Discussions

Today I jumped from 2.9 to 3.0 on my quad and went through the setup routine. I was disappointed (but not surprised) to find my compassmot levels running from 70-73%. My quad is an custom (i.e., homebuilt!) H configuration with the main power lines running beneath the APM. The main body is of composite construction with a top and bottom deck of G-10 separated by about 0.6" balsa spacers. At the time I thought I was being rather clever by installing my power bus wiring into the interior prior to epoxying it all together. Now seeing the stinkin' magnetic fields it's generating, I feel decidedly less clever. Apart from ripping the fuselage open, there's little I can do to improve my power wiring situation.  However....

I did happen to have some thin sheets of Mu metal I was experimenting with a while back (Mu Metal is used for magnetic shielding in civilian aircraft). On my quad the APM sits above the G-10 top plate of the fuselage, riding on double height posts made of Zeal tape (I had hoped the extra distance would be enough to avoid magnetic field issues). So I cut out a small rectangle of the Mu Metal and slipped it between the quad's top plate and the APM. Because of where the Zeal "posts" were, I couldn't have Mu Metal under the entire APM, but I covered about 2/3s to 3/4s of its bottom.

I retested the compassmot level and it dropped to 37 - 38%, not quite half of the initial value. If I was more diligent in the placement of the Mu Metal shield, I could probably get it down to 20%. I was really impressed with how well the Mu Metal shields the magnetic field. So if you're having compassmot issues, consider a thin Mu Metal shield. Placed on the very bottom of the APM might be helpful by adding a bit more mass to it for vibration isolation.