Can an acousto-magnetic system detect an article placed in an aluminum bag?

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In summary, the EAS system uses sensitive magnetometers to detect the presence of articles with tags attached. The system is effective because it is sensitive to the presence of articles near the surveillance zone, even when they are hidden inside an aluminium bag.
  • #1
skazis
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Hi,
I write here for the first time and i hope so far i am doing everything properly :rolleyes:

I am writing about Electronic Article Surveillance (EAS) for my college (University of Latvia), it's popular science homework. This is not very common subject in latvian books so I had to read everything in english and I managed to do it. Except that I couldn't understand how excatly in acousto-magnetic systems bias magnet makes material
(highly magnetostrictive) in tags to oscillate at frequency F (not 2F) which is frequency of used electromagnetic field as I understood.

Thanks!
 
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  • #2
This operates on the same principle as a fluxgate magnetometer. The key to the whole thing is the response of a ferromagnetic material to applied fields, called the B-H curve or hysteresis loop. Sensitive materials like mu-metal have high magnetic permeability (they easily channel magnetic field lines the way a pipe easily channels water) for small fields. However they saturate quickly, putting flat "tails" on either side of the hysteresis curve.

Here's a short description of magnetic hysteresis:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html" [Broken]

If you put your material into an oscillating sinusoidal field at frequency f, the magnetization inside follows but looks like a square wave at f because the material is driven into saturation at each positive and negative peak. A square wave has odd frequency harmonics, that is, it contains frequencies f, 3f, 5f, etc. Their strength decreases with increasing frequency.

Now if you apply a weak static field, you shift the operating point on the hysteresis or B-H curve. It shifts to the left or right along the H (applied field) axis depending on the direction of applied field. The material spends more time saturated in the positive field direction (say), so the magnetization inside is an asymmetric rectangular wave--longer positive, shorter negative. But this kind of waveform adds even harmonics like 2f, 4f, etc. The one at 2f is the strongest even harmonic.

Since another coil can sense the material's magnetization outside the sample, to detect the presence of the material you just look for the presence of a signal at 2f. It's simple and sensitive.

I found the following description of a fluxgate magnetomer (these use ring shaped cores, but the fluxgate principle is the same), starting on p. 21. It's a little technical but I don't see anything simpler on the web.
http://www.meda.com/Data_Sheets/FVM400%20Instruction%20Manual.pdf" [Broken]

Write back if you have questions.
 
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  • #3
Hello there, I also happen to be working on EAS systems for my iB Extended Essay, and I stumbled upon this thread during my research.

I'd like to thank you for this explanation, it made things clear to me now. However, I still have one more question about this system: Why, when people place an article with one of these tags inside an aluminium bag, it goes undetected through the surveillance zone?
 

1. What is an acousto-magnetic system?

An acousto-magnetic system is a type of technology used for security and anti-theft purposes. It utilizes sound waves and magnetic fields to detect and prevent theft in retail stores, libraries, and other public areas.

2. How does an acousto-magnetic system work?

The system works by emitting a high-frequency acoustic signal that is picked up by a tag attached to a product. The tag then responds by producing a unique magnetic signal, which is detected by the system's sensors. If the product is taken out of the designated area without being deactivated, an alarm will sound.

3. What are the advantages of using an acousto-magnetic system?

Compared to other security systems, acousto-magnetic systems offer a longer detection range and higher accuracy. They are also less prone to false alarms and can be easily integrated with other security technologies.

4. Are there any limitations to using an acousto-magnetic system?

One limitation of this system is that it requires special tags to be attached to products. These tags can be expensive and may need to be replaced periodically. Additionally, the system may be affected by metal and electromagnetic interference, which can cause false alarms.

5. Are there any privacy concerns with using an acousto-magnetic system?

No, there are no privacy concerns with using an acousto-magnetic system. The system only detects the presence of a tag and does not collect any personal information. The tags are also deactivated at the point of sale, ensuring that customers are not tracked after making a purchase.

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