On-machine, optical imaging of the layer is bound to play an important role in the development of future in-process monitoring solutions for fused filament fabrication. In addition to detecting layer internal defects, layer imaging offers the possibility to retrieve the outer contours of a layer and thus three-dimensionally reconstruct the side walls of a part by vertically stacking contours belonging to sequential layers. The current open challenges for layer imaging relate to the compromise between imaging resolution and extension of covered area, and to the complexity of image interpretation, exacerbated by optically difficult materials, complex filament patterns and non-uniform environmental conditions. In this work we propose an original optical imaging solution for layer contour identification and stacking. Our solution overcomes the current challenges related to range/resolution and image interpretation by making use of digital twins, i.e. geometric simulations of the layer, based on the current manufacturing process parameters. Thanks to the use of digital twins, our vision system is able to: use high-resolution imaging only where it is needed, by following the predicted positions of the layer contours while acquiring a sequence of images; algorithmically interpret each image by concentrating only on interesting regions as suggested by the simulated prediction, thus reducing the risk for false positives; and finally monitor contour quality by performing a more accurate comparison between the measurement and a reference contour obtained by a simulation which replicates the actual fingerprint of the process, as opposed to simply relying on the edge obtained by slicing the CAD model. The in-process monitoring capability of the computer vision system is demonstrated through the fabrication of acceptable and defected instances of a test geometry. Fabrication issues are effectively and efficiently detected by the system and alarms are triggered that can be used to command early termination of the build process or to initiate corrective actions.
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