梯度-曲面拟合法的亚像素位移测量算法

数字图像相关方法作为光学测量的一种测量方式在结构变形测量中有广泛应用,其中亚像素位移测量精度成为关键。基于亚像素位移测量精度影响因素的分析,研究了计算子区灰度插值、计算窗口大小以及拟合距离对亚像素位移测量的影响。提出了一种梯度-曲面拟合法的亚像素位移测量算法,即,先采用梯度法计算出结构的亚像素位移,再对计算子区进行亚像素插值,然后调整拟合距离进行亚像素曲面拟合。最后利用计算机模拟算例验证了该方法具有良好的计算效率、计算精度及抗噪声能力。     

基于数字图像相关方法的亚像素位移迭代算法性能

在对图像位移进行测量时,不同亚像素位移迭代算法的性能不同,将反向组合对角近似算法和反向组合Dog-Leg算法用于数字图像相关法并进行位移测量,同时对反向组合Levenberg-Marquardt算法的参数更新策略进行简化,以反向组合高斯牛顿（IC-GN）法作为对比,通过模拟散斑图像和真实散斑图像的压缩变形实验,对3种算法的性能进行对比,并进行相应的评估。实验结果表明：在模拟散斑实验中,各个算法在收敛速度、收敛频率和计算速度上各有不同;在真实实验下,小变形实验得到与IC-GN法相似的精度,大变形实验得到的收敛半径更大。     

基于GMA光流网络的二维变形测量数字图像相关法初值估计

提出一种利用深度学习提取散斑变形图像位移场的算法。对比几种主流光流网络模型在散斑图像上的计算精度,将全局运动聚合（GMA）光流网络引入数字图像相关法,使用光流网络模型预测的位移场作为亚像素迭代算法的初值,最后结合逆合成高斯-牛顿法计算散斑图像的亚像素位移场。结果表明,与几种主流的数字图像相关算法相比,GMA光流网络初值的计算精度和计算效率更高,适用于大变形测量,是一种兼顾计算速度和精度的变形测量算法。对木块材料进行压缩实验,结果充分显示了所提方法在散斑变形测量中的有效性。     

数字散斑相关方法亚像素求解的一种混合方法

亚像素求解是数字散斑相关方法(DSCM)中最重要的技术。亚像素求解算法的精度和效率直接影响DSCM的精度和计算速度。目前广泛应用的两种亚像素求解方法在处理高质量散斑图时都存在一定的缺陷:基于灰度插值的散斑场亚像素恢复方法,其计算精度较高,但计算消耗太大;基于相关系数分布的拟合方法,其计算消耗小,但计算精度较低。在分析两种亚像素求解方法各自优缺点的基础上,提出了一种结合两种算法优势的混合算法。混合算法在保证亚像素求解精度的基础上,可以大大降低整个算法的计算消耗。     

梯度算子选择对基于梯度的亚像素位移算法的影响

基于梯度的亚像素位移算法中不同的灰度梯度计算方法对计算结果的影响进行了研究,列出了几种常用灰度梯度计算方法,并用对相邻5像素点的离散灰度进行三次样条插值的方法推导出两种灰度梯度算子。然后对计算机模拟生成的散斑图像对用各种梯度算子计算得出的亚像素位移与预先设定的真实值做了比较,给出了相应的均值误差和均值相对误差。最后的真实试件的刚体平移和单向拉伸实验结果表明,Barron算子是所列的几种梯度算法中最为精确、稳定的梯度算子。     

基于灰度梯度正则化去噪的改进数字图像相关法

为了提高数字图像相关法的抗噪声能力,分析了灰度梯度计算误差对数字图像相关法测量精度的影响,提出采用Tikhonov正则化方法计算图像灰度梯度,进而通过反向组合高斯-牛顿(IC-GN)法计算图像的亚像素位移;基于数值仿真散斑图研究灰度梯度的相对计算误差,并分析采用Tikhonov正则化方法计算图像灰度梯度后,改进数字图像相关法的测量精度,结合实验验证所提方法在实际噪声环境中的抗噪声能力。结果表明:灰度梯度对数字图像相关法测量精度有较大影响,而采用Tikhonov正则化方法后,可以有效提高数字图像相关法的测量精度与抗噪声能力。     

