I'm looking for an advice what should be tuned differently in my otherwise very nicely flying copter. It's very stable, vibrations are well within recommended limits - when recording video with GoPro there is no jello at all. However when I switch from stabilize mode to AltHold and fly with just a bit more dynamics sooner or later the copter shoots up. I mean the throttle is out of sudden set to maximum value and copter ascends rapidly. The only reliable way how to prevent it from flying to completely insane height is to switch back to Stabilize and descend manually.
Any ideas what should I change here?
My setup:
TBS Discovery w/metal arms 250mm
2830 motors
10x3.8 carbon props
APM 2.5.2
FW: 3.1 RC6
External compass
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Just short update from me -- I solved the issue finally by switching to 32bit platform and also by mounting the flight controller on a dumping platform. With these two combined I probably do not reach the clipping and everything is solid. Thanks a lot for all your hints!
Not sure but I think someone already mentioned this, but I got this same problem --- when go to Alt Hold mode the whole thing shoots up (Pixhawk Lite). Apparently its the THR_MID parameter, which is the Throttle setting for Hover. The copter seems to need to know what is the appropriate throttle stick setting (thus RPM) for the copter to Hover, and uses this to do alt hold, probably in addition to baro/gps etc. This parameter is default at 500 ( range between 0-1000), but you will have to find the parameter on your setup (particularly RC Transmitter's throttle).
I did mine like someone mentioned, was to look at APM Planner's Log to see where the RC Control's Throttle was when I was hovering, and use that parameter as THR_MID. For example, my RC Control's Hover is at 1300 microseconds (TX range is 1000 - 2000), so I naturally use 300 for THR_MID parameter, and everthing that requires the copter to hover -- ALT HOLD, RTL is fixed!
Just in case it helps.
So this is a vibration issue?
I did a test where I mounted the Pixhawk right to base plate of the copter. I then flew around a while and everything was just fine. Even in Alt Hold and Position Hold.
I then flew into the wind and low and behold the copter took off and flew 20 to 30 meters into the air in a mater of seconds.
Wow.
I then added two pieces of foam tape to the Pixhawk under body and did the same test again.
The copter flew normally with no issue. It even lost altitude as it flew into the wind. No more excitement.
The thing that confuses me here is it's in Alt Hold mode so its suppose to use the Barometer to determine height. So why does the Accelerometer take over and the Pixhawk not cut the power when it sees it's too high.
Because we use the accelerometers to get the fast reaction times and the barometer to get the slow reaction time. The Barometer will eventually pull it back but by then it will be quite high.
Hi Jakub,
We think we FINALLY figured this one out. We hope this will be fixed in the next release.
We hope so. The vibrations of the frame are causing the accelerometers to saturate. This results in the average acceleration to go from -9.8 m/s to closer to 0. This makes the copter think it is falling and therefore it climbs.
We still get good alt hold because our filtering and other processing seems to be very well done.
To fix the problem we have increased the acceleration range on the accelerometers, this means the vibration needs to be much larger to cause the climb.
So the bad news is, you probably still have a vibration problem. The good news is, we hope your vibrations won't cause the copter to climb any more on 3.3.
Good news Leonard!!, It's going to work on APM too? or only pixhawk?
What you see in the logs is the filtered version.
Don't quote me here but I think we are going from the 8g setting to the 16g setting. That give you some idea how strong the high frequency vibration can be. And remember it is clipping before it goes through the filter so we didn't realise how high the peeks really were.
I think there is something on drones discuss, but I haven't had a look.
@Jakub - I'm wondering if you were able to solve your problem at all? Was it the vibrations?
I have the same problem on a large wooden hex with 15" props. It is rock solid at hover with nil vibrations but if I move relatively fast in Alt Hold it will suddenly shoot up into the sky. My logs look just like yours and it requires switching quickly into stabilize mode in order to control it. I also noticed the same problem (spontaneously shooting upwards) during an auto mission, especially when the hex was moving into a 5-10mph wind. The hex would suddenly shoot up ~5-10m but would settle back down to the proper altitude, yet forward progress was slow since it kept bobbing along. Downwind was fine. I was thinking it might be something with the baro and aerodynamics creating a high pressure situation (???). Anyway, I'd like to hear what you may have found out.
Thanks,
Carl
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