| Electronic Article Surveillance |
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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. For high-value goods that are to be manipulated by the patrons, wired alarm clips may also be used; these, being less common and technologically less mysterious, are not covered by this article. (top) and RF (bottom).]] TYPES OF EAS There are four major types of electronic article surveillance systems :
Magnetic systems These tags are made of a strip of Amorphous Metal (metglas) which has a very low Magnetic Saturation value. Except for permanent tags, this strip is also lined with a strip of Ferromagnetic material with a moderate Coercive Field (magnetic "hardness"). Detection is achieved by sensing harmonics and sum or difference signals generated by the non-linear magnetic response of the material under a mixture of low-frequency (in the 10Hz to 1000Hz range) magnetic fields. When the ferromagnetic material is magnetized, it biases the amorphous metal strip into saturation, where it no longer produces harmonics. Deactivation of these tags is therefore done with magnetization. Activation requires demagnetization. Due to the convenient dimensions of the tags, and their very low cost, this system is popular in Libraries and Retail stores. Magnetic systems are often referred to as 'Electromagnetic' (or EM) systems. EM systems rarely appear in retail environments outside of Europe. Acousto-magnetic systems These are similar to magnetic tags in that they are made of two strips, a strip of Amorphous Metal and a strip of ferromagnetic material. They differ in that these strips are not bound together but free to oscillate mechanically. Also the tag is active when the material is magnetized. The detectors use a 58kHz (or 66kHz) magnetic field which induces mechanical Resonance by Magnetostriction . When the excitating field is turned off, these tags continue to oscillate mechanically, which produces a magnetic signal because of the magnetized second strip. This signal triggers the alarm. 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. Radio-frequency systems These tags are essentially an LC Tank Circuit that has a Resonance peak at 8.2MHz or 2MHz. Sensing is achieved by sweeping around the resonant frequency and detecting the dip. Deactivation is achieved by detuning the circuit by partially destroying the Capacitor . This is done by submitting the tag to a strong Electromagnetic field at the resonant frequency which will induce voltages exceeding the capacitor's Breakdown Voltage , which is artificially reduced by puncturing the tags. Microwave systems These permanent tags are made of a non-linear element (a Diode ) coupled to one microwave and one electrostatic antenna. At the exit, one antenna emits a low-frequency (about 100kHz) field, and another one emits a microwave field. The tag acts as a mixer reemitting a combination of signals from both fields. This modulated signal triggers the alarm. These tags are permanent and somewhat costly. They are mostly used in clothing stores. DISCUSSION Occasional vs. informed shoplifters EAS systems provide a good deterrent against casual theft. The occasional shoplifter, not being familiar with these systems and their mode of operation, will either get caught by them, or will refrain from stealing. Informed shoplifters with appropriate tools can remove or deactivate tags. (See tools.) However they may miss some tags or may be unable to remove or deactivate all of them, especially if concealed or embedded tags are used. (As a service to retailers, some manufacturers embed security tags in the packaging of their products, or even inside.) Also, deactivating or detaching tags may be spotted by the shop staff. Shoplifting tools are illegal in many jurisdictions, and can, in any case, serve as evidence against the perpetrators. Hence, informed shoplifters, although they decrease their risk of being caught by the EAS, expose themselves to much greater judicial risks if they get caught with tools or while trying to remove tags, as this characterizes intent to steal. In summary, while even the least expensive EAS systems will catch most occasional shoplifters, traditional surveillance measures are still required against informed ones. Installation costs A single EAS detector, suitable for a small shop, usually costs on the order of a few thousand dollars or euros. Disposable tags cost on the order of cents. The expense of difficult to circumvent, high-end systems is mostly due to the cost of the tags, which use more sophisticated locks and operate in the microwave (UHF) range. Unlike dummy surveillance cameras, dummy EAS systems are ineffective. As all EAS systems get occasionally tripped, not because of shoplifting, but because of tag deactivation faults, or simply by mindless customers, such dummy EAS systems would be spotted rather quickly. Also, the electromagnetic field of all EAS systems can be easily detected using very simple electronics or with common devices such as radio receivers, or depending on the EAS technology used, portable cassette players. False Alarms A major concern with these systems is when false alarms occur. A false alarm (or false positive) is when the alarms go off when a person passes through the gates without any active tags, in particular without having stolen any merchandise. This most often is due to tags on merchandise not being properly deactivated. In some older systems electronic devices have been known to set off alarms. False