Petri net modeling and real-time control of dual-arm cluster tools with residency time constraint and activity time variations

Because of residency time constraints and activity time variation of cluster tools, it is very difficult to operate them. This paper addresses their real-time control issues. A Petri net (PN) model is developed to model them. This model describes the robot activity sequence with robot waits included. Via this model, to operate a cluster tool is to determine robot wait times and a two-level operation architecture is proposed, which includes an off-line periodic scheduler and real-time controller. By this approach, the system can adapt to random activity time variation. A cluster tool derived as a not-always-schedulable system by the existing methods is shown always-schedulable by using the proposed innovative approach.