I am sure that some of you have experienced the same issue that we have. We are flying a rather heavy hexacopter configuration which pulls a substantial amount of current just to hover. Due to the proximity of the power distribution board and the magnetometer, the magnetic fields generated by the DC current being drawn from the batteries can potentially screw with your heading data. The more current you draw from the batteries, the more interference you encounter. This can cause some nasty issues and in certain flight modes has even caused one of our vehicles to crash. Luckily, the folks I work with are pretty fluent with EM fields and had a simple and reasonably cheap solution that we wanted to share with the community.
There is a material called Mu-metal, which is a nickel alloy, that has an incredibly high magnetic permeability. If positioned correctly, it can isolate standing magnetic fields and greatly increase your magnetometers sensitivity to magnetic field interference. Unfortunately, it also poses the potential threat of blocking Earth's magnetic field if not positioned correctly on the vehicle. With this in mind, it must be used as conservatively as possible. We have installed a sheet of the metal (about the same thickness as shim stock) on our vehicle and flown it without encountering any more issues to date. We will update the community as we progress.
With that being said, here is the data we produced in the lab. First we simply kept the vehicle in the same position (roughly) and ramped up the throttle to 25, 50, 75, and 100 percent. While doing this, we could see the magnetometer data react and at one point we even saturated the sensor. We repeated this process again with the Mu-metal sheet installed. Here are the data sets...
As you can see, a tiny 2 x 3 inch piece of metal had some pretty drastic results but I will only feel at ease about it once we fly it for several hours. For the cost, I would argue that it is a great use of $20 or so considering how much heartache it could save in the end.
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you can order it here: www.mumetals.com or they have a lab-kit too with fully annealed material called Co-NeticAA perfection
Josh
Here is a magnetic shielding Evaluation kits that includes "fully annealed" Co-NETIC magnetic shielding. You can order a full range of MuMETAL sheet and foils here too.
Josh
What is the performance of Mu Metal for shielding, as compared to the shielding product "Giron"? Anyone know which is more effective? Magnetic Shielding: (GIRON)
Hey guys,
found this forum yesterday and read your interesting conversation about shielding. What Adam descibes above is absolutly right. There is no other Material on earth apart from µMetal to shield a magnetic field in the range of 30 - 300 Hz (AC).
Please feel free to have a look at http://www.abschirmungen.eu/english/products/magnetic-shielding/
There you can buy Mumetal sheets which are already heat treated! Heat treatment process is very important once you start bending the Mumetal or putting physical stress on it you have to heat treat it afterwards at arounf 1150 °C. Might also be worth to think about a finished part (round disc) with holes in it to fix it!? Looking forward to discuss with you!
Well, I received my 5" x 12" piece of Mu Metal and had a very weird experience. I wanted to see how good its shielding was so I put a compass on top of it and was going to bring a neodymium magnet up underneath. I was holding the Mu Metal sheet in my hand with the compass on top as I turned to pick up the magnet. To my surprise, as I turned the compass needle stayed fixed in relation to the Mu Metal sheet. The needle should have stayed pointing north but instead turned with the sheet!
I think what was going on is that the magnified compass needle is creating a sort of mirrored dampening field in the Mu Metal beneath it and it just gets locked to itself. If I hold it some distance away it doesn't seem to happen.
This shouldn't be a concern if the magnetometer chip in the APM doesn't have any active magnetic material in it, but is just a passive sensor. I think this is the case. Anyone know for sure? I was planing on putting the Mu sheet directly on the bottom of the APM 2.5 case so it will be very close to the circuit board.,
Are you using the integrated magnetometer on the APM board or a standalone magnetometer? If you're using the APM, was the Mu metal piece simply placed beneath the APM? If someone had placed an APM on Sorbothane-like dampening material, a piece of Mu metal on its bottom would add nice mass dampening.
Could you provide a link(s) for where it can be acquired? I would like to try it as well. I have been working on different mesh shielding and would like to try this in the mix as well.
Thanks!
Peter