Estimating picker blocking in wide-aisle order picking systems

In designing an Order Picking System (OPS) with multiple pickers, the designing (or selection) of several parameters (e.g., width of aisles, storage system and picking strategy) is dependent on the blocking that occurs between pickers. In this paper analytical models to estimate blocking in an OPS that has picking aisles wide enough to allow pickers to pass other pickers in the aisle are developed. In such OPSs, pickers can experience blocking at a pick face when two or more pickers need to pick at the same pick face. The developed models are compared with simulation, with results indicating that the proposed models are sufficiently accurate. Test results suggest that when pickers pick one SKU at a pick face, blocking is less in a wide-aisle OPS compared to that in a narrow-aisle OPS. However, when pickers pick more than one SKU at a pick face, blocking increases monotonically with an increase in the number of SKUs picked. The last result is significant since it highlights the importance of the proposed model that considers the variation in the time the picker is stopped to pick.[Supplementary materials are available for this article. Go to the publisher's online edition of IIE Transactions for the following free supplemental resources: (i) Appendix A, which describes the procedure to obtain a closed-form expression for b1(2); (ii) Appendix B, which describes the derivations of the distributions for the case when pickers pick one SKU and pick:walk time ratio is ∞:1; and (iii) Appendix C, which describes the derivations of the distributions for the case when pickers may pick more than one SKU and pick:walk time ratio is ∞:1.]