三坐标测量机CAD尺寸检测新算法概要Word格式文档下载.docx

1、2000Springer-Verlag London LimitedA New Algorithm for CAD-Directed CMM Dimensional InspectionY.-J. Lin and P. Murugappan*Department of Mechanical Engineering, The University of Akron, Ohio, USAThe recent developments in computer integrated manufacturing （CIMsystems have made the traditional dimensio

2、nal inspec-tions bottlenecks in the production line. To overcome these bottlenecks, computer integrated dimensional inspection was proposed with the coordinate measuring machine （CMMbeing the key device. In this investigation, a framework for integrat-ing the CMM into the computer-aided design/compu

3、ter-aided manufacturing （CAD/CAMenvironment is developed to auto-mate the process of design, manufacturing and inspection. An algorithm to generate an optimum collision-free CMM probe path is proposed. This algorithm uses the ray tracing technique to locate the collision of the possible paths with t

4、he workpiece to be inspected, between the initial probe point and the target point. If there is a collision, the algorithm walks through the topological structure of the part and selects the midpoint of the edge shared by the face with which the path collides and the adjacent face nearest to the tar

5、get point, as the next probe point. This procedure is followed until the target point is reached. The rst half of the proposed algorithm is implemented using Mechanical Desktop as the CAD system and AutoCAD Runtime Extension （ARXas the application programming interface, running on a Windows NT 4.0pl

6、at-form. The effectiveness of the proposed algorithm is veried by the results of the implementation which demonstrate optimum collision-free dimensional inspection path generation for three prismatic parts.Keywords:Computer integrated manufacturing （CIM;Co-ordinate measuring machine （CMM;Dimensional

7、 inspection 1. IntroductionA coordinate measuring machine （CMMis the key device used in computer-integrated dimensional inspection 1.The essential system components of a CMM are, the mechanical *Present address :The Structural Dynamics Research Corporation, Cincinnati, Ohio, USA.Correspondence and o

8、ffprint requests to :Dr Y.-J. Lin, Department of Mechanical Engineering, The University of Akron, Akron, OH 44325 3903, USA. E-mail:YL uakron.edu arrangement with the three axes and the displacement trans-ducers, a probe head with a touch trigger or analogue probe to probe the workpieces in all spat

9、ial directions, a control unit and a computer with peripheral equipment and software to calculate and display the results 2.Although CMM has a considerable level of automation compared with traditional methods, CMMs still have to be manually operated to carry out the rst part inspection, in the case

10、 of on-line inspection planning. The CAD software then generates the DMIS （dimensionalmeasurement interface specication program. DMIS provides a neutral interchange format between CAD systems and the dimensional measuring equipment. The program is then replayed for inspection of the remaining parts,

11、 in a batch production environment. In the case of off-line inspection planning, the whole procedure is simulated by the CAD software. In order to achieve a higher level of automation, CAD-directed inspection planning is essential.In CMM inspection path planning, optimisation of the inspection path

12、plays a major role in increasing the throughput of the CMM. The path planning for a CMM is similar to that of a Cartesian robot path planning, the goal being to generate an optimum collision-free path. In the inspection process, the CMM probe has to move in the workspace to touch different areas of

13、the part for measuring purposes. Since the probe works in the proximity of the part, the geometry of the part and xtures create obstacles that interfere with the movement of the probe. Hence, the generation of an efcient and optimum collision free CMM inspection path forms a major problem in the ins

14、pection planning.A new algorithm is proposed to generate an efcient CMM inspection path by accessing the CAD database for the part geometry and traversing through solid representations in the CAD system for collision detection and path generation. The objects considered are simple prismatic solids.

