Analytical models for an automated storage and retrieval system with multiple in-the-aisle pick positions

An automated storage and retrieval system with multiple in-the-aisle pick positions (MIAPP-AS/RS) is a case-level order fulfillment technology that enables order picking via multiple pick positions (outputs) located in the aisle. This article develops expected travel time models for different operating policies and different physical configurations. These models can be used to analyze MIAPP-AS/RS throughput performance during peak and non-peak hours. Moreover, closed-form approximations are derived for the case of an infinite number of pick positions, which enable the optimal shape configuration that minimizes expected travel times to be derived. The expected travel time models are compared with a simulation model of a discrete rack, and the results validate that the proposed models provide good estimates. Finally, a numerical experiment is conducted to illustrate the trade-offs between performance of operating policies and design configurations. It is found that MIAPP-AS/RS with a dual picking floor and input point is a robust configuration due to the single command operating policy having a comparable throughput performance to a dual-command operating policy.