基于草图的网格曲面变形算法

为了对网格模型进行控制,提出了一种基于草图线和轮廓线的网格曲面变形技术.首先,通过提取模型的侧影轮廓线或在曲面上绘制初始草图线生成变形区域的初始控制线;然后,对原草图线临近区域进行微分特性的操作,或者绘制一条新的草图线以映射初始控制线;最终利用改进的离散拉普拉斯算子建立线性方程组并采用最小二乘法求解,实现变形.这一变形技术较为直观,方便用户对变形效果的控制,在自主开发的软件平台上进行了测试,验证了其效果和工程可行性.     

网格曲面上自由形状特征设计重用

针对目前网格曲面上复杂特征重用困难的问题,提出一种保细节的特征重用方法。借助于拉普拉斯坐标蕴含的曲率、法矢等微分几何信息,提出基于微分坐标的重用特征边界环参数化方法,具有保长度、保曲率和保形状的特性,减小特征边界环从兴趣区域映射至目标区域时所产生的形变。采用离散测地极坐标方法对目标网格进行局部参数化,按照参数坐标一致原则将重用特征的边界环映射至目标网格的指定区域,并以映射后的边界环顶点位置和法矢信息为约束边界条件,基于旋转不变量的改进拉普拉斯变形框架,对重用特征进行变形操作,使重用特征与目标区域实现自然过渡。所提特征设计重用框架无须对重用特征本身参数化,因而可以重用任意复杂的特征,且对特征的亏格数亦无限制。试验结果表明,所介绍方法健壮、有效,可用于复杂特征的迁移式设计重用。     

保持型面特征的网格曲面光顺算法

针对网格曲面光顺存在局部型面细节丢失及整体光顺效果差的问题,结合三角Bézier面片具有精确插值产品型面的优点及G1连续曲面具备整体光顺的特性,提出一种保持型面特征的网格曲面光顺方法,该方法从原始网格曲面提取型面特征网格,将其作为参数域构建G1连续光顺参考曲面,通过将网格曲面顶点调整到参考曲面上,实现网格曲面的光顺处理。实验证明该方法可在有效保留原始网格曲面细节特征的情况下获得理想的整体光顺效果,并且具备较高的运行效率。     

基于网格变形技术的白车身多目标形状优化

市场的竞争压力促使汽车厂商致力于加快车身开发的进程,而基于计算机辅助工程(Computer aided engineering,CAE)的车身结构优化技术由此而成为业内的研究热点。与传统的尺寸优化不同,形状优化在工程优化中具有更大的潜力。将网格变形技术引入形状优化,提出基于近似模型的多目标形状优化方法。利用网格变形技术定义形状变量,并根据灵敏度信息筛选优化变量;采用优化拉丁方试验设计对设计空间均匀分布样本点,进一步拟合高精度的Kriging模型;运用多目标粒子群算法,保持其余性能指标满足预期的前提下,以白车身弯曲刚度和质量为目标进行优化。研究表明,所提出的优化方法成功用于白车身的多目标优化,设计者可根据优化结果权衡各个目标,以指导最终的决策。     

基于网格曲面形状修改的柔性件装配偏差分析

柔性件装配偏差有限元分析方法中的敏感度矩阵来源于产品的理论数学模型,而理论数学模型与实际模型之间存在一定的外形差异,会产生很大的分析误差。针对该问题,提出了一种装配偏差分析的改进方法,即根据产品理论外形和误差源数据,运用网格曲面变形方法获取逼近零件实际形状的数学模型,代替理论数学模型进行装配偏差分析。开展了金属薄板的装配回弹试验以及飞机壁板件的装配偏差分析计算,使用仿真、试验及测量等手段获取了各类偏差分析结果,并进行比较。数据表明,装配偏差分析误差可减小到5%以内。     

基于特征约束的四边形网格划分算法

鉴于有限元分析的特殊需要，提出了一种基于特征约束的四边形网格划分方法。将特征约束分为点约束和线约束，分别采用不同的方法进行处理，然后利用改进的铺砌法进行网格划分。从而生成质量良好的四边形网格。     

基于草绘的网格曲面拉伸变形算法

为提高产品概念设计中三维模型重构的效率,设计了一种基于草绘技术的网格曲面拉伸变形方法。以多模式交互方式选择拉伸区域,系统自适应调整拉伸区域网格模型拓扑结构;利用投影方法将自由形式或非自由形式二维拉伸轨迹曲线转化为空间三维曲线;将拉伸区域边界曲线进行扫掠处理,进而对已形成的相邻扫掠曲线进行连接,最终形成拉伸网格曲面模型。通过在自主开发的软件平台上进行多种工业模型拉伸变形测试,验证了该算法的有效性和工程可行性。     

