Publications Search Results

Now showing 1 - 10 of 32
  • Publication
    Robust approximation of chance constrained DC optimal power flow under decision-dependent uncertainty
    ( 2022)
    Aigner, K.-M.
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    Clarner, J.-P.
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    Liers, F.
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    Martin, A.
    We propose a mathematical optimization model and its solution for joint chance constrained DC Optimal Power Flow. In this application, it is particularly important that there is a high probability of transmission limits being satisfied, even in the case of uncertain or fluctuating feed-in from renewable energy sources. In critical network situations where the network risks overload, renewable energy feed-in has to be curtailed by the transmission system operator (TSO). The TSO can reduce the feed-in in discrete steps at each network node. The proposed optimization model minimizes curtailment while ensuring that there is a high probability of transmission limits being maintained. The latter is modeled via (joint) chance constraints that are computationally challenging. Thus, we propose a solution approach based on the robust safe approximation of these constraints. Hereby, probabilistic constraints are replaced by robust constraints with suitably defined uncertainty sets constructed from historical data. The ability to discretely control the power feed-in then leads to a robust optimization problem with decision-dependent uncertainties, i.e. the uncertainty sets depend on decision variables. We propose an equivalent mixed-integer linear reformulation for box uncertainties with the exact linearization of bilinear terms. Finally, we present numerical results for different test cases from the Nesta archive, as well as for a real network. We consider the discrete curtailment of solar feed-in, for which we use real-world weather and network data. The experimental tests demonstrate the effectiveness of this method and run times are very fast. Moreover, on average the calculated robust solutions only lead to a small increase in curtailment, when compared to nominal solutions.
  • Publication
    Efficient Formulations and Decomposition Approaches for Power Peak Reduction in Railway Traffic via Timetabling
    ( 2021)
    Bärmann, A.
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    Martin, A.
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    Schneider, O.
    Over the last few years, optimization models for the energy-efficient operation of railway traffic have received more and more attention, particularly in connection with timetable design. In this work, we study the effect of load management via timetabling. The idea is to consider trains as time-flexible consumers in the railway power supply network and to use slight shifts in the departure times from the stations to avoid too many simultaneous departures. This limits peak consumption and can help to improve the stability of the power supply. To this end, we derive efficient formulations for the problem of an optimal timetable adjustment based on a given timetable draft, two of which even allow for totally unimodular polyhedral descriptions. The proper choice of the objective function allows the incorporation of the priorities of either the train operating companies or the infrastructure manager. These include the avoidance of large peaks in average or instantaneous consumption and the improved use of recuperated braking energy. To solve the arising optimization models efficiently, we develop specially tailored exact Benders decomposition schemes that allow for the computation of high-quality solutions within a very short time. In an extensive case study for German railway passenger traffic, we show that our methods are capable of solving the problem on a nationwide scale. We see that the optimal adjustment of timetables entails a tremendous potential for reducing energy consumption.
  • Publication
    Prediction of advanced fibrosis in non-alcoholic fatty liver disease using gut microbiota-based approaches compared with simple non-invasive tools
    ( 2020)
    Lang, S.
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    Farowski, F.
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    Martin, A.
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    Wisplinghoff, H.
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    Vehreschild, M.J.G.T.
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    Krawczyk, M.
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    Nowag, A.
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    Kretzschmar, A.
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    Scholz, C.
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    Kasper, P.
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    Roderburg, C.
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    Lammert, F.
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    Goeser, T.
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    Steffen, H.-M.
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    Demir, M.
    Liver fibrosis is the major determinant of liver related complications in patients with non-alcoholic fatty liver disease (NAFLD). A gut microbiota signature has been explored to predict advanced fibrosis in NAFLD patients. The aim of this study was to validate and compare the diagnostic performance of gut microbiota-based approaches to simple non-invasive tools for the prediction of advanced fibrosis in NAFLD. 16S rRNA gene sequencing was performed in a cohort of 83 biopsy-proven NAFLD patients and 13 patients with non-invasively diagnosed NAFLD-cirrhosis. Random Forest models based on clinical data and sequencing results were compared with transient elastography, the NAFLD fibrosis score (NFS) and FIB-4 index. A Random Forest model containing clinical features and bacterial taxa achieved an area under the curve (AUC) of 0.87 which was only marginally superior to a model without microbiota features (AUC 0.85). The model that aimed to validate a published algorithm achieved an AUC of 0.71. AUC's for NFS and FIB-4 index were 0.86 and 0.85. Transient elastography performed best with an AUC of 0.93. Gut microbiota signatures might help to predict advanced fibrosis in NAFLD. However, transient elastography achieved the best diagnostic performance for the detection of NAFLD patients at risk for disease progression.
  • Publication
    AutoIoT: A framework based on user-driven MDE for generating IoT applications
    ( 2020)
    Nepomuceno, T.
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    Carneiro, T.
