Routing policies and cube-per-order index-based storage policies in picker-to-part systems

The paper evaluates and compares the expected travel distance for different routing strategies – namely traversal and return policies — in low-level picker-to-part systems. Items are assigned to storage locations on the basis of the ratio of the required space to the order frequency (cube-per-order index or COI). The focus is on narrow-aisle systems, in which the distance travelled crossing the aisle from one side to the other is negligible compared to the distance travelled along the centreline of the aisle. For both routing policies, an efficient COI-based stock location assignment strategy is first developed. Second, analytical models are derived which relate the expected travel distance required to fill an order to the main system parameters (i.e. the COI-based ABC curve; the number of picks in a tour; the number, length and width of aisles). Simulation results confirming the accuracy of the analytical models are presented. Finally, preference regions as a function of the number of picks in a tour and differently skewed COI-based ABC curves are given for traversal and return policies.