Electronic article surveillance is a technological method for preventing shoplifting from retail stores or pilferage of books from libraries. Special tags are fixed to merchandise or books. These tags are removed or deactivated by the clerks when the item is properly bought or checked out. At the exits of the store, a detection system sounds an alarm or otherwise alerts the staff when it senses active tags.
Acousto-magnetic systems
These are similar to magnetic tags in that they are made of two strips, a strip of magnetostrictive, ferromagnetic amorphous metal and a strip of a magnetically semi-hard metallic strip, which is used as a biasing magnet (to increase signal strength) and to allow deactivation. These strips are not bound together but free to oscillate mechanically.
Amorphous metals are used in such systems due to their good magnetoelastic coupling, which implies that they can efficiently convert magnetic energy to mechanical vibrations.
The detectors for such tags emit periodic tonal bursts at about 58 kHz, the same as the resonance frequency of the amorphous strips. This causes the strip to vibrate longitudinally by magnetostriction, and will continue to oscillate after the burst is over. The vibration causes a change in magnetization in the amorphous strip, which induces an AC voltage in the receiver antenna. If this signal meets the required parameters (correct frequency, repetition etc.) the alarm is activated.
When the semi-hard magnet is magnetized, the tag is activated. The magnetized strip makes the amorphous strip respond much more strongly to the detectors, because the DC magnetic field given off by the strip offsets the magnetic anisotropy within the amorphous metal. The tag can also be deactivated by demagnetizing the strip, making the response small enough to that it will not be detected by the detectors.
These tags are thicker than magnetic tags and are thus seldom used for books. However they are relatively inexpensive and have better detection rates (fewer false positives and false negatives) than magnetic tags.
Shielding
Most systems can be circumvented by placing the tagged goods in a bag lined with aluminum foil. The bag will act as a Faraday cage, isolating the tag from the antennas. Although some vendors claim that their acousto-magnetic systems cannot be defeated by bags shielded with aluminum foil, sufficient amount of shielding, on the order of a 30 layers of standard 20µm foil, will defeat all standard systems.
Although the amount of shielding required depends on the system, its sensitivity, the distance and orientation of the tags with relative to its antennas, total enclosure of tags is not strictly necessary. Indeed, some shoplifters use clothes lined with aluminum foil. Low-frequency magnetic systems will require more shielding than radio-frequency systems due to their use of [near-field] magnetic coupling. Magnetic shielding, for instance with steel or mu-metal, would be more effective, but also cumbersome and expensive.
The shielding technique is well-known amongst shoplifters and store owners. Some states have specific laws against it.
To deter the use of booster bags, some stores have add-on metal detector systems which sense metallic surfaces.
- Post Time: 10-10-17 - By: http://www.rfidang.com