Maintaining Robust Schedules by Fuzzy Reasoning

Practical scheduling usually has to react to many unpredictable events and uncertainties in the production environment. Although often possible in theory, it is undesirable to reschedule from scratch in such cases. Since the supplier of raw materials and clients will be prepared for the predicted schedule it is important to change only those features of the schedule that are necessary.We show how on one side fuzzy logic can be used to support the construction of schedules that are robust with respect to changes due to certain types of events. On the other side we show how a reaction can be restricted to a small environment by means of fuzzy constraints and a repair-based problem-solving strategy.We demonstrate the proposed representation and problem-solving method by introducing a scheduling application in a steelmaking plant. We construct a preliminary schedule by taking into account only the most likely duration of operations. This schedule is iteratively “repaired” until some threshold evaluation is found. A repair is found with a local search procedure based on tabu search. Finally, we show which events can lead to reactive scheduling and how this is supported by the repair strategy.