Dynamic Scheduling for a Flexible Manufacturing System - The Pre-emptive Approach

This paper presents a simulation model of a flexible manufacturing system (FMS) which minimises three performance criteria simultaneously, i.e. mean flow-time, mean tardiness, and mean earliness. The dispatching rule will be changed at a frequency that is varied by the quantity of output produced by the system. Therefore, the dynamic system is an event-trigger rather than triggered by changing the rule at regular time intervals, which is passive. Three indices are used to represent the three criteria being monitored, and the indices are ranked in descending order. The larger the index, the worse is the condition of the criterion in the system. An appropriate rule will be selected for the next operation in order to tackle that criterion with the largest index. This mechanism is called the pre-emptive method. Furthermore, the indices can be biased by the decision maker so that a particular criterion can have larger weighting. Results show that a solution (range of frequency) can always be obtained for changing the dispatching rule so that the system is better than one which just uses fixed FMS scheduling rules.