Study On A Method For Measuring The Large Curved Surface Based On Machine Vision

Measurement techniques has play a very important role in the process of modernindustrial production and scientific research.While the traditional measuring methodoften has many deficiencies, so it is difficult to meet practical needs, therefore it isnecessary for us to find a better and new measuring method. After a comprehensivereview on the indoors and overseas current situation and developing tendency ofcoordinate measurement systems,a protable three-coordinate equipment using formeasuring large curved surface based on machine vision is presented in this subject.We regard the handheld measurement devices with identification points and a sphericalthree coordinates measuring head as a rigid body and research object. Based onmachine vision principle,we establishe the measuring mathematical model, it cancalculated position and attitude of rigid body in the process of measurement, and thenwe can work out coordinate changes of the rigid body’s spherical measuring head.After compensating the error of a series of discrete three coordinates data, we canwork out the coordinates of the point in the surface, and then the size information ofparts is get,the purpose of measurement has been achieved.The main work and creative research in this article are as follows:1.After analyzing the principle of the PNP problem. Based on the coplanar fourpoints P4P,the system’s mathematical model is established, and the analytical equationsof the measured point’s coordinates are derived.The comparative experiment has beenconducted on a Laica laser tracker and the experimental results demonstrate that modelof this system is correct.3.The article studies on a series of image pre-processing methods and controlpoints center extraction method after analysising the characteristics of themeasurement on digital image.The image was smoothed with Gauss filter reducing thenoise of images It can preserve the edge as well as reducing the noise of images.Aappropriate method of threshold value selection is chosen to extract the effective area from the images.The least square method is applied to fitting Ellipses equation, themark point’s center image plane coordinate can be accurately extract4.Because the results of measuring system is actually the probe center’scoordinates, Therefore, In order to find out the object’s suface point coordinates,Radius compensation must be used.After analysing the advantages and disadvantagesof many probe radius compensation methods, A radius compensation method based onDelaunay triangulation is this question in this thesis.5.According building experimental platform, we can prove that this measurementmethod is feasible. In this paper,the algorithm of camera calibration based on threedimensional calibration board are introduced,select the seven vertex on the gage blockas feature points, building the camera calibration model.Because calibration board is ofhigh manufacturing precision, the accuracy of this calibration method can satisfyneed.The measuring system in this paper and the Laica laser tracker are all used tomeasure surfaces,then by means of analysing and comparing the experimental resultsof these two methods. We can confirm the accuracy and stability of equipment.Finally,the feasibility of methods is feasible.