Operations Research 2021

Integrative zoning and item-to-zone assignment in pick-and-pass order picking systems – a basic decision model

Ralf Gössinger and Regina Thelen

Order picking is the logistic warehouse process of consolidating stored items according to customer orders. Since it is one of the most cost- and labor-intensive operations, the management of these activities is an important driver of warehouse performance. Pick-and-pass systems are zoned systems in which each picker only works at one zone and each zone only comprises a sub-set of items. Hence, order-related containers are routed along a sequence of zones to be filled up with the items needed for order fulfillment.

When designing a pick-and-pass system two organizational problems need to be solved. How should the zones be formed and which item should be assigned to which zone? Usually, these problems are tackled in such a way that assignment is done after the zones were formed (sequential approach). This allows for fast solutions, but does not guarantee optimal solutions as soon as both problems are strongly interrelated. For minimizing the makespan of a pick-and-pass system, two time components are relevant: the time it takes to forward a container between zones, and the time it takes in a zone to put the required items in the container. Since both decisions unfold different impact on both time components, these problems are interrelated. However, general statements about the strength of interdependency are not available.

In the intended paper, we propose a decision model that simultaneously decides on zone sizes and item-to-zone assignment in a makespan minimizing way. In order to identify drivers of interdependency, we conduct a numerical study in which the proposed model is compared to a sequential approach. Therefore, factors are varied that are hypothesized to have an impact on the strength of interdependence between the two decision problems.

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