A decomposition approach to dual shuttle automated storage and retrieval systems

Automated storage and retrieval systems (AS/RS) have become vital in today’s distribution and production environments, but it is still necessary to equip them with more efficient operational control policies. Motivated by real situations encountered in companies using AS/RS, the present paper studies a miniload AS/RS system, with a dual shuttle crane in which a set of storage and retrieval requests have to be scheduled in such a way that the prioritized waiting time is minimized. Dual shuttle cranes have been less studied in literature and they pose new problems that have to be solved. We address the problem by decomposing it into a location assignment problem and a sequencing problem. Different heuristic strategies are proposed for making the assignments, while for optimizing the sequence we propose a general mathematical model and an efficient branch and bound procedure. In addition, a fast metaheuristic is also developed to tackle larger instances. A set of real-world based benchmarks with different characteristics is generated to evaluate the proposed methods. Only for very small instances an optimal sequence can be found in reasonable calculation time. The experiments show the effectiveness of the proposed heuristic decomposition method.