A new parametric control method for freeform mesh models

Parametric modeling technology is difficult to apply to freeform mesh models since there is no efficient way to impose geometric constraints. In this paper, we propose a novel method to control freeform mesh models parametrically. Our approach is to construct a control mesh that surrounds an object model and then impose constraints on it. The control mesh is parametrically controlled and the shape of the object model is modified by using an existing freeform deformation method. This paper is mainly concerned with automated construction of a control mesh and treatment of geometric constraints. Procedures for creating a control mesh are as follows: 1) determine the optimal orientation of the model, 2) project the model along three axes and extract contours, 3) create 2D control polygons for the contours, and 4) construct the 3D control mesh from the 2D control polygons. Geometric constraints are imposed on the edges and faces of a control mesh. Types of constraints are given by either a relative relationship between elements or an absolute displacement. A new control mesh is calculated by solving these constraints and the original model is modified accordingly. We tested our algorithms for two freeform models.     

Prediction of surface reconstruction uncertainties for freeform surface inspection

A method is introduced to predict uncertainties of the B-spline freeform surfaces that are reconstructed from the measurement points for inspection of freeform surfaces. The uncertainties of a reconstructed B-spline surface are modeled by variances of coordinates of points on this freeform surface. Prediction of the uncertainty at any location on the reconstructed B-spline surface is carried out in two steps: (1) estimation of variances of the B-spline surface’s control points introduced by the surface reconstruction process, and (2) propagation of the variances from the control points to the points on the B-spline freeform surface. In this research, the variances of the control points of the reconstructed B-spline surface in all three directions are considered to improve the accuracy of uncertainty prediction in the regions with significant changes of geometric shapes. Both the errors and uncertainties at different locations on the reconstructed surface are considered in freeform surface inspection to compare the manufactured surface against the design surface and its tolerance. The developed method has been applied to two case studies to demonstrate its effectiveness.     

锥束ART算法快速图像重建

针对锥束ART算法重建速度慢的问题,提出了一种基于投影的三维射线与体素的快速遍历和求交算法.该算法将三维射线投影到两个互相垂直的平面上,通过计算投影线与投影平面的相交情况来确定三维射线穿过体素的索引及长度.利用该算法在图像重建过程中实时计算权因子,不仅节省了大量的内存空间,而且提高了投影和反投影运算的速度.基于该算法的特点,在重建过程中采用了一种按列优先的策略,减少了不必要的计算,大大提高了重建速度.仿真实验表明,该算法非常有效,与传统的Siddon算法相比取得了17倍以上的重建加速比.     

Efficient analysis-suitable T-spline fitting for freeform surface reconstruction and intelligent sampling

Optical scanning instruments require sampling and reconstruction with high accuracy and low computational cost. T-splines have recently been developed that allow significant reductions in the number of control parameters by overcoming some of the topological constraints of B-splines and NURBS. As a subset of T-splines, analysis-suitable T-splines (ASTS) show promise due to the linear independence and partition of unity of their basis functions. In this paper, a computationally efficient ASTS fitting algorithm for freeform surface reconstruction is proposed. This algorithm starts with adaptive construction of an initial analysis-suitable T-mesh according to the distribution of high-curvature feature points. A local refinement and local optimisation algorithm of the analysis-suitable T-mesh is then iteratively performed until a preset accuracy condition is satisfied. Our experimental results show that the proposed ASTS fitting can produce over 50% root-mean-square reconstruction error reduction compared to NURBS fitting, with the same number of control parameters. The computing efficiency of the proposed algorithm is equivalent to or higher than that for simple T-spline fitting. Fast derivative analysis of the ASTS has also been carried out, where two automatic intelligent sampling design methods have been developed, namely element area sampling and element curvature sampling. Up to 50% reconstruction error reduction is observed when compared to uniform and statistically optimised sampling designs. With the novel reconstruction and compatible intelligent sampling design techniques, freeform surface measurement accuracy and efficiency could be effectively improved using coordinate measuring machines.     

