Hello all,
This is a bit of an odd one, and perhaps a bug, perhaps my foul-up, dunno...
I was out and about today flying my hexa with FPV for the 6th or so time thus far, and while in the middle of flying in Alt hold, I decided I wanted to gain double or so altitude, so I got out of the throttle dead band, and hit the stick up.. The copter happily took its order, climbing to over 200 feet, and after I put the throttle stick back in the dead band, a few moments later, the copter started dropping. I have fat shark goggles, and keep one earbud in to hear the motors when flying, and I heard the motors almost stop, but not quite. The copter dropped fairly fast out of the sky, thankfully onto grass. Killed the rest of the fairly cheap landing gear that comes with the 3DR hexa's.. No excuse to not make the aluminum ones I have stock for in the basement :)
The logs show right at 13.0 that I climb, level off, then drop like a rock, also showing my throttle input jumps up once I notice something is wrong, but the throttle out isn't changing its' mind.
I was wondering if anyone could take a look at the log, and see if you can tell what actually went wrong (other than the obvious gravity reclaiming its' throne..)
I've also got onboard gopro video, and a MP screen cap video with goggle video/HUD overlay if it would be helpful..
Thanks!
Mike
Replies
I had a look at the logs. The alt hold gains weren't great and you basically caused the iterm that is tied to the "throttle cruise" value to oscillate. This make the whole system start to overshoot terribly leading to the crash.
See the overshoot from your Rate_P term? (It's The Red altitude line) That means you're using too much throttle to change altitude. When you change alt set points you go too fast and the lag in the baro sensor lets you sail right by your desired altitude. This starts building up error in the I term which eventually blows up.
Lower throttle_rate_P by 30% and see what happens. You can also lower your throttle_rate_I slightly as well, but one thing at a time.
This sort of thing wont show up until there is significant disturbance in the system such as a major alt change or crosswinds, fast descents, etc. But once it starts your fast gains won't dampen the effects, they amplify them.
Jason
On a side note, I (think) disarmed, armed, and got right back up and flew with alt_hold again, and no issue.. so very strange.. (And there was a 10-15mph straight line wind)
Here is the moment it happened in the log: Alt levels off when going back to dead band in alt_hold (in red) then autopilot throttling drops (blue) even through my input goes up (green)