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Electronic Article Surveillance

Deactivation System

 

 

 

Introduction to EAS

 

In this article we will learn how the Electronic Article Surveillance (EAS) works and walk through the design aspect of this system. The elements of the security system, material, schematics, and firmware will be introduced.

 

All anti-theft EAS systems are made of:

 

1.     Reader (Detection Electronics) or interrogator

2.     Antenna made up of a coil

3.     Tag acting as security element

4.     Deactivation Device (optional)

5.     Activator (optional)

 

The Detection unit has electronics inside it that enables it to interrogate a security tag within a detection zone. The detection zone is a three dimensional spatial zone along the X, Y, and Z that describes the intensity of the detection field at various distances from the antenna connected to the Detection unit. The detection field is inversely proportional to the distance. Depending on the geometry of antenna, tag, and the detection energy the field intensity becomes a inverse proportionality variable of the second order and higher.

 

When the Article (secured merchandise or item) is paid for the tag is then Deactivated. In certain situations the tag may need to be re-activated. If the active tag passes by the detection antenna the EAS system sounds the alarm. Another method of preventing the EAS to sound the alarm, after payment for the merchandise is made by an honest customer, is to remove the security tag (some times called Hard Tag.) There are dedicated tools and equipment used to remove (Detach) the security tag from the secured merchandise. Some of these detachers are hand held simple tools and others are electronically automated. Tags can be either embedded inside the secured merchandise (source tagging) or visible. The also can contain an RFID element to aid in performing certain market transactions and asset management functions. Depending on the secured item (in term of shape, material, and geometry) the system may partially or completely fail to perform to detect or deactivate. Researches are being made to advance the EAS system performance.

 

The guideline VDI 4470 describes an effective benchmark for designing and optimizing the anti-theft systems for customers in the market place.

 

There are various types of EAS security systems that can be categorized based on their operating principle. Among these EAS systems are the Acousto-Magnetic (AM), Radio Frequency (RF), Micro Wave, half-frequency, and Electromagnetic systems.

 

 

Acousto-Magnetic EAS system

 

The system works simply by detecting an active security tag. If tag is active the antennas located at the exit system will detect it and trigger the alarm.

It is the interaction between the antennas at the exit system (pedestals) and the security tag (smart material) that causes the alarm incident. The antennas transmit (XMIT) interrogating signals only within certain protected zone.  If the tag is within the transmit (protected) zone (not allowed to carry unpaid-for-items at exit door) and tag is active the alarm will then sound. This is because the tag re-transmitted the signal (RCV) received from the antennas back to the antennas telling the system there is an active tag leaving the store.

A smart material called magnetostrictive material is used to perform in the AM system. This Magneto-Strictive (MS) material gives a birth to the vibration of the amorphous material (such as Iron) made in a shape of a small strip.

 

This strip is placed close to a bias material (magnet like) that causes the strip to vibrate when the AM field is present.

 

The vibration frequency follows the AM field frequency and the peak of vibration depends on the exact make-up of the amorphous material and the positioning of the strip relative to the bias material. Strip vibrates in such it rapidly elongates and shrinks as a response to the AM 58 KHz field, it dances with the detection AM field. It does that 58000 times per second.

 

As the tag strip vibrates it radiates energy back to the antenna. If the bias material is absent or is demagnetized (Tag is Deactivated – or dead) the strip will not vibrate and thus will not trigger the alarm at the exit door. Also pressing firmly on the tag or bending it will cause it not to vibrate. The strip produces maximum vibration at the tuned resonant frequency of the entire tag design. Many systems have a resonant frequency around the neighborhood of 58 Khz.

 

The electronics inside the EAS unit (Deactivation or Detection unit) sends a stream of pulses modulating the 58 KHz carrier. These pulses resemble someone knocking on the door to find if a reply will be obtained.

 

When the AM field is present the strip vibrates (some one replied as a result of knocking on the door) and keeps vibrating even after the AM field had discontinued, for short period. This phenomenon of the magnetostrictive material makes the tags useful in security application. The vibration decays in a fashion similar to the tuning fork.

 

 

This decay signature (relative in timing to the XMIT knocking pulses) is detected inside the electronics of the detection system and sounds the alarm. The electronics plus the firmware inside the Detection unit looks for the exact timing of these decaying pulses (received from the tag via antennas) and their decay rate to make a decision to trigger the alarm. When both the transmit (XMIT) and receive (RCV) signals above both are put together on the time scale (t) the following will be obtained:

 

 

Since the tag vibrates only when the knocking pulses are present there are periods of times where it is just sitting there waiting for another interrogating knock pulse to vibrate again. It vibrates 58000 times per second for a while (while decaying) and then it rests, it repeats that as long as tag interrogation is needed and until the active tag is found in the detection zone…

 

The following is the tag reply to the “Knock Knock Knock” XMIT-transmit signal from the Detection / Deactivation unit (while interrogating tag only)…

 

After the “Knock” XMIT signal above, from Detection unit, stops completely the following decay is the “I am here” RCV-receive reply signal from the tag…

 

Note: the Detection unit is not allowed to transmit and receive at the same time, otherwise it will fool it self and falsely triggers the alarm. The Detection unit instead transmits XMIT signal and then listens to the RCV reply back signal from the security tag. Time multiplexing is used to synchronize between the XMIT and RCV signals. The firmware inside the Detection unit may select to start listening to the tag reply at any time, and not necessarily immediately after the XMIT signal stops, for performance reasons.

 

The Deactivation system at the cash register produces a strong 500 volts high current magnetic field (with a thumb like sound) to kill (demagnetize) the bias material therefore preventing the strip from vibrating at the designated frequency.

 

It is very difficult for the thieves to generate similar deactivation pulses inside the store. The Deactivation electronics passes a high voltage pulse to an antenna coil, placed on the cash register, that induces this Deactivation field.

 

Every time the merchandise is paid for the item is scanned over the Deactivation antenna element to kill the security tag.

 

So the happy honest customer can pass between the detection antennas, at the exit system, without triggering the stores alarm.

 

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By: Engr. Firas Faham, P. Eng., C. Eng., B. Eng.

20+ years extended experience in the design of Automatic Electronic Systems

 

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