用于BGA共面性检测的激光扫描在线测试系统

一种用于BGA（Ball Grid Array)共面性检测的激光扫描在线测试系统,对扫描中的动态误差进行了实时修正,提出了以三点法为基础的共面性评价方法,并进行了相应的实验研究,取得了满意的结果。     

基于外包络点的最小外接圆误差快速评定方法北大核心CSCD

对大尺寸轴类零件的高精密圆度测量时会生成大样本测量点集,而基于大样本点集的快速最小外接圆(MCC)误差评估已成为急需解决的工程问题。圆度测量产生的点集一般为有序评估点集,其最小外接圆可由该点集的外包络点确定。为提高最小外接圆计算效率,先利用包络向量搜寻外包络点,再进一步将外包络点压缩至常数范围,进而基于压缩后的外包络点快速求取最小外接圆及误差。分别利用文献数据、仿真数据和实测数据验证基于外包络点的最小外接圆误差快速评定方法的通用性、高效性和实用性。基于外包络点的最小外接圆误差快速评定方法具有实现简单、评定速度快的特点,可用于改进现有圆度仪的最小外接圆计算方法。     

倒圆锥面基底蒸发镀膜均匀性理论分析

通过计算得出了蒸发源位于倒圆锥面正下方外部镀膜时锥面上各点的膜厚方程,并对整个锥面上膜厚均匀性进行了理论分析。结果表明：当圆锥面形状固定时,蒸发源与圆锥底圆圆心距离增大使锥面上膜厚均匀性变好;当蒸发源固定时,增大底圆半径导致锥面上膜厚均匀性变差。在同样的配置下,蒸发源为点源或小平面源时锥面上膜厚均匀性的变化趋势一致,小平面源蒸镀比点源蒸镀时圆锥面上膜厚均匀性差。     

万能工具显微镜照明光阑孔径标尺的一种检定方法

万能工具显微镜是用途很广的光学计量仪器,应用它对多种工件实现精密测量。它的一种基本测量方法是影象法。由于成象光线不全是与光轴方向平行的光束,就给测量带来了不允许的误差。通常选择正确的照明光阑孔径(又称“最佳光圈”),来消除这一误差。所以照明光阑孔径标尺的正确否,直接影响到测量     

影象法测量滚刀齿形的一些问题

目前,由于滚刀检查仪的生产和应用尚不多,在许多工厂中仍普遍用影象法来测量滚刀的齿形误差。影象法测量滚刀齿形的方法虽然用得很普遍,但精确度如何,却不多见有分析。本文拟从影象法测量齿形的原理出发,试对这种测量方法的精确度略作分析,笔者希望自己的浅见能引起同志们的批评和讨论,以求对此能得到比较明确的认识。     

模式转换型纵-扭复合超声振动加工系统的设计

将纵-扭复合超声振动在应用于切削和焊接中,能够得到较好的加工质量,并可显著提高加工效率,延长刀具寿命。基于平面简谐波传播理论,推导了螺旋沟槽处反射横波的存在性,并进一步分析了螺旋沟槽结构的振型转换原理;利用数值解析法设计了复合变幅杆,并在其锥面开设螺旋沟槽,从而成功输出纵-扭复合振动;结合有限元分析探究了螺旋沟槽角度θ、沟槽槽宽d及槽深h对扭纵振动转换比e的影响规律,从而优化螺旋沟槽参数,并拟合出变幅杆端面某质点的振动轨迹。利用加工出的实体变幅杆对有限元分析结果进行了验证。     

石墨化对碳纳米管结构与电学性能的影响

在氮气气氛及2700℃温度下,对富含结构缺陷的具有Turbostratic形貌特征的碳纳米管原料进行高温石墨化处理,利用高分辨透射电子显微镜以及自主开发的基于透射电镜的原位性能表征系统对石墨化前后的碳管结构和导电性能进行了研究. 实验结果表明：经过高温石墨化处理后,碳管结构转变为类似于竹节状或管状的锥面结构,锥角为10°~30°,管径为10~40nm. 从锥角数据推算出锥面形成时的旋转位移角中均包含了一个附加的重叠角,说明石墨化后的碳管主要以螺旋的锥面结构为主,且弯曲的螺旋锥面靠∑7、∑13和∑19等重位点阵来稳定. 导电性能测量的结果表明具有螺旋锥面结构的纳米碳管呈半导体特性.     

