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Algorithms for causal discovery have recently undergone rapid advances and increasingly draw on flexible nonparametric methods to process complex data. With these advances comes a need for adequate empirical validation of the causal relationships learned by different algorithms. However, for most real and complex data sources true causal relations remain unknown. This issue is further compounded by privacy concerns surrounding the release of suitable high-quality data. To tackle these challenges, we introduce causalAssembly, a semisynthetic data generator designed to facilitate the benchmarking of causal discovery methods. The tool is built using a complex real-world dataset comprised of measurements collected along an assembly line in a manufacturing setting. For these measurements, we establish a partial set of ground truth causal relationships through a detailed study of the physics underlying the processes carried out in the assembly line. The partial ground truth is sufficiently informative to allow for estimation of a full causal graph by mere nonparametric regression. To overcome potential confounding and privacy concerns, we use distributional random forests to estimate and represent conditional distributions implied by the ground truth causal graph. These conditionals are combined into a joint distribution that strictly adheres to a causal model over the observed variables. Sampling from this distribution, causalAssembly generates data that are guaranteed to be Markovian with respect to the ground truth. Using our tool, we showcase how to benchmark several well-known causal discovery algorithms.
The spectrometer-free chromatic confocal measurement technique enables 3D surface measurements with just one exposure and without scanning. To reduce the need for a spectrometer for the spectral analysis of the reflected light composition and thus the extraction of the local surface height, an optical spectral analysis unit is used. This unit determines the first momentum of the spectral composition reflected from the surface under probe for a large number of lateral measurement points simultaneously. This work investigates the impact of the spectral composition and light power of the light source on the sensitivity and accuracy of this method. A thorough optimization of the light source will be conducted, demonstrating the impact of various spectral compositions and light source power on the system performance, taking into account the system-related etendue. In addition, the optimization of the spectral transmission filter used in the optical spectral analysis unit and its influence on the accuracy and sensitivity of the system over the entire measurement range is shown.
The spectrometer free areal chromatic confocal metrology (ChromaCAM) is an optical 3D surface measurement technology, which allows a simultaneous measurement of a large array of measuring points within a single exposure. In this work, we investigate the accuracy of a first prototype sensor system utilizing this new singleshot 3D measurement technique. It is found that surface height measurement errors smaller 1μm within a total measurement range of about 1000 μm are achievable. Furthermore, several influential factors are investigated showing the advantages and limits of the presented system. Investigating different surface materials it is found that frame rates up to approximately 800 fps for highly reflecting surfaces and up to 30 fps for ceramics, aluminum, and plastics are achievable.
The dissertation presents two approaches for novel and improved measurement techniques, in the area of optical confocal surface metrology.
Firstly, a highly miniaturized endoscopic point distance sensor based on a spatial confocal measurement principle is presented. The sensor utilizes a new technique called spatial confocal point distance measurement. A special feature of the proposed sensor design is the high degree of miniaturization through femto-second direct laser writing and the use of optical fiber bundles, which enable an endoscopic application.
The second part of the dissertation will present a novel method for chromatic confocal metrology, that enables high-speed and high-resolution one-shot areal surface metrology without the need for a spectrometer. After deriving the theoretical basis of the approach, an optical design is conducted and verified using a preliminary optical simulation, After the design of a suitable housing and implementation of a calibration and processing algorithm, the experimental validation and the acquired results will be presented.
In summary, this work adds two novel measurement techniques to the field of fast in-line optical confocal metrology.
