Research On Quasi-CAD Model Based CMM Automatic Measurement

With a unique measuring method, Coordinate Measuring Machine (CMM) can meet the measurement requirements of many parameters like shape error, location error etc. The development of computer digital control made by CMM auto measurement comes true. In setting the measurement path of CMM, the surface can be classified into known mathematical model ones and unknown mathematical model ones according to the features of measured surface. The former can realize CMM automatic measurement easily and is relatively mature and stable. While the latter operation is relatively difficult with a larger measurement error. This paper focuses on CMM measurement of unknown surface and proposes a new model concept—Quasi-CAD Model. Given Laser Scanning's advantage of high measuring speed and convenience, it is used for data acquisition, combination, handling and coordinate establishment to make the Quasi-CAD Model. Quasi-CAD Model is used as a link to make CMM measurement path for unknown model surfaces.Firstly, five CMM system errors including mechanical structure error, temperature error, detection probe error, software algorithms error and measurement error are analyzed; the impact of CMM system parameter settings including probe selection, probe detecting range, probe rotational angle and probe velocity on the precision of CMM is discussed. Three-point circle principle is used to decide the diameter of probe. Five ideas have been proposed for path planning of CMM, in particular, an impeller is used to illustrate the advantage of symmetry measurement based on the characteristics of symmetry and overlap.Secondly, four system errors of Laser Scanning including mechanical system error, calibration error, optical system error, system integration error are analyzed; the impact of system parameter settings on the precision is discussed. Through analysis in principle and lots of experiments, inclination angle of test plane and location of the focus of structured light are proved to be two main factors affecting the measurement error. Based on these two factors, a BP neural network model is established to realize software compensation for the measurement error. As the experiment shows, the model makes compensation for the error and improves measurement precision greatly.Based on the analysis of present measuring path planning algorithms and the character of Quasi-CAD Model, the paper proposes a kind of CMM measuring path on the basis of a vector similarity elimination algorithm of known CAD model. Using this algorithm, measurement points can be optimized and DMIS program of CMM measurement can be generated according to the relationship among curvature variation of nearby points. Then the DMIS program simulation testing platform is established according to the features of DMIS program. As the experiments show, the accuracy, efficiency and precision of measurement can be improved greatly by directly applying the DMIS program that has undergone debug of simulation testing platform on CMM measurement. On the basis of the development of blade measurement both at home and abroad, Quasi-CAD Model is used to realize CMM automatic measurement of an unknown blade mathematical model. Compared with the traditional measurement methods, the measurement efficiency and precision are improved.