Weak Characteristic Grid Node Inversion Method In Metal Sheet Forming Strain Detection

Metal sheet forming is one of the important branches in the field of material processing.It is widely used in automotive,aerospace,home appliances and other fields.In the process of metal sheet forming,it is necessary to detect the strain distribution on the surface of the sheet to analyze its forming performance.The coordinate grid analysis method is the main way for the performance analysis of metal sheets forming.It measures the geometrical changes of the grid pattern prepared on the surface of the sheet before and after forming,calculates the strain distribution on the surface of the metal sheet,and monitors critical deformation parts in the sheet metal forming process,optimized stamping process,etc.Due to the friction between the metal sheet and the mold during the forming process,some grid nodes prepared on the surface of the sheet are worn out,which makes the imaging on the image incomplete or fuzzy,thereby reducing the integrity of the strain detection.To this end,this paper carried out research work on incomplete or blurred imaging of weak feature grid node coordinate inversion technology,and the results achieved are as follows:(1)Research and implement a method of node recognition of lattice grid on the surface of metal sheet.First,filter and binary the acquired lattice image;second,detect the entire pixel edge of the lattice node,and use the spatial moment method to extract the subpixel edge of the lattice node;finally,use least squares ellipse fitting method obtains the two-dimensional coordinates of the lattice grid nodes;(2)A quadrilateral mesh reconstruction technology is studied to realize the construction of lattice topological relations.First,construct a seed quadrilateral grid of better quality in the lattice grid image;second,use the four sides of the seed grid as the seed edges to perform grid expansion to generate up to four new grids;finally,the new grid is used as a seed grid,repeat the previous steps to expand the grid,and establish a lattice topology relationship;(3)A coordinate inversion method for the weak feature grid nodes on the surface of metal sheet is proposed.Firstly,according to the topology relationship of the quadrilateral grid among the lattice grid nodes,the grid nodes in the row direction and column direction at the weak feature grid nodes are searched out;secondly,the B-spline curve interpolation method is used for the grid nodes of two directions respectively;Finally,the intersection of the two interpolation curves is solved and used as the coordinates of the weak feature grid nodes.Finally,on the Visual Studio 2015 platform,combined with the open graphics library named Open CV,the algorithm proposed in this article was implemented using C ++ language programming,and verified with the measured data.The experimental results show that the coordinate error of the weak feature grid nodes retrieved by the method in this paper is within 0.5 pixels,and the effect is good.