Spare zone based hierarchical motion coordination for multi-AGV systems

Abstract

Automated guided vehicles (AGVs) are widely used in various material handling systems due to their high efficiency and low cost, particularly in warehouses. However, deadlock resolution among vehicles is still a thorny issue. Many algorithms re-plan motion paths to avoid deadlocks resulting in greatly increasing the computational complexity, while other deadlock resolution algorithms in which vehicles have fixed paths are less efficient. This paper proposes a spare zone based hierarchical (SZH) motion coordination algorithm supporting time-efficient collision-and-deadlock resolution among vehicles. Unlike previous deadlock resolution methods in which the vehicles can change their paths to avoid deadlocks at any time, SZH avoids deadlocks just by locally adjusting their paths in a decentralized manner. The introduction of a spare zone mechanism ensures reliable resolution of different types of collision and deadlocks while achieving high time efficiency. The allocation of spare zones for each vehicle before performing their missions guarantees that there exist enough free zones to which the vehicles taking part in deadlocks can temporary departs from their current path to avoid deadlocks. In order to improve the efficiency, the vehicle’s spare zones are updated as soon as the vehicle arrives at a zone. The algorithm is realized based on a hierarchical strategy, where the top layer considers the allocation and update of spare zones, while the bottom layer considers the coordination among vehicles within two zones. Extensive simulation results demonstrate the feasibility and efficiency of the proposed motion coordination algorithm.