Severe yaw problems with multiple Pixhawks

Hello everyone!  I have a real tricky problem I desperately need help on.  I've spent almost 40 hours trying to diagnose the issue myself with no luck and am hoping someone on here can point out a stupid mistake I've made (if I could be so lucky).

The vehicle

This is a very large octa-X copter using a 3DR Pixhawk

The problem

On takeoff the vehicle is fine for a few seconds and is relatively stable while hovering (in Stabilize mode).  Shortly after getting in the air it will start to spin in either CW or CCW, and it will gradually accelerate its spinning until I land for safety.  The last test I ran it spun a full 360 with zero rudder command from me.

What I've done to diagnose the problem

I've checked for compass interference, and originally had EKF errors show up in the logs.  After spending a week troubleshooting I decided I must have a hardware failure and ordered a second Pixhawk and new external compass.  It arrived and after setting it up fresh it had the exact same problem.  I've also tried different versions both with the most current (3.4.5), and also 3.3.3 just to make sure it wasn't an odd glitch.

I decided to try a DJI A2 I had and it did not have any yaw problems whatsoever, meaning there's nothing wrong with the drone's hardware from what I can tell.

I've attached the logs, and have verified the following:

  • Everything is mechanically working correctly (e.g. motors spin in correct direction)
  • The spinning is not caused by a rudder input
  • There is a large discrepancy between CW and CCW props, which explains the turning
  • The compass is working correctly and knows the yaw is changing, but the 'Desired Yaw' follows the actual yaw
  • Re-doing all calibrations does not change anything (e.g. compass, accelerometers, level)
  • It sometimes registers magnetic interference and gives me a warning or fail, but other times is less than 20% and does not seem to make a difference
  • When I test it on a small quad I had the quad yaws a tiny bit, but nowhere near as badly as my big one here
  • The external compass is 12" from any source of magnetic interference (also tested at 6" away and no improvement with larger distance)
  • Changing Yaw P or I values does not appear to make an difference

This is driving me absolutely crazy and I'd appreciate any thoughts or suggestions anyone has.  Is there an obvious setting I'm missing?  Given I've tried two separate units and even different versions, but it doesn't have the problem with a DJI controller, I'm sure it's a software or settings problem.

Like I said, I'm stumped.  Thanks in advance for any help!

Test #8.bin

Test #10 - spins almost 360.bin

Test #9.bin

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Replies

  • You're correct,  I do have yaw authority with manual input but it seems like it's not even trying to compensate for the yaw error even though it seems to recognize it's turning.

    I'll try a much higher P and I and see if that helps!  Thanks for your thoughts!


    Andre K. said:

    you can safely assume Yaw rate P of of 0.2 or even 0.3 would make a difference.   , about the same for integral.

    hard to say more unless I "feel" it ,   If I am right, you do have yaw authority with manual input, it's willing to turn, but not precise needing correction all the time  -  it's just drifting all over as AP got too little authority over it. 

  • you can safely assume Yaw rate P of of 0.2 or even 0.3 would make a difference.   , about the same for integral.

    hard to say more unless I "feel" it ,   If I am right, you do have yaw authority with manual input, it's willing to turn, but not precise needing correction all the time  -  it's just drifting all over as AP got too little authority over it. 

  • Yup, I've run into default PID values rarely work before :)

    I tried tuning Yaw P and I values up and down by 50% each way and it seemed to make no difference at all (that may have been a test after the ones I uploaded, I don't happen to recall).

    Do you think that would really cause it to start rotating by 180 degrees on its own?  When I tried flying it with no controls other than throttle (on stabilize mode) it ended up turning a full 360 at an increasing speed before I did an emergency motor stop!

    Thanks for your thoughts Andre, I'll certainly try some more Yaw tuning if no one else has a better idea :)

  • increase RATE_YAW_P  ,it seems that you did not do any tuning to it, one set of default PID's can't fly everything perfectly , you know :) 

    once you get it to fly ok, do autotune.

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