Scheduling flexible manufacturing systems based on timed Petri nets and fuzzy dispatching rules

To explicitly formulate concurrent activities shared resources, precedence constraints and routing flexibility in FMS scheduling, we adopt timed (place) Petri nets for problem representation. The bottom-up method is used to synthesize the system, i.e., the system is partitioned into sub-systems according to the job types, sub-models are constructed for each sub-systems, and a complete net model for the entire process is obtained by merging Petri nets of the sub-systems through the places representing the shared resources. Considering each transition in a conflicting set corresponds to each part type which competes for an available resource for next operation, the fuzzy dispatching rules are employed to select one of the enabled transitions to be fired in each conflicting set. They are derived from ordinarily adopted dispatching rules, such as SPT (shortest processing time) and S/RO (slack per remaining operation). An interpretation algorithm using these rules is given and illustrated through an FMS example.