A Proton Precession Magnetometer - Part 2: Sensor Coils

Somehow two years has passed and this project has languished. But I've finally cleared enough time and made progress on other things to all allow me to get back to it. The fact we've had an excellent auroral display and may be heading into an era of high solar activity gives me more incentive. 

I'd had a false start on the sensor coils - my first attempt resulted in coils that were too large to fit inside the polarisation coil. I'm not sure if I'd misread the instructions or just somehow the coils were larger than expected. Anyway I started again with some slightly thinner acrylic tube (38mm, AliExpress) and had the end pieces laser-cut at the city library (Tūranga) for only $1! 

For winding the coils I made a simple jig out of Meccano. I was never much of a Meccano enthusiast and neither was my son, so this kit that I bought maybe 15 years ago has just been sitting around unused. The gears, motors and framing are ideal for this on-off type of project. I created a holder for the coil former using Polymorph - the moldable plastic that can be softened with hot water.

The rubber band drive was flexible enough to allow constant pressure the winding and since the coils needed to be wound in an opposite sense from each other the reversing motors were very helpful. It's still a fiddly job but easier with two hands. The result was not perfect, especially by the fourth layer but measuring the impedance it came out to 3.44 mH when the instructions suggest approximately 3.5 mH, so pretty close. I weighed each coil and made sure they had the same weight (and presumably then roughly the same number of turns). 

The coils were easy enough to join and with the addition of a small amount of acrylic to brace them they fit neatly in the polarisation coil:

I then carried out the suggested test to figure out the optimal way to wire them. I created a small Arduino sketch using tone() to generate a square wave and passed that through the polarisation coil. At 20 kHz it looked something like a rounded off square wave. The I tried various combinations of the sensor coils. The first one was simply looking at the signal picked up by a single coil. Yellow is the input signal to the polarisation coil, purple is the induced signal on the sensor coil. 

The signal has a lot of "ringing" but it's clearly coming from the polarisation coil.  Then I tried wiring the coils in series:

This amplified the signal, as expected by a factor or about 2X. Finally I wired the coils in anti-series as expected to get the noise reduction effect. This was quite dramatic:

Adjusting the vertical scale showed that the noise reduction was at least a factor of 10x. So hopefully that will work to reduce environmental EM from making it's way into the amplifier. 

Now I'm getting back to the polarisation coil power supply. I have all the parts and will start the mechanical work of assembling the MOSFETS on perfboard and drilling holes in the case.