Huge wobble when yawing and hard throttle up

Hey guys,

I know I'm not the first person to complain of this, but I've been scouring the forums trying to find answer, and none have pointed to a solution that worked for me.  I have a beefy octocopter in an X8 configuration using a Pixhawk and APM firmware 3.3.3.  I've been flying in stabilize mode, and whenever I give it a hard yaw left the drone pitches back (nose up) and rolls to the right.  Yawing hard to the right does not appear to cause a problem.  The same pitching back & up happens when I throttle up very hard as well.

I've tried to address all the possible culprits that other forums have suggested:

  • I'd made sure the fore/aft and left/right center of gravity is right in the center of the pixhawk.  Thought I believe the up/down COG is below the pixhawk.
  • I tuned the RATE_P/I/D for roll and pitch.  The copter actually handles really well if all you're doing is pitching and rolling it (and not yawing or being hard on the throttle)
  • I've done my very best to make sure there's no arm twist, although it could be I didn't do a very good job at that.  So **maybe** that's an issue?

The copter is definitely not lacking in power.  It appears to hover around 30% throttle, which I know is a little low.  Can that cause issues like this?

Here's a link to a log file (sorry 15mb was too big to attach directly to the post, so here it is in Google drive) from a short flight where I intentionally gave several hard yaw and throttle commands that exhibited the behavior.  I tried to analyze the log myself, but I'm no good at this stuff.  Maybe someone can help me uncover what's going on?

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Replies

  • Seeing you log - I think you can try to reduce vibrations, they are awful - be happy it flies at all :)

    • IMU(2)/acc*
      • Actually I just learned about the VIBE set of values in the logs.  According to those I think my x and y vibrations are OK, but the z one is a little high.  I've seen a copter suffer from high vibrations before and this definitely wasn't exhibiting the jumpy behavior you get from that.  It sounds like vibration issues like this have a greater effect when it's trying to do inertial navigation, but it's not doing that in stabilize mode.

        • Developer

          HI Jacob,

          IMU messages are good for looking at low frequency vibrations that mess up your controllers. You should look at both the gyro and accel parts of the message. You can also see these issues as lots of noise on the output to the servoes or in the RATE message. Low frequency noise is makes it very hard to get a good tune using Autotune. Manual tunes with low frequency noise will never be great but also often cause extra motor and esc heating.

          The VIBE message is good for looking at HIGH frequency vibrations that mess up your IMU. Until we added the VIBE message it was very hard to look at how much high frequency vibration people were experiencing. This is the main reason for poor GPS based performance and poor Alt Hold performance.

        • You cannot rely on the VIBE logs to determine if you have a vibe problem.  They go through a high-pass filter, and don't show low-frequency vibrations that can harm the ability of the PID loops to do their job.

          • Rob: Thank you for confirming what I discovered.

            At first(when VIBE was introduced): I thought of it as the ultimate tool to get realtime information if there should be any unusual vibrations.

            Searching for a threshold value to make GCS and/or HoTT warn me at, I discovered that the numbers were very little "useful", not really representing the situation as expected.

            So I gave up using them for such a warning system.

            please consider changes to VIBE to be exectly that,  telemetry usable (low-bandwidth) data that sums up the vibration situation onboard.

            • The VIBE message I believe is primarily intended to detect vibration problems which can cause problems for the EKF inertial nav system.  On smaller copters, it appears to be OK, or at least I haven't seen clear evidence of a problem (yet).  

              But on larger multirotors, and especially helicopters, the VIBE HPF masks problems that affect stabilization and flight control.  I just got done helping another company with a very large X8 who tripped up on this problem.

              • Ok, that's critical information that I shouldn't rely on VIBE for vibration monitoring then.  What is the best way to examine vibrations in that case?

                • I'm not saying the VIBE message is useless.  But you need to look at the IMU messages, GyroX,Y,Z and AccelX,Y,Z

    • Interesting.  The Pixhawk is on a layer of vibration dampening foam.  I'm going to do some propeller balancing and see if it gets better.  Can you tell me which log values you examined to find the vibrations?

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