The use of Radio Frequency Identification (RFID) tags for inventory control is well known. These tags are broadly defined as radio frequency transponders allow tagged inventory items to uniquely self identify themselves to a suitably configured network of RFID readers. These readers interface with a computer network to monitor the movement and/or status of inventory or work-in-process.
Systems tend to fall into two categories, fixed point monitoring and scanning. In the fixed point modality tagged items move passed a fixed reader that localizes the item in time and position. This modality is well suited to monitoring movement of items into and out of a specific area as well as movement along a linear process such as a packaging or assembly line. In a scanning modality, a mobile scanner moves in an area reading and logging, all the RFID tags that are in range of the reader as it moves through the inventory area. This modality is better suited for monitoring static inventory such as materials in a warehouse or books in an archive, in further discussion, it should be understood that warehouse may be taken to mean for holding static inventory.
In many warehouse applications using a scanning or roaming reader, it is also advantageous to know not only that an item is present in the warehouse but also where the item is located, i.e. on which shelf our in which aisle in a storeroom. In order to localize items in a space, the roaming reader may follow a preprogrammed track or may periodically update its location through a number of methods. The mobile reader may have an integrated Real Time Location System (RTLS), the reader may have its location manually updated by an operator, or it may pass certain reference points in its progress along a preplanned route through the warehouse. In general these approaches are adequate for determining the inventory state of a warehouse; however they have the disadvantage that the reader must follow a pre-programmed (deterministic) path or route through the warehouse and they require an initial sweep to determine the initial locations of items in the warehouse. This may be time consuming and may not provide the degree of real time location of items that is required, especially in an active warehouse where items may be moved from storage location to storage location or there may be several stocking locations for similar or associated items with the same warehouse. Further the ease of monitoring, storing and moving data associated with a large operation may be difficult.