Synchronization in order picking

Due to the increase in online retail in recent decades, distribution centers in the supply chains of large e-commerce companies in particular are increasingly operating according to the “parts-to-picker” paradigm: storage bins (e.g., shelves) containing requested items are automatically transported to picking stations. This eliminates unproductive walking distances for warehouse employees, as is the case in conventional warehouses (“picker-to-parts”). Instead, workers remain at the stations to focus exclusively on picking and packing of customer orders. The storage bins are transported either by autonomous mobile robots (usually floor-bound vehicles) or the picking stations are directly connected to an automatic storage and retrieval system. To optimize picking in such a warehouse, incoming storage bins must be coordinated (i.e., synchronized) with outgoing customer orders at the picking stations in order to increase throughput and to reduce the load on the automatic storage and retrieval system. Depending on the (technical) equipment of the picking stations, the storage policy, and the composition of the customer orders, a number of slightly different synchronization problems arise. From an operations research perspective, these synchronization problems are characterized by the combination of elements of known combinatorial optimization problems: set covering, optimal order batching, and sequencing (of storage bins and orders). We analyze the computational complexity of various synchronization problems, develop and evaluate suitable exact and heuristic solution approaches for optimizing order picking (with a focus on application in robotic mobile fulfillment systems), and present business insights for practitioners.