In der vorliegenden Arbeit werden Methoden zur datengetriebenen Qualitäts- und Verschleißbestimmung beim Scherschneiden entwickelt. Zunächst wird ein optisches Überwachungssystem in einen Scherschneidprozess integriert und eine Inline-Überwachung in Bezug auf die Schnittflächenkenngrößen ermöglicht. Für die automatische Qualitätsbestimmung wird am Beispiel der Glattschnitthöhe ein Algorithmus zur automatisierten Bildverarbeitung gezeigt. Die neuen Möglichkeiten, die durch diese Entwicklung entstehen, werden anhand einer Fallstudie analysiert. Um die Schwächen des entwickelten Algorithmus mit Blick auf die Verarbeitungszeit auszugleichen, werden ebenfalls neuronale Netzwerke zur Bildverarbeitung untersucht. Zunächst wird hierzu eine Datenmenge aus dem Fertigungsprozess aufgezeichnet und ein manuelles Labelling sowie eine künstliche Datenerweiterung durchgeführt. Beruhend auf Normen und Expertenwissen, wird eine Methode zur Bewertung der Prognosegüte entwickelt. Diese beinhaltet eine Bewertung anhand von verschiedenen Koeffizienten, die sowohl konturbasierte als auch regionenbasierte Ansätze kombinieren. Durch einen Vergleich unterschiedlicher Netzwerkarchitekturen zur Bildsegmentierung und deren Optimierung wird eine Netzwerkarchitektur ermittelt, die die Qualitätsbewertung im Hinblick auf die Glattschnitthöhe innerhalb des Fertigungsprozesses ermöglicht.
Abschließend wird ein Sensornetzwerk bestehend aus unterschiedlichen Sensoren und dem entwickelten Überwachungssystem an einem Scherschneidprozess installiert. In einer Versuchsreihe werden Daten der unterschiedlichen Datenquellen über die Lebensdauer eines Schneidstempels aufgezeichnet. Zusätzlich wird zu definierten Intervallen der vorliegende Schneidstempelverschleiß ermittelt. Die Daten werden in Verschleißklassen eingeteilt und durch unterschiedliche Datenvorverarbeitung entstehen mehrere Merkmalsräume. Diese unterscheiden sich zum einen durch die Kombination der Datenquellen als auch durch die angewendete Datenreduzierung. Die unterschiedlichen Merkmalsräume dienen zum Training von sowohl klassischen Algorithmen des maschinellen Lernens als auch neuronalen Netzwerken. Eine Bewertung und Analyse erfolgt anhand der jeweiligen Konfusionsmatrix und mittels SHAP-Werten. Es zeigte sich, dass durch eine Vorhersage der Verschleißklassen beruhend auf den Bildaufnahmen der Glattschnittfläche nur minimal schlechtere Ergebnisse erzielt werden als durch ein Sensornetzwerk beruhend auf einer Kombination aller Datenquellen.
Punching is a process that is sensitive to a multitude of parameters. The estimation of part and punch quality is often based on expert knowledge and trial-and-error methods, mostly carried out as a separate offline process analysis. In a previous study, we developed an optical inline monitoring system with subsequent image processing which showed promising results in terms of capturing every manufactured part, but was limited by slow image processing. Here, we present a more efficient image processing technique based on neural networks. For our approach, we manually identify the burnish parts in images based on criteria established via an expert survey in order to generate a training dataset. We then employ a combination of region-based and boundary-based losses to optimize the neural network towards a segmentation of the burnish surface which allows for an accurate measurement of the burnish height. The hyperparameter optimization is based on custom evaluation metrics that reflect the requirements of the burnish surface identification problem as well. After comparing different neural network architectures, we focus on optimizing the backbone of the UNet++ structure for our task. The promising results demonstrate that neural networks are indeed capable of an inline segmentation that can be used for measuring the burnish surface of punching parts.
Chromatic confocal metrology suffers from a limitation in the number of measurement points that can be measured simultaneously in a single frame acquisition. We propose chromatic confocal areal metrology (ChromaCAM), in which the surface height for each point in a 2D grid of measurement spots, generated by a rectangular micro-lens array, is parallely analyzed through the utilization of a pinhole multiplexer unit, an analog optical analysis unit, and postprocessing algorithms. An experiment shows the viability of the simultaneous acquisition of multiple measurement points and the advantages over exisiting areal chromatic confocal approaches. Compared with conventional chromatic confocal metrology, the increase in the acquisition rate is significant and enables one-shot measurements.