Alt hold irratic throttle behaviour with APM 2.5


I would like to share my current problem with my board that I am having for the last couple of weeks. Not sure if I changed something at the time to invoke this behavior, but when in alt hold (or loiter), the copters throttle starts to oscillate, and doesnt maintaine a steady altitude. In some cases starts falling slowly and that time throttle stick is hardly responsive.
I have tryed all the obvious things, like making sure the baro is cleared, and that it has its protective foam in place. The throttle and althold pids are default, and have worked before flawlesly. I did set the hover throttle to match 50% on the stick. Tryed the 2.9  3.0.1 and the new 3.1 firmware with no avail. The eprom has been cleared in between firmwares.
The vibrations are also quite low, I have the external compass and the throttlemot compensation sorted. 
There is no difference with 3 or 4s lipo with the exception that the 3s the oscilating is not as agressive out of the obvious reasons.
I even tryed powering the board from different sources like the dedicated power modul, bec+lc filter, and esc-s. There was no crash or moisture that could damage the board in an obvious way.
When displaying motor outputs you can clearly see when the althold is on from the erratic throttle.
I am also providing the log file.



2013-12-04 16-56 5.log

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  • All,

    I am super confused by this problem but hopeful there are kernels in this thread that can help me with my erratic altitud hold on multiple APM (knockoff) boards of mine.

    I too have very erratic movements of my small quads (2 different 250 size models) where Alt hold it unuseable.  Here's my concern...the board itself, resting on my bench, either enclosed in a case and with foam inserted, or exposed and barometer uncovered, shows wild (trends either up or down and by two tenths or so of a meter on any given change) altitude readings in APM Planner and Mission planner.

    I'm a recovering airline pilot (recovering meaning I don't have that job anymore) and I understand the priciples behind static pressure being used to measure altitude.  What I can't figure out is why a barometer, sitting undisturbed on a bench might give such variable readings in quarter second intervals.

    I've had this with an APM Mini 3.1, the HK Pilot Micro and the APM Mini Pro.  My objective with each of these boards was to set up a small automomous copter and I've never acheived that because of this AltHold issue.

    Is the altitude swing at rest on an unstalled FC normally so variable?

    If so, is the code in the board supposed to "smooth" the readings so that the board isn't always hunting the hold altitude?

    For both of the above, I guessing the answer is "no" but code side of this is a mystery to me.

    It is difficult for me to believe that three flight controllers I've had (I returned the Mini APM 3.1 for this reason) all suffered bad barometers.

    It feels like I'm missing something and I can't find what it is ANYWHERE on any of the forums.

    So, I don't think it has to do with vibes, proximity to other electronics or atmospheric pressure changes. 

    BTW, I have two "regular" APM boards, one 3dr and on knock off, that function well for altitude hold.

    If anyone here has the silver bullet, I'd love to hear it.

    Desperate for help here as I don't want to write off my $100 or so investment in these smaller form factor boards.

    Thanks in advance for any replies.



    • Waltec,

      Guess my last post did not work. I had to wait for better weather to get out and do a test. I caused my props to be imbalanced and show althold (via loiter mode) is very unstable. Then I remove the imbalance and loiter works again.

      in the IMU log (will post later) you can see the impact of the imbalance.

      In the video (attached) you can see (and hear) the result.

      For me, this proves prop balance and the vibration that gets to the IMU can cause AltHold to be un-usable. I suspect I would not need the mass under the APM if I got balance really good. It may also be certain frequencies of vibration that cause bad behavior.

      • Here are some log graph segments of the above flight.

        3702738514?profile=originalAbove is IMU showing both with tape on props and then removed. See much improved Z vibration level.

        3702738659?profile=originalAbove shows desired altitude and actual while props had tape to create vibration.

        3702738627?profile=originalAbove is with props balanced again. See how much better altitude tracks desired (all in loiter mode)

        • Thanks Dale, very helpful. I removed some gem fan 3 blade props in favor of some hq bull nose props. Worlds of difference in vibration. The gemfans are hard brittle junk. The hq's need to be balanced but I now get a steady hold. This was on my hkpilot micro 250 quad. I just sorted a similar issue with a F330 clone using multistar 2206s but my problem there seemed to be the mid throttle parameter setting. That one is an Apm mini 2.6 that I can't get to the log files. It keeps telling me that files couldn't be downloaded or some such. I've been setting mid throttle by trial and error. I'll update this post once I get things further sorted. Thanks for the great guidance. Waltec
    • Waltec,

  • I had a similar problem with a 250 quad running HK micro APM (The little 32mm square one). I originally mounted the board on a thin plastic sheet then bulbs (like camera gimbals use) on each corner to the frame (left in photo). ALT_HOLD was very jumpy and gained/lost altitude randomly. Balancing props helped a bit, but I could never eliminate enough vibration to make it useable. I moved the APM to a 29 gram metal plate (right in photo) thinking the weight would add dampening and this completely solved the issue. ALT_HOLD now is VERY stable along with loiter and other automatic modes.3702737461?profile=original

  • Randy you were right on the vibrations.
    I dampened the controller with a 300g weight for testing purposes and it worked.
    The IAlt and BAlt curves hardly diverge now. Thanks for all your support.

  • Developer


         As you say the vibration looks ok.  The barometer altitude vs the inav altitude is ok although not great which makes me think that it might be a vibration dampening issue.  We've found that four squares at the corners of the board is better than a "bed" of foam.  This may not be the problem however.  One thing is that the CTUN message is disabled which has some important information in it including the pilot's input.  Could you re-enable that and fly again?  Also it's ok to turn off the MOTORs logging, that's not too critical.

    3692904116?profile=originalBy the way, the auto pilot throttle (i.e while in AltHold mode, loiter, etc) is always much more 'active' than when a human controls the throttle so the little jumps up and down on MOT1 in the graph you provided shouldn't be a problem.

  • Update: This is clearly not a hardware issue since it exists on all of my APM's when on F550, and its working fine when running them on my quads.

  • Do you have a video transmitter near your APM or any of the other sensors that are used to determine position like the GPS or compass. I noticed that on mine, having the tx close to either really makes weird things happen, like drifting. I increased my tower and move the gps + compass further away and its back to rock solid. I would suggest that you turn off your video tx and try again and you might be surprised.

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