9,895 research outputs found

    Undergraduate Catalog of Studies, 2023-2024

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    Graduate Catalog of Studies, 2023-2024

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    Design and Evaluation of a Hardware System for Online Signal Processing within Mobile Brain-Computer Interfaces

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    Brain-Computer Interfaces (BCIs) sind innovative Systeme, die eine direkte Kommunikation zwischen dem Gehirn und externen GerĂ€ten ermöglichen. Diese Schnittstellen haben sich zu einer transformativen Lösung nicht nur fĂŒr Menschen mit neurologischen Verletzungen entwickelt, sondern auch fĂŒr ein breiteres Spektrum von Menschen, das sowohl medizinische als auch nicht-medizinische Anwendungen umfasst. In der Vergangenheit hat die Herausforderung, dass neurologische Verletzungen nach einer anfĂ€nglichen Erholungsphase statisch bleiben, die Forscher dazu veranlasst, innovative Wege zu beschreiten. Seit den 1970er Jahren stehen BCIs an vorderster Front dieser BemĂŒhungen. Mit den Fortschritten in der Forschung haben sich die BCI-Anwendungen erweitert und zeigen ein großes Potenzial fĂŒr eine Vielzahl von Anwendungen, auch fĂŒr weniger stark eingeschrĂ€nkte (zum Beispiel im Kontext von Hörelektronik) sowie völlig gesunde Menschen (zum Beispiel in der Unterhaltungsindustrie). Die Zukunft der BCI-Forschung hĂ€ngt jedoch auch von der VerfĂŒgbarkeit zuverlĂ€ssiger BCI-Hardware ab, die den Einsatz in der realen Welt gewĂ€hrleistet. Das im Rahmen dieser Arbeit konzipierte und implementierte CereBridge-System stellt einen bedeutenden Fortschritt in der Brain-Computer-Interface-Technologie dar, da es die gesamte Hardware zur Erfassung und Verarbeitung von EEG-Signalen in ein mobiles System integriert. Die Architektur der Verarbeitungshardware basiert auf einem FPGA mit einem ARM Cortex-M3 innerhalb eines heterogenen ICs, was FlexibilitĂ€t und Effizienz bei der EEG-Signalverarbeitung gewĂ€hrleistet. Der modulare Aufbau des Systems, bestehend aus drei einzelnen Boards, gewĂ€hrleistet die Anpassbarkeit an unterschiedliche Anforderungen. Das komplette System wird an der Kopfhaut befestigt, kann autonom arbeiten, benötigt keine externe Interaktion und wiegt einschließlich der 16-Kanal-EEG-Sensoren nur ca. 56 g. Der Fokus liegt auf voller MobilitĂ€t. Das vorgeschlagene anpassbare Datenflusskonzept erleichtert die Untersuchung und nahtlose Integration von Algorithmen und erhöht die FlexibilitĂ€t des Systems. Dies wird auch durch die Möglichkeit unterstrichen, verschiedene Algorithmen auf EEG-Daten anzuwenden, um unterschiedliche Anwendungsziele zu erreichen. High-Level Synthesis (HLS) wurde verwendet, um die Algorithmen auf das FPGA zu portieren, was den Algorithmenentwicklungsprozess beschleunigt und eine schnelle Implementierung von Algorithmusvarianten ermöglicht. Evaluierungen haben gezeigt, dass das CereBridge-System in der Lage ist, die gesamte Signalverarbeitungskette zu integrieren, die fĂŒr verschiedene BCI-Anwendungen erforderlich ist. DarĂŒber hinaus kann es mit einer Batterie von mehr als 31 Stunden Dauerbetrieb betrieben werden, was es zu einer praktikablen Lösung fĂŒr mobile Langzeit-EEG-Aufzeichnungen und reale BCI-Studien macht. Im Vergleich zu bestehenden Forschungsplattformen bietet das CereBridge-System eine bisher unerreichte LeistungsfĂ€higkeit und Ausstattung fĂŒr ein mobiles BCI. Es erfĂŒllt nicht nur die relevanten Anforderungen an ein mobiles BCI-System, sondern ebnet auch den Weg fĂŒr eine schnelle Übertragung von Algorithmen aus dem Labor in reale Anwendungen. Im Wesentlichen liefert diese Arbeit einen umfassenden Entwurf fĂŒr die Entwicklung und Implementierung eines hochmodernen mobilen EEG-basierten BCI-Systems und setzt damit einen neuen Standard fĂŒr BCI-Hardware, die in der Praxis eingesetzt werden kann.Brain-Computer Interfaces (BCIs) are innovative systems that enable direct communication between the brain and external devices. These interfaces have emerged as a transformative solution not only for individuals with neurological injuries, but also for a broader range of individuals, encompassing both medical and non-medical applications. Historically, the challenge of neurological injury being static after an initial recovery phase has driven researchers to explore innovative avenues. Since the 1970s, BCIs have been at one forefront of these efforts. As research has progressed, BCI applications have expanded, showing potential in a wide range of applications, including those for less severely disabled (e.g. in the context of hearing aids) and completely healthy individuals (e.g. entertainment industry). However, the future of BCI research also depends on the availability of reliable BCI hardware to ensure real-world application. The CereBridge system designed and implemented in this work represents a significant leap forward in brain-computer interface technology by integrating all EEG signal acquisition and processing hardware into a mobile system. The processing hardware architecture is centered around an FPGA with an ARM Cortex-M3 within a heterogeneous IC, ensuring flexibility and efficiency in EEG signal processing. The modular design of the system, consisting of three individual boards, ensures adaptability to different requirements. With a focus on full mobility, the complete system is mounted on the scalp, can operate autonomously, requires no external interaction, and weighs approximately 56g, including 16 channel EEG sensors. The proposed customizable dataflow concept facilitates the exploration and seamless integration of algorithms, increasing the flexibility of the system. This is further underscored by the ability to apply different algorithms to recorded EEG data to meet different application goals. High-Level Synthesis (HLS) was used to port algorithms to the FPGA, accelerating the algorithm development process and facilitating rapid implementation of algorithm variants. Evaluations have shown that the CereBridge system is capable of integrating the complete signal processing chain required for various BCI applications. Furthermore, it can operate continuously for more than 31 hours with a 1800mAh battery, making it a viable solution for long-term mobile EEG recording and real-world BCI studies. Compared to existing research platforms, the CereBridge system offers unprecedented performance and features for a mobile BCI. It not only meets the relevant requirements for a mobile BCI system, but also paves the way for the rapid transition of algorithms from the laboratory to real-world applications. In essence, this work provides a comprehensive blueprint for the development and implementation of a state-of-the-art mobile EEG-based BCI system, setting a new benchmark in BCI hardware for real-world applicability

