Algorithms for on-line order batching in an order picking warehouse

In manual order picking systems, order pickers walk or ride through a distribution warehouse in order to collect items required by (internal or external) customers. Order batching consists of combining these – indivisible – customer orders into picking orders. With respect to order batching, two problem types can be distinguished: in off-line (static) batching, all customer orders are known in advance; in on-line (dynamic) batching, customer orders become available dynamically over time. This paper considers an on-line order batching problem in which the maximum completion time of the customer orders arriving within a certain time period has to be minimized. The author shows how heuristic approaches for off-line order batching can be modified in order to deal with the on-line situation. In a competitive analysis, lower and upper bounds for the competitive ratios of the proposed algorithms are presented. The proposed algorithms are evaluated in a series of extensive numerical experiments. It is demonstrated that the choice of an appropriate batching method can lead to a substantial reduction of the maximum completion time.