基于特征映射的义齿表面三维变形设计方法

义齿表面设计是牙科计算机辅助设计和制造系统的一个重要环节,设计的义齿形状必须符合患者的口腔咬合环境并能保持牙齿的拓扑结构特征。针对这一问题,在此提出一种基于特征映射的义齿表面三维变形设计方法。该方法主要思想是首先识别预备体与同名标准牙的重要特征点,采用径向基函数建立两者的特征映射关系,再通过对应特征点的位置对齐和表面的插值变形,实现义齿表面的变形设计。下颌第一磨牙嵌体的设计案例结果表明,该方法设计程序简单,标准牙曲面变形合理,能有效地避免曲面失真,最终生成的嵌体与预备体匹配良好,为牙科计算机辅助设计和制造系统的临床应用提供一种新的方法。     

基于颌位空间约束的缺损牙体曲面统计重建算法

重建符合牙齿咬合接触关系的缺损牙体曲面是全冠、嵌体、贴面等多类修复体造型过程中均需要解决的关键问题。针对规模有限的标准模板牙冠难以表达牙齿自然形态多样性的不足,提出了基于颌位空间约束的缺损牙齿曲面统计重建算法。采用广义普氏分析构建样本牙冠曲面之间的对应关系,在主要特征近似对齐的基础上计算平均牙冠;结合样本牙冠与平均牙冠对应点间的欧氏距离,建立可以表达样本牙冠与平均牙冠形态区别的差异矩阵,利用主成分分析方法求解影响平均牙冠形态的主要特征矢量,在此基础上建立基于参数表达的统计牙冠模型;根据缺损牙体的残留区域形态及其颌位接触关系采集局部约束特征点,以此反求统计模型的形变参数,构建基于颌位空间约束的统计牙冠,实现缺损牙体曲面的形态重建。下颌第一磨牙的重建试验表明,所建立的统计牙冠不仅具有足够的解剖形态特征,而且能与缺损牙体的残留形态以及邻牙、对颌牙相适应,具有良好的颌位接触关系,后期设计不需要复杂的调颌检测。     

牙齿预备体洞缘线的截面析构提取方法

CAD/CAM技术的嵌体修复方法在修复缺损牙体的临床应用中日益深入和广泛。洞缘线的提取质量决定了嵌体与预备体的配合精度,影响补牙治疗的质量,因而洞缘线的提取具有极为重要的意义。文中提出洞缘线的截面析构提取方法,从牙齿预备体扫描点云数据中提取洞缘线数据点,相对于模型全局特征的提取方法,截面析构法不会产生冗余无意义的特征线,提取准确且快速,再从洞缘线数据点中提取特征进而获得牙齿预备体洞缘线。由此获得的洞缘线不仅能满足误差条件,而且求取速度快,洞缘线的特征点数少,便于口腔临床设计使用。计算结果表明,所提出的截面析构提取方法能够满足临床误差使用要求,也为各种洞缘线的提取提供了一种有效的方法。     

基于Imageware的汽车后大梁反求设计

逆向工程是产品设计与加工的一种新方法,已经成为企业不可或缺的设计手段.以汽车后大梁力研究对象,介绍了反求设计关键技术以及使用Imageware进行自由曲面反求设计的一般方法.通过点云数据的采集、数据预处理、特征线构建以及三维模型重建和曲面质量分析和编辑等设计过程,介绍了利用Imageware实现汽车后大梁这种复杂曲面的反求设计的整个步骤,详细介绍了利用Imageware进行特征线重构和曲面重构的关键技术,以及通过对特征线和曲面的再次编辑,实现了汽车后大梁的反求设计并满足了企业的设计精度要求.并介绍了反求设计的一般设计流程以及在Imageware中进行反求设计的技巧.     

