my own (passive) fluxgate sensor

[joop]

At my job they needed a sensor to measure very small magnetic fields, in the order of 0.1 Gauss. I could not use one of the FGM sensors by speake and co because the magnetic field needs to measured in a radiation environment (roentgen rays), it would damage the electronics inside the sensor. So I needed a fluxgate connected to the electronics through a long cable.

I ordered a sample of METGLAS 2705M (see first picture, in front of the Helmholtz coil), it came in a plastic bag (second picture).

More in second post.

[joop]

From the sheet of METGLAS I cut small strips of 2x30 mm with scissors. It is then glued to a tie-wrap with double sided tape. The tie-wrap will serve as a coil form. Then I take 0.15 mm enamelled coil wire and put 60 winding on the coil form until half way the strip of metglas. Then change the winding direction and put another 60 windings on the form until the metglas is completely covered. See last picture. Wire thickness is not very critical I guess. More in next post.

[joop]

This tie-wrap coil will be the drive coil for the fluxgate. Next we will need to make the sensor coil. The coil form for the sensor coil is made from a material called PEEK (Polyether ether ketone), because I needed radiation-resistant material. But you can use any type of material like PVC, PE or even wood. The sensor coil has about 3500 windings of 0.09 mm enamelled wire, the thinnest wire I could find in our electronics shop. I used an electric drill to wind the coil. This way it will only take about 10 minutes to wind it . More in next post.

[joop]

Now slide the drive coil inside the sensor coil and think of some way to make it easy to connect to the 2 coils. I have used a small piece of PCB and a couple of header pins. See picture, the connection pins can be seen left and right of the sensor coil.

[joop]

Now hook up a cable, I have used a cheap 4-wire microphone cable, each wire having it's own shield.

To see if the fluxgate is working I took a sine-wave generator from the lab and connected it to the drive coil with a 50 Ohm resistor in series. The generator is set to a frequency of 50 kHz with 1 Volt output.

The sensor coil is connected to an oscilloscope with high-ohmic input impedance. The trick now is to look at the second harmonic. Luckily, I found an oscilloscope with som fft capabilities. With the sensor in mid-air I found the 100 kHz amplitude going from 0 to 40 millivolts when rotating the sensor. This is caused by the earth's magnetic field, which is about 0.5 Gauss where I live. If you don't have fft, you will basically see a strong second harmonic component on top of the 50 kHz signal.

I won't go into details about the electronics used, but it should not be to difficult.

This sensor has been used for years in our lab. Can it be used for prospecting? I don't know (never tried), but it may be a good start to build an even more sensitive sensor.

Regards,
Joop

[joop]

Metglass data sheet attached.

Sample request form is at: http://www.metglas.com/support/?q=4&alloy=2705M

regards,
Joop

[Bill512]

Now hook up a cable, I have used a cheap 4-wire microphone cable, each wire having it's own shield.

To see if the fluxgate is working I took a sine-wave generator from the lab and connected it to the drive coil with a 50 Ohm resistor in series. The generator is set to a frequency of 50 kHz with 1 Volt output.

The sensor coil is connected to an oscilloscope with high-ohmic input impedance. The trick now is to look at the second harmonic. Luckily, I found an oscilloscope with som fft capabilities. With the sensor in mid-air I found the 100 kHz amplitude going from 0 to 40 millivolts when rotating the sensor. This is caused by the earth's magnetic field, which is about 0.5 Gauss where I live. If you don't have fft, you will basically see a strong second harmonic component on top of the 50 kHz signal.

I won't go into details about the electronics used, but it should not be to difficult.

This sensor has been used for years in our lab. Can it be used for prospecting? I don't know (never tried), but it may be a good start to build an even more sensitive sensor.



Regards,
Joop

Hi Joop,
thanks for sharing the project.

[golfnut]

Most of us wont be using PEEK unless we robbed a bank

[joop]

Is it expensive? It is used as standard insert material inside multipin LEMO connectors. I know LEMO connectors aren't cheap...