Sequencing optimization of shuttle-based storage and retrieval systems with multiple shuttle carriers

The shuttle-based storage and retrieval system with multiple shuttle carriers is an emerging advancement in automated warehousing. This system is partitioned into multiple disjoint zones, each spanning several tiers and serviced by a dedicated shuttle carrier. Unlike conventional shuttles limited to horizontal movements and bins that must be deposited at their storage tiers, shuttle carriers can move both horizontally and vertically simultaneously, enabling bin pickup and drop-off at any tier. Consequently, executing retrieval requests requires coordinated operations between the lift and shuttle carriers, along with optimal tier selection for bin placement. We focus on the retrieval sequencing problem in this paper, which involves determining the retrieval sequence for each shuttle carrier and lift, and the tier selection for dropping off each request, to minimize the makespan for a given set of retrieval requests. We introduce an adaptive large neighborhood search algorithm tailored to deliver near-optimal solutions efficiently. Compared to existing methods found in both literature and practical applications, our algorithm can reduce the makespan by up to 11.8%. We also implement the proposed algorithm with real data in a practical scenario, showing significant performance gains over the company’s current method. Furthermore, our analysis demonstrates that utilizing fewer shuttle carriers achieves throughput capacity comparable to tier-to-tier shuttles.