金属重氮法PD系统

金属重氮法PD系统最初是Kalle公司于1934年申请的专利。专利介绍重氮化合物的光解产物具有强的还原性,利用这种还原性从金属盐中析出金属而形成影象。然而,此种方法获得的影象很弱,且反差低。1950年报导将物理显影导入该系统。1969年又进一步发展为PD系统。在重氮盐成象体系中PD系统有两种。一种是AM公司提出的压力重氮法(Pressure-Diazo);一种是 Philips公司提出的物理显影法(Physical Development)。     

柯达一步成象彩色照片的工作原理

柯达一步成象彩色加工过程的基础是减色法,其显象是反转加工法。一步成象彩色正片结构如图1和图2。每一张胶片都由一页罩盖片、一片整体式影象接受体,一条内装有活化液的药包以及一个能容纳多余活化液的收集体所构成 (见图3)。     

平面内直线度最小包容区域包络模型与算法实现

根据国家标准GB/T 11 336-2004中给出的平面内直线度误差的几何描述,提出了一种基于评估点集中包络点的平面内直线度最小包容区域模型,并给出了评估点集中包络点的判定原则。根据包络点的判定原则,提出了基于单向搜索构建包络向量的评估点集中包络点搜寻的高效算法,同时利用对比分析的方法说明了所提模型和算法的准确性与完备性。借助仿真数据对万级点集到百万级点集的最小包容区域进行求取,并统计了算法消耗的时间,结果显示算法的时间复杂度为O(N)级别,验证了算法的高效性。     

基本解方法解水下刚性目标三维Helmholtz外散射问题

运用基本解方法求解水下刚性目标三维Helmholtz外散射问题。研究了源点位置分布和数目对基本解方法计算结果的影响,比较了最小二乘配点法和等额配点法的计算精度。结果表明,当源点构成的形状与目标边界的形状差异大时,计算精度差,增加源点的数量可提高计算精度,运用较少的源点也可获得令人满意的精度,从而提高计算效率,但源点不宜距离目标边界过远;最小二乘配点法的计算精度较等额配点法高些。     

A normal offsetting technique for automatic mesh generation in three dimensions

A technique, based on a normal offsetting procedure, for the fully automatic generation of meshes suitable for finite element analysis in three dimensions is presented. The method is completely automatic, requiring no user intervention in the process and no special modelling procedures. The method is applied to three-dimensional solid geometries. The procedure positions nodes in the interior domain of an object by offsetting an initial set of nodes on the object boundary along vectors normal to the boundary to define a layer of new interior point locations. The offset points are processed to ensure good nodal spacing appropriate for generating well-shaped elements. Following processing, the offset points become a new boundary surrounding the remaining unmeshed region in the interior of the geometric domain. The offsetting procedure is applied again to this new boundary layer to form another offset layer farther into the domain interior. The offset-process-offset cycle is repeated until the entire region is filled with nodes. Tetrahedral elements are then formed by triangulation of the nodes. The boundary-based technique ensures good quality element shapes for analysis in critical boundary regions and facilitates applications involving integration of mesh generation with design geometry databases. Calculation of nodal locations are based on local parameters avoiding the higher-order time complexities associated with global calculations.     

三维图像中连续隐边界曲面的逼近计算

三维图像中感兴趣目标的边界曲面的检测与重构是三维图像分析中的一个重要问题.本文提出了从三维图像中计算连续隐边界曲面离散采样点的新方法.基于这些计算得到的离散采样点及其灰度值,可以推导出更合适的等值面或者自适应曲面去近似表示三维图像中的连续隐边界曲面.     

Boundary integral equation solution of viscous flows with free surfaces

Summary solutions of the biharmonic equation governing steady two-dimensional viscous flow of an incompressible Newtonian fluid are obtained by employing a direct biharmonic boundary integral equation (BBIE) method in which Green's theorem is used to reformulate the differential equation as a pair of coupled integral equations which are applied only on the boundary of the solution domain.An iterative modification of the classical BBIE is presented which is able to solve a large class of (nonlinear) viscous free surface flows for a wide range of surface tensions. The method requires a knowledge of the asymptotic behaviour of the free surface profile in the limiting case of infinite surface tension but this can usually be obtained from a perturbation analysis. Unlike space discretisation techniques such as finite difference or finite element, the BBIE evaluates only boundary information on each iteration. Once the solution is evaluated on the boundary the solution at interior points can easily be obtained.     

