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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Interesting.  I read somewhere else that one way to address the efficiency issues on coaxial props is to use a smaller higher pitch prop on the bottom.  Why do you think it would be better the other way?  And why do you suggest 11.47 for both top and bottom?

I'll go give it a try, but I just ordered a huge new battery that will give it a lot more weight, so it may not be too overpowered once I put that one on.

Hey, sorry for the delay in response.  I was out of town previously and just had a chance today to mess with this.  I've attached my exported param file.  I tried setting all the parameters above like you said.  There seems to be less yaw drift (another issue I was having), but no difference on hard throttle or yaw left.

I then tried to run autotune but I was having an issue where I turned it on, it would twitch a little, then I would manually bring it back from drifting away, but then it wouldn't resume the twitching once I centered all the sticks (including throttle).  Taking it out of then back into autotune didn't work either.

Ugh, what should I do?

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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.

Hi Leonard, (replying to this one as the others don't have a reply button to hit)

I ran autotune with the AUTOTUNE_AGGR set to 0.1, the yaw filter set to 5hz and just the yaw axis. I was wondering if you could take a look at the dataflash log of the tune? It came up with rate yaw P=1.280242, I=0.1280242 and D=0. It seems better than the previous tune, and for some reason the quad motors don't sound as though their changing speeds as much (I assume a good thing?). Thanks again for all your help Leonard, I appreciate you taking time out of your day to help me!

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Hi Derek,

The tune looks like it went much better.

It looks like you have your roll over tuned though. I can see that your roll axis is oscillating every time that a yaw test is being done.

How have you tuned roll and pitch? I can see they are both set the same so I assume you have done a manual tune or accidently copied values of an autotune to both axis.

You should fine you get even more improvement out of your yaw tune if you can get your roll and pitch tune set up well.

I would suggest keeping your yaw tune but doing a roll and pitch tune using AGGR of 0.05. I am happy to have a look at that if you like.

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

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.

Hi Leonard,

I completed a roll autotune, was wondering if you could take a look at it. The craft must not have been perfectly level as to get it to keep twitching I had to give it a little roll input each time. Do you think I should do the tune over? It seems to fly well with the roll PIDs it derived.

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