    Performance evaluation and control of an MMC active rectifier with half-bridge and full-bridge submodules for HVDC applications

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    Dissertation (MEng (Electrical Engineering))--University of Pretoria, 2021.The modular multilevel active rectifier was designed and evaluated, whereby the half bridge and the full bridge DC-DC converters as its submodules for the high voltage direct current transmission were compared. It was found that, by taking advantage of the unipolar modulation scheme in the full bridge converter, the switching losses in the two converters are equal when they are both operated in the linear modulation region. Furthermore, operating the full bridge converter in the overmodulation region does not give it a pronounced advantage over the half bridge converter. The conduction losses in the full bridge converter are two times higher than those in the half bridge converter, due to double the number of semiconductor devices. However, using the half bridge converter in the high voltage direct current modular multilevel converter requires an expensive DC-side breaker, while use of the full bridge converter eliminates the need for such a breaker due to the intrinsic DC-side fault current blocking capability. The clear choice between the two requires industry cost data. A design methodology for the submodule capacitor average voltage loop controllers for phase-shifted carrier modulated modular multilevel converters was carried out from first principles. The methodology enables design of such controllers to be carried out in a step by step and straightforward manner without resorting to simulation or guesswork. A simple but effective submodule capacitor sizing method was proposed. The resulting submodule capacitor size was shown to be smaller than those resulting from other sizing methods proposed in the literature while achieving the submodule capacitor voltage ripple specifications. A robust DC bus voltage controller design for modular multilevel rectifiers was presented, whereby a design method for multilevel voltage source converters with DC link capacitors was adopted for modular multilevel rectifiers. Since the modular multilevel converters for HVDC application are designed without the DC-link capacitor to mitigate the effects of a possible DC-side fault current, the submodule capacitors in the modular multilevel converter acted as an equivalent DC link capacitor to accomplish the design.Electrical, Electronic and Computer EngineeringMEng (Electrical Engineering)Unrestricte

