A real life order picking system consisting of a set of unidirectional picking lines is investigated. Batches of stock keeping units (SKUs) are processed in waves defined 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 differ only in size classification, 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. Different 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 investigated in literature. All of these approaches seek to minimise walking distance only and include mathematical formulations and greedy heuristics. Four different 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 effectively. 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.
An 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.
An order picking system that requires pickers to move in a clockwise direction around a picking line with fixed locations is considered. The problem is divided into three tiers. The tier in which orders must be sequenced is addressed. Eight tour construction heuristics are developed and implemented for an order picking system operating in unidirectional picking lines. Two classes of tour construction heuristics -the tour construction starting position (TCS) and the tour construction ending position (TCE) -are developed to sequence orders in a picking line. All algorithms are tested and compared using real life data sets. The best solution quality was obtained by a TCE heuristic with adaptations. OPSOMMING'n Stelsel vir die opmaak van bestellings word ondersoek. Die stelsel vereis dat die werkers in 'n kloksgewyse rigting om 'n uitsoeklyn beweeg. Die probleem is verdeel in drie vlakke van besluite. Die besluit wat handel oor die volgorde waarin bestellings opgemaak word, word ondersoek. Agt toer-konstruksie-heuristieke is ontwikkel en geïmplementeer waarin die bestellings in 'n eenrigting uitsoeklyn opgemaak word. Twee klasse toer-konstruksieheuristieke -die toer-konstruksie-beginposisie (TCS) en die toer-konstruksie-eindposisie (TCE) -is ontwikkel om die volgorde van bestellings in 'n uitsoeklyn te bepaal. Al die algoritmes word getoets en vergelyk vir werklike datastelle. Die beste oplossingskwaliteit is verkry deur 'n TCE-heuristiek met aanpassings.
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