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How Can I Conduct RFID-based Tracking and Localization of Passive Tags Within a 3D Volume?

Posted By RFID Journal, 09.08.2011
I know that it's relatively straightforward to detect a passive-RFID-tagged-object within a container the size of a refrigerator or freezer—and, indeed, such devices are commercially available for tracking inventory. However, the objects being tagged in our application are large, so there aren't many of them. I guess I have two questions:

1) With current technology, what limits the number of passive-RFID-tagged objects that can be identified within such a volume?

2) If the inside of the volume were equipped with fixed reference tags, could information regarding radio signal strength be utilized to identify where such objects were—i.e., to determine the XYZ coordinates within the volume?

Simple example: For a refrigerator being used to store RFID-tagged items, how many such objects could be stored within an average-sized fridge, such that all would be detectable and identifiable? And would it possible to calculate where they were located—that is, on which shelf within the fridge, and so forth?



With both passive high-frequency (HF) and ultrahigh-frequency (UHF) systems, you can read tags in high volume. In a short video, shot at NXP Semiconductors' Application and System Center, in Graz, Austria, Martin Schatzmayer, the head of the center, demonstrated how UHF RFID transponders can be read on 96 vials filled with water. By optimizing the tag and the read field within the tunnel, 100 percent of the tags could be read 100 percent of the time (see Capturing Serialized Data on 96 Drug Vials).

Goods that are not moving can be a problem for UHF systems, because there are "null spots"—holes, essentially—in the read field, in which tags can not be read. But since you can control the environment, there are ways to design the antenna system to achieve complete coverage. What's more, given that the items are not moving within the refrigerator, there is time to read hundreds, or even thousands, of tags.

The question of pinpointing the tags in three dimensions is more challenging, however. There are RFID readers that use phased-array antennas to determine an item's location in 3D space, but the accuracy is usually to within a foot or so, which might not be sufficient for use within a refrigerator. Typically, the way in which companies handle this issue is to place an HF antenna under each location within a refrigerator, and to then utilize the antennas to read each tag and determine its position.

—Mark Roberti, Founder and Editor,
RFID Journal

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