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I bought a new hat
Global warming or not, winter is hear and it is cold. I could not find a hat so I while I was at the local Cal-Ranch store I picked up a knit hat made by Carhartt. Probably not the most interesting topic for a newsletter, but a few days later I noticed one of those adhesive security strips sticking out of the fold. You have seen these before. They are about 1cm by 4cm and are stuck to your product by an adhesive strip on the back.
I got to wondering about how they worked, so I got out an exact-o ‘ka-nif’ and went to town. I wanted to look at the chip I knew had to be inside but I did not find one. As you can see from the picture, there were 4 pieces of metal inside of the strip. Three were very thin and the same size. They lay right next to each other. The 4th piece of metal was encased in plastic and was part of the adhesive strip.
The whole device will set off an alarm when you walk through those hoops by the front door if they don’t deactivate the device first, or if you try to shop using the five-finger discount method.
Black Magic – Magnetostriction
So, what gives here? There is no transmitter, so how does this stuff work. It kind of looks like black magic.
There is a property of ferromagnetic materials know as magnetostriction. It is a property that causes a material to change shape or dimension when exposed to a magnetic field. The material is made up of oblong shaped particles called domains. These domains tend to align with the magnetic field. Because they are not perfectly round, when they rotate, they will change the length of the material along the axis aligned with the magnetic field.
This effect was first identified by James Joule, yes that Joule, in 1842 when looking at a piece of iron material. The material used in the security strips at the store is a Nickel-Iron alloy. They are very cheap to produce. They thickness is only about 25um, but what is really important is the length.
A tuning fork you say
Essentially what the security tag is doing is resonating like a turning fork. The material is known as magnetoacoustic because it resonates when exposed to a specific frequency. That frequency is determined by its length. If you have ever played with a wind chime in the store you are familiar with the length of a metallic bar producing an acoustic pitch. The longer the bar, the lower the pitch when struck. The length of the strip causes the resonant frequency of the device to be about 58khz.
The hoops at the front of the store are pulsed at 58khz. Pulsed because they need to turn on and off, at about 50% duty cycle. When they are on, they cause the magnetoacoustic strip to resonate. The domains align and then misalign with the 58khz AC field. So why do you think they misalign? Why don’t they just stay aligned to the field?
The amount of deflection is proportional to a dc field that must also be present. The maximum deflection of a magnetoacoustic material is when it is in saturation. For the material used in the security device it is about 20ppm. That means it will grow and shrink by about 20um if you have a 1-meter strip. This strip is about 4cm, so it is deforming by 1/25 or about 800nm.
I said this was like a turning fork. When exposed to the AC magnetic field, the strip begins to deform much like pushing a child on a swing. It vibrates at its resonant frequency. When the forcing function, the AC field, is turned off, it takes some time for the energy present to dissipate. It is turned into heat by the domains rubbing against each other and by the bar pushing against the air molecules. The result is the envelop of the vibration decays away in the shape of a damped sinusoid. It is this damped sinusoid that the hoops are looking for during the part of the duty cycle when the transmitter is off. If the magnetic receiver detects a 58khz damped sinusoid that annoying alarm goes off.
Think of the domain pointing in one direction and then the opposite as the domains follow the forcing function. The DC field is like gravity. They domains will eventually point in the same direction as the DC field when all their energy is used up.
What are the 4 pieces of metal?
In the picture, you can see 4 pieces of metal. The 3 loose ones are tuning forks, the 4th, the one encased in plastic, is a magnet. To turn the detector off you simply demagnetize the magnetic strip. Without a DC magnetic field, the magnitude of the resonance is too small to be detected by the receiver.
Technology can seem like magic until you understand how it works. Stay curious.
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