基于特征约束的四边形网格划分算法

鉴于有限元分析的特殊需要,提出了一种基于特征约束的四边形网格划分方法。将特征约束分为点约束和线约束,分别采用不同的方法进行处理,然后利用改进的铺砌法进行网格划分,从而生成质量良好的四边形网格。     

基于原始曲面信息的变形网格的曲面重构

为了对有限元分析后的模型进行干涉检查和再分析,需对变形后的网格重建实体模型。基于原始曲面的信息,对变形后的网格进行区域分割,针对分割后的每个特征区域,分别对二次曲面和B-样条曲面采用了不同的重构方法。对于二次曲面,提出了一种基于等参数线的二次曲面参数提取的方法;对于B-样条曲面,给出了一种误差控制的B-样条曲面拟合的多步迭代方法。基于原始模型的拓扑结构,对重构后的曲面进行裁剪缝合得到变形后的实体模型。由于利用了原始曲面信息,使得曲面重构得以简化。实验表明,基于原始曲面信息的曲面重构方法可以快速有效地重构出任意复杂拓扑结构的变形后的曲面。     

一种保持特征的网格简化算法

针对网格简化算法中出现的细节特征丢失、简化结果过于均匀以及计算复杂等问题,从原始模型的几何特征出发,对待折叠三角形与其邻域内三角形的形状与位置关系进行研究,对三角形折叠点到其三个顶点的偏离程度进行总结,提出了一种基于三角形折叠的保持模型特征的网格简化算法。该方法根据目标三角形各顶点邻域三角形的不同,为其3个顶点分配相应的权值,由各顶点权值的大小最终确定三角形折叠点坐标,然后以二次误差测度为度量标准计算出每个三角形的折叠代价,再结合三角形局部面积比、局部区域不平度以更好地控制简化结果。实验结果表明:该方法能够降低计算的复杂度、提高简化速度,使模型的特征区域和非特征区域均保持一定的简化率,并较好地保持了原始模型的细节特征。     

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.     

一种骨架线驱动的产品曲面变形设计方法

针对复杂产品外形设计中一类由骨架线驱动的曲面变形设计问题,提出基于变分原理和扩散效应控制目标曲面变形的设计方法。通过提取决定产品外形曲面几何特征的骨架线(如脊线、谷线),按照给定的约束条件控制曲面骨架线的几何变形,以变形后的骨架线为几何约束条件控制曲面形状的变化趋势,从而实现产品形状变形设计。此外,利用该方法可以提取原曲面拉普拉斯微分坐标表示的曲面细节,保持曲面变形过程中几何细节特征基本不变。产品曲面变形实例验证了方法的有效性。     

各向异性非刚性形变局部不变特征研究

当前多数局部不变特征算法主要针对刚性形变的图像匹配问题,但非刚性形变普遍存在且多数表现为各向互异.提出一种面向各向异性非刚性形变的局部不变特征算法.本算法对最稳定极值区域进行同性化处理,使其各向形变在一定程度上趋于同性且不受方向性影响,然后将图像投影到三维空间中,并通过调整外观权值获得形变不变的测地距离,最后建立各向异性测地距离灰度直方图(AGIH)描述符,使本算法进一步适应各向异性的微小形变.验证实验表明,在非刚性形变情况下,与SIFT算法、GIH算法相比,本算法在相同的错误匹配率的情况下,正确检测率平均提高了10.65%.     

A real time machining error compensation method based on dynamic features for cutting force induced elastic deformation in flank milling

During the machining process, cutting forces cause deformation of thin-walled parts and cutting tools because of their low rigidity. Such deformation can lead to undercut and may result in defective parts. Since there are various unexpected factors that affect cutting forces during the machining process, the error compensation of cutting force induced deformation is deemed to be a very difficult issue. In order to address this challenge, this article proposes a novel real time deformation error compensation method based on dynamic features. A dynamic feature model is established for the evaluation of feature rigidity as well as the association between geometric information and real time cutting force information. Then the deformations are calculated based on the dynamic feature model. Eventually, the machining error compensation for elastic deformation is realized based on Function Blocks. A thin-walled feature is used as an example to validate the proposed approach. Machining experiment results show that the errors of calculated deformation with the monitored deformation is less than 10%, and the thickness errors were between ?0.05 mm and +0.06 mm, which can well satisfy the accuracy requirement of structural parts by NC (Numerical Control) machining.     

