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Measurement system for capturing object geometries using laser-light-section

David Auzinger

Computer controlled manufacturing as well as 3D-printing allow the realisation of complex object geometries based on digital CAD-models. Vice-Versa 3D-scanners allow the mapping of complex object geometries to CAD-models. This leads to an increased pace of product development --- see 'rapid-prototyping' --- and allows for product monitoring during the production process, also known as 'closed-loop-manufacturing'. Closed-loop-manufacturing embeds 3D-scanners directly into the production process, enabling real-time feedback and/or random sample checks.

Figure 1: Scan of an IP-telefone next to the original.

The light-section method with a line-laser allows a cheap realisation of such a 3D-scanner. The setup created for this thesis combines two approaches, a rotation of the object to be scanned in combination with a fixed line-laser and a linear movement of a second laser to allow the acquisition of complex object geometries by reducing potential shadowing. The setup consists of the aforementioned lasers, a measurement-camera, the kinematics with the necessary control electronics and a computer. The computer, responsible for the computation-intensive data analysis, is connected to the setup via ethernet and USB.
Calibration experiments were developed on the basis of a mathematical model of the setup to identify its parameters. The evaluation of measurement data is also based on this model and the previously identified parameters. The images acquired by the camera are filtered and processed in several steps to obtain a point-cloud representation of the scanned object. This point-cloud can either be used to directly extract information about the object, or it can be further processed using the freely available program CloudCompare to reconstruct the object surface. To close the loop, this surface can be used to create a 3D-printable model which can then be used to create a replica of the original object.
Furthermore, the achievable measurement uncertainty u <= 0.33mm with a 95.5% confidence interval was studied. The camera was identified as a limiting factor and is therefore a starting point for possible future improvements.
After the study of design-inherent limitations of the measurement method as well as the discussion of possible solutions, a number of sample objects are introduced to demonstrate the capabilities of the system.

Keywords: 3d-scanner, 3d-printing, Laser-light-section, line-laser


May 14th, 2018