alarms are embarrassing not only for honest customers who accidentally set off an alarm, but for the business as well - who now have an upset customer on their hands. Tag orientation Except for microwave, the detection rate for all these tags drops when their containing plane is Orthogonal to the axis of the detection loops. When the average Magnetic Flux from the emitting coils that crosses the tags is near zero, detection cannot occur. This is a serious shortcoming of these tags and is well-known and documented in the corresponding patents. The solution would be to place three mutually orthogonal tags on each item. Another solution would be to place a set of three mutually orthogonal pairs of coils (as suggested in the patents) ; however this is cumbersome. Better systems will have special coil dispositions with a more complicated disposition of flux lines, making it difficult but not impossible to find an exit trajectory minimizing the flux crossing the tags. Detaching Detachers can be stolen or built for non-disposable security tags. However, detachers tend to be large and heavy. Some detachers actually have a security tag inside them, to alert store personel of them being removed from (or being brought into) the store. Magnetic activation and deactivation Deactivation of magnetic tags is achieved by straightforward magnetization using a strong magnet. Magneto-acoustic tags require demagnetization. However sticking a powerful magnet on them will bias disposable magnetic tags and prevent resonance in magneto-acoustic tags. Similarly sticking a piece of metal, such as a large coin on a disposable radio-frequency tag will shield it. Non-disposable tags will require stronger magnets or pieces of metal to disable or shield since the strips are inside the casing and thus further away. Reactivating magnetic tags by demagnetizing them can be accomplished as follows. Solidly attach a small magnet (such as a Neodymium-Iron-Boron magnet from an old harddisk, but any magnet will do) to an electric drill. Setting the drill to the slowest speed and taking all appropriate safety measures, make the magnet turn. This will create an alternating magnetic field. Approach the tag, then slowly pull it away. This will demagnetize and thus reactivate it. Shielding By enclosing the tag in a Conductive Material (often called a booster bag), it is possible for shoplifters to carry away goods without triggering many EAS systems, in particular radio-frequency systems. For older systems a single sheet of aluminium foil can be an effective shielding material. Nowadays, multiple or thicker layers of aluminum, or a better (that is, more conductive) shielding material like copper foil or wire mesh is more effective. Fabrics offered to shield RF emissions (like Mobile Phone radiation) are also effective in the 900 MHz range. Magnetic or magneto-acoustic systems operate at very low frequencies. Therefore, they use {Link without Title} magnetic coupling which is immune to this kind of electrostatic shielding. Magnetic shielding, for instance with steel or mu-metal, would be effective, but cumbersome and expensive. To deter the use of booster bags, some stores have detection systems which sense large metallic surfaces. Jamming Like most systems that rely on transmission of electromagnetic signals through a hostile medium, EAS sensors can be rendered inoperative by Jamming . As the signals from tags are very low-power (their cross-section is small, and the exits are wide), jamming requires little power. Evidently, shoplifters will not feel the need to follow radio transmission regulations, hence crude, easy-to-build transmitters will be adequate for them. An amateur can therefore design and build a jammer for magnetic, magneto-acoustic or radio-frequency systems without special equipment. For instance, a square wave near 58kHz driving a coil with less than a watt of power will jam magneto-acoustic systems. As for 8.2MHz radio-frequency systems, since they are sweeping their frequency, building a jammer for them does not require a precise transmission frequency and therefore neither does it require expensive frequency counting equipment, However, due to their high frequency of operation, building a jammer can be difficult for microwave circuits; these systems are therefore less likely to be jammed. Although jamming is easy to perform, it is also easy to detect. A simple firmware upgrade should be adequate for modern DSP-based EAS systems to detect jamming. However, these systems can usually be circumvented by simpler means (shielding, detaching or deactivation). Hence, the incentive for building jammers is low. It is not publically known whether any jamming counter-measures have been implemented. COMPANIES The two largest EAS manufacturers are Checkpoint Systems and Sensormatic . Both these entities are public companies, Sensormatic is a part of Tyco and Checkpoint is independent. 3M also builds EAS systems and has acquired Knogo . Sensormatic systems are usually acousto-magnetic. Checkpoint systems tend to be radio-frequency. 3M systems are usually magneto-harmonic systems found in bookstores and libraries. Due to patent and manufacturing issues, independent EAS manufacturers usually ship radio-frequency systems: Acousto-magnetic systems are still covered by patents, and magneto-harmonic systems, while not anymore covered by patents, require the use of low-saturation material like Metglas, which has few manufacturers. |
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