基于自调整测量基准图的直线电机动子位置测量累积误差消减

为解决直线电机动子位置测量中多帧图像位移叠加而产生的误差累积问题,提出一种加阈值变换基准图（TTBM）的机器视觉累积误差消减算法。首先,根据直线电机动子的一维刚体平动特点,生成并优选灰度变化更加平滑的一维散斑图像作为目标拍摄图。其次,采用相位相关算法（PCA）改进灰度梯度方法,对配准图像进行整像素平移后再计算亚像素位移,以增大基准图像与配准图像之间的位移测量范围。最后,提出自调整基准图方法,根据图像位移的测量范围设定基准图像变换阈值,降低位移叠加次数,减小累积误差,实现直线电机长行程精密位移测量。同时,研究了基准图像变换阈值对测量精度的影响,给出了基准图变换的最佳阈值选取范围。仿真和实验表明,所提方法可以有效地减小直线电机动子测量中叠加而产生的累积误差。     

MEMS动态测试系统及其数据处理

介绍了基于频闪显微干涉技术的MEMS动态测试系统的测量原理,设计了数据分析处理算法。采用基于立方插值的亚像素模板匹配技术实现了1/50像素的面内位移测量精度。利用分割线去包裹算法实现了从干涉图到三维形貌图的重构。     

基于梯度的数字体图像相关方法测量物体内部变形

数字体图像相关方法通过分析两幅以体素为单位的三维体图像获得物体内部的三维变形信息.在利用数字体图像相关方法测量物体内部变形的过程中,整体素的位移场可以通过简单的空域搜索获得,因此如何计算亚体素位移是提高该方法位移测量精度的关键.介绍了一种基于空间灰度梯度的亚体素位移测量算法,并用计算机模拟生成的可精确控制位移的三维散斑...     

基于数字散斑照相术测量刚体面内位移

数字散斑照相术通常测量刚体的面内位移。从基本原理着手,研究了基于散斑照相术的两种常用方法,即数字图像相关法和傅里叶变换法。论述了数字图像相关法中的亚像素梯度算法,讨论了如何选择子区的大小,优化测量结果;傅里叶变换法通过对散斑图进行傅里叶变换和数学形态学等一系列操作,获得精确的条纹间距,从而更好地计算面内位移。实验结果表明,傅里叶变换法的测量精度要高于数字图像相关法。     

A fast digital image correlation method for deformation measurement

Fast and high-accuracy deformation analysis using digital image correlation (DIC) has been increasingly important and highly demanded in recent years. In literature, the DIC method using the Newton-Rapshon (NR) algorithm has been considered as a gold standard for accurate sub-pixel displacement tracking, as it is insensitive to the relative deformation and rotation of the target subset and thus provides highest sub-pixel registration accuracy and widest applicability. A significant drawback of conventional NR-algorithm-based DIC method, however, is its extremely huge computational expense. In this paper, a fast DIC method is proposed deformation measurement by effectively eliminating the repeating redundant calculations involved in the conventional NR-algorithm-based DIC method. Specifically, a reliability-guided displacement scanning strategy is employed to avoid time-consuming integer-pixel displacement searching for each calculation point, and a pre-computed global interpolation coefficient look-up table is utilized to entirely eliminate repetitive interpolation calculation at sub-pixel locations. With these two approaches, the proposed fast DIC method substantially increases the calculation efficiency of the traditional NR-algorithm-based DIC method. The performance of proposed fast DIC method is carefully tested on real experimental images using various calculation parameters. Results reveal that the computational speed of the present fast DIC is about 120-200 times faster than that of the traditional method, without any loss of its measurement accuracy     

