2,209 research outputs found

    Cross-border congestion management in the electricity market

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    EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Full-range Gate-controlled Terahertz Phase Modulations with Graphene Metasurfaces

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    Local phase control of electromagnetic wave, the basis of a diverse set of applications such as hologram imaging, polarization and wave-front manipulation, is of fundamental importance in photonic research. However, the bulky, passive phase modulators currently available remain a hurdle for photonic integration. Here we demonstrate full-range active phase modulations in the Tera-Hertz (THz) regime, realized by gate-tuned ultra-thin reflective metasurfaces based on graphene. A one-port resonator model, backed by our full-wave simulations, reveals the underlying mechanism of our extreme phase modulations, and points to general strategies for the design of tunable photonic devices. As a particular example, we demonstrate a gate-tunable THz polarization modulator based on our graphene metasurface. Our findings pave the road towards exciting photonic applications based on active phase manipulations

    Petri net modeling and performance analysis of can fieldbus

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    The CAN FB (Controller Area Network FieldBus) has been in existence for ten years. It supports automated manufacturing and process control environments to interconnect intelligent devices such as valves, sensors, and actuators. CAN FieldBus has a high bit rate and the ability to detect errors. It is immune to noise and resistant to shock, vibration, and heat. Two recently introduced mechanisms, Distributed Priority Queue (DPQ) and Priority Promotion (PP) enable CAN FieldBus networks to share out the system bandwidth and grant ail upper bound on the transmission times so as to meet the requirements in real-time communications. Modeling and analysis of such networks are an important research area for their wide applications in manufacturing automation. This thesis presents a Petri net methodology which models and analyzes CAN FieldBus access protocol. A Reachability Graph of the Petri net model is -utilized to study the behavioral properties of the protocol. A timed Petri net simulator is used to evaluate the performance of the protocol. Performance measures include the completion time for successful events and operations. Operational parameters investigated using the Petri Net model are FieldBus speed, the length of each frame, and the number of frames in a message

    The Impact of Misspecifying A Higher Level Nesting Structure in Item Response Theory Models: A Monte Carlo Study

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    The advantages of Multilevel Item Response Theory (MLIRT) model have been studied by several researchers, and even the impact of ignoring a higher level of data structure in multilevel analysis has been studied and discussed. However, due to the technical complexity of modeling and the shortage in function of dealing with multilevel data in traditional IRT packages (e.g., BILOG and PARSCALE), researchers may not be able to analyze the multilevel IRT data accurately. The impact of this type of misspecification, especially for MLIRT models, has not yet been thoughtfully examined. This dissertation consists of two studies: one is a Monte Carlo study that investigates the impact of this type of misspecification and the other one is a study with real-world data to validate the results obtaining from the simulation study. In Study One (the simulation study), we investigate the potential impact of several factors, including: intra-class correlation (ICC), sample size, cluster size and test length, on the parameter estimates and corresponding test of significance under two situations: when the higher level nesting structure is appropriately modeled (i.e., true model condition) versus inappropriately modeled (i.e., misspecified model condition). Three-level straightly hierarchical data (i.e., items are nested within students who are further nested within schools) were generated. Two person-related and school-related covariates were added at the second level (i.e., person-level) and the third level (i.e., school-level), respectively. The results of simulation studies showed that both parameter estimates and their corresponding standard errors would be biased if the higher level nesting structure was ignored. In Study Two, a real data from the Programme for International Student Assessment with purely hierarchical structure were analyzed by comparing parameter estimates when inappropriate versus appropriate IRT models are specified. The findings mirrored the results obtained from the first study. The implication of this dissertation to researchers is that it is important to model the multilevel data structure even in item response theory models. Researchers should interpret their results in caution when ignoring a higher level nesting structure in MLIRT models. What's more, the findings may help researchers determine when MLIRT should be used to get an unbiased result. Limitations concerning about some of the constraints of the simulation study could be relaxed. For instance, although this study used only dichotomous items, the MLIRT could also be used with polytomous items. The test length could be longer and more variability could be introduced into the item parameters’ values

    Performance evaluation of an auction-based manufacturing cell using Generalized Stochastic Petri Nets.

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    An auction-based manufacturing system is characterized by the fact that the central control computer is eliminated and all the system entities make decisions locally through the negotiation process. The performance evaluation of such manufacturing systems is a new research topic because the traditional OR scheduling and dispatching models usually assume the existence of a central controller, which is, however, removed from an auction-based system. This thesis deals with the modelling and analysis of an auction-based flexible manufacturing cell, in which multiple machining centres are organized in two stages and are capable of processing multiple part types. The Generalized Stochastic Petri Nets is used as the modelling tool to build general models for the FMC with/without instage buffers. Performance measures such as, machine throughput and utilizations, number of block jobs, and queue lengths in the buffers are obtained by solving the model using SPNP package. The operational behaviour of the heterarchically controlled cell is discussed, i.e., the auction behaviour of downstream machines, the influence of instage buffers, and the effect of the number of part types.Dept. of Industrial and Manufacturing Systems Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .Z554. Source: Masters Abstracts International, Volume: 32-06, page: 1692. Adviser: S. P. Dutta. Thesis (M.A.Sc.)--University of Windsor (Canada), 1993

    Synthesis of Vertically-Aligned Zinc Oxide Nanowires and Their Applications as Photocatalysts

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    Zinc oxide (ZnO) nanostructures, especially nanowires, have been one of the most important semiconductive materials used for photocatalysis due to their unique material properties and remarkable performance. In this project, vertically-aligned ZnO nanowires on glass substrate have been synthesized by using the facile hydrothermal methods with the help of pre-coated ZnO seeding layer. The crystalline structure, morphology and UV-Vis transmission spectra of the as-synthesized sample were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and Ultra-violet Visible (UV-Vis) Spectrophotometer. The photocatalytic activity of the sample was examined for the photocatalytic degradation of methyl orange (MO) as the test dye in aqueous solution under UV-A irradiation. The extent of direct hydrolysis of the MO dye under UV light without the photocatalysts was first measured to eliminate the possible contribution from the undesired variables to the overall efficiency. The effects of pH and initial concentration of the MO solution, as well as the nanowire growth time, on the photocatalytic efficiency have been investigated, in order to determine the optimal conditions for photocatalytic applications of ZnO nanowires in the industry. Furthermore, the reproducibility of the experimental methods used in this project was tested to ensure the reliability of the experimental results obtained; and the reusability of the prepared ZnO nanowire arrays were also evaluated to investigate the stability of the products for photocatalytic applications in a large scale. In addition, a micro-chamber based microfluidic device with integrated ZnO nanowire arrays has been fabricated and used for photodegradation studies of MO solution under continuous-flow conditions. As expected, the micro-chamber based approach exhibited much improved photodegradation efficiency as compared to the conventional method using bulk dye solution. The effects of the flow rate and chamber height of the microfluidic device have also been investigated in order to determine the optimal experimental conditions for photodegradation reactions in microfluidic devices
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