APM2.5 magnetometer issues on DJI FW550

Folks,

I narrowly avoided a flyaway with my newly finished Flame Wheel 550 with an APM2.5.  From what I've determined so far, the issue seems to be related to mounting the APM between the top and bottom plate on the FW550  I had it mounted directly on the bottom plate that is also a PCB with the power distribution traces in it.  I think this confused the compass chip in flight.

I had left the APM in the "learn mag offsets" mode and evidently it got very confused.  After my close call flying waypoints, using MP, I looked at the mag offset settings and found the Z mag offset setting to be way outside of what was acceptable.  It was about -560.

I also tried to set the mag offsets with the manual calibration by flipping the copter every which way, etc....  This seemed to work, but I got a message in MP saying that the calibration failed after I did it.

It seems clear that the APM needs to be mounted up high away from any metal and magnetic fields caused by current flow through the power distribution board.  Either that, or adding an external mag.

For those with more experience with the APM 2.5 and mag issues, does this sound like a good diagnosis? 

Thanks in advance.

Bill

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Replies

  • Honestly I'm not sure if I got this resolved yet or not.

    I have not relocated the APM or bought the remote mag module. I've just turned off the auto learn settings on the mag and did a good manual calibration (dance with the hex).

    I've had no RTL or Auto issues to date, but I haven't flown it much yet either. Work and family stuff has not allowed for much hobby time. :-)

    Honestly, I still don't trust it to behave, and that's not a good place to be...

    Bill
  • Hello Bill,

    Did you get your mag problem solved? If so, what was your solution?

    Dennis

  • I have the same frame and I actually took a mainframe plate from one of my HoverThings HT-450 frames and mounted on 3 inch standoffs above the top plate on the flame wheel. Put the APM 2.5 on it under a witespy dome. This way I can slide the battery underneath so it's still on the top plate. It looks a little weird but it works. The picture kind of gives an idea.

    3692731665?profile=original

  • Actually there is software available to model the magnetic fields produced by circuit board traces... it ain't cheap for professional versions...don't know about trial versions.

    In short, the more current that passes through the board sections, the larger the mag field generated. I have a clone 450 board and there is a lot of real estate for the current to run around on.

    In a circular wire, the field generated is spherical extending from the center of the conductor. For a FW type board, spread all over the place, I would imagine some of the peaks of the field will interact, add, push each other around, etc.

    As Richard B said, 'yes'.

    The general consensus is to move the APM as far away from the generated fields as possible. For a FW, add another deck above and hang the battery under the PDB. That is what I am planning for my DJI Hex chassis. DJI stuff is high quality BTW.

    Shielding? For mag fields you use a high mu metal material. This will add weight to your airframe. Yes an added plate to move the APM will too but now where near the weight that the shielding has. I work with BIG magnets in MRI systems. On some occasions we have to add shielding to the walls, floors, ceilings to knock down the magnet fringe fields to contain them to safety standards. This means additional structural additions to handle the weight of the shielding.

    Just move the APM away to begin with.

    This will not explain all mysteries but certainly seems to be best practice with APM.

    -=Doug 

  • Hi Dennis,

    I'm confused as to why you needed to move the APM up higher to get the Mag to work once you installed an external Mag and already had it up high?

    Could you explain for me?

    ??

  • T3
    Simply put, yes.
This reply was deleted.

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