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。     

牙体预备机器人备牙轨迹规划及实验研究

龋齿作为人类三大重要防治疾病之一,严重影响了人们的口腔健康。牙体修复是治疗龋齿的重要手段,牙体预备是口腔修复龋齿的必要治疗环节。传统牙体预备依赖于医生手工操作并结合丰富的临床经验,需要对患龋牙齿进行大量的重复的微细调整。针对上述情况提出采用机器人辅助医生实现牙体预备可有效地提高备牙质量和口腔治疗的效果。通过分析医生预备过程的操作特点,确定后牙全冠预备的具体需求,明确牙体预备机器人的设计要求;利用曲面映射关系构造曲面片的约束条件进而确定预备体曲面上的型值点,并依据牙齿之间的相邻关系和功能将功能尖斜面分为3个曲面,基于NURBS曲面完成了3个曲面的插值;根据曲面的型值点和曲面片特征,基于等参数法实现了牙体预备机器人的备牙轨迹规划。设计并研制了牙体预备机器人的虚拟样机和物理样机,并进行了牙体预备实验研究,各个特征点在XYZ方向上的最大的相对定点误差分别为0.23、0.15和0.97 mm,系统误差的置信区间均稳定在0.3 mm以内。在不同方向下的各个特征点的随机误差相关性较弱,验证了备牙轨迹规划方法的正确性和机器人牙体预备的可行性。     

牙体预备机器人备牙轨迹规划及实验研究* .txt

摘要：龋齿作为人类三大重要防治疾病之一,严重影响了人们的口腔健康。牙体修复是治疗龋齿的重要手段,牙体预备是口腔修复龋齿的必要治疗环节。传统牙体预备依赖于医生手工操作并结合丰富的临床经验,需要对患龋牙齿进行大量的重复的微细调整。针对上述情况提出采用机器人辅助医生实现牙体预备可有效地提高备牙质量和口腔治疗的效果。通过分析医生预备过程的操作特点,确定后牙全冠预备的具体需求,明确牙体预备机器人的设计要求;利用曲面映射关系构造曲面片的约束条件进而确定预备体曲面上的型值点,并依据牙齿之间的相邻关系和功能将功能尖斜面分为3个曲面,基于NURBS曲面完成了3个曲面的插值;根据曲面的型值点和曲面片特征,基于等参数法实现了牙体预备机器人的备牙轨迹规划。设计并研制了牙体预备机器人的虚拟样机和物理样机,并进行了牙体预备实验研究,各个特征点在XYZ方向上的最大的相对定点误差分别为023、015和097 mm,系统误差的置信区间均稳定在03 mm以内。在不同方向下的各个特征点的随机误差相关性较弱,验证了备牙轨迹规划方法的正确性和机器人牙体预备的可行性。 .txt     

网格曲面上自由形状特征交互式阵列设计

为了提高设计效率、便于后续布局设计优化,提出一种流形网格曲面上自由形状特征可参数驱动的阵列重用设计方法,引入场向参数化方法获得网格曲面上的等值分布线,再由两组不同角度的等值分布线的交点确定特征的阵列位置;以阵列位置点的法矢和方向矢量构建局部坐标标架,用以确定阵列特征的朝向,并交互式确定各特征大小;最后采用基于旋转不变量的拉普拉斯网格变形方法,使得各阵列特征和目标曲面自然过渡,实现自由形状特征的有序多重复制。将位置阵列和特征变形分开处理,使得阵列特征的变形不依赖于场向参数化,仅与其所在的局部区域有关而与曲面整体无关,既保证了各特征和目标网格的融合效果,也为实施并行计算、降低时间成本提供了条件。试验结果表明,所介绍方法可行、有效,能满足自由形状特征在曲面上的阵列式交互设计要求。     

叶片零件的逆向建模与偏差对比分析

通过对叶片自由曲面逆向建模特点与特征建模方法的研究,结合逆向设计软件Geomagic Design X,提出一种以特征点云提取曲面特征截面线的逆向建模方法。详细介绍了如何利用面片草图功能提取特征截面线,并利用提取的截面线实现零件的逆向建模。最后对所建模型进行了偏差对比分析,结果表明:该建模方法有效可行,可提高曲面的建模精度与光顺度,为复杂曲面类零件的逆向设计提供了一种新的建模方法。     

