反求工程中点云数据的曲线拟合技术

针对反求工程中的激光随机扫描所产生的散乱点云数据,提出一种曲线拟合方法。该方法能有效地对点云数据进行三角剖分、精简、平滑去噪处理等操作,并能最终得到满足要求的拟合曲线。     

基于Surfacer的点云数据的预处理研究

“点云”的预处理研究,包括平滑和拓扑关系的建立。利用反求软件——Surfacer,对“点云”进行了平滑,然后直接从空间入手,从一般性的角度来说明了点云数据的三角剖分的算法,建立数据之间的拓扑关系。文中最后指出下一步的工作。     

基于LabVIEW的横截面扫描数据预处理方法研究

数据处理是反求工程中的关键环节,数据处理的准确性直接影响到曲面造型的真实性。利用LabVIEW的数组函数、图形显示模块和滤波器,针对非接触横截面扫描数据线的处理方法进行研究,从滤除的坏点数、阈值的有效范围、数据处理的时间、最大值与最小值之差以及滤除噪声点后的数据结果等各个方面进行综合分析对比,得出曲线检查法去除噪声点并填补坏点是最优方法。同时利用LabVIEW的中值滤波器和巴特沃斯滤波器进行数据光滑处理,取得理想效果。     

反求工程中一种数据平滑的方法

逆向工程中测量获得的点云数据中存在一定的噪声点，必须对其进行数据平滑处理。在比较弦高比阈值滤波、均值滤波和中值滤波的基础上，提出一种数据平滑的方法。该方法充分利用数据点间的相关性和位置信息，对数据点和噪声点采取不同的处理方式，避免了噪声的传播。实验研究表明，该方法既能较好地滤除噪声点又有较好的保边性。     

基于小波包的点云数据平滑处理

点云数据的平滑处理是逆向工程曲面建模前的必要环节.提出了基于小渡包的点云数据平滑处理方法,通过小波包变换将点云数据分解成不同层次的小波系数,而点云数据中的噪声点属于高频信息范围,所以在小波系数域内通过去除高频信息重构点云数据,一方面可以实现点云数据的平滑处理,另一方面则可以针对不同的频谱信息进行噪点云除.同时,由于点云数据的离散间隔信息不均匀,采用离散小波进行处理时在采样方式上存在误差,故根据点云数据的获得原理,采用图像域的二维信息作为索引代替原三维点云数据的x,y坐标.小渡包重构后通过索引信息将原来的坐标信息还原,从而达到点云数据平滑的目的.整个算法通过图像域、小波系数空间和三维欧式空间的相互转换完成点云数据的平滑处理.最后以具体的实例验证所提方法的有效性.     

利用噪声特点与曲面拟合的点云去噪及光顺算法研究

逆向工程中,随着三维扫描测量技术的迅速发展以及基于海量采样点云数据的高精度模型的产生,使得快速有效地处理点云这一技术显得更加重要.点云的去噪和光顺是法矢估计、曲率计算及曲面重建等后续处理工作的重要基础,依据散乱数据点云中的噪声点的特点,对其进行分类,并提出一种新的基于曲面拟合及噪声特点分步去噪的方法.     

基于特征线的复杂曲面反求工程中的CAD建模技术

三维轮廓测量技术是实现反求工程的重要手段 ,复杂曲面建模技术是反求工程研究的重点内容。研究了激光线扫描测量原理以及“点云”数据类型。针对栅格点数据 ,提出了一种快速有效的复杂曲面建模技术 ,并以实例说明“点云”数据平滑处理、特征线提取、曲面分块及曲面构造过程     

一种光学三维型面测量中的阴影去除方法

三维测量获得的点云数据一般须进行去噪处理以保证重构曲面的精度,光学三维测量中这些噪点主要是由阴影形成的,因此阴影去除的效果直接影响到反求工程的效率和精度.采用基于Gray-code投影的方法实现三维面型测量,根据Gray-code唯一、有序的特点,在测量过程中进行阴影处理,可以有效地去除阴影,从而去除测量结果中的噪点.该方法可以避免噪声,保证点云数据的准确性,为后续的曲面重构提供便利,在反求工程中具有重要的实用价值.     

