Real-time control and scheduling of flexible manufacturing systems: An ordinal optimisation based approach

A unique real-time control and scheduling framework for flexible manufacturing systems (FMS) is presented in this paper. The framework enables the adoption of different scheduling policies for short-term intervals when responding to the dynamic changes of the FMS shop floor status. Each time when rescheduling is called for, standard clock (SC) simulation is first employed to evaluate the performance of a set of scheduling policies for a short planning horizon. The ordinal optimisation concept is then used to choose quickly the most desirable scheduling policy. Owing to the use of the standard clock technique and the ordinal optimisation concept, this framework accomplishes a dramatic reduction in the time needed for decision making, the essential requirement for real-time control. It is also found that as the scale of the problem increases, the decision-making time increases linearly rather than exponentially. These two important features indicate that this framework has the potential for being successfully implemented in real FMS settings. Although the framework cannot always guarantee the global best performance, the case study indicates that satisfactory performance results are always achieved by using this framework.