大型定子叶片铸件的数字化测量及误差评定

随着航空、航天、船舶、汽车和模具工业的飞速发展,复杂曲面的数字 化测量与误差评定技术对曲面加工精度起着越来越重要的作用。利用激光跟踪测量设备对大 型定子叶片铸件进行了数字化测量,对测量点云进行了去噪、精简等数据处理,并将测量点 云与理论模型进行了基准匹配,计算出了定子叶片铸件各部位的误差分布,实现了对大型定 子叶片铸件的误差评定;测量及误差分析结果可以用于优化叶片在后续加工中的装夹姿态, 使叶片各部位的加工余量趋于均匀,有效提高产品制造精度和加工效率。     

A virtual inspection framework for precision manufacturing of aerofoil components

The finite element method plays an extremely important role in forging process design as it provides a valid means to quantify forging errors and thereby govern die shape modification to improve the dimensional accuracy of the component. However, this dependency on process simulation could raise significant problems and present a major drawback if the finite element simulation results were inaccurate. This paper presents a novel approach to assess the dimensional accuracy and shape quality of aeroengine blades formed from finite element hot-forging simulation. The proposed virtual inspection system uses conventional algorithms adopted by modern coordinate measurement processes as well as the latest free-form surface evaluation techniques to provide a robust framework for virtual forging error assessment. Established techniques for the physical registration of real components have been adapted to localise virtual models in relation to a nominal design model. Blades are then automatically analysed using a series of intelligent routines to generate measurement data and compute dimensional errors. The results of a comparison study indicate that the virtual inspection results and actual coordinate measurement data are highly comparable and the procedures for registration and virtual inspection are computationally efficient, validating the approach as an effective and accurate means to quantify forging error in a virtual environment. Consequently, this provides adequate justification for the implementation of the virtual inspection system in the virtual process design, modelling and validation of forged aeroengine blades in industry.     

A new convex-hull based approach to evaluating flatness tolerance

In this paper, we consider a minimum-zone evaluation problem for flatness to determine the minimum tolerance zone enclosing all the measurement points obtained from a surface. For the problem, a new geometric approach called the ‘convex-hull edge method’ (CONHEM) is proposed. The method guarantees the minimum zone tolerance for a given set of measurement points. Several examples discussed by other researchers are illustrated to compare the method with some existing approaches.     

叶片模型公差约束条件下的配准方法研究

针对航空发动机叶片检测分析中的模型配准定位问题,结合叶片型面不同区域的设计公差要求不同的特点,以减少配准后超差点的数目为目标,提出了一种公差约束条件下的叶片模型配准方法。在迭代最近点匹配算法（Iterative Closest Point Algorithm,ICP）的基础上,根据公差要求定义约束区域,依据约束区域对点集进行加权处理,并给出了加权后的目标函数及其求解方法。通过实例验证了该方法是有效的,具有实用价值。     

基于蒙特卡罗法的平面度测量不确定度评定

形位误差的测量不确定度评定是目前测量领域研究的热点;但由于其测量的复杂性和测量结果评定的多样性,导致在实际测量结果中形位误差测量的不确定度评定成了难题;为此,根据形状误差评定准则,选取最小二乘法建立数学模型,确定形状误差数学模型中各参数值的传递系数和单点不确定度,并分析具体的测量方法和测量过程中的不确定度来源,根据传统的GUM法对其进行不确定度评定;然后采用蒙特卡罗伪随机数的方法来模拟实际测量数据,从而得到平面度误差的不确定度;通过设置实验对比,验证了蒙特卡罗法评定平面度不确定度的可靠性和准确性;该方法不需要求出数学模型中的传递系数,利用MATLAB软件很容易实现,为平面度误差测量结果不确定度评定提供了更加简便的方法,值得推广和应用。     

利用地面景象信息辅助的无人机自主定位技术

针对无人机自主定位过程中GPS定位系统失效的问题,提出了一种利用地面景象信息辅助的无人机自主定位技术,首先利用无人机所拍摄的实时航拍图像,与预先储存在无人机计算机中地面景象的数字化地形图进行匹配,从匹配结果中提取一个同名像点,结合地面景象数字化地形图所提供的数据信息获取此同名像点的地理位置坐标,根据同名像点位置与无人机位置间的几何关系,结合机载光电测量系统的坐标转换过程,实现无人机的自主定位过程。利用已知的地面同名像点的地理位置信息,反推出无人机的地理位置信息具有一定的创新性。由于整个定位过程存在实际误差,因此利用无人机飞行时记录的数据,采用蒙特卡罗法对定位误差进行仿真试验。试验结果表明该技术能够在误差允许范围内,在GPS定位系统失效的情况下完成无人机的自主定位     

Mean field variational Bayesian inference for nonparametric regression with measurement error

A fast mean field variational Bayes (MFVB) approach to nonparametric regression when the predictors are subject to classical measurement error is investigated. It is shown that the use of such technology to the measurement error setting achieves reasonable accuracy. In tandem with the methodological development, a customized Markov chain Monte Carlo method is developed to facilitate the evaluation of accuracy of the MFVB method.     

