• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1524
  • 363
  • 359
  • 194
  • 78
  • 48
  • 46
  • 39
  • 31
  • 26
  • 20
  • 18
  • 17
  • 13
  • 9
  • Tagged with
  • 3302
  • 1148
  • 436
  • 429
  • 323
  • 320
  • 303
  • 285
  • 269
  • 258
  • 236
  • 234
  • 218
  • 211
  • 205
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Etude locale de systèmes contrôles de type sous-finslerien. / Local study of sub-Finslerian control systems

Ali, Entisar Abdul-Latif 31 January 2017 (has links)
Dans cette thèse j'étudie la géométrie locale des structuresfinslériennes et sous-finslériennes associées à la norme infinien dimension 2 et 3 : géodésiques généralisées courtes, lieu de coupure, lieu conjugué généralisé, lieu de "saut", petites sphères.Pour définir une telle structure au voisinage d'un point $p$ de $mathbb{R}^n$, on se donne une famille de champs de vecteurs $(F_1,dots,F_k)$ et on considère la norme définie sur la distribution$Delta=mbox{vect}{F_1,dots,F_k}$ par $|G|=inf{max{|u_i|} ; | ; G=sum_i u_i F_i} $.En dimension 2, pour $k=2$, si $F_1$ et $F_2$ ne sont pas proportionnels en $p$ alors on obtient une structure finslérienne. Sinon, alors la structure est sous-finslérienne sur une distribution de rang non constant. Nous décrivons les objets géométriques décrits plus haut pour l'ensembles des couples génériques $(F_1,F_2)$.En dimension 3, nous avons étudié la géométrie locale pour les distributions de contact. / In this thesis I study the local geometry of Finslerian and sub-Finslerian structures associated to the maximum norm in dimension 2 and 3 : short generalized geodesics, cut locus, generalized conjugate locus, switching locus, small spheres.To define such a structure in the neighborhood of a point $p$ of $mathbb{R}^n$, we fix a familly of vector fields $(F_1,dots,F_k)$ and consider the norm defined on the distribution $Delta=mbox{vect}{F_1,dots,F_k}$ by $|G|=inf{max{|u_i|} ; | ; G=sum_i u_i F_i} $.In dimension 2, for $k=2$, if $F_1$ and $F_2$ are not proportionnal at $p$ then we obtain a Finslerian structure. If not, the structure is sub-Finslerian on a distribution with non constant rank. We describe the geometric objects for the set of all generic couples $(F_1,F_2)$.In dimension 3, we studied the local geometry for contact distributions.
42

Ordonnancement stochastique avec impatience / Stochastic scheduling with impatience

Salch, Alexandre 29 November 2013 (has links)
Le sujet de cette thèse est l'étude de systèmes de production avec impatience. Ces systèmes sont modélisés comme des problèmes d'ordonnancement stochastiques avec des dates d'échéance. Dans la littérature, peu de résultats existent sur le contrôle optimal de ce genre de systèmes. C'est dans ce cadre que s'inscrit cette thèse. Nous considérons un système générique avec une machine, sur laquelle des tâches sont à exécuter. Les durées d'exécution, les dates d'échéance (ou durées d'impatience) et les dates de disponibilité des tâches sont des variables aléatoires. À chaque tâche est associé un poids et l'objectif est de minimiser l'espérance du nombre pondéré de tâches en retard. Dans notre étude, nous utilisons différentes modélisations, rendant compte des différentes contraintes régissant des systèmes réels. Notamment, nous faisons la différence entre l'impatience, le fait d'avoir attendu trop longtemps, et l'abandon, le fait de quitter le système suite à l'impatience. Dans la classe des politiques statiques, nous donnons des ordonnancements optimaux pour des problèmes avec impatience. Dans la classe des politiques dynamiques avec préemption, nous donnons de nouvelles conditions garantissant l'optimalité d'une politique stricte pour des problèmes avec abandon et nous proposons une heuristique plus efficace que celles que l'on trouve dans la littérature. Enfin, nous explorons des variantes et des extensions de ces problèmes, lorsque le système comporte plusieurs machines et lorsque la préemption n'est pas autorisée. / In this thesis, production systems facing abandonments are studied. These problems are modeled as stochastic scheduling problems with due dates. In the literature, few results exist concerning the optimal control of such systems. This thesis aims at providing optimal control policies for systems with impatience. We consider a generic system with a single machine, on which jobs have to be processed. Processing times, due dates (or patience time) and release dates are random variables. A weight is associated to each job and the objective is to minimize the expected weighted number of late jobs. In our study, we use different models, taking into account the specific features of real life problems. For example, we make a difference between impatience, when a customer has been waiting for too long, and abandonment, when a customer leaves the system after getting impatient. In the class of static list scheduling policies, we provide optimal schedules for problems with impatience. In the class of preemptive dynamic policies, we specify conditions under which a strict priority rule is optimal and we give a new heuristic, both extending previous results from the literature. We study variants and extensions of these problems, when several machines are available or when preemption is not authorized.
43

