# Acousto-magnetic system

1. Mar 3, 2008

### skazis

Hi,
I write here for the first time and i hope so far i am doing everything properly

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!!!!

2. Mar 4, 2008

### marcusl

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.

Last edited by a moderator: May 3, 2017
3. Jan 15, 2011

### Kiquin

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?