Integrating simulation and optimization to schedule a hybrid flow shop with maintenance constraints

The scheduling problems have been discussed in the literature extensively under. the assumption that machines are continuously available. However, in most real life industrial settings a machine can be unavailable for many reasons, such as unforeseen breakdowns (stochastic unavailability) or due to a scheduled preventive maintenance where the periods of unavailability are known in advance (deterministic unavailability). In this paper, we deal with the hybrid flow shop scheduling problem under maintenance constraints to optimize several objectives based on flow time and due date. In this model, we take also on consideration setup, cleaning and transportation times. This paper has three goals. The first is to show how we can integrate simulation and optimization to tackle this practical problem which is NP-hard on the strong sense. The second is to illustrate by an experimentation study that the performance of heuristics applied to this problem can be affected by the percentage of the breakdown times. The last is to show that this approach can perform better than NEH heuristics under certain conditions. (C) 2004 Elsevier Ltd. All rights reserved.