Coordinating Multiple Shuttles in Four-Way AS/RS: Routing, Traffic Control and Deadlock Avoidance

A four-way shuttle automated storage and retrieval system can move pallets both horizontally and vertically within a high-density rack, but adding more shuttles does not raise throughput proportionally. Multi-shuttle scheduling assigns storage and retrieval tasks across the fleet while coordinating routes, priorities and shared resources to keep the system stable. Shuttles competing for aisles, intersections, transfer points and lifts can cause waiting, repeated replanning and lost productive time. Uneven task allocation can leave vehicles idle, and resource conflicts can produce deadlocks even when physical collisions are avoided. Effective control therefore has to coordinate the whole material-handling chain, including lifts and conveyors, rather than optimize shuttle movement alone.