13. Logistik Management Konferenz (LM 2023)
A comparison of alternative approaches to zoning and item-to-zone assignment in pick-and-pass order picking systems
Ralf Gössinger, Regina Thelen
Order picking is the warehousing process of consolidating stored items according to customer orders. Since it is one of the most costly and labor intensive operations, managing these activities is a key driver of warehouse performance. Our focus is on pick-and-pass systems, where the warehouse is divided into zones, with each picker only working in one zone and a sub-set of the items being stored in each zone. Hence, order-related containers are routed along a sequence of zones to be filled with the items needed for order fulfillment.
When designing a pick-and-pass system, two organizational problems need to be solved: While the zoning problem (ZP) deals with the question of how the zones should be formed, the storage location assignment problem (SLAP) refers to the question of which item should be assigned to which zone. The SLAP can be solved at different levels of aggregation. In the most detailed case, the individual items are allocated to bin locations. At the aggregate level relevant to the reasoning in the intended paper, the items are assigned to zones without specifying the bin location within the zone.
The interdependence of zoning and storage location decisions in respect of duration or distance minimization objectives can be traced back to the following relationships. The path length within a zone increases with the number of bin locations (zone size), since the dimensions of the zone increase and the bin locations contained in a zone-internal picking route are more widely dispersed. The strength of this effect is influenced by the item-to-zone assignment, which is typically based on both the frequency of item demand and the cross-correlation between the demand for different items. In this way, items with high demand frequency should be assigned to zones closer to the I/O point. Also, items with high positive correlation should be stored in the same zone or adjacent zones to minimize the number of zones that need to be crossed for order fulfillment. Both principles reduce the path length of cross-zone picking routes, but the zoning unfolds an ambivalent impact: The effectiveness of frequency-based item-to-zone assignment decreases with the zone size. The opposite effect arises with regard to the effectiveness of correlation-based item-to-zone assignment.
In the literature, these two problems are tackled through a sequential approach where the SLAP is solved after the ZP has been solved. This enables short solution times, but does not guarantee optimal solutions, since the neglected influence of storage location decisions on zoning decisions leads to coordination deficits.