双平面镜双相机数字图像相关技术在三维重建中的应用

三维数字图像相关技术在获取工件表面信息方面有重要应用,针对单相机系统在全场测量中的局限性及多相机系统在全场测量中的复杂性,本文提出一种双平面镜辅助的双相机视觉测量方法,并应用在三维重建中。即：分析像素点到三维点的实际映射关系,基于公垂线中点法进行目标点三维重构,确定棋盘格标定板角点的三维坐标;分析角点在镜面的虚实对应关系,标定镜面位置方程,得到反射变换矩阵;通过反射变换矩阵完成物面的虚实转换,最终实现三维全场测量。为验证该方法的可行性和可靠性,分别进行了静态实验和动态实验。结果表明：在游戏币静态实验中,其正反轮廓的三维重建效果良好;在五棱铝柱热变形动态实验中,铝柱外侧表面高度变化均值与Ansys软件的仿真结果基本一致,且优于在镜面上喷涂散斑的重建方法,具有较高的精度。     

基于数字图像相关的水下螺旋桨三维变形测量

提出了一种基于三维数字图像相关(3D-DIC)技术的船用螺旋桨3D变形测量方法.测量系统由双目立体相机、闪光灯和同步装置组成,采用空泡水筒对螺旋桨进行加载.首先,用基于极线约束的自标定方法对系统的外部参数进行标定,以获得桨叶表面的3D位移分布.然后,对重构的桨毂部分进行点云配准,得到变形后螺旋桨叶根部点云的平移向量和旋...     

物体内部三维位移场分析的数字图像相关方法

提出了物体内部三维位移场的数字图像相关分析方法，对物体变形前后，或连续变形的两个相邻状态的内部三维结构的数字图像，通过相关运算获得三维位移场.文中给出了三维相关法的体搜索窗口、相关函数及亚像素运算的相关系数拟合函数.数字模拟结果证明了三维相关法的正确性及可靠性.位移计算精度为0.02像素. 关键词： 数字图像相关 三维相关 亚像素     

用数字图像相关技术预测土壤开裂

介绍使用数字图像相关技术预测土壤开裂的方法。通过对定时得到的一组数字图像进行相关运算得到一组二维位移场，位移的矢量表示展现在计算机屏幕上就成功地展示了开裂裂纹的形态。预测的开裂趋势与实际发生的是一致的。     

基于带通滤波成像的高温数字图像相关方法

提出一种用于高温物体表面全场变形测量的高温数字图像相关方法.该方法采用带通光学滤波成像技术以有效减小高温物体表面热辐射对相机采集图像亮度增强的影响,在物体表面温度高达1000℃时仍可获得可被数字图像相关方法直接分析处理的高质量数字图像.用该方法测量了铬镍奥氏体不锈钢在1000℃高温时的全场热变形以及从室温到1000℃温...     

基于数字图像相关的透明材料厚度测量

提出了一种测定透明材料厚度的新方法。该方法是基于透明材料对光的折射特点,利用光斑点面内位移与材料厚度的变化关系,将厚度的测量转化为对面内位移的测量。将此方法与数字图像相关法相结合,实现了单摄像机对厚度的测定。对轴向拉伸状态下的有机玻璃板的厚度变化进行了测定,将实验结果与理论值进行了对比,两者符合较好,证明了该实验方法的有效性和可行性。     

加权窗口在数字图像相关技术中的应用研究

应用数字图像相关法计算变形前后图像中各点的位移量时,先通过整像素搜索法来确定各点的整像素位移,进而根据整像素搜索得到的相关系数值来计算各点的亚像素位移.提出了一种新的整像素搜索法,即采用计算窗口加权的方法对变形前后图像中的点进行搜索计算.计算结果表明,该方法是可行的,同时可以提高计算结果准确度.     

基于数字图像相关的三维刚体位移测量方法

基于三维位移测量在工程技术领域的必要性和重要性,开展了单摄像机和数字图像相关相结合的三维刚体位移测试方法的研究.基于图像位移场矢量中心和斜率与面内和离面位移的分别对应关系,采用最小二乘拟合法分离图像位移场的常量项与一次项,据此,可实现物体三维位移分量的有效分离.以针孔摄像机成像模型为基础,开展了数值模拟及硅片平移实验,发展了与三维刚体位移对应的散斑图模拟方法,验证了基于仿射变换的相关迭代法的精度和适用性.数值模拟与实验结果验证了数字图像相关方法用于实现物体三维刚体化移重构的可行性和优越性,最大测量误差为5%.     