特征尺寸驱动的鞋楦参数化设计

针对鞋楦数字化设计中对非线性尺寸的约束要求,提出一种基于微分坐标变形的尺寸驱动曲面变形方法,实现围长驱动的楦型设计。基于足-楦尺寸的对应关系修改鞋楦上的非线性尺寸,将围长尺寸变化映射为围线形状的变化,再由变化后的围线形状通过网格变形技术转化为楦型曲面的形状修改。通过迭代计算,可以保证围度曲线长度的精确约束;同时,根据楦型设计规律增加的位置约束和方向约束使变形后的曲面能够继承原有楦型曲面内在的形状特征;拉普拉斯变形拥有的特征保持性特点使变形后的鞋楦仍然保持原有的楦型样式,且其最小二乘意义下的求解框架有利于均布计算误差,使变形更加光顺、自然。试验结果表明,所提方法变形自然,实用性强。     

Aspheric surface finishing by fixed abrasives

This paper presents a method for the finishing of aspheric surfaces using fixed abrasives. The strategy is to remove a specified amount of material from the surface. The method assumes concentric tool paths perpendicular to the axis of the surface. The key parts of the method are: (1) efficient computation of the material removal profile for each tool path and (2) optimisation of the feed rate for the tool paths. Simulation results are included to illustrate the proposed method, which suggest that the method is potentially useful for aspheric surface finishing.     

Any-degrees-of-freedom (anyDOF) registration for the characterization of freeform surfaces

This paper presents an any-degrees-of-freedom (anyDOF) registration method for the characterization of freeform surfaces. The method attempts to fill the research gap regarding traditional surface registration methods which are normally dedicated to solving the global optimization problem with all DOF but they lack flexibility. The proposed anyDOF method is capable of registering surfaces with any specified combination of DOF. This is particularly useful when some of the DOF are known to be unchanged according to the a priori knowledge. The anyDOF surface registration method is regarded as a typical optimization problem of finding the minimum distance from target surface to the reference surface, with constraints of the unwanted DOF. The problem is solved by the Levenberg-Marquardt method. Simulated experiments for a two-dimensional (2D) profile and a three-dimensional (3D) surface were undertaken, together with three measurement experiments including a fluid-jet polished surface, a bonnet polished surface and a diamond machined freeform surface. Experimental results show that the anyDOF registration method is highly flexible in the characterization of freeform surfaces.     

Evaluation of the standard measurement uncertainty due to the ISO50 surface determination method for dimensional computed tomography

Surface determination is the process by which a CT volume is converted from an image-based representation of an object to a surface-based representation. The process of surface determination relies on well-established image processing algorithms, but these algorithms are not exact, surface determination therefore contributes a component of uncertainty to X-ray CT based dimensional measurements. The purpose of this work is to develop a method for quantifying the standard uncertainty due to surface determination such that this standard uncertainty can be combined with other standard uncertainties in order to calculate the combined standard uncertainty for X-ray CT based dimensional measurements. The proposed method relies on evaluating the mode and the 68% dispersion of the air and material grey values of a given CT data-set. These values are propagated through the ISO50 threshold calculation to give the standard uncertainty of the ISO50 threshold value from which the standard uncertainty due to the ISO50 surface determination method can be evaluated. The proposed method is verified numerically and then demonstrated experimentally for dimensional measurements of an aluminium workpiece, a polymer workpiece, and a steel additively manufactured workpiece. The results show that the measurement uncertainty due to the ISO50 surface determination method is largest for dimensions that have both internal and external surfaces such as wall thicknesses, followed by dimensions such as internal and external diameters, whilst dimensions that are not sensitive to surface determination, for example centre-to-centre distances have the smallest uncertainty due to the ISO50 surface determination method.     

A Novel Method of Efficient Machining Error Compensation Based on NURBS Surface Control Points Reconstruction

A novel method to improve the efficiency of error compensation in free-form surface machining based on the Non-Uniform Rational B-Splines (NURBS) surface control points reconstruction is proposed in this article. With the presented method, a relatively small number of inspection points are needed to be measured for error compensation. The machined surface is obtained by reconstructing the control points of the designed surface based on the on-machine measurement data. The machining error of the surface is obtained by calculating the difference between the machined surface and the designed one. Then a compensate surface is achieved using the mirror symmetry model and surface modification method to compensate the machining error. Experimental validation for the milling of a NURBS surface shows that the machining accuracy of the surface is improved by 62.57% through use of the proposed method.     

