Key Techniques And Systems Of Sheet Metal Formability Analysis Based On Machine Vision

With high speed development in manufacturing, the traditional manual strain measurement methods which used mylar tape and tool microscope have gradually exposed some disadvantages such as low efficiency and poor accuracy in most sheet metal stamping situations. In order to overcome various limitations of the manual strain measurement methods and meet the requirements of strain measurement in the filed of sheet metal stamping, theory and application researches on non-contact steel strain measurement based on computer vision are deeply and systematically carried out in this thesis. The key techniques include image analysis, camera calibration, displacement tracking under large deformation, image matching, 3D reconstruction, strain calculation, software & hardware im-plementation, etc. The thesis's main contributions are as follows.1. Key techniques of dynamic strain measurement based on binocular stereo vision have been studied. Binocular calibration and 3D reconstruction in stereo vision theory have been comprehensively discussed. In order to improve efficiency and stability in stereo image matching and tempo-ral-sequential matching, a new technique with Region Growth Initial Matching (RGIM) is proposed. Detailed results and analysis are figured out which demonstrate the effect of different interpolation method, shape function and size of subset on 3D reconstruction accuracy. The premise of DIC(Digital Image Correlation) is that these is no significant difference between two images, aim-ing at this limitation of DIC, a new algorithm call Fractionized Displacement Transfer (FDT) is proposed which can effectively and robustly implement image matching and tracking under large deformation. Experiment demonstrates that even local deformation has reached 170%, the sub-pixel matching still maintains accurate and stable.2. A detail comparative study is carried out on three representative iterative-optimization-based image registration methods, namely DIC using Newton-Raphson iteration, iterative gray-gradient algo-rithm and least square image matching, which are originated and developed in the fields of optical experimental mechanics and stereo vision respectively. The mathematical consistency of the three algorithms is demonstrated. This conclusion not only gives advanced understanding of these meth-ods, but also provides potent theoretical support and technical instruction when the research achievements among these different fields are used for reference and transferred for each other.3. Key techniques of large-scale complex sheet metal forming analysis based on digital camera free-dom taken have been studied. Accuracy 3D reconstruction under multi-view constraint in multiple view geometry theory is proposed. Camera poses calibration is carried out with the combination of coded targets identification and bundle adjustment. On the basis of improvement on the present grid image pre-processing, a coarse-to-fine algorithm is carried out so that the sub-pixel image coordi-nates of grid nodes are extracted and the topological relationship of each node is established. Aim-ing at particularity of grid matching, an automatic matching algorithm for establishing correspon-dence among the grid nodes in multiply images is proposed according to the topological relation and epipolar constraint on the basis of knowing camera position and orientation of every image shot. Experimental results show that the average epipolar distance of all matching points is approxi-mately 0.17 pixel, and the matching method is stable and reliable. Finally, 3D coordinates of grid nods are constructed with the combination of multiple view geometry theory and the proposed grid nodes matching algorithm.4. Determination of sheet mental forming parameters is researched and implemented. A strain calcula-tion with combination of deformation gradient tensor and Green deformation tensor is proposed, and grid node strain is introduced in order to decrease effect of noise. Aiming at FLC determination with dynamic strain measurement, some data-processing skills are proposed,such as elimination of pseudo limit strain according to continuous strain distribution before fracture and selection of criti-cal deformation stage before fracture according to strain gradient change rate. A new technique to determine plastic strain ratio (r value) of sheet metal is proposed utilizing the characteristics of dy-namic strain measurement. With major and minor strain, r value is calculated with conversion of strain in length and width direction be performed by using the property of approximately equality of local strains in the same time of deformation during uniform plastic deformation.5. Dynamic strain measurement system, namely BOSAS (binocular strain analysis measurement sys-tem) based on the binocular stereo vision and strain analysis system, namely MOSAS (monocular strain analysis system) based on multiple view geometry theory are designed and implemented re-spectively. BOSAS has been used for the determination of Forming Limit Curves (FLC) and plastic strain ratio (r value) of aviation aluminum sheet and favorable results have been obtained. MOSAS has been used for the sheet formability analysis of outer plate of the vehicle door, and reliable con-clusion has been achieved.