Barkhausen noise in differential transformers

A treatment is presented of the Barkhausen noise in differential transformers in analogy to our noise calculations on fluxgate magnetometers. A numerical estimate is given which for practical situations leads to an accuracy limit of the order of 10^?10 m.     

Noise in the measurements of magnetic susceptibilities

Three methods of measuring diamagnetic susceptibilities are compared with respect to sensitivity.(1) The balance method is limited in sensitivity by thermal noise with a signal to noise ratio SNR of 3×10⁸.(2) The inductance method is limited by the Nyquist noise with an SNR of 5×10⁴.(3) The magnetometer method is very insensitive with an SNR of 40 due to Barkhausen noise.Department of Physics.Department of Electrical Engineering.     

中国大陆西部强震前超低频磁场时空变化特征

对2015年至2019年期间中国大陆西部的磁通门磁力仪秒采样观测资料开展了5—100 s频段的地磁垂直强度极化分析，并运用一些数学方法对分析结果进行了处理。结果显示，极化高值在经向和纬向均无明显的形态和幅值变化，且极化高值与地磁外源场扰动无关。在此基础上筛选出18次极化高值异常事件，利用插值方法得到了极化高值异常的空间分布图，并分析了18次高值异常与中国大陆西部及周边15次强震的时空关系。分析结果表明：地磁场出现多台同步极化高值现象后的半年内，高值区可能发生M6.0以上强震；高值现象出现后，多个高值区均有可能发生强震；后续强震的震级与高值区面积呈正相关。     

Comparison of three residual-stress measurement methods on a mild steel bar

The objective of this investigation was to ascertain the surface residual-stress condition of a specially thermal-mechanically treated mild steel bar. Three methods of residual-stress measurement were used—two of them accurate, well-verified approaches and the third an experimental one. The first two were stress-relief blind-hole drilling (SRT) and X-ray diffraction (XRD), and the third was Barkhausen noise analysis (BNA). It is difficult to make direct comparisons between these methods since they each sample a different volume of material at different depths into the surface. However, the SRT data, when extrapolated to shallow depths sampled by the XRD, show excellent agreement with that nondestructive method. The BNA results show poor correlation with the XRD and SRT even with extrapolation to similar depths. The major reason for the poor BNA results seems to be its sensitivity to the microstructural conditions of the sample. All of the measurements, with the exception of one set of BNA measurements, indicate that tensile stresses existed in the surface of the specimen.     

水下磁异常定位中两种矢量磁力仪配置方式的比较

在简述水下磁异常定位原理的基础上,提出了八单轴和十单轴两种矢量磁力仪配置方式,推导了两种配置方式下磁场大小及梯度的量测方程,对磁场大小及梯度的解算精度进行了比较分析。数值仿真了磁场大小及梯度、载体相对位置误差与测量基线长△X之间的变化关系,结果表明这些误差都随△X加大而增长。相比于八单轴磁力仪配置,十单轴磁力仪配置方式中磁场大小和梯度计算精确,定位误差很小,在水下磁定位实验中应选用十单轴磁力仪配置方式。     

应力和组分占比对马氏体-铁素体双相钢中磁巴克豪森噪声的影响

磁巴克豪森噪声(MBN)技术在马氏体-铁素体双相钢的微观结构及应力无损评价与表征中具有巨大潜力.为探究拉应力和铁素体占比对磁巴克豪森噪声的影响规律及权重,在0?200MPa范围内实验测得了具有不同铁素体占比的双相钢MBN信号.重点分析了MBN蝶形曲线双峰峰值(分别代表马氏体和铁素体)在应力和铁素体占比两因素耦合条件下的...     

Analysis of relationships between residual magnetic field and residual stress

The impact of stress on changes in magnetisation is one of the most complex issues of magnetism. Magnetic methods make use of the impact of stress on permeability, hysteresis and magnetic Barkhausen noise, which are examined with fields with a high strength and a small frequency. The paper presents an analysis of the impact of residual stress resulting from inhomogeneous plastic deformations in the notch area of the examined samples on the changes in the strength of the residual magnetic field (RMF). The RMF on the surface of the component is the superposition of the simultaneous effect of the shape, the anisotropic magnetic properties of the material, as well as of the values of the components of a weak external magnetic field (most commonly—the magnetic field of the Earth). Distributions of the RMF components were measured on the surface of samples with a various degree of plastic strain. The finite element method was used to model residual stress in the samples. The impact of residual stress on changes in the residual magnetic field was shown. A qualitative correlation was found between places with residual stress and areas with increased values of the gradients of the RMF components. Further research is now in progress in order to develop the quantitative relationships.     

