Felix Weidinger

Scattered Storage: How to Distribute Stock Keeping Units All Around a Mixed-Shelves Warehouse

Scattered storage is a storage assignment strategy where single items are isolated and distributed all around the shelves of a warehouse. This way, the probability of always having some items per s...

Optimizing automated sorting in warehouses: The minimum order spread sequencing problem

In warehouses, order consolidation processes are inevitable whenever picking orders are assembled under a zoning and/or batching policy. In our specific warehouse setting, bins containing partial orders picked under a zoning and batching policy are intermediately stored in an automated storage/retrieval system (ASRS) and, afterwards, released on a conveyor system supplying the consolidation area. Here, a fully-automated sorter merges partial orders dedicated to the same customer in packing stations where, finally, the customers’ cardboard boxes are packed. For sequencing the release of bins from the ASRS we derive an elementary optimization problem, which aims to minimize the spread of orders in the release sequence so that picking orders are quickly assembled at their packing stations. The resulting problem is formalized, computational complexity is proven, and efficient solution procedures are provided. Additionally, managerial aspects are addressed with the help of a comprehensive simulation study.

Truck Scheduling in the Postal Service Industry

The distribution networks of the postal service industry are organized according to the hub-and-spoke paradigm, so that parcel distribution centers play a crucial role to consolidate the parcel flows to full truckloads. In these terminals, inbound trucks are unloaded at gates, shipments are identified, sorted by the central sortation conveyor system, and loaded into outbound trailers, in which they are moved toward their next destination. In this context, the scheduling of inbound trucks, which assigns a gate and a processing interval to each truck, is an essential operational decision problem. We formalize the resulting optimization problem and provide suited solution procedures. Furthermore, we test the impact of truck scheduling on the sortation performance of the central conveyor system with the help of a comprehensive terminal simulation. The online appendix is available at https://doi.org/10.1287/trsc.2016.0722.

Picker routing in the mixed-shelves warehouses of e-commerce retailers

Abstract E-commerce retailers face the challenge to assemble large numbers of time-critical picking orders, of which each typically consists of just a few order lines and low order quantities. To efficiently solve this task, many warehouses in this segment are organized according to the mixed-shelves paradigm. Incoming unit loads are isolated into single units, which are randomly spread all over the shelves of the warehouse. In such a setting, the probability that a picker always finds a demanded stock keeping unit (SKU) close-by is high, irrespective of his/her current position in the warehouse. In spite of this organizational adaption, picker routing, i.e., the sequencing of shelf visits when retrieving a set of picking orders, is still an important optimization problem. In a mixed-shelves warehouse, picker routing is much more complex than in traditional environments: Multiple orders are concurrently assembled by each picker, many alternative depots are available, and items of the same SKU are available in multiple shelves. This paper defines the resulting picker-routing problem in mixed-shelves warehouses and provides efficient solution methods. Furthermore, we use the developed methods to explore important managerial aspects. Specifically, we benchmark mixed-shelves storage against traditional storage policies for scenarios with different ratios between small-sized and large-sized orders. In this way, we investigate whether mixed-shelves storage is also a suitable policy if an omni-channel sales strategy is pursued, and large-sized orders of brick-and-mortar stores as well as small-sized online orders are to be jointly processed by the same warehouse.

Picker routing in rectangular mixed shelves warehouses

Abstract Scattered storage (also denoted as mixed shelves storage) is a warehousing strategy often found in business-to-consumer online retailing. Unit loads are broken down into single items that are spread throughout the warehouse, leading to multiple storage positions per stock keeping unit. This paper investigates the picker routing problem in a rectangular scattered storage warehouse, which differs from classical picker routing problems by being a combined selection and routing problem. A proof of NP-hardness in the strong sense is provided and suited solution procedures are presented. Additionally, the impact of the degree of scatter on the length of picking tours is investigated for differing levels of heterogeneity of the order lines, providing managerial insights on when scattered storage should or should not be applied.

Scheduling last-mile deliveries with truck-based autonomous robots

To reduce the negative impact of excessive traffic in large urban areas, many innovative concepts for intelligent transportation of people and freight have recently been developed. One of these concepts relies on autonomous delivery robots launched from trucks. A truck loads the freight dedicated to a set of customers in a central depot and moves into the city center. Also on board are small autonomous robots which each can be loaded with the freight dedicated to a single customer and launched from the truck. Then, the autonomous robots move to their dedicated customers and, after delivery, autonomously return to some robot depot in the city center. The truck can replenish robots at these decentralized depots to launch further of them until all its customers are supplied. To assess the potential of this innovative concept, this paper develops scheduling procedures which determine the truck route along robot depots and drop-off points where robots are launched, such that the weighted number of late customer deliveries is minimized. We formulate the resulting scheduling problem, investigate computational complexity, and develop suited solution methods. Furthermore, we benchmark the truck-based robot delivery concept with conventional attended home delivery by truck to assess the potential of this novel last-mile concept.

A Precious Mess: On the Scattered Storage Assignment Problem

Induced by the rise of online retailing new storage strategies have evolved, designed to meet the demands of e-commerce warehousing. Although many of these new approaches have established over the last few years, literature on basic planning problems in these environments can be found only rarely. This paper points out the special needs of e-commerce warehousing and details the scattered storage strategy (also known as mixed-shelves storage) where unit loads are unbundled and single items are stored at multiple positions within the warehouse. This way, an item of an ordered product is always close by and the unproductive walking time of pickers is reduced. Based on the paper of Weidinger and Boysen (Scattered storage: how to distribute stock keeping units all around a mixed-shelves warehouse. Working Paper Friedrich-Schiller-University Jena, 2015) [8], the scattered storage assignment problem is presented and the processes in a scattered storage warehouse are described.

Deep-lane storage of time-critical items: one-sided versus two-sided access

Plenty retailers handling fresh produce face the following operational problem in their distribution centers: Pallets of food assembled during the day according to the demands of supermarkets are to be intermediately stored overnight. To save cooling costs, deep-lane storage in some compact storage system is applied, from which the pallets are to be retrieved the next morning when the trucks servicing the supermarkets are to be loaded under great time pressure. To enable an efficient retrieval process, we seek storage assignments defining the put-away of pallets in the deep-lane storage system, such that blockings, i.e., a pallet with later retrieval time is placed in front of another pallet with higher priority in the same lane, are avoided. Most deep-lane storage systems allow only a front access, whereas novel systems can be loaded and unloaded from two sides. We formalize both resulting storage assignment problems, investigate computational complexity, introduce solution procedures, and quantify the increase in flexibility promised by a two-sided access. Our computational results reveal that especially if truck arrivals are hard to anticipate due to a considerable amount of uncertainty, two-sided systems greatly reduce blockings.