Order picking with multiple pickers and due dates - Simultaneous solution of Order Batching, Batch Assignment and Sequencing, and Picker Routing Problems

In manual picker-to-part order picking systems, human operators (order pickers) walk or ride through the warehouse, retrieving items from their storage locations in order to satisfy a given demand specified by customer orders. Each customer order is characterized by a certain due date until which all items included in the order are to be retrieved. For the actual picking process, customer orders may be grouped (batched) into more substantial picking orders (batches). The items of a batch are then collected on a picker tour. Thus, the picking process of each customer order in the batch is completed when the picker returns to the depot after the last item of the batch has been picked. Whether and to what extent due dates are violated depends on how the customer orders are batched, how the batches are assigned to order pickers, how the assigned batches are sequenced and how the pickers are routed. Existing literature has only dealt with specific aspects of this problem so far. In this paper, for the first time, an approach is proposed which considers all subproblems simultaneously. A mathematical model of the problem is introduced that allows for solving small problem instances. For larger instances, a variable neighborhood descent algorithm is presented. By means of numerical experiments, it is demonstrated that the algorithm provides solutions of excellent quality. Furthermore, it is shown that a simultaneous solution approach to the above-mentioned subproblems can be considered as a significant source for improving the efficiency of operations in distribution warehouses.