Browsing by Author "Hofmann, Michael"

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Open Access
Advanced variational methods for dense monocular SLAM
(2016) Hofmann, Michael
Structure from Motion (SfM) denotes one of the central problems in computer vision. It deals with the reconstruction of a static scene from an image sequence of a single moving camera. This task is typically divided into two alternating stages: tracking, which tries to identify the camera’s position and orientation with respect to a global coordinate system, and mapping, which uses this information to create a depth map from the current camera frame. There are already numerous approaches in the literature concerning local reconstruction techniques which attempt to create sparse point clouds from selected image features. However, the resulting scene information is often insufficient for many fields of application like robotics or medicine. Therefore, dense reconstruction has become more and more prominent in recent research. In 2011, Newcombe et al. presented a new technique called DTAM (Dense Tracking and Mapping), which was one of the first to create fully dense depth maps based on variational methods. Since then, most of the follow-up work concentrated on performance rather than on qualitative optimization due to DTAM’s limited real-time capability compared to sparse methods. It is therefore the objective of this thesis to improve the quality and robustness of the original DTAM algorithm and extend it to a generalized and modular mathematical framework. In particular, the influence of different constancy assumptions and regularizers will be evaluated and tested under various conditions using multiple benchmark data sets.
Open Access
Aging‐driven composition and distribution changes of electrolyte and graphite anode in 18650‐type Li‐ion batteries
(2022) Petz, Dominik; Baran, Volodymyr; Peschel, Christoph; Winter, Martin; Nowak, Sascha; Hofmann, Michael; Kostecki, Robert; Niewa, Rainer; Bauer, Michael; Müller‐Buschbaum, Peter; Senyshyn, Anatoliy
A series of low‐temperature studies on LiNi0.80Co0.15Al0.05O2 18650‐type batteries of high‐energy type with different stabilized states of fatigue is carried out using spatially resolved neutron powder diffraction, infrared/thermal imaging, and quasi‐adiabatic calorimetry. In‐plane distribution of lithium in the graphite anode and frozen electrolyte in fully charged state is determined non‐destructively with neutron diffraction and correlated to the introduced state of fatigue. An independent electrolyte characterization is performed via calorimetry studies on variously aged 18650‐type lithium‐ion batteries, where the shape of the thermodynamic signal is evolving with the state of fatigue of the cells. Analyzing the liquid electrolyte extracted/harvested from the studied cells reveals the decomposition of conducting salt to be the main driving factor for fatigue in the electrolyte degradation.
Open Access
Residual stresses in Cu matrix composite surface deposits after laser melt injection
(2023) Zhang, Xingxing; Kornmeier, Joana R.; Hofmann, Michael; Langebeck, Anika; Alameddin, Shadi; Alessio, Renan Pereira; Fritzen, Felix; Bunn, Jeffrey R.; Cabeza, Sandra
Tungsten carbide particles reinforced metal matrix composite (MMC) coatings can significantly improve surface wear resistance owing to their increased surface hardness. However, the presence of macro‐ and micro‐residual stresses in MMC coatings can have detrimental effects, such as reducing service life. In this study, neutron diffraction was used to determine the residual stresses in spherical fused tungsten carbide (sFTC) reinforced Cu matrix composite surface deposits after laser melt injection. We also developed a thermo‐mechanical coupled finite element model to predict residual stresses. Our findings reveal that sFTC/Cu composite deposits produced with a preheating temperature of 400°C have low residual stresses, with a maximum tensile residual stress of 98 MPa in the Cu matrix on the top surface. In contrast, the sFTC/bronze (CuAl10Ni5Fe4) composite deposit exhibits very high residual stresses, with a maximum tensile residual stress in the Cu matrix on the top surface reaching 651 MPa. These results provide a better understanding of the magnitudes and distributions of residual stresses in sFTC‐reinforced Cu matrix composite surface deposits manufactured via laser melt injection.