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RE: TLDs as Anti-Theft Devices?
I found an explanation (at howstuffworks.com) of the physics for these
tag-and-alarm systems, better known as electronic article surveillance
(EAS). All EAS systems work in essentially the same way: Tags or labels
with certain characteristics are affixed to product and activated; at store
exit areas, a transmitter sends a signal to a receiver, creating a zone
that detects tags that have not been deactivated at a register; when an
active tag enters the zone, it sets off an alarm. But the systems use
varying methods and levels of detection. I've seen a report that says
WalMart uses EAS system #3 listed below, the acoustomagnetic system (I
haven't been able to find the specific EAS system used by Kmart and Home
Depot). Acoustomagnetic systems use a transmitter that sends a signal at
58 kHz in pulses. The tag responds to the pulse. If certain characteristics
are present, it creates an alarm. Anyone care to venture what
characteristics are present in a TLD that would create an alarm on an
acoustomagnetic EAS system?
John M. Sukosky, CHP
Dominion
Surry Power Station
(757)-365-2594 (Tieline: 8-798-2594)
Three types of EAS systems dominate the retail industry:
1) Radio Frequency (RF) Systems are the most widely used systems in the
United States today and RF tags and labels are getting smaller all the
time. A label that contains a miniature, disposable electronic circuit and
antenna is attached to a product. The label responds to a specific
frequency emitted by a transmitter antenna (usually one pedestal of the
entry/exit gate). The response from the label is then picked up by an
adjacent receiver antenna (the other pedestal). This processes the label
response signal and will trigger an alarm when it matches specific
criteria. The distance between the two gates, or pedestals, can be up to 80
inches wide. Operating frequencies for RF systems generally range from 2 to
10 MHz (millions of cycles per second); this has become standard in many
countries. Most of the time, RF systems use a frequency sweep technique in
order to deal with different label frequencies.
2) The Electromagnetic (EM) system, which is dominant in Europe, is used by
many retail chain stores, supermarkets and libraries around the world. In
this technology, a magnetic, iron-containing strip with an adhesive layer
is attached to the merchandise. This strip is not removed at checkout --
it's simply deactivated by a scanner that uses a specific highly intense
magnetic field. One of the advantages of the EM strip is that it can be
reactivated and used at a low cost. What most people refer to as an
electromagnetic tag is actually a metal wire or ribbon that has high
permeability, making it easy for magnetic signals to flow through it. A
magnetized piece of semi-hard magnetic material (basically, a weak magnet)
is put up next to the active material to deactivate it. When you magnetize
the semi-hard material, it saturates the tag and puts it in its inactive
saturated state.
The EM system works by applying intensive low frequency magnetic fields
generated by the transmitter antenna. When the strip passes through the
gate, it will transmit a unique frequency pattern. This pattern is, in
turn, being picked up by an adjacent receiver antenna. The small signal is
processed and will trigger the alarm when the specific pattern is
recognized. Because of the weak response of the strip, the low frequency
(typically between 70 Hz and 1 kHz) and intensive field required by the EM
system, EM antennas are larger than those used by most other EAS systems.
The maximum distance between entry pedestals is 40 inches. Also, because of
the low frequency here, the strips can be directly attached to metal
surfaces. That's why EM systems are popular with hardware, book and record
stores. (Check out the patent for more details!)
Magnet-based systems have a small wire or metal strip hidden in the tag.
3) Another magnetic technology is the acousto-magnetic system, which has
the ability to protect wide exits and allows for high-speed label
application. It uses a transmitter to create a surveillance area where tags
and labels are detected. The transmitter sends a radio frequency signal (of
about 58 kHz) in pulses, which energize a tag in the surveillance zone.
When the pulse ends, the tag responds, emitting a single frequency signal
like a tuning fork. While the transmitter is off between pulses, the tag
signal is detected by a receiver. A microcomputer checks the tag signal
detected by the receiver to ensure it is at the right frequency, is
time-synchronized to the transmitter, and that it is at the proper level
and the correct repetition rate. If all these criteria are met, the alarm
occurs.
In each case, an EAS tag or label is attached to an item. The tag is then
deactivated, or taken from an active state where it will alarm an EAS
system to an inactive state where it will not flag the alarm. The
disposable tag is deactivated by swiping it over a pad or with a handheld
scanner that "tells" the tag it's been authorized to leave the store. If
the item has not been deactivated or detached by the clerk, when it is
carried through the gates, an alarm will sound.
howstuffworks.com
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