Domain-specific Gaussian process-based intelligent sampling for inspection planning of complex surfaces

Precision measurement of complex surfaces requires intensive sampling for fully characterising the surface geometry and reducing the measurement uncertainty, which is, however, less efficient when the data are costly to acquire. This paper presents a Gaussian process (GP)-based intelligent sampling method for achieving well balance between the measurement efficiency and accuracy. The method makes use of GP to model the surface with domain-specific composite covariance kernel functions. The statistical nature of the GP makes it capable of giving credibility to the arbitrary prediction over the entire established model which can be used in a critical criterion to perform intelligent sampling of the surfaces. The method is independent from the coordinate frames, which makes the sampling plan easily utilised without accurate pre-positioning in actual measurement. The effectiveness of the method is verified through a series of comparison study and actual application in measuring a multi-scaled complex mould insert on coordinate measuring machine.     

Designing coupled free-form surfaces

The problem of designing optical systems that contain free-form surfaces is a challenging one, even in the case of designing a single surface. Here we present a method for the coupled design of two free-form reflective surfaces that will have a prescribed distortion. On one hand, the method can be described using traditional vectors and matrices, which we do, but it is motivated by viewing the problem in the language of distributions from differential geometry and makes use of the exterior differential systems, which we relegate to an appendix. Example applications are given to the design of a mirror pair that increases the field of view of an observer, a similar mirror pair that also rotates the observer's view, and a pair of mirrors that give the observer a traditional panoramic strip view of the scene.     

Automated print inspection for cylindrical surfaces

The objective of this research was to determine the criteria for automated inspection of a string of alphanumeric characters printed directly to the surfaces of cylindrical containers. The technical problems encountered, because a curved surface was used instead of a flat surface, were container positioning for locating surface features, character position compensation due to curvature, resolution limits defined for sensor and reflected illumination. Automated print inspection requires the use of machine vision to perform the registration of characters, template comparison for correct text being produced and the interrogation of the individual characters for correctness, position, size and type.     

Automatic quadrilateral/triangular free-form mesh generation for planar regions

Automation of finite element mesh generation holds great benefits for mechanical product development and analysis. In addition to freeing engineers from mundane tasks, automation of mesh generation reduces product cycle design and eliminates human-related errors. Most of the existing mesh generation methods are either semi-automatic or require specific topological information. A fully automatic free-form mesh generation method is described in this paper to alleviate some of these problems. The method is capable of meshing singly or multiply connected convex/concave planar regions. These regions can be viewed as crosssectional areas of 2 1/2 D objects analysed as plane stress, plane strain or axisymmetric stress problems. In addition to being fully automatic, the method produces quadrilateral or triangular elements with aspect rations near one. Moreover, it does not require any topological constraints on the regions to be meshed; i.e. it provides free-form mesh generation. The input to the method includes the region's boundary curves, the element size and the mesh grading information. The method begins by decomposing the planar region to be meshed into convex subregions. Each subregion is meshed by first generating nodes on its boundaries using the input element size. The boundary nodes are then offset to mesh the subregion. The resulting meshes are merged together to form the final mesh. The paper describes the method in detail, algorithms developed to implement it and sample numerical examples. Results on parametric studies of the method performance are also discussed.     

Concurrent process planning for finish milling and dimensional inspection of sculptured surfaces in die and mould manufacturing

With the introduction of computer-aided tools, traditional manufacturing tasks such as design, machining and inspection are now highly automated. However, due to the complexity and enormous knowledge involved in each process, most of these activities are still dealt with separately. Recent development of concurrent engineering emphasizes the importance of bringing manufacturing knowledge into the early design stage for optimum product and process design. In this paper, a knowledge-based CAD/CAM system which integrates process planning for finish milling and dimensional inspection of sculptured surfaces in die and mould manufacturing is presented. Optimum production plans are determined by minimizing the integral cost of machining and inspection. NC path generation and inspection planning are then verified by dynamic geometric simulations which provide the designer with the evaluations of machinability and inspectability. The implied significance is that strong inter-dependency may exist among various design life-cycle activities and that optimum solutions can be obtained by taking into account the interactions of the life-cycle events.     

Automatic registration for inspection of complex shapes

With the increase in computing power, new ways of inspecting manufactured parts can be realized. The availability of computing power allows computational alignment between measurements and computer aided design (CAD) models using registration algorithms. The present paper proposes a new inspection approach that removes a constraint of traditional inspection processes related to part alignment. Traditional techniques require a fixture to align the part so the inspection machine can establish a common coordinate system between the measurement and the CAD model. Removing the need of a fixture aids automating the inspection process. The proposed approach employs an automated registration methodology based on two main stages. First, a part measurement in an arbitrary coordinate system is transformed to approximately fit the shape of the CAD model. Then, this approximation is iteratively refined until its convergence. To demonstrate the feasibility of the proposed method, results are demonstrated on measurements obtained from three rapid prototyped parts with complex geometry.     

