SKU arrangement on a unidirectional picking line

Abstract

AbstractAn order picking system consisting of independent unidirectional picking lines has been investigated in this study. A picking line forms a type of unidirectional carousel as multiple pickers walk in a clockwise direction around a conveyor belt running down the centre of the picking line. There are three decision tiers in this order picking system, namely the assignment of stock keeping units (SKUs) to different waves, the arrangement of SKUs into locations on a picking line for a wave of picking and the sequencing of orders. Each tier has an objective to minimise the total travel distance of pickers. The focus of this paper is on the arrangement of SKUs on a picking line for a wave of picking. An exact mathematical formulation is introduced which is not solvable to optimality for large real‐life instances. Two heuristics known to be optimal for certain carousel systems as well as two further heuristics using SKU correlations are therefore tested. The heuristic approaches are compared against lower bounds generated by relaxing the exact formulation and a set of random solutions. Known algorithms for carousel systems are not optimal in this carousel system and all heuristic approaches perform approximately equally well. It is shown that the gains from addressing the order sequencing decision tier outweighs that of the SKU arrangement decision tier. Moreover, the complexity of addressing the entire system of decision can be reduced by arranging SKUs with a simple heuristic having a negligible impact on solution quality.