高温数字图像相关法变形测量中玻璃介质误差校正

为了校正玻璃介质在高温变形测量中引起的测量误差,本文将玻璃介质作为相机标定模型的一部分,基于摄影测量技术和数字图像相关法,提出一种复杂环境下的双目相机标定方法,将其应用在高温变形测量中.首先,针对复杂环境下图像质量差引起的标定困难问题,采用带畸变校正的相机成像模型,通过捆绑调整的相机标定方法完成双目相机标定,提高了标定...     

高精度数字图像相关测量系统及其技术研究

研制双CCD长距显微数字图像相关测量系统,它融合光电子、数字图像相关方法和散斑技术于一体,具有非接触式、高精度、宽量程、准实时、测试方法简便等优点,它可用于材料力学性能(特别是柔性高分子材料力学性能)测量、低膨胀系数新型材料线胀系数等的测量。若将其用于应变测量,灵敏度优于1με,而传统数字图像相关方法测量应变的理论灵敏度为20με。利用列阵光学元件解决了0.05 mm高衬比度微小标记的方便设置;提出消除了数字图像灰度噪声的时间平均法,使相关测量精度稳定地达到0.01 pixel。还给出金属材料拉应变的测量结果,实验结果与经典理论值相当接近。此外,还阐述数字图像相关测量技术是一种超光学系统衍射极限分辨力的测量技术。     

A particle image velocimetry study of dual-rotor counter-rotating wind turbine near wake

Journal of Visualization - This experimental work studied the flow characteristics in the near wake region behind dual-rotor wind turbines using two-dimensional particle image velocimetry. Two...     

A note on tower wake/blade interaction noise of a wind turbine

Experimental observations on wind turbine tower wake/blade interaction noise are presented. Results for circular, elliptical and square sections of the support tower legs are compared and quantified with respect to the acoustic frequency spectra produced. The results are preliminary ones obtained in a continuing study aimed at constructing a shape that may yield a lower interaction noise.     

Flapwise dynamic response of a wind turbine blade in super-harmonic resonance

The flapwise dynamic response of a rotating wind turbine blade in super-harmonic resonance is studied in this paper, while the blade is subjected to unsteady aerodynamic loads. Coupled extensional–bending vibrations of the blade are considered; the governing equations which are coupled through linear and quadratic terms arising from rotating and geometric effects respectively are obtained by applying the Hamiltonian principle. The lth flapwise linear frequency and the rotational frequency are assumed to be in an almost 3:1 ratio, so super-harmonic resonance occurs when this linear frequency is close to the associated nonlinear frequency. By using the first n, no less than l, linear undamped modal functions as a functional basis and applying the Galerkin procedure, a 2n-degree-of-freedom discrete model with quadratic and cubic terms owing to geometric effect is derived. The generalized displacements corresponding to the discrete system are disintegrated into static and dynamic displacements. Perturbation method is adopted to get analytical solutions of the discrete dynamic system for positive aerodynamic dampings. The coning angle and the inflow ratio are chosen as two control parameters to analyze aeroelastic behaviors of the blade. By assuming that the static and dynamic displacements are of the same order in resonance region, and there is no other resonance except the super-harmonic resonance, the multiple-scales method is employed to obtain a set of amplitude modulation equations whose coefficients depend on two control parameters. The frequency-response equation is derived from the amplitude modulation equations. A method to estimate the functional dependence of the detuning parameter on two control parameters is introduced. The amplitude of the harmonic response is derived from the frequency-response equation after knowing the detuning parameter. Then the stability of the steady motion with respect to control parameters can be determined. The evolution of the dynamic response of the resonance mode with decreasing aerodynamic damping is discussed by means of both perturbation and numerical methods.     

On damage diagnosis for a wind turbine blade using pattern recognition

With the increased interest in implementation of wind turbine power plants in remote areas, structural health monitoring (SHM) will be one of the key cards in the efficient establishment of wind turbines in the energy arena. Detection of blade damage at an early stage is a critical problem, as blade failure can lead to a catastrophic outcome for the entire wind turbine system. Experimental measurements from vibration analysis were extracted from a 9 m CX-100 blade by researchers at Los Alamos National Laboratory (LANL) throughout a full-scale fatigue test conducted at the National Renewable Energy Laboratory (NREL) and National Wind Technology Center (NWTC). The blade was harmonically excited at its first natural frequency using a Universal Resonant EXcitation (UREX) system. In the current study, machine learning algorithms based on Artificial Neural Networks (ANNs), including an Auto-Associative Neural Network (AANN) based on a standard ANN form and a novel approach to auto-association with Radial Basis Functions (RBFs) networks are used, which are optimised for fast and efficient runs. This paper introduces such pattern recognition methods into the wind energy field and attempts to address the effectiveness of such methods by combining vibration response data with novelty detection techniques.     

