MuMetal Testing

I’ve been trying to clean up magnetic interference from the motors on my quad. When I run compassmot I get any where from 100% to 130%. I ordered some MuMetal and did some testing to figure out how to best use the metal and to see exactly where interference was coming from.

All of the flight electronics were removed and temporarily installed under the frame.

Here are the specs of the quad used:
3DR Quad frame Rev C
APM 2.5 running 3.0.1rc1
3DR AC2830-358 850KV motors
3DR 20A ESCs
APC 10x4.7 props
3S 6400mAH battery
Spektrum AR8000 Rx
3DR 900Mhz Telemetry

My transmitter is a Spektrum DX8

I used an old compass I have left over from my boy scout days and the magnetometer within my cell phone. (The app is Sensor List Pro).

The fields generated by the wiring, ESCs and battery all seemed to be about the same relative strength.

The MuMetal does have a small magnetic field of it’s own. It’s not very strong but it is there. (Notice how the compass point angles toward the MuMetal). In later tests I put a small amount of space under the compass.

I think I’m going to end up with my APM about 22mm above my wiring with a MuMetal sheet at about 19mm. I may consider also attaching a small MuMetal sheet to each ESC on the top as they seem to be quite magnetically noisy.

@NikTheGreek - Unfortunately my bench supply can't handle the amp load of my Quad so my only testing option is the battery. Since the test runs were so short the battery only lost about 15% during testing.

@Monroe Lee King Jr. - I didn't think to try a different starting environment/alignment. I'm a bit restricted on space so I just chose the arrangement that was easiest to fit everything in. I've already rebuilt the quad and I'm down to about 70% with Compassmot and the APM mounted at about half the distance from the wiring. I know I'm not using MuMetal "right" but it is working. Unfortunately I don't have a gauss meter at this time so the tools I have will have to work. I did order a large sheet of MuMetal and the first test I did was to wrap the primary harness with a cylinder of it (perpendicular to the wire). This had an effect but the compass still danced around. That was what prompted me to test all of the other random parts of the quad. I don't think its feasible to wrap every magnetic field producing item on the quad. I'm going to try a few tests with wrapping the APM today. My quad flys great. Loiters in a sub 1 meter sphere and has no problems with RTL and auto missions. This was just something fun to do and to see if I could get my electronics stack a little shorter and cleaner looking.

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Comments

J B July 7, 2013 at 12:19pm

Just finished some flight testing. MuMetal directly under the APM or even within 5mm of the APM dampened the earths mag field to the point that the Quad had trouble which way was north.

I've relocated the MuMetal to directly above the wiring and the APM a short distance above that. (The bottom of the case is about 30mm above the MuMetal.)

You can just see the white line of the MuMetal under the bottom plate.

Compassmot now returns a 53%. Loiter is very tight and yaw behavior is nice and sharp. I don't have the space for RTL and Auto testing at home so that will happen later.
J B July 7, 2013 at 6:52am

@NikTheGreek - Unfortunately my bench supply can't handle the amp load of my Quad so my only testing option is the battery. Since the test runs were so short the battery only lost about 15% during testing.

@Monroe Lee King Jr. - I didn't think to try a different starting environment/alignment. I'm a bit restricted on space so I just chose the arrangement that was easiest to fit everything in. I've already rebuilt the quad and I'm down to about 70% with Compassmot and the APM mounted at about half the distance from the wiring. I know I'm not using MuMetal "right" but it is working. Unfortunately I don't have a gauss meter at this time so the tools I have will have to work. I did order a large sheet of MuMetal and the first test I did was to wrap the primary harness with a cylinder of it (perpendicular to the wire). This had an effect but the compass still danced around. That was what prompted me to test all of the other random parts of the quad. I don't think its feasible to wrap every magnetic field producing item on the quad. I'm going to try a few tests with wrapping the APM today. My quad flys great. Loiters in a sub 1 meter sphere and has no problems with RTL and auto missions. This was just something fun to do and to see if I could get my electronics stack a little shorter and cleaner looking.
R. D. Starwalt July 7, 2013 at 6:51am

Interesting experiment, thanks for posting it.

+1 with Monroe in that the high mu sheets do not bounce back the field, but becomes part of the field plot. In practical applications, we do as Monroe mentions and shape the shielding to push the field where it does not cause a problem.

I have a few MRI installations with hundreds of square feet of the stuff, layered, and installed around the magnet. The intent is to reduce the noted gauss lines to prevent interaction with other systems...or pacemakers...that happen to come nearby.

A calibrated gauss meter would be nice. There are several relative gauss meter circuits (online) that use hall effect sensors. Another approach would be to use the APM as the sensor and plot X,Y,Z position from a point against the compass indication.

<sigh> too many idea and not enough hours in the day...

-=Doug
NikTheGreek July 7, 2013 at 2:25am

IMHO think that you have to use power supply to make the tests more accurate...because the power of the battery constantly decreased during the test.
Jiro Hattori July 6, 2013 at 11:26pm

It's nice practice of measurement of magnetic field interference from each components on the copter.
I would like to know what is happening on the real flight:-)

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