Automatic registration for inspection of complex shapes

With the increase in computing power, new ways of inspecting manufactured parts can be realized. The availability of computing power allows computational alignment between measurements and computer aided design (CAD) models using registration algorithms. The present paper proposes a new inspection approach that removes a constraint of traditional inspection processes related to part alignment. Traditional techniques require a fixture to align the part so the inspection machine can establish a common coordinate system between the measurement and the CAD model. Removing the need of a fixture aids automating the inspection process. The proposed approach employs an automated registration methodology based on two main stages. First, a part measurement in an arbitrary coordinate system is transformed to approximately fit the shape of the CAD model. Then, this approximation is iteratively refined until its convergence. To demonstrate the feasibility of the proposed method, results are demonstrated on measurements obtained from three rapid prototyped parts with complex geometry.     

Multiple deferred state sampling inspection

Multiple deferred state sampling is on attribute inspection procedure in which the decision criterion for each lot dictates one of three decisions: (1) accept the lot; (2) reject the lot; or (3) conditionally accept or reject the lot based on the disposition of future related lots. It is intended to supplement existing sampling procedures such as chain sampling, dependent stage sampling, exponential smoothed sampling, and fixed deferred state sampling.     

The effects of inspection error on single sampling inspection plans

Acceptance sampling plans are designed under the assumption of perfect inspection. However, inspection tasks are not, even under ideal inspection conditions, free of error. In this paper we consider the effects of inspection error on probability of acceptance, average outgoing quality, and average total inspection. These measures are examined under both replacement and non -replacement assumptions. Also, a method is presented whereby an acceptance sampling plan may be designed which explicitly considers inspection error.     

Combining physical shell mapping and reverse-compensation optimisation for spiral machining of free-form surfaces

Machining of free-form surfaces has an important role in industrial manufacturing, but conventional tool-path generation strategies for free-form surfaces machining have the drawbacks of serious flattening distortion and poor tool-path continuity. Therefore, a novel method is developed to generate a spiral tool path for the machining of free-form surfaces by improving surface-flattening distortion and tool-path continuity. First, physical shell mapping is presented to flatten a free-form surface into a plane, which takes stretching energy, bending energy, and global energy into account. Then, the spatial spiral polyline is rounded to generate a spiral path by proposing reverse-compensation optimisation. Therefore, the free-form surfaces can be quickly flattened with less distortion, remaining free of overlap, and can in addition be machined at high speed along a C² continuous spiral tool path. Further, the flattening error, tool-path length, mean curvature, mean scallop-height error of the spiral path, machining time and surface roughness are obviously reduced. Finally, simulation results are given to show the effectiveness and feasibility of the presented strategy.     

Double-pass oblique-incidence interferometer for the inspection of nonoptical surfaces

Interferometric testing of the flatness or straightness of ground surfaces of metals, granite, ceramics, and glasses has always proved difficult because of the extremely low specular reflection from these optically rough surfaces. At oblique angles of incidence the specular reflection coefficient of nonoptical surfaces increases rapidly, typically from near 0.01 at 75 degrees up to, say, 0.50 at 85 degrees and near 1.00 at 90 degrees angle of incidence. By taking advantage of this enhanced reflection at oblique incidence and the brightness and spatial coherence of visible gas lasers, it becomes practical to use interferometric inspection of nonoptical surfaces. A novel double-pass oblique-incidence interferometer is described which enables testing of large pieces having a specular reflection as low as 1%.     

Comprehensive design of experiments-based framework for optimal CMM inspection and uncertainty analysis of form tolerances

In the coordinate measuring machine (CMM) measurement environment, uncertainty can arise from various sources. Previous literature has focused on how to measure uncertainty in a CMM system using segregated approaches. We developed a framework to verify bias and variance tendencies deriving from form errors. Two types of form tolerances, namely flatness and circularity, are addressed here. By studying the nature of the CMM measurement uncertainty impact, this paper targets three objectives: (1) developing a design of experiments (DOE)-based measurement performance approach to integrate critical measurement strategy factors; (2) optimizing identified controllable measurement factors; and (3) synthesizing the impact of CMM uncertainty issues. Consequently, the paper fully explores the influence and interplay of these contributing factors, resulting in robust and optimal guidelines for the CMM flatness and circularity measurement strategy. Extensions of the framework for other form tolerances are discussed.     

