Petri Net Modeling and Deadlock-Free Scheduling of Attachable Heterogeneous AGV Systems

The increasing demand for automation and flexibility drives the widespread adoption of heterogeneous automated guided vehicles (AGVs). This work intends to investigate a new scheduling problem in a material transportation system consisting of attachable heterogeneous AGVs, namely carriers and shuttles. They can flexibly attach to and detach from each other to cooperatively execute complex transportation tasks. While such collaboration enhances operational efficiency, the attachment-induced synchronization and interdependence render the scheduling coupled and susceptible to deadlock. To tackle this challenge, Petri nets are introduced to model AGV schedules, well describing the concurrent and sequential task execution and carrier-shuttle synchronization. Based on Petri net theory, a firing-driven decoding method is proposed, along with deadlock detection and prevention strategies to ensure deadlock-free schedules. Furthermore, a Petri net-based metaheuristic is developed in an adaptive large neighborhood search framework and incorporates an effective acceleration method to enhance computational efficiency. Finally, numerical experiments using real-world industrial data validate the effectiveness of the proposed algorithm against the scheduling policy applied in engineering practice, an exact solver, and four state-of-the-art metaheuristics. A sensitivity analysis is also conducted to provide managerial insights.