Roll instability/vibration and Loiter?

Newbie, just starting out and confused by the results of a fresh build of a 350 Quad copter with a Pixhawk flight controller. Once in the air, in stabalise mode, the quad vibrated back and forward badly in the roll axis. I fixed that by going into the advanced config/tune settings and reducing the Roll Angle P factor by 10%.

One point I would make about that was that the nice GUI interface documented on the web site bares zero resemblance to what was displayed in APM Planner 2 (Installed a deb for Ubuntu).

That’s the Roll vibration sorted but I then switch into Loiter mode and the quad dropped out of the sky at a fairly constant rate to land on the ground. I’d say it was from about 10m (I’ve not sorted out log files as yet, have to read up and enable that) So that made me wonder do I have to configure Loiter mode.

That made me wonder what is the lazy flight controller doing? Surely the whole point of the flight controller is in the title. The Pixhawk has access to barametric pressure, and with a GPS lock it knows it’s altitude. It also has accelerometers so why would I have to configure Loiter? Surely it has more then enough info to be able to set throttle to required settings to maintain its position.

That line of thinking brought me back to the vibration in the roll axis. The quad should know that it’s oscillating back and forward in the roll axis. It should also know that this is not due to user input, as there is no oscillation in the transmitter input. No hand on that control so centered by default. So given that info surely it could decide that it needed to adjust it’s control settings?

OK very basic questions and given I’m only starting out I know nothing so I’m just imagining how the system would work, in my head. Which is obviously totally wrong. Thank the maker that RTL worked as expected :wink:

@johnfwhitmore did you performed calibration of accels, compasses, radio, ESC?

Should have given that info in the original post. Yes Full calibration of everything and GPS lock before arming. Actually would you be allowed to arm the quad if you’d not done all that?

Ok, so answering to your questions

There can be a lot of reasons, unfortunately it is hard to say without logs.
My first copter dropped out of the sky too when I switched to Loiter. Later I realized that the hover thrust level for Stabilize differs from Loiter. I just forgot to add more thrust thus crashed.

What type of configuration do you mean? There is no mandatory configuration for Loiter. If your copter is calibrated and flies well in stabilize then it should fly well in Loiter too.

ArduCopter has AutoTune mode for that

Oops your response sort of suggests that the word “Loiter” doesn’t mean what I think it means. I thought that that mode would keep the craft positioned where it was. Sort of holding its current position. I may have been mistake in that understanding.

By “configuring” Loiter mode I meant more or less what you’d said in the previous paragraph. You wrote that hover thrust level for Stabalise differs from Loiter. So should I have been configuring a thrust level for Loiter mode. As opposed to the flight controller just holding the position. If I did have to do that it makes no sense to me at all. If I calibrate the thrust level for Loiter and then change the circumstances of my flight I’d have to recalibrate. So for example I’d imagine that if I flew at sea level with a certain air pressure and then flew at the top of a mountain there’d be different air density so different thrust levels required. If I later added FPV gear I’d have to re-calibrate the thrust level I wanted. That just seems wrong to me. Like I say new to this so my expectations might be a bit askew.

I’d not heard of Auto Tune so I’ll go look into that. Thanks for that pointer.

Auto Tune looks brilliant sorry I’d not found that mode

Hi, after stabilize mode ,you should try alt hold. Alt hold mode has a dead band (40 to 60 %, adjustable) in throttle stick ,where it holds the same altitude. Above and below the dead band it is commanded to rise or sink. Loiter mode keeps the position and altitude ,when there is no stick input.
So your copter came down in loiter ,why?
If you have an overpowered copter , that hovers at 35% throttle in stabilize, when u switch to loiter or alt hold it is commanded to go down.!.
You should adjust throttle mid . ,switch between stabilize and alt hold ,confirm, and then try loiter.

See Here as well. Look at the Automatic learning for 3.4 and above versions.

Hi, thank you for reminding it.

Overpowered and Underpowered… Note to self. Thanks for the link to that page I think that might well be the problem so I’ll have to go through that to sort out the throttle.