Introducing split orders and optimizing operational policies in robotic mobile fulfillment systems

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In robotic mobile fulfillment systems, human pickers don’t go to the inventory area to search for and pick the ordered items. Instead, robots carry shelves (called “pods”) containing ordered items from the inventory area to picking stations. At the picking stations, pickers put ordered items into totes; then these items are transported to the packing stations. This type of warehousing system relieves the human pickers and improves the picking process. In this paper, we concentrate on decisions about the assignment of pods to stations and orders to stations to fulfill picking for each incoming customer’s order. In previous research for an RMFS with multiple picking stations, these decisions are made sequentially with heuristics. Instead, we present a new MIP-model to integrate both decision problems. To improve the system performance even more, we extend our model by splitting orders. This means parts of an order are allowed to be picked at different stations. To the best of the authors’ knowledge, this is the first publication on split orders in an RMFS. And we prove the computational complexity of our models. We analyze different performance metrics, such as pile-on, pod-station visits, robot moving distance and throughput. We compare the results of our models in different instances with the sequential method in our open-source simulation framework RAWSim-O. The integration of the decisions brings better performances, and allowing split orders further improves the performances (for example: increasing throughput by 46%). In order to reduce the computational time for a real-world application, we have proposed a heuristic.
Original languageEnglish
JournalEuropean Journal of Operational Research
Issue number1
Pages (from-to)80-97
Number of pages18
Publication statusPublished - 01.01.2021

Bibliographical note

The authors would like to thank two anonymous referees for their insightful comments and suggestions. Nils Thieme and Ruslan Krenzler are funded by the industrial project “Robotic Mobile Fulfillment System”, which is financially supported by Ecopti GmbH (Paderborn, Germany) and Beijing Hanning Tech Co. Ltd. (Beijing, China). We would like to thank the Paderborn Center for Parallel Computing for providing their clusters for our numerical experiments.

Publisher Copyright:
© 2020 The Author(s)

    Research areas

  • Business informatics - logistics, MIP models, Integrated operational optimization, Robotic mobile fulfillment systems, Split orders