US Patent 9487356 Managing low-frequency inventory items in a fulfillment center

The disclosed inventory system and methods can be used to improve access time to less frequently accessed inventory items. Specifically, a mobile drive unit may be selected from a plurality of mobile drive units to retrieve or store an inventory item based on a density of the storage location of the inventory item and/or an estimated cost (e.g., estimated time) associated with the retrieval or storage of the inventory item. For instance, an overhead drive unit may be selected over a ground drive unit to retrieve inventory items from or store inventory items to densely-packed storage areas. As another example, an overhead drive unit may be selected over a ground drive unit when the cost of using the ground drive unit exceeds a threshold value.