SKU assignment to unidirectional picking lines using correlations

J Matthews,Se Visagie

doi:10.5784/31-2-531

Abstract

A real life order picking system consisting of a set of unidirectional picking lines is inves- tigated. Batches of stock keeping units (SKUs) are processed in waves dened as a set of SKUs and their corresponding store requirements. Each wave is processed independently on one of the parallel picking lines as pickers walk in a clockwise direction picking stock. Once all the orders for a wave are completed a new mutually exclusive set of SKUs are brought to the picking line for a new wave. SKUs which dier only in size classication, for example small, medium and large shirts, are grouped together into distributions (DBNs) and must be picked in the same wave. The assignment of DBNs to available picking lines for a single day of picking is considered in this paper. Dierent assignments of DBNs to picking lines are evaluated using three measures, namely total walking distance, the number of resulting small cartons and work balance. Several approaches to assign DBNs to picking lines have been in- vestigated in literature. All of these approaches seek to minimise walking distance only and include mathematical formulations and greedy heuristics. Four dierent correlation measure are introduced in this paper to reduce the number of small cartons produced and reduce walking distance simultaneously. These correlation measures are used in a greedy insertion algorithm. The correlation measures were compared to historical assignments as well as a greedy approach which is known to address walking distances eectively. Using correlation measures to assign DBNs to picking lines reduces the total walking distance of pickers by 20% compared to the historical assignments. This is similar to the greedy approach which only considers walking distance as an objective, however, using correlations reduced the number of small cartons produced by the greedy approach. Key words : SKU assignment, order picking, assignment problems, combinatorial optimisation.

Highlights

Warehouses form a central part of supply chains
A real life order picking system where re-slotting is performed on a daily basis as implemented by PEP was investigated
The order picking system consisted of unidirectional picking lines in a forward pick area where all the piece picking is processed

Summary

Introduction

Warehouses form a central part of supply chains. The role of warehouses is typically to match supply with demand and to consolidate product from multiple suppliers [2]. All the SKUs associated with the same DBN are placed on the same picking line ensuring that all the SKUs in the same DBN arrive at the store at the same time This process of populating, picking and clearing stock on a picking line may take anything from four hours to two days depending on the number and size of orders associated with, and the characteristics of, the SKUs assigned to that wave on the picking line. Significant improvements were made on the historical method by using both integer programming (IP) and heuristic approaches They pointed out that focusing on walking distance alone resulted increased the number of small cartons produced, as many stores required a small volume of stock from certain picking lines.

Literature review

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Conclusion