Order scheduling of multiple stacker cranes on common rails in an automated storage/retrieval system

Abstract

An order scheduling method for improving the work efficiency of multiple cranes on a common rail in an automated storage/retrieval system (AS/RS) is proposed in this paper. Most AS/RSs are equipped with one stacker crane. However, this is logistically challenging, and higher work efficiency in warehouses, such as those using more than one stacker crane, is required. In this paper, a warehouse with multiple stacker cranes working simultaneously is proposed. Unlike warehouses with only one crane, order scheduling is very difficult because there are multiple cranes working simultaneously and collisions are possible. Warehouse order must be maintained while avoiding collisions. Sometimes, detours are necessary to avoid collisions. As a result, work efficiency will decrease if a sequence of orders is not properly planned. Nevertheless, as transport works in AS/RSs are occurring randomly, scheduling cannot be conducted in advance, which makes the order scheduling of multiple cranes difficult. A fast order scheduling method is proposed here. Several orders are combined into an order cluster, and dynamic programming is used to obtain the best combination of order clustering. By utilising this method, cranes can work cooperatively, improving work efficiency to 0.95 with a two-crane AS/RS and to 0.78 with a three-crane AS/RS.