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    Maia, P.H.
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    Adnan, M.
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    Nepomuceno, T.
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    Martin, A.
    Developing an Internet of Things (IoT) system requires knowledge in many different technologies like embedded programming, web technologies, and data science. Model-Driven Engineering (MDE) techniques have been used as a concrete alternative to boost IoT application development. However, the current MDE-to-IoT solutions require expertise from the end-users in MDE concepts and sometimes even in specific tools, such as the Eclipse Modelling Framework, which may hinder their adoption in a broader context. To tackle this problem, this work proposes AutoIoT, a framework for creating IoT applications based on a user-driven MDE approach. The proposed framework allows users to model their IoT systems using a simple JSON file and, through internal model-to-model and model-to-text transformations, generates a ready-to-use IoT server-side application. The proposed approach was evaluated through an experiment, in which 54 developers used AutoIoT to create a server-side application for a real-world IoT scenario and answered a post-study questionnaire. The experiment reports the efficacy of AutoIoT and user satisfaction of more than 80% through 6 out of 7 evaluated criteria.
  • Publication
    Design and fabrication of liquid crystal microring resonator-based tunable optical add-drop filters
    ( 2019) ;
    Hartwig, H.
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    Martin, A.
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    Blasl, M.
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    Rieck, A.
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    Namdari, M.
    In fiber optic communication systems, tunable optical add-drop filters are needed for wavelength selection and wavelength channel (de)multiplexing. Ideally filters should possess wide dynamic tuning range, narrow bandwidth and high sidelobe-suppression capabilities. In this contribution, we show that such filters can be obtained with microring resonators incorporating liquid crystals as core materials. Special thermotropic liquid crystal blends were developed such that, in their isotropic phase, to be able to provide large Kerr constants, low loss from visible to infrared and sub-microsecond response time. These blends were used in the design of Vernier type filters with 2 to 4 serially coupled ring resonators. The filters were designed by means of FEM simulations, to operate in C-band. Parameters such as the number of rings, ring geometry and light coupling geometry were optimized to obtain large free spectral range and wide tuning range, narrow bandwidth, high sidelobe-suppression and low insertion loss. Moreover, the influence of fabrication tolerances on the device optical loss was considered in the design. The designed filters were fabricated on silicon wafers by micromachining processes. In particular, a special wafer assembly process was developed to ensure improved wafer bonding and low insertion loss. Experimental results revealed loss optimized tunable wavelength reconfigurable filters with add / drop functions of large free spectral range (FSR) of about 50 nm, wide dynamic tuning range of about 1 FSR and fast (sub-microsecond) tuning capabilities. Additionally, we show that these devices can be useful for other WDM applications, e.g., switching in C-band or visible range.
  • Publication
    Derivation of Guidelines for Reliable Finishing of Aluminium Matrix Composites by Jet Electrochemical Machining
    ( 2018)
    Lehnert, N.
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    Hackert-Oschätzchen, M.
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    Martin, A.
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    Schubert, A.
    The Collaborative Research Centre SFB 692 at Chemnitz University of Technology is focused on aluminium-based light-weight materials in safety-related parts and components. One field of activity are aluminium matrix composites (AMCs). Thereby, the manufacturing, the mechanical properties, the economic opportunities and challenges as well as the machinability are investigated. One of the research focusses is the finish-machining of AMCs by Jet Electrochemical Machining (Jet-ECM). Therefore, one objective is to achieve guidelines for reliable machining of AMCs by Jet-ECM. In a first step the dissolution characteristic of EN AW 2017 with 10% of SiC particles, which was manufactured within the SFB 692, was analysed by help of a microcapillary cell, applying a sodium nitrate electrolyte. Furthermore, Jet-ECM was used to generate point erosions using different pH-neutral electrolytes, namely NaNO3 and NaCl. AMCs made of the aluminium alloy EN AW 2017 with 0%, 5% and 10% SiC particles were machined with a maximal voltage of 25 V. In the current study the parameter field is extended further. The dissolution characteristic by using the pH-neutral electrolyte sodium-bromide is analysed and the voltage range is set to 60 V. Those experiments show, that each electrolyte causes a specific dissolution characteristic. Based on the experimental data, guidelines for a reliable finishing of AMCs by Jet-ECM are derived. The resulting guidelines can be used to predict the depth of the point erosions as function of particle fraction, the applied voltage and the machining time, with respect to the used electrolyte. The statistical evaluation of the guidelines show, that the applied voltage and the machining time are the dominant factors to control the dimensions of the point erosions. Whereas the particle fraction has a minor effect on depth and width.
  • Publication
    A GUI-based platform for quickly prototyping server-side IoT applications
    ( 2018)
    Nepomuceno, T.
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    Carneiro, T.
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    Korn, C.
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    Martin, A.
