Analysis of a 3D optical scanner based on photogrammetry suitable for industrial applications in close and micro-range

The new paradigm of Industry 4.0 encompasses the manufacturing metrology and the necessity for fast, flexible, reliable and holistic systems arises, in order to accompany the more advanced manufacturing technologies. The more extensive use of additive manufacturing techniques, which entail the realization of complex structures and freeform geometries, as well as, the use of new materials, enhances this concept. These driving forces are the basis for the great interest towards 3D scanning systems because they perfectly fit the key factors of the new manufacturing metrology 4.0. Among other things, they have the capacity to reconstruct complete and detailed 3D models in a very short time, which makes them suitable for on-machine verification. In this context, close-range photogrammetry is recognized as a simple, versatile, and effective methodology for 3D measurements of components, even if they are characterized by a prevailing dimension respect to the others (e.g. height is much higher than length or vice versa), complex free form geometry, and under-cuts. Moreover, it is able to provide accurate measurements and 3D photorealistic (thanks to the computation of the texture) surface reconstructions in a simple and inexpensive way, as well as in very short time. Photogrammetry-based systems and, generally, optical-based techniques, are flexible and holistic systems, but their strengths are also their weaknesses, because this complexity results in more variables involved and more sources of error affecting the results. The present thesis is focused on the development and the analysis of an optical 3D scanner based on photogrammetry, suitable for measurements of complex parts in close and micro range. The analysis started from the identification of the main sources of error affecting the measuring system, with the final goal to include them in a proper uncertainty assessment. In particular, there are errors due to the measuring system itself, errors due to the object under measurement, i.e. errors due to the manufacturing process, as well as, errors due to the interaction of the specific system with the object (materials, colours and surface texture). Thus, the uncertainty evaluation of such systems is still an open issue. The first chapters are dedicated to the state of the art of currently available measuring techniques, highlighting the main advantages and drawbacks, in order to explain the importance of developing a photogrammetry-based system for industrial application. The third chapter is of fundamental importance since it describes the state of the art of the currently available standards in 3D optical scanning. They mainly refer to the VDI/VDE 2634 series, in the form of acceptance and reverification test. In addition, the standards usually used for the uncertainty assessment of the more reliable Coordinate Measuring Machine (CMMs), such as the ISO 15530-3, were described, highlighting the main criticalities and the possible adaptation for optical-based scanners. The forth chapter is entirely dedicated to photogrammetry-based systems, with a brief introduction to the state of the art when applying photogrammetry in close and micro range, a description of the measuring principle through the mathematical models behind and the main advances carried out in the development of the reconstruction software algorithms. Then, the photogrammetry-based system is presented together with the sensors and the optical equipment used throughout the thesis. Finally, a summary of the main criticalities is reported. The experimental investigations carried out during the PhD course are collected in the chapters from 5 to 11. Each chapter is dedicated to a specific measuring task, with the aim to analyse a specific aspect or a criticality of the photogrammetry- based system under exam. The fifth chapter is focused on the analysis of repeatability of the photogrammetric reconstruction software used, which has proved to be a fundamental part of the system. The study was conducted on a pyramidal artefact already used in previous experiments. In the sixth chapter, a new three-dimensional reference artefact was presented. The purpose was double: for the estimation of the external orientation, scale adjustment, and for the uncertainty assessment, calibration. The effectiveness of this reference artefact was proved through the reconstruction of the test object used in the previous chapter, the pyramidal artefact. The capacity of the presented system to reconstruct free form geometries was analysed through a preliminary test in Chapter 7. The tests were performed through the use of three artefacts produced by additive manufacturing techniques, which were a customized version of the NPL free form artefact designed and developed by NPL institute. The need for the customization was mainly due to the necessity to resize the artefact to make it measurable with the optical equipment under exam. In Chapter 8, the application of the photogrammetry-based system for measuring additive manufactured biomedical devices was reported, highlighting critical aspects due to their object and surface textures characteristics. Chapters 9 and 10 report the work conducted during the external stay period at the Department of Mechanical Engineering of the Denmark Technical University. The introduction of a step gauge reference artefact was analysed, and the photogrammetry-based system was then compared with other non-contact measuring techniques, such as structured light scanner, laser based scanner and a computed tomography scanner. The performance verification of all those 3D non-contact measuring techniques was conducted through the step gauge reference artefact. In chapter 10, the investigation was focused on the analysis of the interaction between the 3D optical scanning systems and the materials and colours of the objects under measurements. For the purpose, five miniature step gauges made of different polymeric materials and colours were scanned and analyzed. Finally, Chapter 11 is dedicated to the application of the photogrammetry to the microscopic range, for the acquisition of components realized through µEDM (Electro Discharge Machining). The optical equipment, used in this investigation, allowed to reach magnification levels higher than 2x, with optical resolutions up to 2,4 µm. The investigation was mainly focused on the verification of the reproducibility of the internal parameters estimated through the traditional mathematical models, for such magnification levels.