TBS Discovery with Pixracer 2.8.4 altitude hold problem

Hi Guys,

I have a big problem with my TBS Discovery DIY builded drone. It is based on Pixhawk 2.4.8 FC, 920kv motors, and 4s 4k mAh battery, with 9540 props, and Mission Planner as Ground Control Station.

Problem concerns altitude hold mode and hold position mode. When im entering this modes my drone is constantly slightly changing its altitude by 0,25 – 0,5 m in up and down direction – changes are irregular and sudden. Attached video shows the problem.

https://youtu.be/ql21jTX5iNA

What I have tried yet:

  • Middle throttle position setup
  • Different PID settings, including auto tune ones.
  • Putting foam on barometer
  • Damping whole pixhawk by putting on foam.
  • Updating firmware

I also uploaded logs.

My newest clue is IMU2, I have checked logs and found considerable noises on Accel(X, Y, Z) signal while on first IMU they were marginal. I decided to check it and I turned off IMU2 by EK2_IMU_MASK parameter. It was much better, not ideal but close. 

Here is comparsion of altitude hold with only IMU working (higher graph) and with both IMU's (lower graph). What do you think? Shoud i leave it with IMU2 turned off? 

And photo of Pixhawk mounting (the foam adhesive is holding tight):

Here are logs of flight with two IMU's active (second graph): https://we.tl/Luv99b0ora

Cheers from sunny Warsaw! 

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