Fast tool servo diamond turning of optical freeform surfaces for rear-view mirrors

The International Journal of Advanced Manufacturing Technology - Optical freeform surfaces have been applied in optoelectronic systems more and more widely, but it is difficult to fabricate this...     

Novel hybrid robot and its processes for precision polishing of freeform surfaces

Freeform surfaces have important applications in various industrial fields. However, achieving a high level surface finish on freeform surfaces using polishing is always a challenging task. Hybrid robots are promising alternatives to conventional computer numerical control (CNC) machines and industrial robots for ultra-precision machining. Herein, we present a novel custom-built hybrid robot for freeform polishing. After the laboratory prototype was successfully developed, its automation for a specific freeform surface was a major obstacle preventing its application. This critical issue was addressed by presenting a process to deal with tedious robot programming. Random tool path planning was performed in the task space, and hybrid robot motion planning was conducted in the joint space. By integrating the process flows, a robot programming toolkit was developed to directly output the control program for a specific robot controller. An illustrative example with a freeform surface is provided to verify the functionality of the developed hybrid robot and the proposed control processes, and the corresponding experimental results verify their effectiveness.     

基于FESS的混合模型脑图像分割方法

提出一种基于FESS的混合模型脑图像分割方法.其特点在于生成模型与判别模型得到了有效结合.生成模型通过条件随机场融合体素点的灰度信息、形状信息以及区域相邻关系,实现对脑子结构外观特征的描述.在此基础上,利用生成模型将训练样本映射到FESS特征空间;判别模型中采用LS-SVM分类器并将数据场应用于混合分割模型的训练过程中,降低了判别模型由于训练数据不平衡而引起的性能波动并提高其泛化能力.实验结果表明,与若干前沿的脑图像分割方法相比,该方法具有更好的分割质量和性能.     

基于网格分割的自适应细分曲面算法研究

细分曲面中,随着对模型细分次数的增多,模型的面片数迅速增长,巨大的数据量加大了模型后处理的难度。为此,结合网格分割技术,提出了一种自适应细分曲面算法。该算法利用控制网格每个顶点与其对应的极限位置之间的欧氏距离不同、采用K-均值算法和区域合并技术对控制网格进行分割,随后对用户指定的网格区域或不满足精度的区域进行局部自适应细分。实验结果表明,该算法在给定精度范围内,有效地以相对较少的面片数获得了理想光滑的细分曲面,且大大提高了模型的后处理速度。     

具有复杂型面机架的快速曲面重构技术

零件的曲面模型是获得其逆向设计数据等后续处理的关键环节。为了快速获得具有复杂型面机架的设计数据,研究了该机架点云曲面重构的获取方法。鉴于机架不同部位对逆向设计精度要求不同,采用Imageware和Geomagic Studio软件对机架的销孔和外形点云分别进行曲面重构之后,再实施整个模型的曲面整合策略。介绍了应用两种软件完成机架曲面模型的主要处理过程及关键环节。该方法能快速获得符合精度要求的机架曲面模型,可行且高效,为机架曲面模型的实体化及其后续处理提供了可靠依据。     

A robust surface fitting and reconstruction algorithm for form characterization of ultra-precision freeform surfaces

Ultra-precision freeform surfaces are non-rotational symmetric surfaces possessing sub-micrometer form accuracy and nanometric surface finish. Although they can be fabricated accurately by ultra-precision machining technology, their surface quality is difficult to be characterized. Surface reconstruction is a vital task in the form characterization of ultra-precision freeform surfaces. This paper presents a robust surface fitting algorithm to reconstruct a high fidelity surface from measured discrete points while the surface smoothness can be ensured as well. A fitting threshold named confidence interval of fitting error is proposed to strike the balance between fitting accuracy and surface smoothness in the fitting process. The fitting algorithm is in two steps. In the first step, bidirectional sampling method is developed to extract a curve network from measured points cloud to construct an initial surface. In the second step, the fitting error of the initial surface is minimized to meet the prescribed fitting error threshold. A series of experimental work has been conducted and the results show that the proposed algorithm is able to provide effective means for increasing the accuracy in the form characterization of ultra-precision freeform surfaces.     