    This research work presents the AutoIoT, a platform conceived to generate server-side IoT applications. The AutoIoT platform provides a Graphical User Interface (GUI) to model an IoT system, and it applies a model-to-text transformation to generate a server-side application. The generated system is a complete Flask project, which follows the MVC pattern and is written in Python. Developers have full access to the source code of the generated system, which can be used as it is or extended to fulfil further requirements. The AutoIoT platform was used to model a real-world IoT scenario: an Assembly Line. According to the results, it is possible to conclude that AutoIoT is an effective tool for prototyping IoT systems.
  • Publication
    Surface structuring of boron doped CVD diamond by micro electrical discharge machining
    ( 2018)
    Schubert, A.
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    Berger, T.
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    Martin, A.
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    Hackert-Oschätzchen, M.
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    Treffkorn, N.
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    Kühn, R.
    Boron doped diamond materials, which are generated by Chemical Vapor Deposition (CVD), offer a great potential for the application on highly stressed tools, e. g. in cutting or forming processes. As a result of the CVD process rough surfaces arise, which require a finishing treatment in particular for the application in forming tools. Cutting techniques such as milling and grinding are hardly applicable for the finish machining because of the high strength of diamond. Due to its process principle of ablating material by melting and evaporating, Electrical Discharge Machining (EDM) is independent of hardness, brittleness or toughness of the workpiece material. EDM is a suitable technology for machining and structuring CVD diamond, since boron doped CVD diamond is electrically conductive. In this study the ablation characteristics of boron doped CVD diamond by micro electrical discharge machining are investigated. Experiments were carried out to investigate the influence of different process parameters on the machining result. The impact of tool-polarity, voltage and discharge energy on the resulting erosion geometry and the tool wear was analyzed. A variation in path overlapping during the erosion of planar areas leads to different microstructures. The results show that micro EDM is a suitable technology for finishing of boron doped CVD diamond.
  • Publication
    Application of laser scanning as a pre-machining metrology technique in Jet-ECM
    ( 2017)
    Zanjani, M.Y.
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    Zeidler, H.
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    Martin, A.
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    Schubert, A.
    In Electrochemical Machining (ECM), where the material removal takes place based on the anodic dissolution of the workpiece material, the working distance is one of the most important parameters. Especially in Jet Electrochemical Machining (Jet-ECM), where a micro nozzle is moved over the initial surface of the workpiece in order to apply an electrolytic free jet to produce the desired shapes, the distance between the nozzle and the workpiece becomes even more important. On the one hand a small working distance is aspired to achieve high current densities resulting in a high efficiency of the process. On the other hand the working distance needs to be large enough to avoid damages on the micro nozzle caused by electrical discharges or mechanical contact. Hence, the adjustment of the working gap is essential to realize a precise, effective and secure Jet-ECM process. The control of the gap size is done based on the data gathered before machining by surface measurement. Until now, the initial surface has been detected by electrostatic probing through moving the nozzle stepwise to the work piece surface and detect the voltage drop between the nozzle and the work piece. With this strategy, only a limited number of points can be detected within adequate time. Hence, in most cases only three points of the initial surface are detected in order to adjust the working distance according to the planar inclination of the workpiece. The coordinates of the three detected points are used to calculate the normal vector of the initial surface. In recent studies, another strategy was analysed, which is realized by dividing the surface into smaller areas and respectively calculating the normal vector of each area in order to obtain more accurate data of the initial surface. A further strategy is to use probing along the machining path of the tool and to gather the coordinates of a number of points along the path.
  • Publication
    Thermal conductivity of silver loaded conductive epoxy from cryogenic to ambient temperature and its application for precision cryogenic noise measurements
    ( 2016)
    Amils, R.I.
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    Gallego, J.D.
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    Sebastian, J.L.
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    Munoz, S.
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    Martin, A.
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    Leuther, A.
    The pressure to increase the sensitivity of instrumentation has pushed the use of cryogenic Low Noise Amplifier (LNA) technology into a growing number of fields. These areas range from radio astronomy and deep space communications to fundamental physics. In this context manufacturing for cryogenic environments requires a proper thermal knowledge of the materials to be able to achieve adequate design behavior. In this work, we present experimental measurements of the thermal conductivity of a silver filled conductive epoxy (EPO-TEK H20E) which is widely used in cryogenic electronics applications. The characterization has been made using a sample preparation which mimics the practical use of this adhesive in the fabrication of cryogenic devices. We apply the data obtained to a detailed analysis of the effects of the conductive epoxy in a monolithic thermal noise source used for high accuracy cryogenic microwave noise measurements. In this application the epoxy plays a fundamental role since its limited thermal conductivity allows heating the chip with relatively low power. To our knowledge, the cryogenic thermal conductivity data of this epoxy has not been reported before in the literature in the 4-300 K temperature range. A second non-conductive epoxy (Gray Scotch-Weld 2216 B/A), also widely used in cryogenic applications, has been measured in order to validate the method by comparing with previous published data.