    Graduate Catalog of Studies, 2023-2024

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    A dual source fed eleven level switched capacitor multilevel inverter with voltage boosting capability

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    This work introduces an 11-level switched-capacitor multilevel inverter (SCMLI) designed for solar photo-voltaic (PV) applications, capitalizing on the growing popularity of multilevel inverters due to their superior power quality. With a 1.67-times boosting capability, the proposed SCMLI employs 10 switches, 2 DC supplies, and 2 capacitors to achieve an 11-level output voltage waveform. The topology requires only seven driver circuits, incorporating 2 bidirectional switches and 3 complementary pairs of switches. The proposed inverter has intrinsic capacitor self-balancing features since the capacitors are connected across the DC voltage source at different times throughout a basic cycle to charge the capacitors at a level of input voltage. A thorough comparison between the topology and recently developed SCMLI’s has been presented. The comparison demonstrates the effectiveness in terms of switches, capacitors, sources, efficiency, total standing voltage (TSV), and boosting capacity. To experimentally validate its performance, the suggested SCMLI undergoes testing using a frequency-based switching method. The topology exhibits low total harmonic distortion (THD) of 7.65% in its output voltage waveform and 0.89% in the output current waveform

    Exploiting the potential of chemical looping processes for industrial decarbonization and waste to energy conversion. Process design and experimental evaluations

