Search results for "CONSTRAINT"
showing 10 items of 361 documents
Toward Approximate GML Retrieval Based on Structural and Semantic Characteristics
2010
International audience; GML is emerging as the new standard for representing geographic information in GISs on the Web, allowing the encoding of structurally and semantically rich geographic data in self describing XML-based geographic entities. In this study, we address the problem of approximate querying and ranked results for GML data and provide a method for GML query evaluation. Our method consists of two main contributions. First, we propose a tree model for representing GML queries and data collections. Then, we introduce a GML retrieval method based on the concept of tree edit distance as an efficient means for comparing semi-structured data. Our approach allows the evaluation of bo…
Extensions of the witness method to characterize under-, over- and well-constrained geometric constraint systems
2011
International audience; This paper describes new ways to tackle several important problems encountered in geometric constraint solving, in the context of CAD, and which are linked to the handling of under- and over-constrained systems. It presents a powerful decomposition algorithm of such systems. Our methods are based on the witness principle whose theoretical background is recalled in a first step. A method to generate a witness is then explained. We show that having a witness can be used to incrementally detect over-constrainedness and thus to compute a well-constrained boundary system. An algorithm is introduced to check if anchoring a given subset of the coordinates brings the number …
Méthodes géometriques en mécanique spatiale et aspects numériques
2005
We present in this thesis two research projectson the optimal control of the space vehicles.In the first, we have dealt with the orbit transferproblem. We study the minimum time control of a satellite that we want to reach a geostationary orbit. Our contribution is of two kinds. Geometric, first, since we study the controllability of the system together with the geometry of the transfer (structure of the command) by means of geometric control without state constraint tools (minimum principle). Then we present shootingalgorithm and homotopy method. These approaches allow the numerical resolution of problems with strong or low thrust satellites.The second project concerns to the calculation o…
Optimal control of the atmospheric arc of a space shuttle and numerical simulations with multiple-shooting method
2005
This article, continuation of previous works, presents the applications of geometric optimal control theory to the analysis of the Earth re-entry problem for a space shuttle where the control is the angle of bank, the cost is the total amount of thermal flux, and the system is subject to state constraints on the thermal flux, the normal acceleration and the dynamic pressure. Our analysis is based on the evaluation of the reachable set using the maximum principle and direct computations with the boundary conditions according to the CNES research project\footnote{The project is partially supported by the Centre National d'Etude Spatiales.}. The optimal solution is approximated by a concatenat…
Asymptotics of accessibility sets along an abnormal trajectory
2001
We describe precisely, under generic conditions, the contact of the accessibility set at time $T$ with an abnormal direction, first for a single-input affine control system with constraint on the control, and then as an application for a sub-Riemannian system of rank 2. As a consequence we obtain in sub-Riemannian geometry a new splitting-up of the sphere near an abnormal minimizer $\gamma$ into two sectors, bordered by the first Pontryagin's cone along $\gamma$, called the $\xLinfty$-sector and the $\xLtwo$-sector. Moreover we find again necessary and sufficient conditions of optimality of an abnormal trajectory for such systems, for any optimization problem.
Regularization of chattering phenomena via bounded variation controls
2018
In control theory, the term chattering is used to refer to strong oscillations of controls, such as an infinite number of switchings over a compact interval of times. In this paper we focus on three typical occurences of chattering: the Fuller phenomenon, referring to situations where an optimal control switches an infinite number of times over a compact set; the Robbins phenomenon, concerning optimal control problems with state constraints, meaning that the optimal trajectory touches the boundary of the constraint set an infinite number of times over a compact time interval; the Zeno phenomenon, referring as well to an infinite number of switchings over a compact set, for hybrid optimal co…
Optimal control with state constraints and the space shuttle re-entry problem
2003
In this article, we initialize the analysis under generic assumptions of the small \textit{time optimal synthesis} for single input systems with \textit{state constraints}. We use geometric methods to evaluate \textit{the small time reachable set} and necessary optimality conditions. Our work is motivated by the \textit{optimal control of the atmospheric arc for the re-entry of a space shuttle}, where the vehicle is subject to constraints on the thermal flux and on the normal acceleration. A \textit{multiple shooting technique} is finally applied to compute the optimal longitudinal arc.
Scheduling independent stochastic tasks under deadline and budget constraints
2018
This article discusses scheduling strategies for the problem of maximizing the expected number of tasks that can be executed on a cloud platform within a given budget and under a deadline constraint. The execution times of tasks follow independent and identically distributed probability laws. The main questions are how many processors to enroll and whether and when to interrupt tasks that have been executing for some time. We provide complexity results and an asymptotically optimal strategy for the problem instance with discrete probability distributions and without deadline. We extend the latter strategy for the general case with continuous distributions and a deadline and we design an ef…
Where Alonso meets Sierpinski: an urban economic model of a fractal metropolitan area
2004
International audience; La coexistence d'activités résidentielles et agricoles dans des ceintures périurbaines caractérise de nombreuses aires métropolitaines modernes. Malheureusement, peu de modèles théoriques de l'économie urbaine prennent en compte ce type de mixité spatiale. L'article contribue à combler cette lacune : les auteurs présentent un modèle de localisation résidentielle (standard en économie urbaine) opérant dans un espace structuré par la géométrie spatiale. Plus précisément, d'un côté, un tapis de Sierpinski est utilisé pour rendre compte de l'organisation hiérarchisée de sites urbains et ruraux dans une aire métropolitaine. D'un autre côté, un ménage maximise, sous contra…
Administrative Capacity as a Constraint to Fiscal Decentralization. The Case of Romania and Poland
2020
This paper considers the problem of administrative capacity as one of the main requirements that the accession countries seeking EU membership had to meet, and as a prerequisite to the decentralization of their public sectors and public finances. The selected decentralization problems are analyzed using the cases of two countries: Romania and Poland. The results of a theoretical and practical evaluation of administrative capacity as a likely obstacle to fiscal decentralization are presented taking account of the different levels of decentralization in Poland and Romania. A comparative analysis of Romania and Poland shows that administrative capacity can be a constraint for fiscal decentrali…