Optimization and optimal control of plant growth : application of GreenLab model for decision aid in agriculture. / Optimisation et contrôle optimale de modes culturaux : application du modèle GreenLab pour l’aide à la décision en agriculture.

Qi, Rui 10 March 2010 (has links)
L'objectif de cette thèse est de développer des méthodes d'optimisation et de contrôle optimal pour l'amélioration du rendement des cultures en utilisant le modèle de croissance de plantes GreenLab. Les méthodes proposées doivent se placer dans le contexte suivant: (1) la recherche d'amélioration du rendement se fait par des simulations basées sur le modèle structure-fonction GreenLab et (2) les méthodes utilisées sont des algorithmes d'optimisation heuristiques et des techniques de contrôle optimal. L'application de ces méthodes à plusieurs espèces de plantes, allant des plantes agronomiques aux arbres, et avec des différents objectifs a permis d'identifier certaines caractéristiques associées à des plantes ayant un bon rendement. En particulier, les résultats d'optimisation ont révélé la dynamique des relations source-puits au sein de la plante durant sa croissance. Ces résultats peuvent être considérés comme des références pour guider la sélection génétique pour l'amélioration variétale, et également pour l'amélioration des itinéraires culturaux. La perspective à long terme de cette thèse est l'intégration de ses résultats dans des outils d'aide à la décision pour l'agriculture. Pour atteindre les objectifs de cette thèse, nous avons analysé successivement les effets de facteurs endogènes et exogènes (environnementaux) sur la croissance de la plante et sur son rendement. Plus précisément, l'effet des facteurs endogènes a été étudié à conditions environnementales fixées, puis des méthodes de contrôle optimal ont été appliquées sur les variables environnementales, pour un génotype de plante fixé. En conséquence, les problèmes traités dans cette thèse relèvent à la fois de la théorie de l'optimisation et du contrôle optimal. Les principales contributions de cette thèse incluent les points suivants : Des problèmes d'optimisation simple objectif, d'optimisation multi-objectif et d'optimisation sous contrainte ont été formulés et résolus, dans le but de trouver les paramètres endogènes associés à la plante ayant le plus haut rendement, ce qui correspond à la définition d'un idéotype, pour une plante d'espèce donnée. Pour la plupart des problèmes d'optimisation présentés, la méthode la plus appropriée est celle d'un algorithme basé sur une population. Plusieurs algorithmes de ce type ont été comparés et celui ayant les meilleures performances est un algorithme heuristique nommé Particle Swarm Optimization (PSO). Du contrôle optimal a été appliqué pour définir la stratégie d'élagage optimale (application aux feuilles de thé). Come GreenLab peut être formulé comme un système dynamique discret et que la fonction objectif est analytique, une méthode basée sur le gradient, basée sur une approche variationnelle et sur la théorie de Lagrange, a été utilisée. La solution trouvée à été comparée à celle obtenue par la méthode PSO afin de valider cette dernière. Un modèle de dynamique de population d'insectes a été développé à des échelles spatiales et temporelles compatibles avec le modèle GreenLab, afin d'étudier l'interaction plante-insectes. Plus précisément, un écosystème tritrophique a été modélisé, incluant les interactions entre les plantes, les insectes ravageurs et des prédateurs des ravageurs, appelés insectes auxiliaires. un modèle des interaction. L'originalité de ce travail est la rétro-action entre la dynamique de population des insectes et la croissance de la plante, ainsi que la prise en compte de la répartition spatiale des insectes sur chacun des organes de la plantes. Une analyse de sensibilité basée sur la méthode de Morris a été appliquée pour identifier les paramètres les plus ou moins influents sur les sorties d'intérêt. Cela a permis de calculer des stratégies optimales pour l'application des techniques d'éradication des ravageurs. Les paramètres de GreenLab ont été estimés sur environ 400 jeux de données correspondant à 44 génotypes de tomates, à l'aide d'un algorithme des moindres carrés non linéaires généralisés. Considérant l'ensemble des valeurs estimées comme l'espace des paramètres possibles, nous avons calculé les valeurs optimisant le rendement en fruits des tomates. Nous avons analysé les corrélations entre les paramètres estimés ou optimaux et le rendement à l'aide de méthodes statistiques, ce qui a permis d'identifier les plus importants paramètres