23. International Working Seminar on Production Economics 2024

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 logistic warehouse process of consolidating stored items according to customer orders. Pick-and-pass systems are zoned systems in which each picker only works in one storage 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 questions need to be answered: How should the zones be formed and which item should be assigned to which zone? Answers to these questions can be given by solving corresponding optimization problems. Since both problems are strongly interrelated, we developed an integrative decision model that simultaneously determines zone size and item-to-zone assignment. The decision model is solved for a set of problem instances using both an exact approach and variants of the fix-and-optimize heuristic. The heuristic, starting from an initial solution, iteratively solves different sub-problems where a part of the decision variables is fixed.

In the intended paper, we analyze how the performance of the heuristic can be improved by generating the initial solution, specifying a fixation strategy, and limiting the number of iterations. To this end, we develop variants of the fix-and-optimize approach and conduct a full-factorial numerical study in which the performance of the variants is compared with each other and with the exact approach. Therefore we evaluate the model and the solution approaches in terms of solution quality, solution time and solution structure.

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