LISE Magnetometer

3-axis DC magnetometer for monitoring fields in LISE

Magnetometer choice at the moment is Stefan-Mayer 3-axis fluxgate FLC3-70:

https://stefan-mayer.com/en/products/magnetometers-and-sensors/magnetic-field-sensor-flc3-70.html

The coders want to use a Labjack, controlled by a Raspberry Pi, so I built a transition board from the FLC3-70 to the Labjack inputs.  Schematic:

The signal path is a passive 2-pole Bessel filter with Fc at a few hundred Hz, necessary to remove the prominent 14 KHz excitation, because Labjack provides no specs on its anti-aliasing capabilities.

Power path is a low-noise 10V voltage regulator with input from a 12V wall wart.  We'll run the Labjack in differential mode +/-10V full scale, even though the mag output is only +/-5V, because Labjack doesn't have a +/-5V range option.

This photo shows the magnetometer, transition board, and Labjack, but not the Raspberry Pi.  The Labjack is powered from the USB host.

The connection from the mag to the transition board is currently a few inches of loose wires, but could become up to several feet of shielded cable, if the installation requires it.

Here's a closeup of the mag, showing mounting screw holes.  Details of hole locations are on the datasheet from the SM website.

SM recommends plastic screws, going no deeper than 5mm.  The mag should be mounted at least a few inches away from any iron or nickel in order to avoid warping the magnetic field.

Noise performance:

Hard to tell on my bench.   Once the excitation noise was tamed, it appeared that the dominant source of noise was ground noise from the Labjack caused by activity on the NON-ISOLATED USB interface.  Since this effectively ties the magnetometer ground to the PC ground, the total noise will depend mostly on the USB ground noise, which will be different for a Raspberry Pi than for a PC-based system.  Therefore, we will need to test with the RPi acquiring data from the Labjack over USB to get any meaningful noise data.