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Please Help! APM Hex crash in RTL. Need to fix by tomorrow.

Posted by Parker Stephen on June 18, 2014 at 5:48pm

I finally thought I had my copter all tuned up and working well, but today I had a wreck that cost me a motor mount and prop.  I engaged RTL a few feet off the ground and the copter immediately pitched hard into the wind, overshooting apparently, then pitched back violently the other way.  The first time, I was able to recover in stabilize mode, but the second time the rocking was so violent it continued to overshoot all the way to the ground.

What do I change to make the copter not overshoot in autopilot modes?  I don't use the copter in actual auto mode, but when I did try one time it seemed to do the same thing (in heavy wind only).  Please help if you have any ideas.  Would it help if I turned on the braking feature of my ESC's?

I'm really hoping to get this resolved tonight as I have videos to shoot the next two days.  Unfortunately, I had no extra CF props and now am forced to use one plastic one.  If logs are needed, please let me know and I'll post them.

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Replies

  • Parker Stephen June 19, 2014 at 8:38am

    I think I've found the source of the problem, or possibly sources.  My board mount has some slop in it which was obviously causing overcorrection.  I plan to use this information found in a post from Leonard Hall to attempt to fix the problem.

    We choose ATC_RATE_RP_MAX to be a rate we know the copter can safely stop from and again I can calculate this from Autotune. Here I will probably go to something like 30 to 50% the maximum value calculated by Autotune.
     
    This addresses the problem with large copters that fly fine until they are commanded to go from full right 45 degrees to full left. The copter then overshoots and flips. This is being caused by the Stab controller asking for a higher de-acceleration than the copter can achieve because of a large Stab_P value. When correcting small angle errors, say 45 degrees, the copter doesn't build up too much speed. However, when the angle error becomes large the copter is able to get a real head of steam up and it is like comparing the stopping distance of a car from 60 km/h to doing 120 km/h (the stopping distance isn't double, it is quadruple).
    I'll be testing out this change this morning and hopefully it will be fixed.
    • Parker Stephen > Parker Stephen June 19, 2014 at 9:57am

      With ATC_RATE_RP_MAX set to 9000 centidegrees/s, the problem seems to have subsided.  I also havled the ATC_ACCEL_RP_MAX to half of the default value.  The copter seems to move around a little in the wind, but it RTL's properly now with no crashing.

  • 3D Robotics
    Chris Anderson June 18, 2014 at 5:56pm

    What version of the code are you using?

    • Parker Stephen > Chris Anderson June 19, 2014 at 4:34pm

      Chris, I'm sure you're a busy guy but I really could use some help here.  This problem surfaced again while in stabilize mode and I'm at a loss.  Wrecked my videography copter two times over the last two days and now my gimbal is broken and I can't film tomorrow like I was scheduled to.  Today I was flying on the beta firmware 3.2 rc2 and had adjusted the parameters as mentioned below.  The first battery went fine with no problems at all and then about 1 minute into my second flight, the oscillations of death started as I descended at a normal rate.  I'm very discouraged by my failings with Arducopter and think it may just be too complex for me.  If I can't get my copter to fly reliably with an APM, I'll be forced to get a different flight controller, maybe even a NAZA :(

      Please tell me what you think is going on here, I'm desperate.

    • Parker Stephen > Chris Anderson June 18, 2014 at 6:01pm

      It's the latest version, I just updated it a few days ago.  This has been an ongoing problem with the hex.  It is quite overpowered, which may contribute. Let me know if logs are needed.

      EDIT:  It's the latest stable version BTW, not beta.

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