US Patent 9714139 Managing inventory items via overhead drive units

The disclosed inventory system and methods can be used to improve storage time for storing less frequency accessed inventory items. Specifically, a mobile drive unit may be selected from a plurality of mobile drive units to store an inventory item by obtaining storage densities of available storage locations and selecting a storage location for an inventory item based on the projected frequency of access of the inventory item. An overhead drive unit may be selected over a ground drive unit to store less-frequently accessed inventory items in densely-packed storage areas. Inventory holders may be selected based on projected frequency of access of other inventory items stored therein.