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Faraday Magnetometer

Johannes Egger

In addition to the conventional methods of magnetic field measurement, there are other physical effects which allow to spatially resolved image a magnetic field, which are rarely instrumentalized for measurement purpose. One of them is the Faraday effect.

The objective of this work lies in the magnetic field measurement of a polyethylene foil, which is partially magnetically coated and is in a translational motion.
For a visualization of the magnetic fields the Faraday effect is used, which rotates a polarized electromagnetic wave with an angle dependent on the local magnetic flux density.
An optical system is presented which uses the Faraday effect to convert the local magnetic flux density into light intensity and image it on a line sensor.
The video output signal of the line sensor is digitized and the measured data is stored on an external hard disk with the help of a Raspberry Pis for further examinations.

After a brief description of the Faraday effect, a polyethylene foil with a partial magnetic colour coating is described, which has to be measured for quality statements during production.
This coating is provided with a periodic pattern and partially also has microstructures in the sub millimetre range, which has to be resolved laterally with the measuring instrument at a production speed of 3 m/s.
It is shown with a presented line sensor, which has a pixel area of 63.5 µm x 63.5 µm, that this resolution, in the row direction and in the direction of the polyethylene foil, with a 1:1 imaging optics, is achieved.
The video output voltage of the line sensor is amplified by a factor of 2 with an amplifier circuit and with an offset signal, controlled by the Raspberry Pi, the video signal is moved into the measurement range of the 8-bit ADC.
The sampling timing of the ADC is controlled exactly at the designated timing by means of a CPLD and a time delay chip.
After digitizing at a 63.378 kHz line scan rate, with 64 pixels per line, the measurement data are cached in a 32 kB FIFO memory where they are read from a Raspberry Pi.
In order to be able to achieve a streaming speed of 4 MB/s on the Raspberry Pi, a system consisting of various developed kernel modules and programs is presented.
The Raspberry Pi streams the measurement data, after minimal signal processing, to an external hard drive, where it is then available for offline analysis.

For the proof of functionality, a magnetic field measurement of a coated polyethylene foil in the production was carried out on a trial basis, a section of the measurement data can be seen in Figure 1, and the results are briefly discussed.
It is shown that the microstructures can also be resolved by the measuring instrument at full line speed and based on these values, a qualitative statement about the coating can be made and therefore also provides empirical data for a signal processing development.

Figure 1: Measurement data of a polyethylene foil with partially coating, which is magnetically excited.

Figure 1: Measurement data of a polyethylene foil with partially coating, which is magnetically excited.

Keywords: Faraday effekt, Raspberry Pi, Magnetometer, magnetic fields

The circulation of the project work is restricted from February 25th, 2018 for a period of 5 years.

February 2nd, 2018