Warning about APM2.x performance on Octocopters

I have read this http://www.rcgroups.com/forums/showthread.php?t=2012435&highlight=octocopt with dismay as I have an octo that I am in the process of tuning. Does this mean it is likely to risk the 'copter to try full waypoint control? 

What is the take on this forum about the issues and possible solutions?

Peter

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  • I didn't really want this to be handled in this way here, I was planning on doing a blog post or something, but since you got the discussion going, I'll have to respond.

     

    At this point, I don't really know how it will fly in Auto mode.  It might do OK, it might fly the same way it does in Loiter mode *without* simple mode turned on.  I hadn't got to test that part, but I suppose I should so that we can know.

     

    It certainly is possible to fly an Octo, and I did a fair amount.  In fact, I shot this video with my Octo, most of it done in Alt Hold mode:

     

     

    This was done in Loiter, IIRC:

     

     

    So, it is certainly possible to fly an Octo in Loiter with APM.  The issue really is just that the performance does not meet my (very high) standards. And there is a risk that if you use Simple Mode, and Mount, and/or who knows what else that you will have a problem.  That is the main message I'm trying to send.  Do not load up the processor with extra features beyond just the basics.

     

    Usage of Simple mode in Loiter results in quite unacceptable performance, but I didn't find it dangerous.  It's the addition of Mount that just pushes it over the edge and it goes critical.

     

    Mike, I am using a brushless gimbal.  The only thing I am doing with the Mount code is to use it as a PWM pass-through.  I use PPM Sum, so I cannot simply put my Ch8 to the pitch control input on the AlexMos board.  So, I am using the Mount code to take the Channel 8 input, and send it out Ch11, with no stabilization.  It's straight pass-through.

     

     

    PX4 and Pixhawk is of course the solution to the problem. But Pixhawk is not yet ready for public use, so you'll probably have to wait a bit yet.

  • If you read through the mentioned thread, and the dev discussions as well, you will see that it is not a terminal problem but one of being aware of the load limitations of the APM2.x boards.

    So if you are running an Octo, do not run your gimbal through the board (Brushless is the way to go anyway) turn logging down to your absolute minimum, and avoid simple mode if you can. I believe Mount was also a factor in the problems arising.

    There is definitely a limit, but there are also many Octo fliers out there quite happy with their setup.

  • Pixhawk won't have the same CPU bottleneck, and will run 8 motors no probs.

    Your risk with Pixhawk is it's a version 1.0 hardware. If you work in IT, you'll know that 1.0 is *never* bug free. Wait until 1.0.2 or 1.0.3,

  • Interesting. I was just yesterday out flying my big Octo and I'm happy with it.

    Loiter works well, I think. Did some Auto-missions too, and while it does change heading quite abruptly and sloppy at the same time, I blame the standard PID's, I haven't really fiddled with those yet, so strange behaviour is to be expected.

    I'm not doing gimbal control from the Octo yet, couldn't get it to work. 

    Lefebre may have a point, but it seems to be working well for me so far. I have a 2.5+.

    /Magnus, Sweden

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