Optimal routing of multiple-load automated guided vehicles subject to last-in-first-out loading constraints

When simple automated guided vehicles (AGVs) having no random access load transfer mechanism are used for carrying multiple loads between workstations, the loads cannot be handled independently. This paper considers the case when loads are placed in flat pallets and each new picked up pallet is loaded on the top of batch of pallets already carried by the AGV. To avoid use of excessive space and time needed to reorder pallets in the batch, the loading–unloading procedures should be performed in accordance with last-in-first-out (LIFO) rule. In this paper we formulate the condition of existence of AGV routes in which it visits each workstation only once and meets LIFO constraint. We also suggest an algorithm for finding the shortest one among such routes. Examples are provided to illsutrate the performance of the algorithm.