responsables des différences observées parmi les rendements des différentes plantes. Ainsi, à partir de ces résultat d'optimisation et d'analyse des corrélation, les différences phénotypiques entre différents génotypes ont pu être expliquées d'un point de vue physiologique. / The objective of the thesis is to improve plant yield through optimization and optimal control based on the GreenLab plant growth model. Therefore, the thesis proposed a methodology for investigation of plant yield improvement,whose characteristics are that (1) investigations are all based on the functional-structural plant growth model GreenLab and (2) heuristic optimization algorithm and optimal control techniques are applied to the plant growth model in order to improve plant yield. By applying optimization techniques on different species of plants (crops or trees) and for different kinds of optimization problems, common characteristics that a plant with high yield should possess were obtained. The optimal results in the thesis revealed the source-sink dynamics during the plant growth. The optimization results can be considered as references to guide breeding for ideotype and to improve cultivation modes. The optimization application of GreenLab could thus be possibly used to the agricultural decision support system.To achieve the aims of the thesis, the thesis investigated the effects of endogenous factors and exogenous environmental factors of plant growth on plant yield separately. First, given environmental conditions, the thesis investigated endogenous factors, and then the thesis did optimal control on exogenous environmental factors given plant genotype. Therefore, the problems investigated in the thesis consist of general optimization problems and optimal control problems.The main contributions of the thesis include following issues: According to the species of plants, single optimization problems, multi-objective optimization problems and optimization problems with constraints with respect to plant endogenous factors were formulated and investigated, in order to find the ideotype of plants with high plant yield. A population based algorithm is more suitable for the optimization problems in this thesis. Due to its better performance compared with other heuristic optimization algorithms, all optimization problems were solved by a population-based, heuristic optimization algorithm, namely Particle Swarm Optimization (PSO). Optimal control on the pruning strategy was formulated and investigated in the thesis. As GreenLab can be considered as discrete dynamic system and the objective function of the optimal control problem is analytical, the gradient based method, which is based on the variational approach and Lagrange theory, was used to solve the optimal control problem. Moreover, the optimal solutions were compared with the ones found by PSO, in order to validate the PSO method. The insect population dynamics was modeled mathematically, which was compatible with the plant model GreenLab in terms of spatial and temporal scales, to study the effect of biotic factors on plant growth. The interaction among plants, pests and auxiliaries was implemented, and the ecosystem model, which involves the three tri-trophic components, was thus developed in the thesis. The tri-trophic ecosystem model can simulate the insect population dynamics and the plant growth with consideration of the interaction of insects. Moreover, the tri-trophic ecosystem model considered the partition of individuals in the insect population among plant organs, which is not taken into account in the previous works. A global sensitivity analysis method Morris method was used to analyze the most important parameters and the least influential parameters to model outputs of interest. Through optimization on pest management techniques, the optimal strategies of the application of the pest management techniques were obtained. Estimation of GreenLab parameters with about 400 sets of observation data of 44 tomato genotypes was done in the thesis, by using a generalized non-linear least square algorithm. Taking the estimated parameter values as parameter space, the GreenLab model parameters were optimized, in order to maximize the fruit yield. Through the analysis of the correlation of estimated and optimal parameters with the fruit yield by statistical analysis methods, the most important parameters that result in the difference of fruit yield were found. According to the correlation and optimization results, the phenotypic differences among genotypes were explained from the physiological point of view.
44