基于局部基面参数化的点云数据边界自动提取

提出了一种反求工程中基于局部基面参数化方法的点云数据边界特征的自动提取方法。首先选择合适的局部基面 ,然后用点云垂直投影于局部基面投影点的参数化代替空间点的参数化 ,二次参数曲面逼近点云 ,再利用曲面的微分特性估计点云数据的曲率值 ,求出曲率极值点 ,从中提取边界点。通过这些边界点可以进一步拟合边界曲线 ,达到对点云数据进行自动分片的目的。该方法具有较强的可操作性和实用性 ,对于反求工程的自动化和智能化研究具有实际意义     

航空发动机外形点云的保特征去噪方法

三维激光扫描设备可以提供航空发动机外形实测点云,但其中包含的噪声会直接影响后期外形几何模型的重建精度。为保证在去除噪声的同时不模糊或破坏掉发动机复杂的外形几何特征,提出了一种基于深度学习的点云保特征去噪方法。将航空发动机外形噪声点云分割成特征数据和非特征数据之后,分别设计了特征去噪网络和非特征去噪网络,用于预测特征噪声点和非特征噪声点的位置修正向量,噪声点沿预测向量移动后被投影回模型真实的底层表面上,实现去噪。构建了用于特征去噪学习和非特征去噪学习的数据集。验证结果表明,在将该方法应用于各种噪声尺度的发动机外形点云时,相比现有的学习基方法,去噪效果得到提高,且有更好的几何特征保护能力,可以为后续重建提供高质量点云。     

Design and application of a small size SAFT imaging system for concrete structure

A method of ultrasonic imaging detection is presented for quick non-destructive testing (NDT) of concrete structures using synthesized aperture focusing technology (SAFT). A low cost ultrasonic sensor array consisting of 12 market available low frequency ultrasonic transducers is designed and manufactured. A channel compensation method is proposed to improve the consistency of different transducers. The controlling devices for array scan as well as the virtual instrument for SAFT imaging are designed. In the coarse scan mode with the scan step of 50 mm, the system can quickly give an image display of a cross section of 600 mm (L) × 300 mm (D) by one measurement. In the refined scan model, the system can reduce the scan step and give an image display of the same cross section by moving the sensor array several times. Experiments on staircase specimen, concrete slab with embedded target, and building floor with underground pipe line all verify the efficiency of the proposed method.     

微位移机构中变截面柔性铰链等效刚度的求解方法研究

对变截面柔性铰链的等效刚度k进行研究,提出了k值的两种求解方法:修正系数法与变截面法。通过实例计算,两种方法求得的结果基本一致,而变截面法更具有通用性且计算简便。采用变截面法计算了柔性微位移放大机构中变截面柔性铰链的等效刚度k,在此基础上,对机构进行了分析,求得不同载荷作用下机构的输入、输出位移及其放大比,同时对其进行有限元分析,两种结果具有较好的一致性,验证了k值计算的正确性,表明了变截面柔性铰链等效刚度计算方法的有效性。     

Analysis of diametral strain in uniaxial tensile and compression testing of round specimens of anisotropic materials

The development of elliptical cross sections in initially round test pieces of anisotropic materials makes it difficult to measure the true strain on a cross section using a transverse extensometer. In this paper, an analysis is given of the diametral strain and strain rate on a round cross section. Based on the experimental observation that static and dynamic microstructural changes of many anisotropic materials was too small to cause a noticeable change in anisotropic factor, R, up to relatively low strain levels, it is assumed that R is independent of strain in this analysis. Analytical results suggest that, when both the imposed axial strain rate and the anisotropic factor R remain constant and R0.5, the change in diametral strain rate is negligible no matter which orientation a transverse strain extensometer is positioned on a cross section. The analysis includes two methods of measuring the diametral strain: (i) measurement of the distance between two diametral-intercept points on the circumference of a cross section; and (ii) measurement of the distance between two parallel tangent lines of the cross section. Based on the analytical and experimental results, a new method has been proposed for obtaining true stress–strain data in uniaxial tensile or compression testing of anisotropic materials with constant or varying anisotropic factor.     

压杆截面设计的简便算法

压杆的稳定性设计是通过计算压杆截面面积并选用截面形状来达到的。压杆的截面设计一般采用试算法，计算量较大；图解法避免了试算法中的重复计算过程，但精度较差，且不能进行型钢截面的设计。本文对不同的截面形状，提出几种直接的优化、计算方法，它比试算法简单，比图解法精确，适用于大、中、小各种柔度的压杆，也适用于截面形状因子为常数的截面和常用的型钢截面；导出了一些简便实用的计算公式，能够直接得出截面面积，选取型钢型号，应用范围广泛。     

Low-energy electron/atom elastic scattering cross sections from 0.1–30 keV

Empirical forms for electron/atom scattering cross sections predict backscattering factors that compare well with those calculated using tabulated Mott data from 0.1 to 30 keV. The form of the empirical total cross section is similar to the screened Rutherford cross section. The fit to the tabulated differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section (σ_R), and the second part being an isotropic distribution (σ_I). The ratio of the total cross sections (σ_R/σ_I) between the screened Rutherford part of the differential scattering cross section and the isotropic part of the distribution is fitted to give the same ratio of forward to backscattered currents as the tabulated Mott differential cross sections. The three equations, one for the total elastic cross section and two describing the differential cross section—one for the Rutherford screening parameter and one for the ratio σ_R/σ_I—give backscattering results covering all the major trends with energy and atomic number compared with the backscattering coefficients calculated using tabulated Mott cross sections. However, agreement with experiment is poor for some well-researched examples such as Au. Monte Carlo calculations using the empirical cross sections show that surface effects may be critical in interpreting experimental results.     

