See also: Near-Field Communication (NFC)
Another type of magnetic field sensing device is the magnetic tag. Basically, the technology behind magnetic tags is the same as the one used in anti-theft alarm systems in shops, where simple inductor-capacitor circuits or magnetostrictive strips are excited by a fixed search coil. Magnetostriction is the change in the dimension of a material when subjected to an external magnetic field.
The tags have specific frequencies (the “ID” of each tag) determined by their design characteristics, therefore several tags of various frequencies can be detected individually when sweeping the frequency of the coil. Whenever the sweeping frequency of the reader coil equals the center frequency of a specific tag, this tag will resonate, i.e. will be remotely excited by the coil, and the reader can identify its characteristics.
The main advantages of magnetic coupled tags include the fact that they do not require line-of-sight; that is, the tag does not need to be “seen” by the reader coil, i.e. some non- metal materials may be part of the interface without influencing the measurement.
Another interesting feature is that each tag can be uniquely determined – its presence in the system, as well as, depending on the tag design and complexity, its position and its orientation. Limitations include the maximum number of tags that can be identified by a system, a number dependent on the sweeping frequency range and resonance width of the tags, and the limited speed of reading.
(From Sensors and Sensor-to-Computer Interfaces, book chapter by Marcelo M. Wanderley)