回转曲面及变形在花朵造型中的应用

花朵各部分的形状大多接近于回转曲面;提出了通过回转曲面变形获得所需曲面。对回转曲面的变形,主要是通过对其母线和轴线加变形函数实现;这种方法构造曲面参数方程较为简单,易于实现且执行速度快,是花朵造型的一种实用方法。     

基于自适应采样的曲面加工误差 在机测量方法

基于模具在机测量的自适应采样结果,提出一种新的获取自由曲面加工误差的方法。该方法首先基于自适应采样获取加工曲面上少量测点的坐标数据,利用NURBS曲面重构来拟合加工曲面;然后基于广义牛顿法计算重构的实际曲面和理论曲面的法向距离,获得自由曲面的加工误差,并对实验加工的模具模型曲面的轮廓度误差进行分析。实验结果表明,基于自适应采样的加工曲面重构方法能够在机测量且有效地获得自由曲面加工误差。     

Constrained deformation of freeform surfaces using surface features for interactive design

There is an increasing demand in conceptual design for more intuitive methods for creating and modifying freeform curves and surfaces in CAD modeling systems. The methods should be based not only on the change of the mathematical parameters but also on the user's specified constraints and shapes. This paper presents a new surface representation model for freeform surface deformation representation. The model is a combination of two functions: a displacement function and a function for representing an existing NURBS surface called a parent surface. Based on the surface model, the authors develop several novel deformation methods which are named SingleDef (Single-point constraint based deformation method), MultiDef (Multiple-points constraints based deformation method), CurDef (Curve constraints based deformation method) and FeatDef (Feature constraint based deformation method). The techniques for freeform surface deformation allow conceptual designers to modify a parent surface by directly applying point constraints, curve constraint or a surface constraint to the parent surface. The deformation methods are implemented in an experimental CAD system. The results show that designers can easily and intuitively control the surface shape.     

Basic concepts in surface shaping by cutting tools

Different concepts regarding the initial instrumental surface and production surface of cutting tools are outlined. The definition of the production surface in State Standard GOST 16530–83 includes two different surfaces. That poses serious problems for the theory of shaping. Definitions are proposed for the production line and the production surface. In addition, the cutting production surface is defined as the envelope of the production surface of a tool in cutting. Depending on the type of contact between the cutting production surface and the machined surface (all points, a single line, a single point), three general methods of surface shaping by a cutting tool are described. Specific tools are considered. For each tool, the general shaping method that best describes its operation is noted. The distinction between rolling cutters for gear turning and gear milling that employ the same general shaping method is established.     

模拟板带轧制三维变形的流面条元法

提出模拟板带轧制过程三维变形的一种新的数值方法——流面条元法。沿着金属的流动轨迹,将变形区划分为若干流面(曲面)条元,为方便分析和计算,又将其映射为平面条元。横向位移和高向位移的纵向分布被分别构造为四次曲线和二次曲线,其横向分布均用三次样条插值函数表示,高向分布均用二次曲线拟合。根据塑性力学流动理论,分析推导了变形区三维变形和应力的数学模型。与曾经提出的流线条元法相比,考虑了应力与变形沿高向的不均匀分布,实现了精确的三维分析和计算。仿真实例表明,该方法和模型符合实际,为板带轧制过程的三维力学仿真提供了一个新的实用工程数值方法。     

基于动态轮廓采样法的轴向超声振动辅助磨削工件表面形貌预测与试验验证

提出了一种基于动态轮廓采样法的轴向超声振动辅助磨削的工件表面形貌预测方法。假设磨粒直径服从正态分布,磨粒位置服从随机分布,生成砂轮表面形貌的模型,从运动学角度建立了轴向超声振动辅助磨削过程中任意磨粒的轨迹方程,针对磨粒运动轨迹的特点,提出了动态轮廓采样方法。通过建立磨削沟槽变宽模型,引入了磨削弹性变形模型和塑性堆积模型,对动态轮廓采样方法进行了修正,最终得出工件表面形貌的预测结果。对预测结果进行了试验验证,对比分析了工件表面形貌的预测结果和实测结果,两者特征相似,且比较工件表面粗糙度的预测值和实测值平均误差为5.3%,从而验证了该预测方法的准确性。