2005 Volume 18 Issue 4 Pages 156-163
We discuss an optimal input and output scheduling problem for automated warehouses (referred to as AS/RS) of big scale where stacker cranes which hold a few items at a time can simultaneously and automatically input and output items to and from racks, respectively. There is a lot of items to input and output in a given planning horizon and hence the set of items must be partitioned into more than one sub-group to assign stacker cranes. Each stacker crane can input items in a sub-group into designated cells in a rack and outputs items in a sub-group from cells stored in the same rack by touring the rack. Then, we consider a simultaneous optimization of partition of the items and routing for each stacker crane to minimize the mean tour time (equivalently, to maximize the mean throughput rate). We discuss the computational complexity of the problem including the size of solution space and propose an approximation algorithm based on tabu search and heuristics. We demonstrate performances of the proposed algorithm and the stacker cranes by means of numerical experiment based on practical conditions.