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?
Replies
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.
8.BIN
Hi Jacob
Your main cause of the problem is that you have the larger props on the top (12.5) and the (11.47) on the bottom.
if you change the larger props to the bottom layer you will certainly see a positive behavior change.
In your case, since you are seriously overpowered, I would run 11,47 on top and bottom, and it will behave a lot better.
I know it sounds so easy ....
oh, you might have to go back to stock PIDs for the test.
cheers
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.
Jacob I had a similar behavior with a large quad , that lead to a crash .
After autotuning roll and pitch were smooth but Yaw move very fast and I did not noticed.
During a long flight test I gave by mistake full Yaw stick command and two motors went out of sync and then the quad crashed. With Yaw command in a quad or a coaxial octo two (four) motors slow down a lot .
Try to raise the minimum motor speed and see if the problem continues.
Hi Jacob,
Could you provide the .bin log. The one you provided doesn't have all the parameters in it. If you could also send your .param file after exporting your parameters that would be great.
It looks like your motors are slightly off in angle but not too bad.
I suspect this is largely a tuning issue. I can't see your parameters but I suspect you have not run autotune and are running without input conditioning turned on.
Am I crazy, or is there no way to export a .param file from a .bin file using APM planner (I'm on a mac, so don't have Mission Planner right now). Here is a link to the .bin log. https://drive.google.com/file/d/0BxHZtVL_kDQnV1hobXZIRXBkcXc/view?u...
I thought the .bin file was just a compressed version of the .log file. What are the differences between them.
You are correct that I have not run autotune. I used the Ch6_opt to manually tune the P,I & D rate values, but perhaps I should go do autotune.
I don't know what input conditioning really is (Google wasn't very helpful), so I suspect that I am, indeed, not using it. Care to educate me on what that is?
Hi Jacob,
You need to do this when connecting to the pixhawk and in the parameter list. There seems to be some parameter values missing from your .bin and .log file.
You should set:
RATE_YAW_I = 0.1 * RATE_YAW_P
RATE_YAW_IMAX = 1000
Unless your tuning is terrible you should be able to set STB_YAW_P = 3 safely.
RC_FEEL_RP = 25
ATC_RATE_FF_ENAB = 1
The last parameter turns on your input shaping. What this does is smooth out your rate and angle requests to the controllers. The throttle surge and bounce when you apply yaw is because your yaw rate is attempting to increase with a high acceleration. The parameters I have given you above smooth this out. Autotune sets your ATC_ACCEL_xxx_MAX to values your copter is capable after tuning.
Let me know how you go with this and we can go from there.
Good luck!!
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?
params.param
Leonard,
Did you do some changes in RC 3.4 for Auto Tune? I installed 3.4 rc1 and redid the AT. This is a Alien H680 similar to the discussion.
It flies so much better I don't hate this frame now. In Position Hold, at say 12 m/s speed, I let go of the right stick and the copter makes a nice even (big quick) stop and holds. It is also more steady in wind @ hover. It used to dance around a lot.
Also, I've noticed a decrease in altitude drop on this copter during acceleration.
Could be I just got a better tune, but since loading 3.4 it seems to be more stable overall and flying missions even in high wind there are no overshoots on the splined wayponts; ran three tonight and can't tell the difference between them from the flight logs on the map.
Hi DG,
I am glad your copter is flying well!!
I loose track of exactly when I made small changes or tweaks. Every now and again I find a corner case I can catch in one way or another. You may have benefited from one of those improvements.
Thanks, it is always good to hear when things are going well!!
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