基于广义反向粒子群与引力搜索混合算法的多阈值图像分割

提出了基于粒子群优化(PSO)与引力搜索(GSA)混合算法(PSOGSA)的多阈值图像分割方法来解决图像阈值搜寻过程中单一优化算法局部搜索能力不强的问题。提出了图像阈值分割领域中的广义反向学习策略,在阈值寻优过程中提高群体多样性,增强了全局搜索能力;采用了全局最优解的正态变异策略,扩展了全局最优的搜索区域,避免了算法的早熟收敛。在此基础上,实现了基于广义反向粒子群与引力搜索混合算法的多阈值图像分割方法。最后,使用本方法对复杂多目标图像进行了多阈值分割实验,并与引力搜索算法和萤火虫算法进行了比较。实验结果表明,本文方法的分割精度优于引力搜索算法与萤火虫算法,其分割目标函数值在连续运行时的标准差降低了90%以上,是一种精度高、稳定性强的多阈值图像分割方法。     

一种基于特征拟合的网格分割方法及其在反求工程中应用

随着3D数字化技术的逐步普及,越来越多的形体通过光学扫描设备输入计算机,形成网格模型,然后借助网格分割技术从中提取有用的结构、特征信息.当前基于曲率的分割方法占据主导地位,但常不能取得满意的分割结果.介绍了一种基于分层特征拟合的网格分割方法,适用于工业产品的反求设计,且可以处理扫描数据残缺的情形.详述了算法思路、流程,并给出了反求工程应用实例.     

Embryo zebrafish segmentation using an improved hybrid method

Zebrafish is an invaluable vertebrate model in life science research and has been widely used in biological pathway analysis, molecular screening and disease modelling, among others. As a result, microscopic imaging has become an essential step in zebrafish phenotype analysis, and image segmentation thus plays an important role in the zebrafish microscopy analysis. Due to the nonuniform distribution of intensity and weak boundary in zebrafish microscope images, the traditionally used segmentation methods may lead to unsatisfactory result. Here, a novel hybrid method that integrates region and boundary information into active contour model is proposed to segment zebrafish embryos from the background, which performs better than traditional segmentation models. Meanwhile, how to utilize the gradient information effectively in image segmentation is still an open problem. In this paper, we propose to improve the aforementioned hybrid method in two aspects. Firstly, the mean grey value of background is estimated by the expectation maximization (EM) algorithm to constrain the active curve evolution. Secondly, an edge stopping function sensitive to gradient information is designed to stop curve evolution when the active curve reaches the embryo boundary. Experimental results show that the proposed methods can provide superior segmentation results compared to existing algorithms.     

基于萤火虫算法的二维熵多阈值快速图像分割

提出了基于萤火虫算法的二维熵多阈值快速图像分割方法以改善分割复杂图像和多目标图像时存在计算量大、计算时间长的问题。首先,分析了二维熵阈值分割原理,将二维熵单阈值分割扩展到二维熵多阈值分割。然后,引入萤火虫算法的思想,研究了萤火虫算法的仿生原理和寻优过程;提出了基于萤火虫算法的二维熵多阈值快速图像分割方法。最后,使用该方法对典型图像进行阈值分割实验,并与二维熵穷举分割法、粒子群算法(PSO)二维熵多阈值分割法进行比较。实验结果表明:该方法在单阈值分割、双阈值分割和三阈值分割时分别比二维熵穷举分割法快3.91倍,1040.32倍和8128.85倍;另外,在阈值选取的准确性和计算时间方面均优于PSO二维熵多阈值分割法。结果显示,基于萤火虫算法的二维熵多阈值快速图像分割方法能快速有效地解决复杂图像和多目标图像的分割问题。     

