Patent 9487356 was granted and assigned to Amazon on November, 2016 by the United States Patent and Trademark Office.
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.