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- Abenius, Erik
(författare)
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Time-Domain Inverse Electromagnetic Scattering using FDTD and Gradient-based Minimization
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis addresses time-domain inverse electromagneticscattering for determining unknown characteristics of an objectfrom observations of the scattered .eld. Applications includenon-destructive characterization of media and optimization ofmaterial properties, for example the design of radar absorbingmaterials.A nother interesting application is the parameteroptimization of subcell models to avoid detailed modeling ofcomplex geometries. The inverse problem is formulated as an optimal controlproblem where the cost function to be minimized is thedi.erence between the estimated and observed .elds, and thecontrol parameters are the unknown object characteristics. Theproblem is solved in a deterministic gradient-basedoptimization algorithm using a parallel 2D FDTD scheme for thedirect problem.This approach is computationally intensive sincethe direct problem needs to be solved in every optimizationiteration in order to compute an estimated .eld.H ighlyaccurate analytical gradients are computed from the adjointformulation.In addition to giving better accuracy than .nitedi.erences, the analytical gradients also have the advantage ofonly requiring one direct and one adjoint problem to be solvedregardless of the number of parameters. When absorbing boundary conditions are used to truncate thecomputational domain, the equations are non-reversible and theentire time-history of the direct solution needs to be storedfor the gradient computation.Ho wever, using an additionaldirect simulation and a restart procedure it is possible tokeep the storage at an acceptable level. The inverse method has been successfully applied to a widerange of industrial problems within the European project,IMPACT (Inverse Methods for Wave Propagation Applications inTime-Domain).T he results presented here includecharacterization of layered dispersive media, determination ofparameters in subcell models for thin sheets and narrow slotsand optimization problems where the observed .eld is given bydesign objectives.
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- af Klinteberg, Ludvig
(författare)
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Computational methods for microfluidics
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with computational methods for fluid flows on the microscale, also known as microfluidics. This is motivated by current research in biological physics and miniaturization technology, where there is a need to understand complex flows involving microscale structures. Numerical simulations are an important tool for doing this.The first paper of the thesis presents a numerical method for simulating multiphase flows involving insoluble surfactants and moving contact lines. The method is based on an explicit interface tracking method, wherein the interface between two fluids is decomposed into segments, which are represented locally on an Eulerian grid. The framework of this method provides a natural setting for solving the advection-diffusion equation governing the surfactant concentration on the interface. Open interfaces and moving contact lines are also incorporated into the method in a natural way, though we show that care must be taken when regularizing interface forces to the grid near the boundary of the computational domain.In the second paper we present a boundary integral formulation for sedimenting particles in periodic Stokes flow, using the completed double layer boundary integral formulation. The long-range nature of the particle-particle interactions lead to the formulation containing sums which are not absolutely convergent if computed directly. This is solved by applying the method of Ewald summation, which in turn is computed in a fast manner by using the FFT-based spectral Ewald method. The complexity of the resulting method is O(N log N), as the system size is scaled up with the number of discretization points N. We apply the method to systems of sedimenting spheroids, which are discretized using the Nyström method and a basic quadrature rule.The Ewald summation method used in the boundary integral method of the second paper requires a decomposition of the potential being summed. In the introductory chapters of the thesis we present an overview of the available methods for creating Ewald decompositions, and show how the methods and decompositions can be related to each other.
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| 3. |
- Althaus, Philipp, 1972-
(författare)
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Indoor Navigation for Mobile Robots : Control and Representations
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with various aspects of indoor navigationfor mobile robots. For a system that moves around in ahousehold or office environment,two major problems must betackled. First, an appropriate control scheme has to bedesigned in order to navigate the platform. Second, the form ofrepresentations of the environment must be chosen.Behaviour based approaches have become the dominantmethodologies for designing control schemes for robotnavigation. One of them is the dynamical systems approach,which is based on the mathematical theory of nonlineardynamics. It provides a sound theoretical framework for bothbehaviour design and behaviour coordination. In the workpresented in this thesis, the approach has been used for thefirst time to construct a navigation system for realistic tasksin large-scale real-world environments. In particular, thecoordination scheme was exploited in order to combinecontinuous sensory signals and discrete events for decisionmaking processes. In addition, this coordination frameworkassures a continuous control signal at all times and permitsthe robot to deal with unexpected events.In order to act in the real world, the control system makesuse of representations of the environment. On the one hand,local geometrical representations parameterise the behaviours.On the other hand, context information and a predefined worldmodel enable the coordination scheme to switchbetweensubtasks. These representations constitute symbols, on thebasis of which the system makes decisions. These symbols mustbe anchored in the real world, requiring the capability ofrelating to sensory data. A general framework for theseanchoring processes in hybrid deliberative architectures isproposed. A distinction of anchoring on two different levels ofabstraction reduces the complexity of the problemsignificantly.A topological map was chosen as a world model. Through theadvanced behaviour coordination system and a proper choice ofrepresentations,the complexity of this map can be kept at aminimum. This allows the development of simple algorithms forautomatic map acquisition. When the robot is guided through theenvironment, it creates such a map of the area online. Theresulting map is precise enough for subsequent use innavigation.In addition, initial studies on navigation in human-robotinteraction tasks are presented. These kinds of tasks posedifferent constraints on a robotic system than, for example,delivery missions. It is shown that the methods developed inthis thesis can easily be applied to interactive navigation.Results show a personal robot maintaining formations with agroup of persons during social interaction.Keywords:mobile robots, robot navigation, indoornavigation, behaviour based robotics, hybrid deliberativesystems, dynamical systems approach, topological maps, symbolanchoring, autonomous mapping, human-robot interaction
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| 10. |
- Appelgren, Ester, 1977-
(författare)
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The influence of media convergence on strategies in newspaper production
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Convergence implies that previously unalike areas come together, approaching a common goal. A subordinate concept of convergence, i.e., media convergence, is a concept that has become common when denoting a range of processes within the production of media content, its distribution and consumption. The concept of media convergence has achieved buzzword status in many contexts due to its widespread use.The concept is not new and has been discussed by researchers in many academic fields and from several different points of views. This thesis will discuss media convergence as an ongoing process and not an end state.Newspapers are one of many so-called publishing channels that provide information and entertainment. They have traditionally been printed on paper, but today’s digital technology makes it possible to provide newspapers through a number of different channels. The current strategy used by newspaper companies involves a process of convergence mainly regarding multiple publishing. A newspaper company interested in publishing content through multiple channels has to adapt its production workflow to produce content not only for the traditional printed edition, but also for the other channels.In this thesis, a generalized value chain involving four main stages illustrates the production workflow at a newspaper company in relation to the convergence processes. The four stages are creation, packaging, distribution and consumption of content.The findings of the thesis are based on studies of the newspaper industry in Sweden and reflect specific newspaper companies, their strategies, production workflow and ventures from 2002 to 2005. The methods used have been case studies, literature studies and scenarios.Some of the conclusions of the thesis indicate that convergence processes have steered the newspaper companies’ development towards multiple channel publishing. Advancing technology and mergers between companies have contributed to the processes of convergence. However, the new publishing channels have been described as threatening to the traditional printed editions since they compete for consumers’ time and advertising revenues. Convergence of technology has made it possible to store, edit and publish material over many different networks using the same tools and the same database system. If the content is stored in a neutral format, it can be packaged and used in many different types of publishing channels. However, according to the studied newspapers, a fully automated workflow for all publishing channels is undesirable and impossible to achieve with the existing technology, standards and organizational structure.This licentiate thesis will discuss some of the strategies behind multiple channel publishing, production workflows and market conditions to detect how the newspaper industry is coping with media convergence.
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