15、To lay the ground for the presentation of this new algorithm, past related work on path planning is briey reviewed and the overall framework and the goal of the proposed algorithm are described in Section 2. The proposed algorithm and its verication are discussed in Section 3. The results are shown

16、in Section 4. Finally the conclusions and recommendations for future study are given in Section 6.108Y.-J. Lin and P. Murugappan2. Review of Related WorkCMM inspection path planning has received wide attention in recent years. There has been research using CAD databases for CMM inspection path gener

17、ation. A computer-integrated environment for automated dimensional inspection was pro-posed 1which plans the inspection path by searching for functional features from the CAD database using the speci-cation module, and generates the path using the planning module. The path generated is checked for c

18、ollisions using the verication module and is executed using the execution module. The measured data is then compared with the specied dimen-sions and tolerances using the analysis module.The specication module accepts various inspection attributes from users to generate the inspection specication wh

19、ich is used by the inspection planning module to search for the features and divide them into single features and related fea-tures. Based on the geometry of each individual feature and its relationship with the inspection specication, the number of inspection points, point locations, probing vector

20、s, backoff distance, the probing sequence is determined. These compo-nents are checked for interference with part surfaces and a collision-free inspection path is generated. In this approach much effort is related to searching the CAD database using the specication given, rather than using the part

21、geometry or the solid representation of the features and generating a colli-sion-free path.Graphic representation methods can be classied into sweep representation methods, conguration space methods and space subdivision methods. Sweep representation denes a new object by sweeping an object along a

22、trajectory through space. Then the overlap or intersection between the swept volumes and the obstacles is determined, and a new path is proposed. Since simple surfaces are used to model the object accurately, it is difcult to determine whether two such models overlap. More-over, the proposed path pr

23、ovides only local information about the potential collision, for example, the shape of the intersec-tions of the volumes, or the identity of the obstacle giving rise to the collision. This information suggests only local path changes, but not a radically different path if that is required 3.This res

24、ults in an expensive search of the space of possible paths.In a conguration space method 46the objects considered are polyhedrons. Conguration space characterises the position and orientation of an object. This is formed by shrinking the object to a point while expanding the obstacles to account for

25、 the object dimensions. The remaining free space is left to translate the point object between the starting and nal pos-itions. This method uses the vertices of all the transformed obstacle polyhedrons to form every possible visible path. A vertex graph is then used to search for the shortest vertex

26、 path. The computational time increases exponentially as the number of obstacles increases 7.In a CMM, the part is xtured to its base, so this leads to a long search time as the xtures become obstacles.The space subdivision method can be divided into two approaches, namely, spatial enumeration 8and

27、octree database 9,10methods. Many algorithms have been developed based on an octree data structure. Octree is a recursive method of representing 3D objects in a hierarchical eight-array tree struc-ture. The moving object and the obstacles are modelled by sweeping volumes in octree methods, which mak

28、es collision detection difcult. A common problem in these methods is the zigzag nature of the path which involves acceleration and deceleration of the probe. This, in turn, leads to low path efciency due to vibration induced measurement errors and longer travelling time.Lu et al. 11proposed an algor

29、ithm which uses a modied 3D octree ray tracing technique to search for the colliding obstacles in an octree database on a selected path. It also uses the global information about the obstacle vertices to reduce the zigzag nature of the path generated by the octree based methods. This approach repres

30、ents the part using an octree data structure, which is not widely used in CAD solid model-ling, since it approximates the geometry of the part with cells. The commonly used solid representations are B-reps （boundary representations and CSG （constructivesolid geometry. More-over, since the CAD model

31、contains complete information about the part dimensions, tolerances, geometry, orientation, etc., this information can be used directly for inspection path planning. Therefore, it avoids the process of representing the workspace and object in either special functions or a graphic database, which is

32、time consuming.This research develops a framework for CAD directed inspection planning and implements the basic structure of the framework 12,13.3. Proposed FrameworkThe main objectives of the proposed framework are:1. To integrate the CMM into the CAD/CAMenvironment, so that the part designed and manufactured can also be inspected in a single cycle.2. To incorporate feature based inspection planning at the design stage.3. To generate an efcient and optimum collision-free CMM inspection path.4. To generate a DMIS （dimensionalm