Modelling approaches for optimal liquidation under a limit-order book structure

Blair, James January 2016 (has links)
This thesis introduces a selection of models for optimal execution of financial assets at the tactical level. As opposed to optimal scheduling, which defines a trading schedule for the trader, this thesis investigates how the trader should interact with the order book. If a trader is aggressive he will execute his order using market orders, which will negatively feedback on his execution price through market impact. Alternatively, the models we focus on consider a passive trader who places limit orders into the limit-order book and waits for these orders to be filled by market orders from other traders. We assume these models do not exhibit market impact. However, given we await market orders from other participants to fill our limit orders a new risk is borne: execution risk. We begin with an extension of Guéant et al. (2012b) who through the use of an exponential utility, standard Brownian motion, and an absolute decay parameter were able to cleverly build symmetry into their model which significantly reduced the complexity. Our model consists of geometric Brownian motion (and mean-reverting processes) for the asset price, a proportional control parameter (the additional amount we ask for the asset), and a proportional decay parameter, implying that the symmetry found in Guéant et al. (2012b) no longer exists. This novel combination results in asset-dependent trading strategies, which to our knowledge is a unique concept in this framework of literature. Detailed asymptotic analyses, coupled with advanced numerical techniques (informing the asymptotics) are exploited to extract the relevant dynamics, before looking at further extensions using similar methods. We examine our above mentioned framework, as well as that of Guéant et al. (2012), for a trader who has a basket of correlated assets to liquidate. This leads to a higher-dimensional model which increases the complexity of both numerically solving the problem and asymptotically examining it. The solutions we present are of interest, and comparable with Markowitz portfolio theory. We return to our framework of a single underlying and consider four extensions: a stochastic volatility model which results in an added dimension to the problem, a constrained optimisation problem in which the control has an explicit lower bound, changing the exponential intensity to a power intensity which results in a reformulation as a singular stochastic control problem, and allowing the trader to trade using both market orders and limit orders resulting in a free-boundary problem. We complete the study with an empirical analysis using limit-order book data which contains multiple levels of the book. This involves a novel calibration of the intensity functions which represent the limit-order book, before backtesting and analysing the performance of the strategies.
45

Entropy-regularized Optimal Transport for Machine Learning / Transport Optimal pour l'Apprentissage Automatique

Genevay, Aude 13 March 2019 (has links)
Le Transport Optimal régularisé par l’Entropie (TOE) permet de définir les Divergences de Sinkhorn (DS), une nouvelle classe de distance entre mesures de probabilités basées sur le TOE. Celles-ci permettentd’interpolerentredeuxautresdistancesconnues: leTransport Optimal(TO)etl’EcartMoyenMaximal(EMM).LesDSpeuventêtre utilisées pour apprendre des modèles probabilistes avec de meilleures performances que les algorithmes existants pour une régularisation adéquate. Ceci est justifié par un théorème sur l’approximation des SDpardeséchantillons, prouvantqu’unerégularisationsusantepermet de se débarrasser de la malédiction de la dimension du TO, et l’on retrouve à l’infini le taux de convergence des EMM. Enfin, nous présentons de nouveaux algorithmes de résolution pour le TOE basés surl’optimisationstochastique‘en-ligne’qui,contrairementàl’étatde l’art, ne se restreignent pas aux mesures discrètes et s’adaptent bien aux problèmes de grande dimension. / This thesis proposes theoretical and numerical contributions to use Entropy-regularized Optimal Transport (EOT) for machine learning. We introduce Sinkhorn Divergences (SD), a class of discrepancies betweenprobabilitymeasuresbasedonEOTwhichinterpolatesbetween two other well-known discrepancies: Optimal Transport (OT) and Maximum Mean Discrepancies (MMD). We develop an ecient numerical method to use SD for density fitting tasks, showing that a suitable choice of regularization can improve performance over existing methods. We derive a sample complexity theorem for SD which proves that choosing a large enough regularization parameter allows to break the curse of dimensionality from OT, and recover asymptotic ratessimilartoMMD.Weproposeandanalyzestochasticoptimization solvers for EOT, which yield online methods that can cope with arbitrary measures and are well suited to large scale problems, contrarily to existing discrete batch solvers.
46

