Task scheduling in a full roaming shuttle system

A new concept in automated storage and retrieval systems is the full roaming shuttle system, the distinguishing feature of which is that its material handling shuttles are not aisle-captive, but can easily switch aisles and levels in the storage area. A consequence of this flexibility is that shuttles more often overtake each other and deliver product totes in a different sequence than they are requested. In case of strict sequence requirements for delivered totes, this leads to more waiting time of shuttles and thus to loss of throughput capacity. In this paper we propose heuristics to assign tasks to shuttles that aim at minimizing the number of out-of-sequence occurrences and at maximizing the throughput capacity. These heuristics are evaluated through simulation. The results suggest that, in comparison to first-in first-out task assignment, substantial throughput improvement can be achieved by employing smart task assignment heuristics.