University of Twente Student Theses


Are detailed decisions better decisions? Improving the performance of high-capacity sorter systems using inbound container assignment algorithms

Fikse, K. (2011) Are detailed decisions better decisions? Improving the performance of high-capacity sorter systems using inbound container assignment algorithms.

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Abstract:High-capacity sorter systems are used worldwide in the express parcel and aviation industry. The ability to sort thousands of items per hour enables express parcel companies to deliver items overnight at the same continent. Furthermore, they provide airports and airlines with the ability to sort the baggage items for thousands of passengers to dozens of flights within the three hours that are available for check in. Express parcel companies as well as airports frequently use a first-come-first-served (FCFS) policy when deciding which container should be unloaded on a sorter system infeed. However, often the contents of individual containers are known, and this information could be used when selecting a container. This thesis therefore focusses on how knowledge about the contents of containers could be used when selecting the next container to be unloaded, in order to improve the performance of highcapacity sorter systems. The literature review shows that this topic has received little attention from researchers. There are a few papers that focus on the development of detailed and complicated (un)loading schedules, but these approaches are time consuming. Their use is therefore limited to offline scheduling, i.e. creating fixed unloading schemes. The dynamic load balancing algorithm (DLBA), however, is specifically developed for online inbound container scheduling at parcel and postal hubs. Each time an infeed becomes available, the approach selects that container from the queue that minimises the workload imbalance on the sorter. More specifically, the approach ensures that the number of items (i.e. workload) per destination on the sorter system is more or less equal. Unfortunately, the DLBA has some shortcomings. It does, for instance, not take the internal transport times into account. Especially in larger sorter systems this could severely affect the selection decision being made. Therefore, an adapted version, the advanced dynamic load balancing algorithm (ADLBA), has been developed in this research. This algorithm estimates when items will arrive at the outfeed, and selects containers in such a way that the workload at each of the outfeeds is, at all times, more or less equal. Furthermore, two extensions for the baggage handling industry have been developed. The priority extension acknowledges that some containers might have priority over others. Priority should, for instance, be given to a container that mainly contain items that have to make a flight that departs soon. The delayability extension, on the other hand, sends containers holding only items that currently cannot be sorted, because their destinations have not yet been assigned to an outfeed, to a remote container park. This enables the dispatcher to select only containers that can be unloaded and significantly reduces the number of items that are sent to the early baggage system (EBS). Additionally, this could reduce the workload at the main sorter, thereby improving the performance. In order to determine the performance of each of the scheduling approaches, three simulation models and four simulation scenarios are developed. The simulation studies show that especially the DLBA is able to improve the performance of sorter systems, whereas the ADLBA performs equal to, or even worse than, current practice FCFS. In fact, results show that the DLBA is able to increase the throughput of sorter systems up to 4.5% for parcel and postal. Furthermore reductions in missort rate (number of items per thousand that arrive too late) of several permillage points, depending on the workload on the sorter system, are possible. The results for the extensions are twofold. The priority extension proves to be of little use, for it reduces the missort rate only marginally. The delayability extension, on the other hand, performs extremely well and is able to reduce the missort rate by percentage points, again depending on the workload of the sorter system. We therefore conclude that the workload balancing approach can be very interesting. The DLBA is able to improve the results for both the baggage handling and the parcel and postal industry. Because the ADLBA performs rather poorly, we conclude that a more detailed approach does not necessarily result in better scheduling decisions. The results for the extensions do, however, show that a suitable extension can result in an increasing performance as well. Further, we recommend to continue research on workload balancing approaches, but focus should be on more general, less detailed, approaches. The results for the delayability extension show that elementary approaches developed for either of the industries can contribute significantly to the performance, but are easier to implement. Future research should therefore not fixate itself on finding a single solution for both trades, but aim at finding good solutions for either of them.
Item Type:Essay (Master)
VanDerLande Industries, Veghel, the Netherlands
Faculty:BMS: Behavioural, Management and Social Sciences
Subject:85 business administration, organizational science
Programme:Industrial Engineering and Management MSc (60029)
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