非接触采集手形相对特征识别性能分析

因非接触采集方式而产生的成像缩放与倾斜变形现象,使得成像后手形各特征的尺寸发生变化,从而造成绝对特征识别性能的显著下降。针对该问题作出了光学成像分析,证明了因倾斜变形现象产生的影响远小于缩放现象。因此在选用相对特征避免缩放现象的干扰后,利用类内方差均值和Fisher判别率,对变形后相对特征的稳定性和识别性能进行分析。实验结果表明,定义的相对特征可以作为稳定特征用于识别,同时经过小样本实验得到34个相对特征组成的向量,其识别率达到88.93%,说明该特征向量可以在小样本范围达到手形特征的有效识别,并可以作为手部识别的部分有效特征。     

Calculation method of radial stress and deformation on conic threaded connections with interference fit

This paper presents a calculation method for radial stress distribution and deformation on conic threaded connections with interference fit. Based on elastic mechanics, a new calculation model is established using the thick-walled-cylindrical theory. A sample calculation for API 88.9 mm conic threaded connection indicates that the method proposed in this paper is reasonable, and the finite element analysis (FEA) method is used to validate the proposed method. The results obtained by the proposed method and FEA method are identical. The model offers a new way of calculating the radial stress and deformation on conic thread connections with interference fit.     

Free vibration of multi-layered circular cylindrical shell with cross-ply walls,including shear deformation by using spline function method

The spline function technique is used to analyze the vibration of multi-layered circular cylindrical shells with cross-ply walls including first-order shear deformation theory. Both antisymmetric and symmetric cross-ply laminations are considered in this analysis. The governing equilibrium equations are obtained in terms of displacement and rotational functions. A system of coupled ordinary differential equations in terms of displacement and rotational functions are obtained by assuming the solution in a separable form. These functions are approximated by using Bickley-type splines of suitable order to obtain the generalized eigenvalue problem by applying point collocation techniques with appropriate boundary conditions. Parametric studies are performed to analyze the frequency response of the shell with reference to the material properties, number of layers, fiber orientation, thickness to radius ratio, length to radius ratio and circumferential node number. Reasonable agreement is found with existing results obtained by FEM and other methods. Valuable results are presented as graphs and discussed. This paper was recommended for publication in revised form by Associate Editor Maenghyo Cho Dr. K. K. Viswanathan was born in 1962 in Vellore District, India. He received his B.Sc. in Mathematics from University of Madras and M.Sc. in 1992 and Ph.D. in 1999 from Anna University, India. Later he was a Project Associate in Indian Institute of Science, Bangalore. He served as lecturer in Crescent Engg. College and as Asst. Professor in SRM University, India. He did his post doctoral research in Korea for three years. At present he serves as Professor in the Dept. of Naval Architecture, Inha University, Incheon, Korea. His research areas of interest includes vibration of plates, shells and the application of numerical techniques in Engineering problems. Dr. Kyung Su Kim was born in Korea in 1954. He is a professor in Naval Architecture and Ocean Engineering at Inha University, Korea. He obtained his B.Sc. degree in Naval Architecture and Ocean Engineering from Seoul National University, Korea, in 1981. He worked for KR (Korean Register of Shipping) from 1981 to 1983. He obtained M.Sc. degree in Naval Architecture and Ocean Engineering in 1986, and Ph.D. degree in Structural Mechanics in 1991 from Rheinisch — Westfaelische Technische Hoch-schule Aachen, Germany. From 1986 to 1992, he was a Post Doctoral Research Engineer of Engineering Research Institute at Rheinisch — Westfaelische Technische Hochschule Aachen. He was appointed as a professor of Inha University, Korea, in 1994. His major area of study is Impact and Fatigue Fracture. Dr. Jang Hyun Lee was born in Korea in 1969. Currently, he is an Assistant professor of the Department of Naval Architecture and Ocean Engineering at Inha University, Korea. He obtained his B.Sc., M.Sc. and Ph.D. degrees in Naval Architecture and Ocean Engineering from Seoul National University, Korea, in 1993, 1995 and 1999 respectively. From 1999 to 2002, he was a Post Doctoral Research Engineer of Engineering Research Institute at Seoul National University. He joined the Inha University in 2005 after holding the Chief Technology Officer at Xinnos for four years. His research interests include press forming of thick plates and shells, computational welding mechanics and Product Lifecycle Management.