ORDONNANCEMENT EN TEMPS REEL DANS LES PROBLEMES A EN-COURS LIMITES

In this thesis, a production system with two characteristic is considered. First, the processing time of an operation to be performed in a given time interval can be selected. Such a production system is said to have controllable processing times. Second, the operations of a product follow each other without waiting. A production system having this characteristic is said to be no-wait. The two characteristics are key in scheduling of many different types of industrial systems. The proposed methodology allows one to control the work-in-process and production cycle time. The system productivity is optimised with original algorithms developed in this research. The analysis of computational complexity of the seventeen algorithms shows that they can be used for on-line scheduling.Dans cette thèse, nous montrons l'intérêt d'étudier des systèmes de production présentant les deux caractéristiques suivantes. Tout d'abord, les durées des opérations à réaliser sont choisies dans des intervalles donnés -de tels systèmes de gestion de fabrication sont dits à temps opératoires contrôlables-. De plus, les opérations successives réalisées sur chaque produit se suivent sans temps d'attente -de tels systèmes sont dits sans attente-.Ces deux caractéristiques permettent de gérer en temps réel un large éventail de systèmes de fabrication. En outre, cette approche permet le contrôle des en-cours et des temps de fabrication.La production du système de fabrication est optimisée grâce à des algorithmes originaux. L'analyse de la complexité des dix-sept algorithmes proposés prouvent leur compatibilité avec leur utilisation en temps réel

High intensity behavior of pyroelectric photorefractive self-focusing in LiNbO3

International audienceThe formation of self-confined beams using pyroelectric effect is numerically and experimentally studied in photorefractive LiNbO3. For a given crystal temperature change, the trapped beam width is shown to be less efficient as intensity is increased. Numerical calculations reveal that the induced refractive-index profile varies along propagation for large intensities due to a nonlinear photovoltaic effect. Moreover, it eventually gives beam splitting for intensities greater than a threshold intensity that depends on LiNbO3 composition

The PERT Problem with Alternatives: Modelisation and Optimisation

Management of projects often requires decisions concerning the choice of alternative activities. The completion time of the whole project (i.e. the makerpan) is computed subsequently. In this paper, we aim at selecting the activities and computing the makespan simultaneously. This problem is referred to as PERT Problem with Alternatives (PPA). The corresponding model is similar to a conventional PERT graph, except that two types of nodes are introduced to represent either the choice between activities, or the fact that a set of activities should be completed before starting a subsequent set of activities. In this paper, we analyse the PPA and we propose a pseudo-polynomial algorithm to solve it

Scheduling and Controlling Work-in-Process : An on Line Study for Shop Problems

In this paper, we address the problem of production systems having two characteristics. First, the manufacturing times can be chosen between given bounds. Such a production system is said to have controllable processing times. Second, an operation must start as soon as the previous operation on the same part (if any) is completed. A production system having this characteristic is said to be no-wait. Several on-line schedules are considered to minimize the makespan in flow shop and job shop situations. We prove that in the worst case, the makespan provided by these schedules is m times longer than the optimal one (for different flow shops and job shops), m being the number of machines. We give several related results on competitive ratio

On-line Scheduling in Assembly Processes

The assembly system under consideration is composed with several machines, and some of these machines may be identical or able to perform the same operations. The manufacturing system is fully automated and, semi-finished products or components are not stored during the process. A limited flexibility exist since the manufacturing times can be extended within certain limits at the expense of the unavailability of the resource. There are no conflicts between the resources; in other words, the same machine cannot be used to perform different operations for a same product. Due to the intensity of the flow of products to be manufactured. It is not allowed to reschedule products which have been previously scheduled. Thus, when a new product requirement arrives in the system, we have to take advantage of the idle time windows. The goal is to complete the product as soon as possible. We give a real-time scheduling algorithm which guarantees an optimal makespan to any product which arrives in the assembly system. A numerical example is provided to illustrate this approach

Towards Optimization of Cyclic Production Systems

In this paper, the expression "production systems" refers to flow-shops, job-shops, assembly systems, Kanban systems and, in general, to any Discrete Event System (DES) which transforms raw material and/or components into products and/or components. Such a system is said to be cyclic if it provides indefinitely the same sequence of products. A schedule of a cyclic production system is defined as soon as the starting time of each operation on the related resource is known. It has been showed that, whatever the feasible schedule applied to the cyclic production system, it is always possible to fully utilize the bottleneck resource. In other words, it is always possible to maximize the productivity of such a system. As a consequence, we aim at finding the schedule which permits to maximize the productivity with a Work-In-Process (WIP) as small as possible. We propose a heuristic approach based on Petri nets to find a near-optimal, if not optimal, solution. We also give a sufficient condition for a solution to be optimal

Production Planning Using Mixed Lots

In this paper, we consider the production planning of a manufacturing system capable of producing several types of part. The demands are known at the end of each elementary period (at the end of each day or week for instance). The objective function (i.e. the criterion) is the sum of inventory, backlogging and sep-up costs on a finite horizon. We use the concept of lot to simplify management decisions: instead of launching in production parts type by type, we launch lots. A lot is a mix of parts of different types. The drawback is the definition of the manufacturing times of the lots. We consider that the manufacturing time of a lot is its manufacturing time in steady state: it is a way to associate a manufacturing time to each lot, independently from the work-in-process and the set-up times. As a consequence, we have to launch identical lots in series to reduce the error made by introducing this hypothesis. In this paper, we propose a heuristic algorithm and a branch-and-bound approach to solve this problem. Numerical examples are provided

Electric Vehicles: Effect of the Availability Threshold on the Transportation Cost

This paper addresses the transportation problem using public electric cars. At each car station, a decision, concerning the cars which should become available to customers, has to be taken. We assume that a vehicle is available when its charge is greater than a given threshold. Our goal is to optimize this threshold

On PERT Networks with Alternatives

Management of projects often requires decisions concerning the choice of alternative activities. Then, the completion time of the whole project (i.e. the makerpan) is computed. In this paper, we aim at selecting the required activities simultaneously with the computation of the makespan. This problem is referred to as PERT Problem with Alternatives (PPA). The corresponding model is similar to a conventional PERT graph, except that two types of nodes are involved to represent either the choice between activities, or the fact that a set of activities should be completed before starting another set of activities. A formalization of the problem and some important properties concerning the optimal solution are given. Several well- solvable cases of the problem and a powerful decomposition algorithm running in polynomial time are presented. This decomposition is applicable for solving many real-life problems