Discussions

When moving in AltHold or Loiter copter shoots up - looking for advice

Posted by Jakub on November 22, 2013 at 8:30am

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

log.graph.althold.shoots.up.png

copter.pids.png

basic.pids.png

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31rc6, apm, althold, tbs discovery, ascend, full, rapid, shoots, throttle, up

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Replies

Jakub November 30, 2015 at 2:21am

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!
Kuson November 29, 2015 at 11:09pm

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.
Michael Burmeister March 23, 2015 at 3:02pm

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.
- Developer
  
  leonardthall > Michael Burmeister March 23, 2015 at 3:16pm
  
  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.
Developer

leonardthall March 23, 2015 at 4:53am

Hi Jakub,

We think we FINALLY figured this one out. We hope this will be fixed in the next release.
- Alex > leonardthall March 23, 2015 at 7:48am
  
  Leonard what was it? I've been battling with this problem for the past three weeks.
  
  Are you saying that it will be fixed in 3.3?
  - Developer
    
    leonardthall > Alex March 23, 2015 at 2:49pm
    
    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.
    - Jun Yamog > leonardthall March 26, 2015 at 11:36am
      
      Is this issue only on 3.2.x? I have an APM and after downgrading to 3.1.5 it works much better on alt hold and loiter. Otherwise it would shoot up, tries go down and keeps on bobbing and motors goes fast and slow.
      
      On the pixhawk I did not see this, but maybe it just reacts faster.
    - Cala > leonardthall March 24, 2015 at 2:48pm
      
      Good news Leonard!!, It's going to work on APM too? or only pixhawk?
    - Alex > leonardthall March 23, 2015 at 11:23pm
      
      Leonard,
      
      If the accelerometer is saturating, how do you possibly see the spikes that we see on landing etc? I wouldn't think the range could be that limited
      
      Is what we get in the log file the actual output of the accelerometer or is it a filtered version?
      
      Last but not least, if I may, what's the current and the new range that you program the accelerometer for?
      
      --Alex