The optimization of mixed block stacking requiring relocations

This paper addresses the optimization of a block stacking storage system (BSSS) in which unit loads are stored vertically. One of the important problems in a BSSS is in the relocation required when unit loads are located on top of the next unit load to be picked. Relocations are bound to occur when multiple types of unit loads are mixed in the same stacking area. Relocation is a major source of inefficiency during a BSSS handling operation. This study shows how the number of relocations can be reduced by utilizing the information regarding the arriving unit load type when determining its storage location. For the case where the information is not available, statistical models have been developed that estimate the expected number of relocations. For the case where the information is available and utilized, a method based on a genetic algorithm is suggested for use in determining the storage location for each arriving unit load in such a way that minimizes the expected number of relocations. A discussion is presented regarding how to determine the optimal number of stacks allocated to a set of unit load types which will share the same storage area considering the expected number of relocations.