Using a pseudo-functionally graded interlayer in order to improve the static and dynamic behavior of wind turbine blade T-bolt root joints

The large wind turbines blades with multi-ton composite structures are mostly connected to the peach-bearings flanges using T-bolt joints which induce shear and bearing stress fields around the cross bolts. The significant differences between the modulus of elasticity of metallic bolts and composite surrounding materials cause stress concentration around interfaced zones and, also, limit the load capacity of the joints. In the present research, a pseudo functionally graded material (PFGM) as an interlayer is used around the cross bolts to examine the reduction of the stress concentration. Some radial variation of the mechanical properties would be considered for this interlayer. The finite element method is used to analyze the structures. Loadings are applied to the center of the cross bolts analogous to the real cases. Both the static and dynamic loadings are studied. For the finite element of the functionally graded material interlayer, a multilayer alternative material with constant properties in each layer is used. The results show that using an isotropic single layer with an average modulus of elasticity and specific thickness decreases the stress concentration of the composite part up to 47%. The various property models for the interlayer also show that an appropriated model can decrease the stress concentration up to 55%. Dynamic transient analyses would be implemented over the joint structure and improved considering to the practical cases. Using the PFGM interlayer decreases the constant and variable parts of the stresses up to 55% and also causes significant increasing of the joint fatigue life.     

Multi-spectral temperature measurement method for gas turbine blade

One of the basic methods to improve both the thermal efficiency and power output of a gas turbine is to increase the firing temperature. However, gas turbine blades are easily damaged in harsh high-temperature and high-pressure environments. Therefore, ensuring that the blade temperature remains within the design limits is very important. There are unsolved problems in blade temperature measurement, relating to the emissivity of the blade surface, influences of the combustion gases, and reflections of radiant energy from the surroundings. In this study, the emissivity of blade surfaces has been measured, with errors reduced by a fitting method, influences of the combustion gases have been calculated for different operational conditions, and a reflection model has been built. An iterative computing method is proposed for calculating blade temperatures, and the experimental results show that this method has high precision.     

Mössbauer study of turbine blade steels

Mössbauer investigations were carried out on low carbon steels containing 12–13.5% Cr and 3–5% Ni in order to get information about the reason of cracking and fracture which take place during the use of turbine blade wheels. The quantity of retained austenite determined from the Mössbauer spectra of steels was low (<1%) in the cracked and fractured basic materials. Comparing this value with those being considerable in quenched (≈11%) as well as in annealed state (≈5%) of the same sample, we can conclude that the transformation of the austenite taking place during the working of turbine blade wheel can be associated with the cracking and the fracture. We found an anomalous increase of the quantity of the austenite in steel samples (quenched from different temperature between 700 and 1000 °C and aged at 450–600 °C) aged again at 450–550 °C. On the basis of the evaluation of Mössbauer spectra of the steels, information can be obtained about the changes in the concentration of alloying elements being in martensite at the various heat treatments.     

Detecting salt deposition on a wind turbine blade using laser induced breakdown spectroscopy technique

The study of pollution performance on a wind turbine blade due to lightning is important, as it can cause major damage to wind turbine blades. In the present work, optical emission spectroscopy (OES) technique is used to understand the influence of pollutant deposited on a wind turbine blade in an off-shore environment. A methodical experimental study was carried out by adopting IEC 60507 standards, and it was observed that the lightning discharge propagates at the interface between the pollutant and the glass fiber reinforced plastic (Material used in manufacturing of wind turbine blades). In addition, as a diagnostic condition monitoring technique, laser-induced breakdown spectroscopy (LIBS) is proposed and demonstrated to rank the severity of pollutant on the wind turbine blades from a remote area. Optical emission spectra observed during surface discharge process induced by lightning impulse voltage is in agreement with the spectra observed during LIBS.     

Experimental study on the micromechanical behavior of a PBX simulant using SEM and digital image correlation method

The micro-scale mechanical behavior of a polymer-bonded explosive (PBX) simulant was experimentally studied using a scanning electron microscope (SEM) imaging system and digital image correlation (DIC) method. The semi-circular bend (SCB) test was chosen for the study. During the testing, a series of SEM images of the specimen was acquired in situ. The natural micro-structural features of the specimen were used as random speckle pattern for DIC analysis. The displacement and strain fields at the area of interest were obtained by DIC. The deformation and damage of PBX were analyzed. Heterogeneous strain fields demonstrated the damage evolution underneath the specimen surface and predicted possible micro-crack growth. Based on the contour plots of the correlation coefficient, the formation and extension of microscopic cracks were quantitatively analyzed.