Scheduling of storage/retrieval orders under a just-in-time environment

The authors consider the problem of scheduling storage and retrieval orders under dual-command operations in a unit-load automated storage/retrieval system. The objective is to minimize the weighted sum of earliness and tardiness penalties about a common due date. The authors propose four heuristics based upon a two-step approach where dual-command cycles are formed first and then optimally scheduled. A mixed-integer programming model is also formulated to obtain an optimal solution. Performances of the heuristics are compared in the simulation tests. Computational results show that some heuristics perform favourably.