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    The impact of anthropogenic activities on the environment is leading to climate changes and exceptional meteorological phenomena all over the world. To address this negative trend, the scientific community agrees that the environmental impact from fossil fuels-based power production must be mitigated by the integration with alternative and sustainable technologies, such as renewable energy. However, the time required for the complete development and diffusion of such technology poses the urgency of finding a midterm solution to significantly reduce CO2 emissions. Carbon capture, utilization, and storage (CCUS) technologies represent an interesting option to mitigate CO2 emissions. CCUS involves (among other possible applications) the separation of the CO2 content from industrial off-gases, its transport and storage or its reconversion to a chemical/fuel. Chemical looping can be considered as an oxyfuel combustion where the oxygen supply comes from the lattice oxygen atoms of a solid. It is based on gas-solid reactions where a solid also known as oxygen carrier, generally a metal oxide, undergoes successive reduction and oxidation steps. In the reduction step, normally occurring at high temperatures (700-1000 °C), the oxygen carrier interacts with a reducing agent, such as coal, natural gas, syngas etc. and loses part of its oxygen atoms. By controlling the degree of reduction of the oxygen carrier is thus possible to achieve a complete oxidation of the reducing agent (the fuel) to CO2 and H2O (chemical looping combustion) or a partial oxidation to a syngas (chemical looping reforming and gasification). In these latter case, the introduction of external CO2 and H2O can be of help to support the reforming or gasification processes. The oxygen carrier in the reduced phase is then sent to an air reactor, where it reacquires the oxygen atoms by an exothermic reaction with air. This process presents several advantages according to the specific application. In chemical looping combustion, intrinsic separation of N2 and CO2 is achieved, because the two streams are involved in two different reaction steps. This largely simplifies the CO2 separation effort for storage or utilization purposes. On the other hand, in chemical looping reforming it is possible to achieve autothermal operation thanks to the exothermicity of the oxidation step in the air reactor, as well as high reforming efficiencies. Similarly, in chemical looping gasification the resulting syngas is characterized by no N2 dilution, lower tar release and possibility of autothermal operation. These benefits enhance the energy efficiency of the process, leading to a better energy utilisation. In this work, strategies for the decarbonisation and circularity of the industrial and power sector are proposed based on the synthesis of hydrogen and hydrogen-derived fuels. In particular, the potential of chemical looping technology is deeply studied aiming at exploiting its ability to reconvert or valorise CO2 or waste streams to a syngas and then to a liquid fuel/chemical, such as methanol or ammonia. This task is carried out through modelling and experimental evaluations. The modelling activities mainly concern design of process schemes involving the chemical looping section for waste or CO2 reconversion and the liquid fuel synthesis section. The experimental evaluations are focused on two crucial that have been limitedly discussed in the literature: the thermochemical syngas production step by oxidation with CO2 and H2O streams, the effect of high-pressure operation on the redox abilities of a typical iron and nickel-based oxygen carrier. In Chapter 1, a general overview on the main research developments on chemical looping technology is provided. A section is reserved for each chemical looping variant, i.e. combustion, reforming and gasification, and a general description of each process is provided along with the summary of the main research achievements. Subsequently, the technology is divided by application in power production and chemicals production. Main findings from techno-economic assessment and process designs are discussed in comparison with benchmark technologies and other clean pathways. In Chapter 2 steel mills are taken as an example of the hard-to-abate industry. A H2-based decarbonization strategy is proposed and assessed by Aspen Plus simulation. The strategy starts from an initial configuration that is characterized by a typical blast furnace-basic oxygen furnace steel mill and consider the introduction of direct reduction – electric arc furnace lines, that are more efficient and involve natural gas as reducing agent rather than coke. Sensitivity analyses are carried out to assess the effect of the introduction of H2/CH4 blendings in the direct reduction plant and of the utilization of scrap material in the electric arc furnace. The impact of each configuration on the CO2 emissions and the energy flows of the plant is assessed by mass and energy balances. The results indicate a promising decarbonization potential of the introduced technologies but require large investments to increase the renewable sources penetration in the energy mix and large availability of H2. Therefore, alternative pathways for an earlier decarbonization of hard-to-abate industries and for large scale syngas/H2 production need to be considered. In Chapter 3, a novel process scheme is proposed involving chemical looping for syngas production. The CO2 content in blast furnace gases is separated with a calcium looping cycle and subsequently injected with H2O into the oxidation reactor of a chemical looping cycle. Assuming an inlet stream of pure CO2, mass balances on the chemical looping plant are carried out to compare the performance of nickel ferrites and iron oxides in terms of required oxygen carrier flow rate to process 1 t/h of CO2. Computational fluid dynamics simulations with integrated reaction kinetics are then carried out to validate the assumptions on the oxygen carrier conversion and syngas compositions. In Chapter 4 and 5, experimental evaluations are carried out on two crucial aspects for the successful operation of a chemical looping plant aiming at syngas production. In Chapter 4, the syngas productivity by CO2 and H2O splitting over a Fe bed is investigated. This is a very important step, and the effect of various parameters was considered. Firstly, the CO2 splitting is analysed for different temperatures with an inlet flow rate of 1 NL/min to ensure a substantial dissociation of the CO2. Subsequently, combined streams of CO2 and H2O are evolved in the reactor. The effect of the total flow rate, reactants molar ratio and bed height is investigated and from the results, the optimal syngas composition is identified. SEM and XRD are used to assess the morphological evolution and the phase changes of the material during the test. On the contrary, in Chapter 5 the effect of high-pressure operation on the redox abilities of two NiFe aluminates is assessed. The aluminates present similar Fe loadings, but different Ni loadings. High pressure operation is crucial for the development of this technology because it facilitates downstream processing of the syngas to liquid fuels. For a comparative analysis, preliminary tests at low pressure are carried out at three temperatures. Subsequently, the effect of reactants flow rate, temperature, total pressure, gas composition is analysed at high pressure conditions. Finally, long term tests are performed both at ambient and high-pressure conditions. Material characterization by SEM, XRD and H2-TPR is used to support the comparative analysis. In Chapter 6, a techno-economic analysis on a process scheme encompassing methanol and ammonia production from chemical looping gases is carried out. Chemical looping hydrogen production is a very versatile technology and allows for the combined production of power and H2 or syngas. With proper calibration of the flow rates, a stream of high purity N2 can also be obtained at the air reactor outlet and used for ammonia synthesis. Back up with an alkaline electrolyser is considered for the supply of the required amount of hydrogen. Sensitivity analyses are carried out on the chemical looping plant to evaluate the effect of fuel flow rate, steam flow rate, and oxygen carrier inlet temperature to the fuel reactor. Subsequently, a techno-economic analysis is carried out evaluating several parameters among which: the specific CO2 emissions, the energy intensity, and the levelized cost of methanol and ammonia. Finally, a comparison with benchmark technologies and other clean alternatives is presented. In this way, the benefits as well as the drawbacks of chemical looping in terms of environmental and economic parameters are assessed and the missing elements to reach industrial competitivity are clarified

    Design and Implementation of a single switch high gain boost topology : Structure, Ripple Control and ZCS