Inverse Optimal Control : theoretical study / Contrôle Optimal Inverse : étude théorique

Maslovskaya, Sofya 11 October 2018 (has links)
Cette thèse s'insère dans un projet plus vaste, dont le but est de s'attaquer aux fondements mathématiques du problème inverse en contrôle optimal afin de dégager une méthodologie générale utilisable en neurophysiologie. Les deux questions essentielles sont : (a) l'unicité d'un coût pour une synthèse optimale donnée (injectivité); (b) la reconstruction du coût à partir de la synthèse. Pour des classes de coût générales, le problème apparaît très difficile même avec une dynamique triviale. On a donc attaqué l'injectivité pour des classes de problèmes spéciales : avec un coût quadratique, la dynamique étant soit non-holonome, soit affine en le contrôle. Les résultats obtenus ont permis de traiter la reconstruction pour le problème linéaire-quadratique. / This PhD thesis is part of a larger project, whose aim is to address the mathematical foundations of the inverse problem in optimal control in order to reach a general methodology usable in neurophysiology. The two key questions are : (a) the uniqueness of a cost for a given optimal synthesis (injectivity) ; (b) the reconstruction of the cost from the synthesis. For general classes of costs, the problem seems very difficult even with a trivial dynamics. Therefore, the injectivity question was treated for special classes of problems, namely, the problems with quadratic cost and a dynamics, which is either non-holonomic (sub-Riemannian geometry) or control-affine. Based on the obtained results, we propose a reconstruction algorithm for the linear-quadratic problem.
47

Modeling and Uncertainty Analysis of CCHP systems

Smith, Joshua Aaron 15 December 2012 (has links)
Combined Cooling Heating and Power (CCHP) systems have been recognized as a viable alternative to conventional electrical and thermal energy generation in buildings because of their high efficiency, low environmental impact, and power grid independence. Many researchers have presented models for comparing CCHP systems to conventional systems and for optimizing CCHP systems. However, many of the errors and uncertainties that affect these modeling efforts have not been adequately addressed in the literature. This dissertation will focus on the following key issues related to errors and uncertainty in CCHP system modeling: (a) detailed uncertainty analysis of a CCHP system model with novel characterization of weather patterns, fuel prices and component efficiencies; (b) sensitivity analysis of a method for estimating the hourly energy demands of a building using Department of Energy (DOE) reference building models in combination with monthly utility bills; (c) development of a practical technique for selecting the optimal Power Generation Unit (PGU) for a given building that is robust with respect to fuel cost and weather uncertainty; (d) development of a systematic method for integrated calibration and parameter estimation of thermal system models. The results from the detailed uncertainty analysis show that CCHP operational strategies can effectively be assessed using steady state models with typical year weather data. The results of the sensitivity analysis reveal that the DOE reference buildings can be adjusted using monthly utility bills to represent the hourly energy demands of actual buildings. The optimal PGU sizing study illustrates that the PGU can be selected for a given building in consideration of weather and fuel cost uncertainty. The results of the integrated parameter estimation study reveal that using the integrated approach can reduce the effect of measurement error on the accuracy of predictive thermal system models.
48

