How can an object's movement (for example, if an item is relocated from one shelf to another) be updated by a reader? Do RFID applications use a grid of the space covered? Or is some other technique used?
The answer really depends on the type of RFID system you are talking about. Real-time location systems (RTLS) employ active RFID transponders that broadcast their unique ID number every few seconds or minutes, depending on how often you need to know an object's location. That signal is received by three or more readers around the location's perimeter, and software is used to determine the object's location on a grid or map. If the item is moved, the system triangulates on its location when it again beacons, and its map position is updated.
Updating location information with passive systems is more of a challenge. Passive RFID systems usually tell you only that a tagged item is a specific reader's field. You could put an RFID interrogator in each bay if you had a shelf unit with eight bays, for instance, or install a reader antenna in each bay if you had a multi-port reader, and you could set it up so that each interrogator or antenna reads the transponders within that bay. Then, if an item is moved from one bay to another, the system would indicate that a different reader or antenna picked up the tag, and you would know it moved.
It can get expensive to have readers or antennas on every single shelf, so you need to think about the business value of knowing an item has moved from one shelf bay to another, and how urgent it is to know this information. Is it enough to know an item is in the shelf unit? If so, that can be determined with a handheld.
Passive systems can also tell you in which room an item is located if a reader and antennas are set up in a portal configuration around a doorway. The reader will pick up each time the object moves into or out of the room, and its location can be updated in a back-end system, which knows which reader is associated with which particular room.
—Mark Roberti, Editor, RFID Journal