Routing orderpickers in a warehouse: A comparison between optimal and heuristic solutions

In this paper the problem of finding efficient orderpicking routes is studied for both conventional warehouses, where pickers have a central depot for picking up and depositing carts and pick lists, and modern warehouses, where orderpicking trucks can pick up and deposit pallets at the head of every aisle without returning to the depot. Such environments can be found in many warehouses where paperless picking is performed from pallet locations with pickers having mobile terminals receiving instructions one by one. In order to find orderpicking routes with a minimal length in both the situations of a central depot or decentralized depositing, we extend the well-known polynomial algorithm of Ratliff and Rosenthal that considered warehouses with a central depot. In practice, the problem is mainly solved by using the so-called S-shape heuristic in which orderpickers move in a S-shape curve along the pick locations. The performance of the new algorithm and the S-shape heuristic are compared in three realistic orderpicking systems: (1) narrow-aisle high-bay pallet warehouse; (2) picking in shelf area with decentralized depositing of picked items; and (3) conventional orderpicking from wide-aisle pallet locations. The new algorithm gives a reduction in travel time per route of between 7 and 34%. It turns out that the reduction in travel time strongly depends on the layout and operation of the warehouse.