形态学算法的去噪处理在自由曲面重构中的应用

介绍了形态学中开操作和闭操作在曲面去噪中的应用,在所设计的原始曲面上随机设置噪声,对含噪声的原始曲面型值点数据进行开操作和闭操作,减少或消除噪声。应用Unigraphics软件中的NURBS完成曲面重构,并对曲面重构的精度和效率进行了分析、验证。     

基于小波变换和NURBS的自由曲面重构的去噪处理

小波分析理论是一种新兴的信号处理理论,在时间和频率上都有很好的局部性。借助时一频局部分析特性,小波分析理论已成为信号去噪处理的一种重要的工具。介绍了小波分析理论在曲面去噪中的应用,在所设计的原始曲面上随机设置噪声,对含噪声的原始曲面型值点数据进行小波分解,提取低频分量,再应用UG软件中的NURBS完成曲面重构,并对曲面重构精度进行了分析。     

基于平面曲率提取产品特征线的散乱数据建模

提出了一种基于平面曲率从散乱数据中提取特征线的方法,它包括三个主要步骤：首先对散乱数据截取平面点云,利用平面曲率法提取特,正点;然后,把取得的特征点作为NURBS曲线的节点,求出NURBS曲线,用能量法光顺,生成产品特征线;最后,利用生成的特征线分割测量数据,然后分块拟合。该方法把复杂的三维散乱数据的产品特征线提取问题变为简单的二维计算,降低了运算复杂度。     

光学扫描方法在曲面重构中的应用

介绍了基于光学扫描方法的曲面重构,详细分析了曲面重构及曲面建模的理论,在理论的基础上,利用逆向工程软件Imageware处理光学坐标仪器采集的点云数据并拟合曲面,同时利用机械设计软件UG将特征曲线及曲面参数化,建立相应的数字模型。     

在逆向工程中复杂曲面重构技术的开发应用

介绍了一种在逆向工程中基于三角域上NURBS复杂曲面重构新方法,并利用在VC环境下逆向工程操作平台与多个软件相结合,从而实现复杂曲面从点云数据到三维实体模型的重构。     

飞机叶片焊接修复中的三维测量重构技术

提出一种基于NURBS理论的叶片点云处理和曲面重构的方法.飞机涡轮叶片是结构、气动、强度等多学科优化设计的结果,其外形为复杂的自由参数曲面.用最小二乘法将拓扑后的点云局部曲面拟合对异常点去噪,依据曲率阈值点云分割.将双三次Coons的B-spline形式曲面作为基面,散乱点云无约束曲面逼近,利用优化理论迭代得到曲面控制点从而确定B-spline曲面方程.根据NURBS理论对曲面拼接修剪,复原磨损叶片的三维模型,为后续的焊接修复提供精确三维模型.     

面向薄壁平坦曲面三维法向补偿方法的研究

在分析薄壁平坦曲面加工变形特点的基础上,提出基于曲面三维法向补偿的曲面重构法,首先以逆向工程将加工变形后的曲面导入计算机辅助设计软件,再以原始自由曲面为基准获得变形点在曲面法向上的反向补偿点,然后由反向补偿点插值生成相应的NURBS曲线,由NURBS曲线构建变形补偿网格曲面,并以变形补偿网格曲面代替原始曲面进行数控自动编程加工,从而实现误差补偿和数控编程一体化,进一步提高了误差补偿精度,可以有效解决薄壁平坦曲面的加工变形问题。通过实践验证了方案的有效性,此法在生产中具有很强的实用性。     

基于特征的汽车覆盖件反求建模方法

汽车覆盖件具有结构比较复杂的特点,针对某一汽车覆盖件的实测散乱点云数据实例,采用基于特征的反求工程CAD建模系统RE-SOFT,根据基于特征的反求工程建模策略,按照曲率分析、点云分块、特征提取、曲面重构、模型生成等步骤,完整地进行了模型重构的过程。使整个模型准确地再现了原始设计的特征结构,完全实现了特征建模的模型重构过程。     

基于STEP-NC的自由曲线曲面插补技术的研究

为了满足在STEP-NC数控系统下对自由曲线曲面插补的要求,在对现有插补方法分析后,提出通过带节点的B样条曲线曲面实体和有理B样条曲线曲面实体在STEP标准中的定义,按照STEP标准中对新增实体命名的规定,给出了一个包含NURBS曲线曲面所有参数的新实体.根据曲线的几何特性与刀具运动特性之间的关系列出方程,并用泰勒展开式进行插补点密化,且在MATLAB6.5环境下进行仿真实验,结果表明基于STEP-NC的NURBS曲线曲面插补在保证精度的同时,提高了插补速度.     

复杂曲面边界线自动提取技术研究

复杂曲面零件的几何模型重构是逆向工程的研究重点之一,由零件表面的数字化数据提取边界线是构造几何模型的重要步骤。针对由CMM获取的三维数据“点云”,提出了经纬线扫描算法自动提取单值曲面边界线,提出了基于局部增量网格扩张的三维散乱数据三角剖分算法,实现了任意非封闭曲面边界线的自动提取。实践证明,通过该算法得到的复杂曲面的边界线能够满足模型重建的工程需求。     

基于双NURBS曲线的叶轮叶片工业机器人GMAW增材制造

以双NURBS曲线获得的直纹面广泛存在于各行业复杂零件中,如何实现复杂直纹面的增材制造一直是增材制造领域中极为关心的问题。以双NURBS曲线描述叶片模型,进一步获得直纹面,采用优化工艺参数,以GMAW方法实现了叶轮叶片的增材制造,增材获得的叶片完整,成形良好,表明以GMAW工艺实现双NURBS曲线叶片增材制造是可行的。为叶片模型构建、复杂形状叶片的电弧增材制造提供了新方法,对复杂形状零件增材制造过程控制具有借鉴意义。     