Efficient five-axis machining of free-form surfaces with constant scallop height tool paths

Generation of efficient tool paths is essential for the cost-effective machining of parts with complex free-form surfaces. A new method to generate constant scallop height tool paths for the efficient five-axis machining of free-form surfaces using flat-end mills is presented. The tool orientations along the tool paths are optimized to maximize material removal and avoid local gouging. The distances between adjacent tool paths are further optimized according to the specified scallop height constraint to maximize machining efficiency. The constant scallop height tool paths are generated successively across the design surface from the immediate previous tool path and its corresponding scallop curve. The scallop surface, an offset surface of the three-dimensional design surface based on the specified scallop height, is used to establish accurately the scallop curve with the constant scallop height. The present method was implemented and validated through the five-axis machining of a typical free-form surface. The results showed that the conventional isoparametric tool paths were over 36% longer in the total tool path length and less efficient than the constant scallop height tool paths generated by the present method.     

A note on economic sampling inspection plans

The determination of economic sampling inspection plans is discussed. A computational procedure that allows beta prior information is outlined.     

基于3D数模的三坐标测量机曲面检测

计算机辅助设计／制造技术的发展，对曲面工件的检测提出了更高的要求，这一任务的完成通常依靠三坐标测量机。本文结合ZCRMDT三坐标测量机测量软件的研发，从数据转换、对齐、测尖补偿、理论值捕获等四个关键方面，对基于3D数模的三坐标测量机曲面检测技术进行了深入剖析。同时对三坐标测量机测量软件现状和发展趋势进行了探讨。     

Repeat inspection planning using dynamic programming

In this paper we propose a dynamic programming approach to the problem of determination of the inspection sequence of multi-characteristic critical components, and the number of repeat inspection for each characteristic. The model presented here considers the case of several classification of a product by an inspector. An inspector could classify a product as non-defective, to be reworked, or to be scrapped, with respect to a certain characteristic. The model accounts as well for possible misclassification by the inspector. The dynamic programming algorithm searches for a solution that minimizes the total cost of inspection per accepted component. The total cost includes the cost of false rejection of good items, the cost due to false acceptance of an item which is either reworkable or to be scrapped, the cost of inspection, and the cost of rework.     

CNC tool path planning for multi-patch sculptured surfaces

A new CNC tool path generation method for a multi-patch sculptured surface in the parametric plane is developed to obtain a minimum number of cutter location points while maintaining the required machining accuracy. In this study, a method to obtain intersecting points is suggested to generate the continuous tool path among different patches. In addition, a method of selecting a reference plane and a simple error analysis method are proposed to determine the step and side-step sizes. The effectiveness of the proposed method is demonstrated through simulation and experimental study.     

Prediction of toolpath redundancy for NC machining of free-form surfaces based on automatic recognition of steep-wall features

The degree of toolpath redundancy is a critical concern when looking for an appropriate toolpath strategy for free-form surface machining. Hence, quantitative analysis of toolpath redundancy is important to CAM applications. In this work, a novel approach for prediction of toolpath redundancy for free-form surface machining is proposed. Firstly, a general mathematical model to represent toolpath redundancy rate is proposed based on the analysis of local toolpath intervals and their difference from the optimal values. And then, taking the most widely used iso-planar machining as case study, the steep-wall features that bring in the variation of surface slope rates alone machining strips are identified as the main cause of the generation of toolpath redundancy, so a method to automatic recognising steep-wall features from free-form surface is developed. At last, based on the steep-wall feature segmentation, an algorithm is presented to quantitatively predict the toolpath redundancy rate for free-form surface machining. A comparison study is made between the predicted redundancy rates and the experimental results by a number of case studies. The results have validated that the proposed approach can effectively predict the redundancy rate for a surface machining case before the real toolpaths to be generated.     

Isophote-based ruled surface approximation of free-form surfaces and its application in NC machining

This paper presents a method to approximate free-form surfaces using piecewise ruled surface and its application in five-axis NC machining. New concepts of isophote, iso-inclination curve and iso-inclination angle are introduced to facilitate the generation of the piecewise ruled surfaces. The resulting ruled surfaces are adaptive to the surface features, such as peaks and valleys. Adjusting the isoinclination angle controls the error of this approximation. The application of the isophote-based ruled surface approximation in five-axis NC machining is also studied. The tapered tools are suggested to cut the ruled surfaces. Methods for selecting appropriate tools and generating tool paths are presented. The present case studies show that the new approach may lead to the integration of rough, semi-finish and finish machining of free-form surfaces.     

三坐标测量机的升级改造

三坐标测量机升级改造的各个关键环节和关键技术包括机械改造、控制系统升级改造、三维测头升级改造和测量软件升级改造等。通过这些升级改造,三坐标测量机大大提高了测量速度和准确度,并可以实现逆向制造,为单位创造良好的经济效益和社会效益。