薄壁空间螺旋形曲线梁的约束扭转理论分析及结构计算方法（第一部分：基本理论）

本文应用矢量运算及薄壁曲线梁翘曲扭转理论，建立了空间螺旋形的薄壁曲线梁基本微分方程，用初参数法解出翘曲扭转微分方程，为进一步进行薄壁螺旋曲线梁的空间结构分析提供理论基础。     

Improving the performance of the boundary element method with time-dependent fundamental solutions by the use of a wavelet expansion in the time domain

The object of this paper is a wavelet-based formulation of the boundary element method (BEM) for diffusion problems, characterized by time-dependent fundamental solution. While the BEM is a well known and often used technique, its time-dependent formulation for diffusion problems is very rarely used in practical applications, due to the high computational cost which characterizes it. Here, a new formulation is proposed, which, through the use of the wavelet expansion of the time behaviour of the boundary elements, is characterized by a lower CPU time consumption when compared with the standard BEM diffusion formulation. The problem to be solved is transformed into an algebraic system (of higher dimension) and its solution gives the time domain behaviour of the desired quantities; in this way, the time stepping procedure is avoided. Together with the formulation, the analysis of the computational cost, and two examples are given in the paper. Copyright © 2006 John Wiley & Sons, Ltd.     

A Novel Contour Tracing Algorithm for Object Shape Reconstruction Using Parametric Curves

Parametric curves such as Bézier and B-splines, originally developed for the design of automobile bodies, are now also used in image processing and computer vision. For example, reconstructing an object shape in an image, including different translations, scales, and orientations, can be performed using these parametric curves. For this, Bézier and B-spline curves can be generated using a point set that belongs to the outer boundary of the object. The resulting object shape can be used in computer vision fields, such as searching and segmentation methods and training machine learning algorithms. The prerequisite for reconstructing the shape with parametric curves is to obtain sequentially the points in the point set. In this study, a novel algorithm has been developed that sequentially obtains the pixel locations constituting the outer boundary of the object. The proposed algorithm, unlike the methods in the literature, is implemented using a filter containing weights and an outer circle surrounding the object. In a binary format image, the starting point of the tracing is determined using the outer circle, and the next tracing movement and the pixel to be labeled as the boundary point is found by the filter weights. Then, control points that define the curve shape are selected by reducing the number of sequential points. Thus, the Bézier and B-spline curve equations describing the shape are obtained using these points. In addition, different translations, scales, and rotations of the object shape are easily provided by changing the positions of the control points. It has also been shown that the missing part of the object can be completed thanks to the parametric curves.     

Saddle-node equilibrium points on the stability boundary of nonlinear autonomous dynamical systems

A complete characterization of the stability boundary of an asymptotically stable equilibrium point in the presence of type-k saddle-node non-hyperbolic equilibrium points, with k ≥ 0, on the stability boundary is developed in this paper. Under the transversality condition, it is shown that the stability boundary is composed of the stable manifolds of the hyperbolic equilibrium points on the stability boundary, the stable manifolds of type-0 saddle-node equilibrium points on the stability boundary and the stable centre and centre manifolds of the type-r saddle-node equilibrium points with r ≥ 1 on the stability boundary. This characterization is the first step to understanding the behaviour of stability regions and stability boundaries in the occurrence of saddle-node bifurcations on the stability boundary.     

A Dirichlet problem for the biharmonic equation in a semi-infinite strip

This paper addresses the two-dimensional biharmonic problem for a semi-infinite strip with Dirichlet boundary conditions. The method of superposition is used to solve the problem. The object of this paper is to clarify mathematical questions connected with the solution of a special integral equation and to provide a rigorous justification of the applicability of the method of superposition. Mellin's transform technique of investigating the asymptotic behaviour of unknown density when the argument tends to infinity is used.     

激光扫描测量直径

提出一种应用光电共轭原理实现边界检出并进行激光扫描测量直径的新方法。采用该方法 的测量系统中,参考信号是测量信号的反向共轭函数,两函数的一对交点与被测工件边界相对应,工件直径可通过测量两函数交点之间的距离获得。实践验证了该方法的可行性,其边界检出精度±1μm,连续工作漂移不大于±5μm。