Abstract
The shuttle-based storage and retrieval system (SBS/RS) is composed of a shuttle sub-system that is responsible for horizontal movements and a lift sub-system that is responsible for vertical movements. As the combination of the two sub-systems yields high flexibility, low operating cost and large storage capacity, SBS/RS is becoming more and more popular, but also raises managerial issue of how to coordinate the shuttle and the lift. Based on its operational processes, this study models the tier-to-tier SBS/RS system as a semi-open queuing network (SOQN). By removing or adding the synchronization nodes, the SOQN is further transformed into two different closed queuing networks (CQNs), and applies the approximate mean value analysis (AMVA) algorithm to solve the model and estimate its performance. The system performance is measured by the utilization of shuttles, the utilization of lift, and the task cycle time, under various design configurations. Simulation is carried out to validate the effectiveness of the analytical model and algorithm. Compared with the simulation results, the established semi-open queuing network can accurately estimate task cycle time for different rack configurations. The proposed solution method can help to identify the optimal number of shuttles and guide the design of the SBS/RS system.
Highlights
The shuttle-based storage and retrieval system (SBS/RS) is a new type of automated warehousing systems, which is mainly applied to stock keeping units (SKUs) with high usage frequency
An semi-open queuing network (SOQN) is established based on a detailed analysis of the movement processes of shuttles and the lift
By removing or adding the synchronization node, the SOQN is further transformed into two different closed queuing networks (CQNs), and the approximate mean value analysis (AMVA) algorithm is used to solve the model and obtain the system performance
Summary
The shuttle-based storage and retrieval system (SBS/RS) is a new type of automated warehousing systems, which is mainly applied to stock keeping units (SKUs) with high usage frequency. According to whether shuttles can perform cross-tier tasks with the help of a lift, two configurations can be defined: SBS/RS with tier-to-tier shuttles and that with tier-captive shuttles. If one shuttle breaks down, for the tier-captive system, no SKUs of the tier with the failed shuttle can be stored or retrieved. In the tier-to-tier SBS/RS, lifts serve the vertical movement, while shuttles serve the horizontal movement of SKUs. Each shuttle can reach any tier of the storage rack with the help. For the tier-to-tier system, even if one shuttle does not work, other shuttles can be dispatched to perform storage or retrieval tasks. The aim of this paper is to establish such an analytical model for the tier-to-tier SBS/RS and to provide guidance for warehousing system designers. Section six concludes the paper with a summary of major findings and the direction of further research
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