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    The need for high gain DC-DC converters has lately increased in tandem with the utilization of renewable energy supplies. Particularly appealing are high gain converters that do not require the inclusion of extra power switches and/or other passive elements to the system. As a result, this study proposes a non-isolated single switch converter with ultra-high voltage gain (UHG) that is appropriate for most renewable energy conversion systems, like solar installations. With only a single MOSFET working within a suitable duty cycle region, the proposed converter provides significant voltage gain and around 95% efficiency. Moreover, the MOSFET in this UHG converter is turned on in zero current switching (ZCS) mode, resolving the diode recovery issue. The recommended UHG converter’s working modes, steady-state parametric study, circuit variables like voltage stress on switching devices, and converter gain are all thoroughly explained. Comparisons have been done with comparable topologies presented in the literature, and lastly, experimental results depending on 200W (20V input, 320V output voltage) are given to validate the operation of the proposed UHG design.publishedVersionPeer reviewe

    Laser Technologies for Applications in Quantum Information Science

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    Scientific progress in experimental physics is inevitably dependent on continuing advances in the underlying technologies. Laser technologies enable controlled coherent and dissipative atom-light interactions and micro-optical technologies allow for the implementation of versatile optical systems not accessible with standard optics. This thesis reports on important advances in both technologies with targeted applications ranging from Rydberg-state mediated quantum simulation and computation with individual atoms in arrays of optical tweezers to high-resolution spectroscopy of highly-charged ions. A wide range of advances in laser technologies are reported: The long-term stability and maintainability of external-cavity diode laser systems is improved significantly by introducing a mechanically adjustable lens mount. Tapered-amplifier modules based on a similar lens mount are developed. The diode laser systems are complemented by digital controllers for laser frequency and intensity stabilisation. The controllers offer a bandwidth of up to 1.25 MHz and a noise performance set by the commercial STEMlab platform. In addition, shot-noise limited photodetectors optimised for intensity stabilisation and Pound-Drever-Hall frequency stabilisation as well as a fiber based detector for beat notes in the MHz-regime are developed. The capabilities of the presented techniques are demonstrated by analysing the performance of a laser system used for laser cooling of Rb85 at a wavelength of 780 nm. A reference laser system is stabilised to a spectroscopic reference provided by modulation transfer spectroscopy. This spectroscopy scheme is analysed finding optimal operation at high modulation indices. A suitable signal is generated with a compact and cost-efficient module. A scheme for laser offset-frequency stabilisation based on an optical phase-locked loop is realised. All frequency locks derived from the reference laser system offer a Lorentzian linewidth of 60 kHz (FWHM) in combination with a long-term stability of 130 kHz peak-to-peak within 10 days. Intensity stabilisation based on acousto-optic modulators in combination with the digital controller allows for real-time intensity control on microsecond time scales complemented by a sample and hold feature with a response time of 150 ns. High demands on the spectral properties of the laser systems are put forward for the coherent excitation of quantum states. In this thesis, the performance of active frequency stabilisation is enhanced by introducing a novel current modulation technique for diode lasers. A flat response from DC to 100 MHz and a phase lag below 90° up to 25 MHz are achieved extending the bandwidth available for laserfrequency stabilisation. Applying this technique in combination with a fast proportional-derivative controller, two laser fields with a relative phase noise of 42 mrad for driving rubidium ground state transitions are realised. A laser system for coherent Rydberg excitation via a two-photon scheme provides light at 780 nm and at 480 nm via frequency-doubling from 960 nm. An output power of 0.6 W at 480 nm from a single-mode optical fiber is obtained . The frequencies of both laser systems are stabilised to a high-finesse reference cavity resulting in a linewidth of 1.02 kHz (FWHM) at 960 nm. Numerical simulations quantify the effect of the finite linewidth on the coherence of Rydberg Rabi-oscillations. A laser system similar to the 480 nm Rydberg system is developed for spectroscopy on highly charged bismuth. Advanced optical technologies are also at the heart of the micro-optical generation of tweezer arrays that offer unprecedented scalability of the system size. By using an optimised lens system in combination with an automatic evaluation routine, a tweezer array with several thousand sites and trap waists below 1 Όm is demonstrated. A similar performance is achieved with a microlens array produced in an additive manufacturing process. The microlens design is optimised for the manufacturing process. Furthermore, scattering rates in dipole traps due to suppressed resonant light are analysed proving the feasibility of dipole trap generation using tapered amplifier systems
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