实物测量造型中的测量数据重定位方法

根据几何图形的坐标变换原理,提出了一种工于基准点的数据转换对齐方法,通过重复测量三个基准点,求出坐标旋转变换和平移变换短阵,在误差分布不均匀的情况下,可采用最小二乘法计算点的最佳重合值,最后将此方法应用于摩托车外形塑件的测量造型中,用以检验转换精度,得到的产品造型准确,配合轮廓事,达到了设计要求。     

Strain measurement of structures with curved surface by means of personal computer-based picture processing

An optical strain measurement system was developed that includes computer processing of stereoscopic picture data of marks painted on the specimen. This system allows us to measure large strains in specimens with curved surfaces as well as plane surfaces even at elevated temperatures. The results of a verification test performed on a bellows specimen at room temperature have shown that the strain error of this measurement system is allowable considering the limitation of the apparatus.     

A closed-loop error compensation method for robotic flank milling

Errors of diverse sources prevented industrial robots from being adopted into milling applications. This paper proposes a closed-loop error compensation method for robotic flank milling of complex shaped surfaces. First, the finished surface is measured in situ by a laser tracker based measuring system without unclamping the fixture. Then, the sampled points are mapped into the model reference coordinate system and a deterministic bicubic B-spline surface is fitted to extract the systematic components of the machining errors. Finally, the compensation tool path is directly generated for the mirror symmetry points of the only-systematic-error-contained sample points. The robot motion program is converted accordingly for further machining. The experiment shows that the surface accuracy is improved significantly in terms of the profile error via the proposed error compensation process, which well validates the effectiveness of the method.     

非接触式三坐标测量短圆弧方法研究

非接触式三坐标测量机是把光学、机械和计算机控制技术融为一体的高精度、高效率、功能强大的测量设备,对于加工中心、数控机床加工的零件检测和制造过程中对形状复杂、公差要求严格的零件检测都特别有效,给数控机床的工装夹具和刀具位置的调整及加工程序补偿提供最有效且精确的数据。实际工作中发现,由于短圆弧工件选取测试采集点比较集中,按照传统的测试方法效果不尽如人意。为此,研究测量短圆弧方法,解决实际测试误差显得至关重要。着重对短圆弧测量误差理论数据进行分析,提出工作中切实可行的测量方法。     

Weld-seam identification and model reconstruction of remanufacturing blade based on three-dimensional vision

Remanufacturing technique has been widely used for repairing the damaged regions of aero-engine blades. As the irregularity of weld-seam shape and uneven residual height pose great challenges for the subsequent precision grinding and polishing, it is necessary to use the high efficiency and precision measurement method to obtain a true and accurate weld-seam model, especially in the boundary areas. Therefore, a measurement approach for weld-seam identification and model reconstruction of the remanufacturing blade based on self-developed binocular vision system is presented in this work. The calculation of three-dimensional reconstruction and the complexity of subsequent point cloud processing were reduced by coarse positioning firstly, and then the weld-seam was located precisely by point cloud segmentation based on the region growth algorithm and point cloud normal filtering method. On this basis, a true weld-seam model with precise boundary was obtained. The average error of the boundary extracted by proposed method is about 0.263 mm lower than that of the traditional method. In addition, the B-spline surface was fitted according to the point cloud without weld-seam feature, and a theoretical machining model was obtained by cutting of B-spline surface along the weld-seam boundary. Furthermore, the results of verification experiment indicated that the smoothness error of machined blade surface was less than 0.025 mm and the machining error was less than 0.07 mm. This method has obvious advantages in calculation efficiency and reconstruction accuracy of theoretical machining model while compared with traditional measurement methods for the remanufacturing blade, which is beneficial to improve the machining quality and efficiency of the remanufacturing parts.     

Computer-aided measurement plan for an impeller on a coordinate measurement machine with a rotating and tilting probe

This paper presents a computer-aided measurement plan (CAMP) for an impeller on a coordinate measurement machine (CMM) with a rotating/tilting probe. The blade surfaces of a machined impeller were measured exactly to secure the machining tolerance and surface finish of the impeller. CMM with a rotating and tilting probe is commonly used for this measurement. However, it is not easy to evaluate all the points on the impeller surfaces due to the time-consuming nature of the CMM measurement. In addition, many points cannot be easily accessed by the CMM probe due to the interference between the probe and the impeller blades. Furthermore, the inherent complexity of the existing teaching operation of a probe using a joystick suggests the necessity of developing a new method for the measurement planning of an impeller. Thus, this paper proposes a CAMP for effectively gauging the inspection points based on the ruled line information of the impeller blade surfaces. The proposed plan partitions the surfaces into several measurement regions based on the ruled lines of a blade surface and the approach vectors of the probe. Finally, a case example for an impeller is presented to demonstrate the effectiveness of the proposed strategy.     