Noise analysis for sensitivity-based structural damage detection

As vibration-based structural damage detection methods are easily affected by environmental noise, a new statistic-based noise analysis method is proposed together with the Monte Carlo technique to investigate the influence of experimental noise of modal data on sensitivity-based damage detection methods. Different from the commonly used random perturbation technique, the proposed technique is deduced directly by Moore-Penrose generalized inverse of the sensitivity matrix, which does not only make the analysis process more efficient but also can analyze the influence of noise on both frequencies and mode shapes for three commonly used sensitivity-based damage detection methods in a similar way. A one-story portal frame is adopted to evaluate the efficiency of the proposed noise analysis technique.     

Surface profiling using sequential sampling and inverse methods. Part II: Implementation and experimental results

Sequential sampling with inverse calculation is demonstrated to provide a practical and reliable method for profiling surfaces, independent of rigid-body motions. Examples are given where various arrangements of displacement sensors can provide single-sided, double-sided, and parallel profilling. Double-sided measurements also give specimen thickness, and parallel measurements give surface twist. In common with other inverse calculation methods, there is a tradeoff between achievable spatial resolution and sensitivity to measurement noise. Such sensitivity, which typically doubles the measurement noise in the computed profiles, can be minimized by appropriate choice of sensor configuration, measurement method, and use of regularization. Practical guidelines for all these features are given, together with example experimental results for four typical sensor configurations.     

A method for the identification of dynamic constraint parameters in multi-supported flexible structures

In this paper, a new method for identifying the dynamical parameters of local constraining supports such as mass, stiffness, and damping was developed through combining the measured frequency transfer functions and structural modification techniques. Since measurement noise often leads to erroneous identifications, regularization techniques have been implemented to reduce noise amplification in the inverse problem. The developed technique has been validated by numerical tests on a multi-supported flexible structure, which can be seen as an idealized electricity generator rotor shaft. The results are satisfactory for noise-free data as well as under realistic noise levels. The sensitivity of the identified support features to noise levels is asserted through a parametric study     

微小型航姿测量系统及其数据融合方法

以小型无人机航姿测量系统的微小型化为背景,利用MEMS惯性测量元件研制了一种低成本微型航姿测量系统.针对MEMS器件用于载体航姿测量时精度低、易发散的问题,提出一种计算量小、实时性强的加速度信息、磁场强度信息、陀螺信息的融合方法.采用卡尔曼滤波器对系统的俯仰角、滚转角和航向角的误差进行最优估计;设计数据融合的判别准则,并根据判据的判断结果调整卡尔曼滤波器中的量测信息,使系统可用于小型无人机的定高自主飞行.实验结果表明,系统输出航姿的更新频率可达100Hz,航姿测量误差小于0.6°,航姿标准差小于0.09°;将其应用于某小型固定翼飞行器的飞行控制系统中进行自主飞行实验,完成了预定的飞行任务.     

基于自适应UKF算法的MEMS陀螺空中在线标定技术

为保证微型卫星定位应用中系统精度与稳定性,需要对姿态传感器进行实时在线标定.在无外界姿态参考时,提出一种用三轴磁强计测量值来实时估计MEMs陀螺的零漂误差的方法,采用UKF滤波算法,将陀螺漂移作为滤波状态向量,通过建立三轴磁强计测量微分方程,作为系统量测方程实现陀螺漂移的最优估计.针对磁强计测量信息易受干扰导致滤波量测模型不准确的问题,将自适应因子引入到UKF中,通过在线监控和调整测量误差,减少陀螺标定的估计误差,增强系统性能.实验结果表明,经过标定,MEMS陀螺精度提高约30%,并且在磁强计有外界干扰时,陀螺的标定结果收敛.将标定后的MEMS陀螺进行姿态解算,其动态误差小于2°.     

Identification of orthotropic elastic constants using the Eigenfunction Virtual Fields Method

The Virtual Fields Method (VFM – Pierron and Grediac, 2012), an inverse method based on the principle of virtual work (PVW), is being increasingly used to estimate mechanical properties of materials from full-field deformations obtained from techniques such as Digital Image Correlation, moiré and speckle interferometry and grid methods. By making specific choices for virtual fields (VFs) in PVW, one obtains a system of algebraic equations, which is then solved for the unknown material constants. Recently, a new variant of VFM, known as the Eigenfunction Virtual Fields Method (EVFM) has been proposed (Subramanian, 2013). In EVFM, principal components of the measured (i.e. true) strain fields are used to systematically generate VFs. We extend EVFM to orthotropic elastic materials in this work, and estimate the relevant material parameters from full-field strain data generated from a finite-element model of an unnotched Iosipescu test. Varying levels of Gaussian white noise are added to the synthetic strain data to evaluate the sensitivity of EVFM to input noise. It is observed that for low to moderate noise, the material properties estimated by the proposed method are relatively insensitive to noise. However, when noise levels are high, the proposed method yields large variance in some of the computed properties when compared to the state-of-the-art optimized piecewise continuous VFM (Toussaint et al., 2006; Pierron and Grediac, 2012). Some of the large variance in properties estimated from noisy data using EVFM is traced to the sensitivity of the third dominant eigenfunction and modifications to the proposed method to address this issue are suggested.     

