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Searching for maximum flux density.

Submitter: Michel Arts
Description: Over the last month, a new time registration system has been installed at ASTRON. As with many things one has to become familiar with it. Instead of sliding a metal key through a slit, one has to put a special kind of key hanger in front of the terminal. But how should you hold the key hanger to execute registration most efficiently? Maybe the first days you had to play a little bit with the key hanger until it was recognized by the terminal. It turned out that the position as shown on the photograph above is a good position. If you hold the key hanger this way, it is immediately recognized by the terminal. Can this be explained by physics? To explain this, you should first know how such a system works. Both the terminal and the key hanger contain a coil. The coil in the key hanger is connected to an integrated circuit. The coil in the terminal is fed by an rf-current (I don't know the frequency of this system but 125 kHz and 13.56 MHz are commonly used frequencies for these kind of systems). If you hold the key hanger in front of the terminal, the magnetic field produced by the coil of the terminal induces a voltage in the coil of the key hanger. This voltage feeds the integrated circuit and induces a current in the coil of the key hanger. The magnetic field due to this current, induces a voltage in the coil of the terminal. For the electronics of the terminal, the induced voltage is recognized as a change in the input impedance of the coil. The current in the coil of the key hanger is modulated by the integrated circuit. This modulation leads to a variation of the input impedance seen by the electronics of the terminal. Each individual key hanger produces a unique modulation pattern which in turn causes a unique variation of the input impedance of the coil in the terminal. This is how individual key hangers are distinguished.

The approximate positions of the coils of both the key hanger and the terminal are depicted by the red lines in the photograph. The picture on top at the right shows the magnetic field lines of a coil carrying a current. One can see that the center of the coil has the largest flux density. This explains why you have to put the key hanger at the position as shown in the photograph. Also the maximum magnetic flux goes through the coil of the key hanger if the key hanger is in this position. From Faraday's law we know that the induced voltage is proportional with the magnetic flux going through the coil. This causes a stronger magnetic coupling between the two coils. Due to this stronger coupling the modulation by the integrated circuit in the key hanger causes larger variations of the input impedance of the magnetic coil of the terminal. This makes it easier for the electronics of the terminal to recognize the key hanger.
Copyright: Michel Arts
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