Information Technology, Measuring and Reverse Engineering for the Optimization of the Process Chain (CRC 708 – SP C4)

The Collaborative Research Center ‘3D-Surface Engineering for Tool Systems of Sheet Metal Production’ (SFB 708) of the German Research Foundation (DFG) is a research project, in which several departments of the TU Dortmund University are included. The main aim of this project is the analysis of complex technological processes of sheet metal production by means of the simulation of single processes in the production chain.

The superordinate task of the project is to keep the production of coated forming tools as efficient as possible. Thus, process-related form errors of the workpiece must be detected and compensated by the manipulation of CAD/CAM representation at the earliest stage in the process chain. To achieve this aim, the results of the simulations can be used to modify the CAD data so that these modifications compensate the form deviations. The comparison of the as-built parts with the desired shape poses an important challenge. The information about the deformations of the workpiece, which will be obtained at this stage, can help to develop tailor-made rules for error compensation.

The procedure model for the analysis and compensation of deformations is shown in figure 1. First, the produced workpiece will be digitized and transformed into the coordinate system of the CAD model. For this transformation the best alignment between the model and the digitized data has to be found. This process is also called global registration or best fit. A standard method is to calculate a rigid transformation (rotation and translation) that minimizes the distance between two shapes. To accomplish this, a lot of best-fit algorithms like ICP (Iterative Closest Point) [1] minimize the least squares distance between the corresponding points. Therefore, first, the correspondence problem has to be solved by matching each data point to a corresponding model point. In general, such matching is calculated by nearest-neighbour search or point projection. Furthermore, the corresponding points set up a deformation field, which can be used for the description or the visualization of shape deviations.

Figure 1: System for the analysis and compensation of deformations.

Consequently, the common best-fit methods are not the only and in particular not the best solution for comparison of two shapes. In case of strong deformations, the alignment calculated by the global and rigid registration could be erroneous and may also lead to the conclusion that the whole workpiece is deformed. Thus, methods of discrete optimization and mathematical programming are used to improve the calculation of the corresponding pairs. For example the calculation of a maximum weighted bipartite matching generates an accurate deformation field for point clouds with similar point distribution. Furthermore, specific subsets of the data can be fitted exactly into the model by local best fits and the deformations between the different subsets can be analyzed more precisely. The method of multiple local best fits [2] is also used to detect the rotations inside the workpiece, as often are caused by springback. The detailed description of the deformation can help to improve the conclusions for the process of error compensation. The future research should be focused on the development of non-rigid, isometric registration methods in order to improve the accuracy of the deformation field [3].

The compensation of form error is achieved by manipulation of the CAD/CAM model according to the analyzed deformations. Therefore, a continuous deformation function (free-form deformation), which approximates the calculated deformation field, is used to manipulate the geometrical representations of forming tools, e.g., meshes. In particular, the original NC programs of the designed tools can be manipulated directly [4, 5]. In order to improve the accuracy and the efficiency of the deformation method an adaptive approach was developed, which refines and re-optimizes the deformation space with respect to approximation errors [6]. In doing so, the process of reverse engineering can be eliminated. Furthermore, the generated form-error descriptions can provide objectives for the optimization of the forming process by changing the parameters like blankholder force or punch displacement.

References

[1]
Besl, P. J.; McKay, N. D.: A Method for Registration of 3-D Shapes. IEEE Trans. Pattern Analysis and Machine Intelligence 14 (2) (1992) pp. 239-256.
[2]
Biermann, D.; Surmann, T.; Sacharow, A.; Skutella, M.; Theile, M.: Automated analysis of the form error caused by springback in metal sheet forming. In: Proceedings of the 3rd International Conference on Manufacturing Engineering, 1.-3. October 2008, Kallithea of Chalkidiki, Greece, pp. 737-746.
[3]
Sacharow A.; Balzer J.; Biermann D.; Surmann T.: Non-rigid isometric ICP: A practical registration method for the analysis and compensation of form errors in production engineering. Computer-Aided Design, 2011, 43(12), S. 1758-1768.
[4]
Biermann, D.; Sacharow, A.; Surmann, T.: Kompensation von Formfehlern durch Raumverzerrung. In: Tillmann, W. (Hg.): SFB 708 - 3. öffentliches Kolloquium. Dortmund: Praxiswissen (3D-Surface Engineering für Werkzeugsysteme der Blechformteilefertigung), (2009) S. 171–179.
[5]
Biermann, D.; Sacharow, A.; Surmann, T.; Wagner, T.: Direct free-form deformation of NC programs for surface reconstruction and form-error compensation. Production Engineering. Research and Development, 4 (2010) 5, S. 501-507.
[6]
Sacharow, A.; Surmann, T.; Biermann, D.: Adaptive Free-form Deformation for the Modification of CAD/CAM Data. In: ADVCOMP 2011, S. 27-31.

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