A throughput model for a dual-tray Vertical Lift Module with a human order-picker

Dual-tray Vertical Lift Modules (VLMs) are relatively new systems not yet addressed in automated storage and retrieval (AS/R) systems research. This paper aims to contribute in this field both for theory and practice. A throughput model for single dual-tray Vertical Lift Module automated storage and retrieval system with a human order-picker is presented. The model is developed on the basis of two previous models of mini-load AS/RS and single-tray VLM presented in literature. Results of testing in comparison with simulation models indicate that the proposed model performs very well in estimating system throughput. Analysis of throughput for various VLM configurations and for both assumed deterministic and exponentially distributed pick time per tray showed significant differences, leading to the conclusion that real distributions of pick times have a non-negligible influence on system throughput. This parameter should be taken with caution when estimating system cycle time and throughput. The model׳s results provide guidelines for throughput estimation for designers and managers of warehousing systems. Together with previously developed throughput models for single-tray VLM and VLM pods, they are able to evaluate different VLM-based solutions for order-picking.