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Methods for Stochastic Optimal Control under State Constraints
No full textThis thesis looks at a few different approaches to solving stochas-tic optimal control problems with state constraints. The motivatingproblem is optimal control of an energy buffer in a hybrid vehicle,although applications are abundant in a number of areas.Stochastic optimal control problems can be solved via the so-called Hamilton-Jacobi-Bellman (HJB) equation. State constraintsresult in boundary conditions for the HJB equation causing the valuefunction to go to infinity as the state approaches the boundary, whichmakes it difficult to solve this partial differential equation numerically.Different approaches to avoiding infinite values on the boundaryare investigated. First, we consider a logarithmic transformation ofthe value function. This results in an exact linearization, turningtheHJB equation into an eigenvalue problem in the one-dimensional case,and also in higher dimensions, but then with certain restrictions onthe relation between noise and control cost. Then, for a more generalproblem formulation, we introduce a different transform which yieldsa nonlinear problem. It is investigated under what conditions theboundary constraints will be well-behaved, and example problemsare solved using a collocation method, demonstrating how a smallnumber of collocation points is sufficient to yield good solutions inthose cases. Finally, we consider a method starting from the Fokker-Planck equation. This yields an equivalent problem, but where thevalue function of the HJB equation need not be computed explic-itly, but the probability density function of the closed-loop system iscomputed instead. This fact can be utilized to focus computationalresources on the parts of the state-space that are the most relevant
On Infinite-Horizon State-Constrained Optimal Control - With Application to a Fuel Cell APU
No full textThis thesis presents a method for computation of optimal control policies for state-constrained, time continuous, infinite-horizon systems. It is applicable to input-affine systems where the stochastic contribution can be modeled as a vector of white noise signals that is added to the control input, with the intensity of the noise being inversely proportional to the control cost.
A distinctive advantage of the method is that it can treat problems where the interaction between stochastic disturbances and hard constraints are of importance to the optimal control policy.
The method is demonstrated with the example of control of a fuel cell driven auxiliary power unit. This is a unit that would produce on-board electricity for a truck or other vehicle by reforming the hydrocarbon fuel and feeding the obtained hydrogen to a fuel cell stack. Since energy is stored in several locations in the system, control is required to ensure that the stochastic power demand can be met at optimum fuel efficiency
On Infinite-Horizon State-Constrained Optimal Control - With Application to a Fuel Cell APU
No full textThis thesis presents a method for computation of optimal control policies for state-constrained, time continuous, infinite-horizon systems. It is applicable to input-affine systems where the stochastic contribution can be modeled as a vector of white noise signals that is added to the control input, with the intensity of the noise being inversely proportional to the control cost. A distinctive advantage of the method is that it can treat problems where the interaction between stochastic disturbances and hard constraints are of importance to the optimal control policy. The method is demonstrated with the example of control of a fuel cell driven auxiliary power unit. This is a unit that would produce on-board electricity for a truck or other vehicle by reforming the hydrocarbon fuel and feeding the obtained hydrogen to a fuel cell stack. Since energy is stored in several locations in the system, control is required to ensure that the stochastic power demand can be met at optimum fuel efficiency
Methods for Stochastic Optimal Control under State Constraints [Elektronisk resurs]
No full textThis thesis looks at a few different approaches to solving stochas-tic optimal control problems with state constraints. The motivatingproblem is optimal control of an energy buffer in a hybrid vehicle,although applications are abundant in a number of areas.Stochastic optimal control problems can be solved via the so-called Hamilton-Jacobi-Bellman (HJB) equation. State constraintsresult in boundary conditions for the HJB equation causing the valuefunction to go to infinity as the state approaches the boundary, whichmakes it difficult to solve this partial differential equation numerically.Different approaches to avoiding infinite values on the boundaryare investigated. First, we consider a logarithmic transformation ofthe value function. This results in an exact linearization, turningtheHJB equation into an eigenvalue problem in the one-dimensional case,and also in higher dimensions, but then with certain restrictions onthe relation between noise and control cost. Then, for a more generalproblem formulation, we introduce a different transform which yieldsa nonlinear problem. It is investigated under what conditions theboundary constraints will be well-behaved, and example problemsare solved using a collocation method, demonstrating how a smallnumber of collocation points is sufficient to yield good solutions inthose cases. Finally, we consider a method starting from the Fokker-Planck equation. This yields an equivalent problem, but where thevalue function of the HJB equation need not be computed explic-itly, but the probability density function of the closed-loop system iscomputed instead. This fact can be utilized to focus computationalresources on the parts of the state-space that are the most relevant
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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