Free-form surface machining error compensation applying 3D CAD machining pattern model

The article presents a methodology for compensating for systematic influences of computer numerical control (CNC) machining processes on free-form surfaces. The proposed procedure is performed off-line by introducing corrections compensating for these influences to machining programs. The effect of systematic influences of machining are deterministic deviations of surfaces. CAD models of these deviations, averaged for a number of surfaces machined under repeatable conditions, represent a machining pattern model (MPM) which serves as the basis for performing compensation. The basis for developing such models is surface deviations determined during coordinate measurements carried out along a regular grid of points. For estimating surface MPMs, a methodology is proposed in which regression analysis, spatial statistics methods, an iterative procedure, and NURBS modeling are applied. An MPM with the opposite sign was used for compensating systematic influences of the ball-end milling process by modifying the nominal geometry data and correcting the machining program. The results of machining error compensation carried out on the basis of a previously developed MPM were compared to the results of compensation performed on the basis of raw measurement data as well as to the results these after compensation on the basis of a model of deterministic deviations for the surface under study.     

Tolerance Optimization for Mechanisms with Lubricated Joints

This paper addresses an analytical approach to tolerance optimization for planar mechanisms with lubricated joints based on mechanical error analysis. The mobility method is applied to consider the lubrication effects at joints and planar mechanisms are stochastically defined by using the clearance vector model for mechanical error analysis. The uncertainties considered in the analysis are tolerances on link lengths and radial clearances and these are selected as design variables. To show the validity of the proposed method for mechanical error analysis, it is applied to two examples, and the results obtained are compared with those of Monte Carlo simulations. Based on the mechanical error analysis, tolerance optimizations are applied to the examples.     

Point set augmentation through fitting for enhanced ICP registration of point clouds in multisensor coordinate metrology

In multisensor coordinate metrology scenarios involving the fusion of homogenous data, specifically 3D point clouds like those originated by CMMs and structured light scanners, the problem of registration, i.e. the proper localization of the clouds in the same coordinate system, is of central importance. For fine registration, known variants of the Iterative Closest Point (ICP) algorithm are commonly adopted; however, no attempt seems to be done to tweak such algorithms to better suit the distinctive multisensor nature of the data. This work investigates an original approach that targets issues which are specific to multisensor coordinate metrology scenarios, such as coexistence of point sets with different densities, different spatial arrangements (e.g. sparse CMM points vs. gridded sets from light scanners), and different noise levels associated to the point sets depending on the metrological performances of the sensors involved. The proposed approach is based on combining known ICP variants with novel point set augmentation techniques, where new points are added to existing sets with the purpose of improving registration performance and robustness to measurement error. In particular, augmentation techniques based on advanced fitting solutions promote a paradigm shift for registration, which is not seen as a geometric problem consisting in moving point sets as close as possible to each other, but as a problem where it is not the original points, but the underlying geometries that must be brought together. In this work, promising combinations of ICP and point augmentation techniques are investigated through the application to virtual scenarios involving synthetic geometries and simulated measurements. Guidelines for approaching registration problems in industrial scenarios involving multisensor data fusion are also provided.     

基于拟蒙特卡罗方法的三维医学重建模型体积测量方法研究

测量医学图像三维重建病灶组织与器官的体积, 为临床诊疗与医学研究提供更可靠的数据。通过先对系列二维医学图像进行预处理, 并对处理后的图像进行三维重建, 可以获得较好的只有表面三角网格的三维重建模型; 然后使用拟蒙特卡罗方法在构造的包围盒内生成低差异分布的随机点, 通过计算模型内的点数量与全部随机点数量的比例进行体积测量。分别对四组重建的三维模型进行体积测量并与蒙特卡罗方法相比, 拟蒙特卡罗方法在测量体积方面具有较好的效果。拟蒙特卡罗方法对三维医学重建模型体积测量可以得到较准确的体积数据, 具有一定的实际应用与理论研究价值。     

Decision theoretic approach to real-time robust identification

A decision-theoretic approach to the estimation of unknown parameters from a linear discrete-time dynamic measurement model in the presence of disturbance uncertainty is considered. The unknown disturbance statistics are characterized by a certain class of distributions to which the real disturbance distribution is confined. Using game theory and the asymptotic estimation error covariance matrix as the criteria of how good an estimator is, the stochastic gradient-type algorithm is shown to be optimal in the min-max sense. Since the optimal solution is not tractable in practice, several suboptimal procedures are derived on the basis of suitable approximations. The convergence of the derived algorithms is established theoretically using the ordinary differential equation approach. Monte Carlo simulation results are presented for the quantitative performance evaluation of the algorithms.     

基于Matlab的复杂曲面形状误差评定

提出了一种满足最小区域法的曲面形状误差评定方法。利用NURBS曲面插值构造出理论轮廓曲面的数学模型,根据曲面形状误差的定义建立了误差评定的数学模型,采用分割逼近法快速求取测点到理论曲面的最小距离,阐述了曲面形状误差评定的步骤。以汽轮机叶片曲面形状误差评定为例,证实了该评定方法的优越性。