An accurate approximation for the highly efficient sampling of polar scattering angle of electron elastic single-scattering events

In single-event Monte Carlo electron transport simulations, elastic scattering events dominate the changes in electron trajectories due to collisions. Classically, the polar scattering angle due to an elastic collision can be sampled efficiently from the screened Rutherford cross section. However, the screened Rutherford cross section fails for both high Z materials and when the incident electron energy becomes too low. Alternatively, improved simulation accuracy for electrons in all energy ranges and through all materials may be obtained by sampling directly from differential data derived from partial-wave-expansion method (PWEM) calculations based on theoretical atomic potential models. While sampling directly from wave calculations will yield simulation results to the best known physical accuracy, it comes at the cost of simulation time. This is due to a sampling process that is typically more involved when compared with using the screened Rutherford cross section. In this work we present a relationship capable of reproducing the moments of the differential cross section derived from PWEM calculations, resulting in good preservation of forward and backscattering peaks. The relationship is directly invertible and is as easily sampled as the Rutherford cross section. Most important, the data presented in this paper in combination with this relationship produce Monte Carlo simulation results which are comparable with those using the exact differential cross section from PWEM calculations for elements Z = 1 to 96 and for incident electron energies from 300,000 down to 50 eV.     

Effects of the introduction of the discrete energy loss process into Monte Carlo simulation of electron scattering

First, the single scattering model is briefly described. Next, the hybrid model is explained, which takes into consideration a part of the discrete energy loss processes. The Vriens or the Gryzinski cross section is used for core electron ionizations, and the Moller cross section for free electron excitations. The model is applied to the calculations of the energy distribution of transmitted electrons through a thin film and the depth distribution of generated x-rays. From comparisons among the calculated results with and without energy straggling and experimental data, it is found that the Gryzinski cross section shows the best result.     

Comparison of different laser-assisted electrochemical methods based on surface morphology characteristics

This paper presents two different laser electrochemical machining (LECM) methods: point-by-point method and scan method. Experiments about the two LECM methods were performed on the aluminum alloy plates placed in a shallow container filled with NaNO₃ electrolyte 2 mm above. Then, the scanning electron microscopy and the optical profiling system were used to analyze machining quality and morphology characteristics of two different machining methods. For the point-by-point method, the shape accuracy at the starting point, finishing point, and node of line cross is lower. Compared with the point-by-point method, the better etching morphology was gained by the scan method in the experiments. The influence of the scan speed on machining quality and groove width was investigated. The better etching surface quality and the higher etching efficiency were gained when the scan speed varied from 0.1 to 0.15 mm/s. These results demonstrate that laser electrochemical machining by the scan method is a promising method to achieve complex profiles.     

Using cross‐correlation for automated stitching of two‐dimensional multi‐tile electron backscatter diffraction data

A method for automatically aligning consecutive data sets of large, two‐dimensional multi‐tile electron backscatter diffraction (EBSD) scans with high accuracy was developed. The method involved first locating grain and phase boundaries within search regions containing overlapping data in adjacent scan tiles, and subsequently using cross‐correlation algorithms to determine the relative position of the individual scan tiles which maximizes the fraction of overlapping boundaries. Savitzky‐Golay filtering in two dimensions was used to estimate the background, which was then subtracted from the cross‐correlation to enhance the peak signal in samples with a high density of interfaces. The technique was demonstrated on data sets with a range of interface densities. The equations were implemented as enhancements to a recently published open source code for stitching of multi‐tile EBSD data sets.     

Assessment of Scattered Damage in Structural Materials

Methods for assessing scattered damage to materials are considered. Ultrasonic methods based on recording backscattered ultrasonic signals are among the most practically feasible methods for assessing scattered damage to materials at the mesolevel (the size of the probing wavelength). We describe methods for determining scattered damage in the volume of a material based on scanning the object surface with a normal double-crystal piezoelectric transducer, recording and statistically processing the backscattered signal in the form of an A-scan, constructing the spatial distribution of backscattering cross section in the form of a B-scan or tomographic image, and assessing damage in the material volume based on the relative change in the backscattering cross section or the “disorder” of its spatial image.