Gaussian pre-filtering for uncertainty minimization in digital image correlation using numerically-designed speckle patterns

This paper discusses the effect of pre-processing image blurring on the uncertainty of two-dimensional digital image correlation (DIC) measurements for the specific case of numerically-designed speckle patterns having particles with well-defined and consistent shape, size and spacing. Such patterns are more suitable for large measurement surfaces on large-scale specimens than traditional spray-painted random patterns without well-defined particles. The methodology consists of numerical simulations where Gaussian digital filters with varying standard deviation are applied to a reference speckle pattern. To simplify the pattern application process for large areas and increase contrast to reduce measurement uncertainty, the speckle shape, mean size and on-center spacing were selected to be representative of numerically-designed patterns that can be applied on large surfaces through different techniques (e.g., spray-painting through stencils). Such “designer patterns” are characterized by well-defined regions of non-zero frequency content and non-zero peaks, and are fundamentally different from typical spray-painted patterns whose frequency content exhibits near-zero peaks. The effect of blurring filters is examined for constant, linear, quadratic and cubic displacement fields. Maximum strains between ±250 and ±20,000 µε are simulated, thus covering a relevant range for structural materials subjected to service and ultimate stresses. The robustness of the simulation procedure is verified experimentally using a physical speckle pattern subjected to constant displacements. The stability of the relation between standard deviation of the Gaussian filter and measurement uncertainty is assessed for linear displacement fields at varying image noise levels, subset size, and frequency content of the speckle pattern. It is shown that bias error as well as measurement uncertainty are minimized through Gaussian pre-filtering. This finding does not apply to typical spray-painted patterns without well-defined particles, for which image blurring is only beneficial in reducing bias errors.     

Performance of iterative gradient-based algorithms with different intensity change models in digital image correlation

Digital image correlation (DIC) is a non-contact and powerful tool for whole-field displacement and strain measurement in modern optical metrology. In the DIC technique, the gradient-based algorithm is one of the most commonly used sub-pixel registration algorithms due to its effectiveness and accuracy. Thus, this algorithm has been further investigated and improved by many researchers to increase its accuracy. In the existing gradient-based algorithms, there are three relation models for the gray level intensity of a point in the undeformed and deformed images: namely the constant intensity model, the linear intensity change model and the non-linear intensity change model. However, little quantitative investigation has been conducted to compare their performance. In this paper, three iterative gradient-based algorithms are given using the corresponding intensity change model and the iterative least squares (ILS) solving method. The accuracy, robustness and computational efficiency of the three algorithms for intensity variation are investigated through numerical simulation experiments. The experimental results reveal that the algorithm generated using the non-linear intensity change model is the most accurate, robust and efficient of the three algorithms and the algorithms generated using the linear and non-linear intensity change models have approximately the same accuracy when the intensity variation is small between undeformed and deformed images.     

数字散斑亚像素小角位移测量的曲面拟合法

与传统测量方法相比,数字散斑相关法由于其目标特征单元网格划分的灵活性,能够更好地满足不同场合小角位移的测量需求。针对该方法亚像素小角位移测量的曲面拟合参数选择问题,研究了亚像素测量图像小角旋转前后的九点二次曲面拟合法,并根据计算机生成模拟散斑进行模拟实验分析,得到最佳误差效率优化条件下的曲面拟合法求解亚像素小角位移的最佳散斑尺寸3.5 pixel、计算窗口尺寸41×41 pixel和拟合窗口尺寸3×3 pixel。实验验证了上述测量参数的有效性,为进一步的曲面拟合法数字散斑成像角位移测量提供参考。     

