Sn

The relationship between the distribution of the critical temperature, the percolation function, and the resistive transition of the critical temperature is explored for polycrystalline Nb₃Sn. In the neighborhood of the critical temperature, Nb₃Sn is assumed to be a random mixture of superconducting and normal grains. Percolation concepts are applied to a study of the resistivity. A general analysis is made showing that the onset and shape of the resistive transition for composite conductors are determined by the percolation function and the distribution of the critical temperature. An approximate form of the percolation function is determined based on a linear FEM analysis. Example resistive transitions are calculated for an assumed normal distribution of the critical temperature. An argument is presented that relates grain orientation and strain dependence in Nb₃Sn. It is noted that a dependence of the distribution of T_c with strain, in addition to the usual shift in T_c with strain, would be the result of a strain dependence that is a function of grain orientation. The analysis shows the extent to which the slope of the resistive transition is a measure of the distribution of the critical temperature, and therefore a measure of the grain orientation strain sensitivity. Finally, a method is described to determine the percolation function experimentally.     

Liquid helium cryostat for SQUID-based MRI receivers

We describe a liquid helium cryostat, developed to cool SQUID-based receivers in low field MRI systems. The cryostat has a 4 L liquid helium capacity, a hold time of over 3 days and accommodates 10 cm diameter receiver coils. New vacuum insulation methods reduce the noise level by at least an order of magnitude compared to existing commercial designs. The minimum detectable field at 425 kHz, with a 5 cm diameter circular coil, was estimated to be 0.018 fT/Hz^1/2 from Q-factor measurements and 0.035 fT/Hz^1/2 by direct measurement with a SQUID amplifier. Further measurements indicated that most of this field noise probably originates with dielectric losses in the cryostat’s fibreglass shells.     

长余辉材料Sr_4Al_(14)O_(25)∶Eu~(2+),Nd~(3+)的溶胶凝胶法制备及性能研究

首次研究了以Nd~(3+)离子为辅助激活剂,对Eu~(2+)掺杂的发光材料Sr_4Al_(14)O_(25):Eu~(2+)余辉性能的影响.用溶胶凝胶法合成了Eu~(2+), Nd~(3+)共掺杂的Sr_4Al_(14)O_(25):Eu~(2+),Nd~(3+)发光粉末,并用扫描电镜、X射线衍射计、荧光分光光度计、余辉亮度测试仪、热释光剂量计等手段对粉末样品进行了表征.结果表明,在1350℃得到了单一的Sr_4Al_(14)O_(25)相,粉末颗粒平均粒度在1μm左右.Eu~(2+), Nd~(3+)共掺杂的Sr_4Al_(14)O_(25):Eu~(2+),Nd~(3+)发光粉末有402和485nm两个发射峰,与Eu~(2+)单掺杂的Sr_4Al_(14)O_(25):Eu~(2+)相比,发射峰位置没有变化,但适量的掺杂可以大大提高余辉时间和余辉亮度,余辉时间可达18h以上.最后通过对热释光谱的分析解释了双掺杂发光粉余辉性能增强的原因,适宜深度的陷阱可以有效存储光能,增强余辉的时间和强度.     

Development of No-Vent™ liquid hydrogen storage system for space applications

A number of technological advances required to store and maintain normal-boiling-point and densified cryogenic liquids, including liquid hydrogen, under zero boil-off conditions in-space, for long periods of time, have been developed. These technologies include (1) thermally optimized compact cryogen storage systems that reduce environmental heat leak to the lowest-temperature cryogen, which minimizes cryocooler size and input power, and (2) actively-cooled shields that surround the storage systems and intercept heat leak. The processes and tools used to develop these technologies are discussed. A zero boil-off liquid hydrogen storage system technology demonstrator for validating the actively-cooled shield technology is presented.