Algorithms to support point-based cadcam

This paper presents a system that supports a point-based approach to CADCAM that was first suggested by McLaughlin. The core to the approach is the ability to interpolate a grid of points in a geometrically meaningful way that does not suffer from the disadvantages of the standard interpolation schemes based on NURBS. The essential features of the approach are based on recursive subdivision between existing points to generate intermediate points that are consistent with a smooth curvature profile. There are many advantages to this approach. Time-consuming surface fitting is not necessary; there are no problems with ensuring continuity between surfaces; data exchange can be carried out exactly. By chosing to work entirely with points, all of the standard interrogations available in CAE systems, including visualisations, FE analysis and NC machining, which are dependent on NURBS curves and surfaces have to be replicated. The feasibility of the point-based approach is illustrated by showing how geometric interrogations and machine tool paths can be accurately generated using just points.     

基于产品件展开后毛坯网格的边界外延方法

产品件用一步逆成形初始场预示方法展开得到的坯料形状,没有考虑成形过程中除零件本身以外所需要的板料面积,因此,需要对展开后的坯料网格再进行一定尺寸的延展,才能保证得到最终坯料尺寸的正确性。文章提出了一种将离散数据连续化后,利用非均匀有理B样条(NURBS)的几何性能完成延展再离散成网格数据的方法,能够很好地预示出满足冲压需要的最终坯料形状。这种方法具有计算效率高、优化效果好等优点。     

NURBS曲线与圆弧求交的剪刀迭代法

提出了一种剪刀迭代算法,用于CAD／CAM／CNC系统中NURBS曲线段与圆弧之交点的求取。该方法从几何的观点出发,像剪刀一样将两段曲线的两端不断剪短,经多次迭代后,最终得到交点。该方法从数学上将高次方程降阶为低次方程,特别适用于3次NURBS曲线与圆弧的求交。     

螺旋锥齿轮离散齿面数字仿真加工方法研究

基于螺旋锥齿轮HFT加工方法,建立了螺旋锥齿轮的仿真加工模型,通过编制仿真加工程序,获取了螺旋锥齿轮齿面离散点,在此基础上实现了螺旋锥齿轮齿面的NURBS表示,并通过实例验证了该方法的可行性.     

Adaptive interpolation scheme for NURBS curves with the integration of machining dynamics

This paper develops a comprehensive interpolation scheme for non-uniform rational B-spline (NURBS) curves, which does not only simultaneously meet the requirements of both constant feedrate and chord accuracy, but also real-time integrates machining dynamics in the interpolation stage. Although the existing work in this regard has realized the importance to simultaneously consider chord error and machining dynamics, none has really incorporated these in one complete interpolation scheme. In this paper, machining dynamics is considered for three aspects: sharp corners or feedrate sensitive corners on the curves, components with high frequencies or frequencies matching machine natural ones and high jerks. A look-ahead module was developed for detecting and adaptively adjusting the feedrate at the sharp corners. By Fast Fourier Transform (FFT) analysis with a moving window in the interpolation stage identified were some special frequency components such as those containing high frequencies or with frequencies matching machine natural ones. Then, the notch filtering or time spacing method was used to eliminate these components. To more completely reduce feedrate and acceleration fluctuations, the jerk-limited algorithm was also developed. Finally, the interpolated feedrate was further smoothened with B-spline fitting method and the NURBS curves were re-interpolated with the smoothened feedrate. During the interpolation, the chord error was repeatedly checked and confined in the prescribed tolerance. Two NURBS curves were used as examples to test the feasibility of the developed interpolation scheme.     

Trajectory generation in high-speed,high-precision micromilling using subdivision curves

Motion control in high-speed micromilling processes requires fast, accurate following of a specified curvilinear path. The accuracy with which the path can be followed is determined by the speed at which individual trajectories can be generated and sent to the control system. The time required to generate the trajectory is dependent on the representations used for the curvilinear trajectory path. In this study, we introduce the use of subdivision curves as a method for generating high-speed micromilling trajectories. Subdivision curves are discretized curves which are specified as a series of recursive refinements of a coarse mesh. By applying these recursive properties, machining trajectories can be computed very efficiently. Using a set of representative test curves, we show that with subdivision curves, trajectories can be generated significantly faster than with NURBS curves, which is the most common method currently used in generating high-speed machining trajectories. Trajectories are computed efficiently with subdivision curves as they are natively discretized, and do not require additional evaluation steps, unlike in the case of NURBS curves. The reduced trajectory generation time allows for improved performance in high-speed, high-precision micromilling. We discuss the use of several metrics to quantify the quality of the subdivision interpolation, and apply them in calculating the error during trajectory generation for the test curves.     

基于NURBS曲面的五轴联动插补算法

文章提出了一种基于NURBS曲面的五轴联动插补算法。对于NURBS曲面,确定一个参数方向的一系列参数值,就可确定一系列的另一个参数方向的NURBS曲线。沿这些曲线逐条进行插补,就可实现对整个曲面的插补。在每个插补点,求出两个参数方向的切矢,确定出该点的曲面法矢,在假设刀控方向垂直于插补曲面的情况下求出两个旋转轴的运动,实现五轴联动插补。同时,利用NURBS曲线的局部性质来保证插补的实时性。