An analysis of noise in PIV images

Particle Image Velocimetry can provide detailed velocity field information that has unparalleled value to both experimentalists and computational fluid dynamicists. As with any measurement technique, it is important to assess the error associated with the measurements. The PIV measurement chain contains several stages where error is introduced. The dynamics of the seed particles determine how well they represent the local fluid velocity. Acquiring the image, whether photographically or electronically, will introduce aberrations, distortion, diffraction, and positioning uncertainties. The image analysis procedure to determine particle displacements is the final step in the measurement chain. This paper considers the error in this step of the measurement chain. Synthetic images were generated with controlled levels of the following quantities: background mean illumination, background illumination standard deviation, mean particle diameter, number of paired particles, the ratio of unpaired particles to paired particles, displacement magnitude, and displacement direction. The analysis method tested is a generic autocorrelation technique using a two-dimensional forward FFT, PSD calculation, inverse FFT, and a peak detection algorithm based on a decreasing threshold search with subsequent sub-pixel interpolation. The error analysis principles demonstrated, however, could easily be applied to other algorithms including those implementing cross-correlation techniques. The error in the analysis technique is characterised by four quantities: the bias in the displacement magnitude, the precision index of the displacement magnitude, the bias in the displacement direction, and the precision index of the displacement direction. Each independent variable was varied over a specified range and the behaviour of the four dependent variables observed. The results showed a clear ability for this error assessment technique to illustrate the reliable operating range for each parameter. The variation in error with parameter level tended to be similar in all cases. There was a significant portion of the range where the error was very low. Then, at some critical value, the analysis technique broke down and the error became quite high.     

在电子散斑干涉中利用反相位法进行三维变形测量

提出一种可将离面位移与面内位移分离的三维位移计算方法。在双光束电子散斑干涉系统中增加一路参考光,使这一路参考光为两光束所共用。两束光各自独立地对变形物体进行测量,分别计算相位分布,并对其中之一进行反相位计算。理论分析表明,对二路检测光所得到的相位进行相减运算,就能够较好地减少电子噪声的影响,分离面内位移场与离面位移,实现物体变形的三维测量。介绍该方法的原理,并利用典型实验证实了该方法的可行性。     

Evaluation of the quality of a speckle pattern in the digital image correlation method by mean subset fluctuation

The quality of the speckle pattern used in digital image correlation is studied in this paper using a parameter called mean subset fluctuation. A numerical translation test is performed on four speckle patterns captured from actual experiments. The translated images are analyzed and the results show that the mean bias error of the calculated displacement is linear with the value of the mean subset fluctuation. The results from the numerical calculation illustrate that speckle size and density have an influence on the quality of speckle pattern.     

两种插值亚像素定位法在数字散斑测量中的应用

利用数字散斑相关测量法只能得到试件的整像素位移量,不能满足高精度的测量要求。探讨了两种插值法在数字散斑亚像素级测量中的应用,并对相关系数多项式插值法和对数插值法的测量结果进行了比较,为在不同亚像素级位移条件下选择较为恰当的测量方法提供了依据。     

三维时域散斑动态干涉测量系统的集成化设计

三维电子散斑干涉技术（3D ESPI）具有非接触、高精度、高灵敏度和全场测量等优点，被广泛应用于许多领域。为了实现非接触动态全场三维测量，设计并建立了一个紧凑、完备的三维测量系统。用一个多波长光纤耦合激光器代替3个独立光源，产生的离面、面内散斑干涉图仅用一台彩色CCD相机就能捕捉和处理；整个测量系统采用笼式结构，具有高度的灵活性和稳定性；对基于小波变换的相位展开算法进行了编程，实现了被测物体三维位移信息的完整提取。实验证明该测量系统可以实时获取被测物体的三维位移，在测量实验中，获得的三维位移值17.68 μm、36.23 μm、13.85 μm，相比于实际位移值18.1 μm、36.4 μm、14.0 μm它们的绝对误差分别为0.42 μm、0.17 μm、0.15 μm，相对误差分别为2.3%、0.5%、1.1%。