Cooperative search strategies for multi-vehicle teams

Oğraş, Ümit Yusuf January 2002 (has links)
No description available.
49

Analytical and Numerical Optimal Motion Planning for an Underwater Glider

Kraus, Robert J. 06 May 2010 (has links)
The use of autonomous underwater vehicles (AUVs) for oceanic observation and research is becoming more common. Underwater gliders are a specific class of AUV that do not use conventional propulsion. Instead they change their buoyancy and center of mass location to control attitude and trajectory. The vehicles spend most of their time in long, steady glides, so even minor improvements in glide range can be magnified over multiple dives. This dissertation presents a rigid-body dynamic system for a generic vehicle operating in a moving fluid (ocean current or wind). The model is then reduced to apply to underwater gliders. A reduced-order point-mass model is analyzed for optimal gliding in the presence of a current. Different numerical method solutions are compared while attempting to achieve maximum glide range. The result, although approximate, provides good insight into how the vehicles may be operated more effectively. At the end of each dive, the gliders must change their buoyancy and pitch to transition to a climb. Improper scheduling of the buoyancy and pitch change may cause the vehicle to stall and lose directional stability. Optimal control theory is applied to the buoyancy and angle of attack scheduling of a point-mass model. A rigid-body model is analyzed on a singular arc steady glide. An analytical solution for the control required to stay on the arc is calculated. The model is linearized to calculate possible perturbation directions while remaining on the arc. The nonlinear model is then propagated in forward and reverse time with the perturbations and analyzed. Lastly, one of the numerical solutions is analyzed using the singular arc equations for verification. This work received support from the Office of Naval Research under Grant Number N00014-08-1-0012. / Ph. D.
50

Optimal and Adaptive Subband Beamforming / Optimal och Adaptiv Delbandsbeamforming

Grbic, Nedelko January 2001 (has links)
The increased use of personal communication devices, personal computers and wireless cellular telephones enables the development of new inter-personal communication systems. The merge between computers and telephony technologies brings up the demand for convenient hands-free communications. In such systems the users wish to lead a conversation in much the same way as in a normal person-to-person conversation. The advantages of hands-free telephones are safety, convenience and greater flexibility. In many countries and regions, hand held telephony in cars is prohibited by legislation. By placing the microphone far away from the user a number of disadvantages are introduced, which results in substantial speech distortion and poor sound quality. These disturbances are mainly caused by room reverberation and background noise. Furthermore, acoustic feedback generated at the near-end side is a problem for the far-end side talker, who will hear his/her own voice echoed with 100-200 ms delay, making speech conversation substantially more difficult. Digital filtering may be used to obtain a similar sound quality as for hand held telephony. Three major tasks must be addressed in order to improve the quality of hands-free communication systems; noise suppression, room reverberation suppression, and acoustic feedback cancellation of the hands-free loudspeaker. The filtering operation must perform the above mentioned tasks without causing severe near-end speech distortion. A properly designed broad-band microphone array is able to perform all the given tasks, i.e. speech enhancement, echo cancellation and reverberation suppression, in a concise and effective manner. This is due to the fact that the spatial domain may be utilized as well as the temporal domain. This thesis deals with the problem of specification and design of beamformers used to extract the source signal information. A new subband adaptive beamforming algorithm is proposed, where many of the drawbacks embedded in conventional adaptive beamforming are eliminated. Evaluation in a car hands-free situation show the benefits of the proposed method. Blind signal separation is discussed and a new structure based on frequency domain inverse channel identification and time domain separation, is proposed. Further, filter-bank properties and design are discussed together with performance limitations in subband beamforming structures. / Avhandlingen behandlar specifikation och konstruktion av mikrofon-arrayer för att extrahera talinformation. En ny adaptiv delbands beamforming-algoritm föreslås där många av nackdelarna hos konventionella adaptiva beamformers är eliminerade. En utvärdering i en bil med ett frihands-system bekräftar fördelarna med den föreslagna metoden. Blind signal-separation diskuteras och en ny struktur föreslås, baserad på en inverterande kanalidentifiering utförd i frekvensdomän med en kontinuerlig separation utförd i tidsdomän. Filterbanks-egenskaper och designmetoder diskuteras tillsammans med begränsningar som finns i beamforming-strukturer utförda i delband.

Page generated in 0.0734 seconds