Regenerative braking control strategy in mild hybrid electric vehicles equipped with automatic manual transmission

The actual regenerative braking force of an integrated starter/generator (ISG), which is varied with desired braking deceleration and vehicle speed, is calculated based on an analysis of the required deceleration, maximum braking force of ISG, engine braking force and state of charge (SOC) of battery. Braking force distribution strategies are presented according to the actual regenerative braking force of ISG. To recover the vehicle’s kinetic energy maximally, braking shift rules for a mild hybrid electric vehicle (HEV) equipped with automatic manual transmission (AMT) are brought forward and effects of transmission ratios are considered. A test-bed is built up and regenerative braking tests are carried out. The results show that power recovered by the braking shift rules is more than that recovered by the normal braking control rules. __________ Translated from Chinese Journal of Mechanical Engineering, 2006, 42(10): 156–160 [译自: 机械工程学报]     

大型有限元网格模型的快速简化与交互绘制

随着有限元分析计算的复杂化,计算结果的数据量大大增加甚至超出内存限制.提出一种基于外存的多分辨率构建和绘制算法,采用自适应性二叉树对模型的包围盒进行划分,自底层向上简化网格数据及物理数据,构建模型的多分辨率层次结构,较好地保持了原始细节分布;在实时绘制时,采用基于视点的细节层次选择策略进行模型的跟新.实验结果表明,该算法计算速度较快,可以对大型有限元网格模型进行快速绘制并显示其物理计算结果.     

An image reconstruction algorithm based on total variation with adaptive mesh refinement for ECT

In this paper, a robust image reconstruction algorithm for electrical capacitance tomography (ECT) is proposed. The key feature of the algorithm is the use of adaptive mesh refinement based on total variation (TV) in solving the inverse problem. It keeps the edge preserving and scale-dependent properties of total variation regularization, and enhances the distinguishability by using adaptive mesh refinement. This strategy improves the spatial resolution efficiently with less calculation and is less underdetermined than uniform refinement. Simulation and experimental results show that the algorithm performs better than both standard Tikhonov regularization and the conventional total variation method.     

基于色彩分割和自适应窗口的快速立体匹配

针对现有立体匹配算法难以兼顾匹配精度和速度的不足,提出了一种基于联合匹配代价的局部方法.首先,根据视差在同一色彩分割区域内平滑变化的假设,提出了一种利用参考图像和目标图像的色彩分割信息获得的基于任意形状和大小支持区域的匹配代价;然后在RGB色彩空间中,通过由窗口内的平均匹配误差、误差方差及较大窗口的偏向误差构成的窗口选择评价函数,获得基于自适应矩形窗口的匹配代价;最后,将这两种匹配代价进行有机的结合构成联合匹配代价,并通过局部优化方法获得稠密视差图.采用Middlebury dataset进行的实验结果表明,本文算法不仅可以提高视差不连续区域和低纹理区域的匹配精度,而且获得的视差与当前主流算法具有可比性.此外,所提算法的处理时间较之当前优秀的局部方法提高了约19～35倍.     

基于SIMD技术的锥束ART算法快速并行图像重建

ART(algebraic reconstruction technique)算法是一种迭代图像重建方法,适合于大型工业构件的无损检测,其缺点是计算量大、重建时间长。为了提高锥束ART算法的重建速度,本文提出一种快速并行图像重建方法。首先根据锥束CT扫描方式下三维射线的对称性提出一种权因子和体素索引的并行计算方法,通过一次计算可同时得到两条射线的权因子和体素索引;然后采用Intel处理器的单指令多数据(single instruction multiple data,SIMD)技术,一次性加载多个打包数据,利用SSE(streamingSIMD extension)指令实现了投影、计算图像校正和反投影的并行运算。实验结果表明本文提出的方法非常有效,在保证图像重建精度的同时取得了约1.5倍的重建加速比。