Chebyshev spectral method and Chebyshev noise processing procedure for vorticity calculation in PIV post-processing

Chebyshev spectral method and Chebyshev noise processing procedure are proposed for the calculation of vorticity from PIV experimental data. The Chebyshev spectral method offers superior intrinsic accuracy of derivative calculations. To overcome its noise sensitivity, the Chebyshev noise processing procedure can be applied prior to the derivative calculation to remove the high-frequency noise in the Chebyshev transform space. We compare the Chebyshev spectral method against the least-squares approach and test their performance in the calculation of vorticity with an Oseen vortex and with PIV data of the wake of a trapezoidal mixing tab. It is found that for clean velocity data the Chebyshev spectral method is extremely accurate. However, the Chebyshev spectral method alone is found to be more sensitive to noise than the least-squares method. When the Chebyshev noise processing procedure is applied together with the Chebyshev spectral method it greatly reduces the error and makes the Chebyshev spectral method more accurate than the least-squares method for a wide range of vorticity values. A special requirement imposed by the Chebyshev spectral method is that the PIV velocity processing must be carried out on special grids such as Gauss–Lobatto points.     

CABARET solutions on graphics processing units for NASA jets: Grid sensitivity and unsteady inflow condition effect

The GPU CABARET method for solving the Navier–Stokes equations coupled with the Ffowcs Williams–Hawkings scheme for far-field noise predictions is applied for conditions of the NASA SHJAR experiment corresponding to Set Point 3 and 7 in accordance with Tanna's classification. The questions addressed include the sensitivity of the flow and noise spectra solutions to the grid resolution and the inflow condition at the nozzle exit. To study the grid sensitivity, several “hand-made” multi-block curvilinear grids are considered along with a simple hanging-nodes-type grid that was automatically generated with OpenFOAM, whose solutions are cross-verified. To study the effect of the inflow jet condition, the flow and noise solutions based on the laminar inflow condition for Set Point 7 case are compared with the same based on modifying the interior nozzle geometry with a turbulence grid to generate the initial unsteadiness inside the nozzle so that both the centerline velocity fluctuations and the jet Mach number at the nozzle exit are preserved in accordance with the experiment. The numerical solutions obtained are compared with the experimental data and reference LES solutions available in the literature.     

结构随机激励的响应灵敏度分析

本文建立了有色噪声随机激励的响应协方差矩阵方程,推导了响应灵敏度的计算式,给出了进行灵敏度计算的方法,并作了实例计算。     

高斯牛顿技术求解偶应力反问题

建立了便于敏度分析的偶应力反问题数值求解模型,给出了直接法和伴随法两种敏度计算格式.在反演计算中采用了高斯牛顿技术对未知本构参数进行识别,探讨了测点数目、初值选取和数据噪音对反演结果的影响,数值算例给出了令人满意的结果.     

小波分析在悬臂梁裂纹识别中的应用

基于空间信号的小波分析理论,将含裂纹悬臂梁前四阶振型信息直接用于小波变换,小波系数在空间域上的突变反映了裂纹的存在并指出了裂纹的位置.本文分析了前四阶振型对小波识别结果的敏感性,利用小波系数模极大值在尺度上的表现与Lipschitz指数之闻的关系建立了集中因子和裂纹深度之间的关系,以此来估计裂纹深度.鉴于实测信号往往是含噪声信号,分析了噪声对识别结果的影响规律.数值算例表明利用sym4小波对含裂纹梁的四阶振型信息进行小波分析可以准确地识别出裂纹的位置和深度;高阶振型对结构损伤较为敏感,高阶振型更适合于微裂纹和含噪声信息的处理,但高阶振型的非线性也会给裂纹识别带来一定的困难.使用本文方法进行结构裂纹参数识别,噪声对裂纹位置的影响只是指示清晰度的影响,基本不会产生错误的识别,而对裂纹深度的影响远比对位置的影响复杂,由于小波系数混入了噪声成分,从而增加了集中因子的取值,致使识别结果总是比真实结果偏大.     

Local sensitivity of spatiotemporal structures

We present an index for the local sensitivity of spatiotemporal structures in coupled oscillatory systems based on the properties of local-in-space, finite-time Lyapunov exponents. For a system of nonlocally coupled Rössler oscillators, we show that variations of this index for different oscillators reflect the sensitivity to noise and the onset of spatial chaos for the patterns where coherence and incoherence regions coexist.     

基于振动响应的耦合弦系统损伤检测

为解决具有相近频率和相重频率特征的多跨耦合弦系统的损伤检测问题,建立了耦合弦系统在外激励作用下的有限元运动方程.利用直接积分法计算了系统强迫振动响应,将弦的局部损伤模拟为单元面积的减少,推导了振动响应对弦单元面积的灵敏度,并利用此响应灵敏度进行弦的局部损伤识别.数值算例表明:此方法能快速准确地识别出耦合弦的局部损伤,并且对模拟的人工噪声不敏感.