Research On Grinding/Polishing Robotic Vision Measurement Technology For Weld Beads Of Large Structural Parts

With the great development of industry and manufacture business，the demand forlarge-scale parts used in high speed train，steamer body and aircraft body is increasing. As animportant technical process in manufacturing field, welding has been widely employed in theflat and curved surfaces moulding and connecting of large-scale structures. Unnecessarymetal materials should be removed after weld. Grinding and polishing unnecessary materialscan not only meet the demand for smooth joint part, but also reduce the welding stress. Theprocess of removing is critical.Currently, the process is still manually performed. Skilled workers grand and polishlarge-scale structures by abrasive cloth flap wheel. The whole process needs high labourintensity, but comes inefficiency, no guarantee of working accuracy. The damage of basemetal happens a lot during grinding and polishing, more importantly, prolonged contact withdust during the process can harm workers’ health. In some cases, workers need to operatewithin narrow space or at high altitude, where the environment could be execrable.Automatic grinding and polishing of large-scale structures is in pressing need. Automaticgrinding and polishing by machine tools needs the size of large-scale structure smaller thanmachine tool. The machine tools of required size are usually difficult to assemble or process,with low flexibility, but expensive. Based on difficulties discussed, a new thought ofgrinding and polishing large-scale structures by micro-robot is proposed in the paper. It isonly the geometry and position information of three-dimensional space at actual time can beobtained that the parameters to be used will be programmed to test grinding and polishingand machine allowance. Focusing on critical technical problems discussed above, the papershows the research works including vision system of grinding and polishing robot, parameterprogramming, mathematical modeling of vision system, weld bead image processing, weldbead feature extracting, featured point sub-pixel coordinates extracting，and grindingallowance detection.According to specific structural characters and working conditions of large-scale structuregrinding and polishing, taking requirements of robotic vision system into consideration,vision system of grinding and polishing robot is designed by combining binocular stereovision system, laser-assisted source and P4P robot featured points. The change of pictureshape projected by laser structure light indicates change of3D geometric information ofweld bead space. Sub-pixel coordinates are obtained by image processing, and the featuredpoints’ coordinates of weld bead are extracted. Based on binocular stereo visionsystem, parallax computation is performed in corresponding feature points of the sameimage pair so that the three-dimensional space geometric information is obtained. Thecomputation avoids massive calculation for image match, and saves lots of resource and time.Since four LEDs on the same plane are as feature point，we can obtain correct coordinateinformation of weld featured points and azimuth information of weld bead.After detailed analysis of plan for grinding and polishing parameters based on weld bead information（geometrical information&position information）in vision system, grindingmould for weld bead is built. According to quantitative relationship between grinding forceremoved material in weld bead, a strategy of grinding and polishing control is proposed inthe paper, and the workflow of grinding and polishing robotic vision system is determined.Based on the obtained data, the type of hardware is confirmed and relative calculationperformed, resolving power of vision system at Z direction preliminary calculated andtestified. Mathematical model of polishing and grinding robotic vision system is developedand performed solution. Basing on the traits of different calibration methods, calibrateintrinsic parameter of camera and calibration method of camera is determined.Detailed analysis of noise source and noise characteristic in weld bead structure lightimage is performed, furthermore, noise is removed by adaptive filtering method, andadjusted image is obtained by gray level interpolation，Edge shaping，contrast stretch andbinary image processing are performed in weld image. Search pixel of structure light edge,then the pixel coordinates of structure light bar center is found. In order to improve theaccuracy and speed of weld bead processing, aiming at nearly straight weld bead, a methodof location algorithm for ROI (region of interest) is proposed in the paper. By this method,the size of processing image area is within2%of the original area, the amount of calculationreduced, while the speed improving. Column differential Gauss algorithm specifically forROI is proposed: a new way of extracting coordinates of sub pixel in the center of structurelight without pre-processing. For the weld bead of general shape, a dynamic ROI containingweld bead feature points can be accurately and quickly obtained. In the foundation of the factthat luminance of structure light spreads approximately Gaussian distribution, the paper putsforward Gaussian model and parabolic model for luminance distribution. Sub-pixelcoordinates are extracted by partial differential detection algorithm (on basis of HESSIANmatrix) and by length threshold removal algorithm. Moreover, the excessive branch line isremoved. Basing on extraction of sub pixel coordinates of weld bead structure light bar,analysis algorithm of threshold value of slope and distance is proposed. According to theproposed method, the information of turning point and the highest point of weld bead arecorrectly extracted. The area of section where laser projection bar locates is calculated by themethod of numerical integration.The mathematic relation between robot and camera coordinates proposed in the paper iscalculated by4LEDs of robot body on the same plane. Considering the approximate ellipseshape LED appeared in the image, an extraction algorithm for ellipse sub-pixel center isproposed which applying adaptive threshold segmentation and gray weighted interpolation.According to the image processing tests, the precision of extraction by force algorithm isverified by comparison. Collected data includes all relative parameters necessary for visionsystem. Error compensation is presented on the background of analysis of vision systemmeasurement error and of positioning error.On the basis of theoretical analysis, the paper discusses the method used in vision systemmeasurement and positioning, also makes comparison for image processing precision of Search fitting algorithm，differential algorithm，Steger algorithm based on HESSIAN. Underdifferent algorithms, the spatial geometrical information of the same section in weld bead ismeasured. The result indicates that Steger light center extraction algorithm combiningSlope-distance threshold analysis can yield to good measurement precision of within0.09mm. Weld bead allowance detection algorithm is verified, and the method with highprecision and efficiency. In case of Steger algorithm, tests are performed for precision ofvision system repeatability. The tests yield to precision of repeatability within0.04mm.Grinding and polishing on robot test indicates the reliability and validity of robotic visionsystem. Grinding and polishing programming on basis of the weld bead information and thebuilt grinding and polishing mould leads to the same conclusion.The paper research work indicates that the thought of weld bead grinding and polishingstructure part by micro mobile robot is reasonable, which could improves the precision andprocessing quality of grinding and polishing. The grinding and polishing robot vision systemis with stability and reliability. The image processing methods have high robustness,precision and speed including structure light center detection algorithm, featured point centersub-pixel extraction algorithm and weld bead featured point extraction algorithm. Thesystem meets requirement for real time detection and positioning in weld bead. The researchwork discussed in the paper provides a new technical protocol and resolution for large-scalestructure piece robotic grinding and polishing.