Order Batching and Picker Routing in manual order picking systems: the benefits of integrated routing

Order Batching and Picker Routing Problems arise in warehouses when items specified by customer orders have to be retrieved from their storage locations. The Order Batching Problem includes the grouping of a given set of customer orders into feasible picking orders such that the total length of all picker tours is minimized. In order to calculate the length of a picker tour, the sequence has to be determined according to which the items contained in the picking order will be picked. This problem is known as the Picker Routing Problem. Although quite sophisticated heuristics and even efficient exact solution approaches exist to the Picker Routing Problem in warehouse with up to two blocks, the routing problem does not get much attention when dealing with the Order Batching Problem. Instead, the order pickers are assumed to follow a certain, simple routing strategy when making their ways through the warehouse. The advantage of this approach can be seen in the fact that—in particular for single-block warehouse layouts—the corresponding picker tours are very straightforward and can be memorized easily by the order pickers. This advantage diminishes, however, when more complex, multi-block layouts have to be dealt with. Furthermore, in such case, the approach may result in picker tours which are far from optimal. For multi-block layouts, we integrate different routing algorithms into an iterated local search approach for the batching in order to demonstrate what the benefits are from solving the Order Batching and the Picker Routing Problem in a more integrated way. By means of numerical experiments it is shown that paying more attention to the Picker Routing Problem results in a substantial improvement of the solution quality without increasing computing times.