3 research outputs found
Analytical evaluation of the output variability in production systems with general Markovian structure
Performance evaluation models are used by companies to design, adapt, manage and control their production systems. In the literature, most of the effort has been dedicated to the development of efficient methodologies to estimate the first moment performance measures of production systems, such as the expected production rate, the buffer levels and the mean completion time. However, there is industrial evidence that the variability of the production output may drastically impact on the capability of managing the system operations, causing the observed system performance to be highly different from what expected. This paper presents a general methodology to analyze the variability of the output of unreliable single machines and small-scale multi-stage production systems modeled as General Markovian structure. The generality of the approach allows modeling and studying performance measures such as the variance of the cumulated output and the variance of the inter-departure time under many system configurations within a unique framework. The proposed method is based on the characterization of the autocorrelation structure of the system output. The impact of different system parameters on the output variability is investigated and characterized. Moreover, managerial actions that allow reducing the output variability are identified. The computational complexity of the method is studied on an extensive set of computer experiments. Finally, the limits of this approach while studying long multi-stage production lines are highlighted. © 2013 Springer-Verlag Berlin Heidelberg
Commande optimale stochastique appliquée aux systÚmes manufacturiers avec des sauts semi-Markoviens
Les travaux de ce mĂ©moire sont constituĂ©s de deux parties principales. La premiĂšre partie tente de formuler un nouveau modĂšle du problĂšme de commande optimale stochastique de systĂšmes sur un horizon fini. Les systĂšmes considĂ©rĂ©s sont soumis Ă des phĂ©nomĂšnes alĂ©atoires dits sauts de perturbation qui sont modĂ©lisĂ©s par un processus semi-Markovien. Ces sauts de perturbation traduits par des taux de transition dĂ©pendent de lâĂ©tat du systĂšme et du temps. Par consĂ©quent, le problĂšme de commande est formulĂ© comme un problĂšme dâoptimisation dans un environnement stochastique. La deuxiĂšme partie vise Ă modĂ©liser des systĂšmes de production flexible (SPF). Dans ce mĂ©moire, ces SPF se composent de plusieurs machines en parallĂšles, ou en sĂ©rie, ou dâune station de travail (une machine reprĂ©sentative). Ces machines sont sujettes Ă des pannes et Ă des rĂ©parations alĂ©atoires. Lâobjectif de la modĂ©lisation est de dĂ©terminer les taux de production u(t) de ces machines en satisfaisant les fluctuations de demande d(t) sur un horizon fini.
Dans ce mémoire, nous avons :
(a) proposĂ© un nouveau modĂšle du problĂšme dâoptimisation dans un environnement stochastique sur un horizon fini pour deux cas; avec taux dâactualisation (Ï > 0) et sans taux dâactualisation (Ï = 0);
(b) modélisé des SPF en déterminant une stratégie de commande plus réaliste incluant stratégie de production;
(c) prĂ©sentĂ© des exemples numĂ©riques Ă lâaide dâune mĂ©thode de Kushner et Dupuis (2001)
Fluctuations of the production output of transfer lines
The fluctuations around the average throughput delivered by simple production system were investigated using a fluid modeling approach. A special consideration is paid to the buffered production dipole for which an explicit estimation for an stationary variance of the throughput is calculated