Scheduling shipments in closed-loop sortation conveyors
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At the very core of most automated sorting systems— for example, at airports for baggage handling and in parcel distribution centers for sorting mail—we find closed-loop tilt tray sortation conveyors. In such a system, trays are loaded with cargo as they pass through loading stations, and are later tilted upon reaching the outbound container dedicated to a shipment’s destination. This paper addresses the question of whether the simple decision rules typically applied in the real world when deciding which parcel should be loaded onto what tray are, indeed, a good choice. We formulate a short-term deterministic scheduling problem where a finite set of shipments must be loaded onto trays such that the makespan is minimized. We consider different levels of flexibility in how to arrange shipments on the feeding conveyors, and distinguish between unidirectional and bidirectional systems. In a comprehensive computational study, we compare these sophisticated optimization procedures with widespread rules of thumb, and find that the latter perform surprisingly well. For almost all problem settings, some priority rule can be identified which leads to a low-single-digit optimality gap. In addition, we systematically evaluate the performance gains promised by different sorter layouts.
KeywordsLogistics Transshipment Sortation conveyor Scheduling
This research has been supported by the German Science Foundation (DFG) through the grant “Planning and operating sortation conveyor systems” BO 3148/5-1 and BR 3873/6-1.
- Clausen, U., Diekmann, D., Baudach, J., Kaffka, J., & Pöting, M. (2015). Improving parcel transshipment operations – impact of different objective functions in a combined simulation and optimization approach. In 2015 Winter Simulation Conference (WSC), pages 1924–1935 .Google Scholar
- DHL. (2008). Daten & Fakten. http://www.dp-dhl.com/content/dam/logistik_populaer/leipzig_hub/daten_fakten_hub_leipzig_de.pdf.
- Fedtke, S., & Boysen, N. Layout planning of sortation conveyors in parcel distribution centers. Transportation Science, (to appear).Google Scholar
- Haneyah, S., Schutten, J., & Fikse, K. (2014). Throughput maximization of parcel sorter systems by scheduling inbound containers. In U. Clausen, M. ten Hompel, & F. Meier (Eds.), Efficiency and innovation in logistics (pp. 147–159). Berlin, Heidelberg: Springer International Publishing.Google Scholar
- Lowerre, B. (1976). The Harpy speech recognition system. Ph.D. thesis, Carnegie Mellon University.Google Scholar
- Meller, R. D. (1997). Optimal order-to-lane assignments in an order accumulation/sortation system. IIE Transactions, 29(4), 293–301.Google Scholar
- Muth, E. J., & White, J. A. (1979). Conveyor theory: A survey. AIIE. Transactions, 11(4), 270–277.Google Scholar
- Nazzal, D., & El-Nashar, A. (2007). Survey of research in modeling conveyor-based automated material handling systems in wafer fabs. In Simulation Conference, 2007 Winter (pp. 1781–1788).Google Scholar
- Tsui, L. Y., & Chang, C.-H. (1990). A microcomputer based decision support tool for assigning dock doors in freight yards. Computers & Industrial Engineering, 19, 309–312.Google Scholar
- U.S. Department of Transportation. (2009). Air travel consumer report. http://airconsumer.ost.dot.gov/reports/2009/June/200906ATCR.PDF.
- Vanderlande. (2014). Bagstore, the flexible, reliable and efficient storage solution. http://www.vanderlande.com/en/Baggage-Handling/Products-and-Solutions/Early-Bag-Storage/BAGSTORE.htm.