Hi there,
I'm writing to report a very likely bug affecting at least firmwares 3.1 rc8 to 3.1.2 (), at least for hex X frames. The bug manifests when RTL is the last step of an AUTO mission and RTL return altitude is different from the current altitude. The drone crashes consistently in these conditions. Additionally, the drone crashes from LOITER after a non-deterministic time - sometimes is quick (tens of seconds), sometimes it can loiter for minutes. In both cases the symptoms are the same: the drone goes from hover holding output (motors ~ 1600) to shutting down 1-2 motors in 100-200ms, and then shutting down the others in another 100-200ms. The way it shots down the motors is very specific: it reduces throttle to 12xy where xy is the same for all motors. In the attached logs for example, after LOITER the motor commands are as follows:
RCOU, 487396, 1695, 1645, 1672, 1667, 1443, 1874, 32767, 32767
RCOU, 487495, 1523, 1597, 1517, 1602, 1231, 1824, 32767, 32767
RCOU, 487595, 1398, 1421, 1368, 1451, 1231, 1581, 32767, 32767
RCOU, 487696, 1231, 1231, 1231, 1231, 1231, 1231, 32767, 32767
RCOU, 487796, 1231, 1231, 1231, 1231, 1231, 1231, 32767, 32767
RCOU, 487897, 1231, 1231, 1231, 1231, 1231, 1231, 32767, 32767
RCOU, 487996, 1231, 1231, 1231, 1231, 1231, 1231, 32767, 32767
RCOU, 488096, 1329, 1444, 1401, 1372, 1231, 1545, 32767, 32767
RCOU, 488195, 1392, 1380, 1545, 1231, 1299, 1474, 32767, 32767
RCOU, 488297, 1404, 1369, 1501, 1271, 1231, 1545, 32767, 32767
Obviously 1231 is not sufficient to sustain flight - the fact that all motors have the exact same value (1231) reinforces the likelihood of this being the output of a bug.
The RTL example is similar - the RTL command is executed right after the first RCOU:
RCOU, 742799, 1697, 1603, 1542, 1756, 1571, 1728, 32767, 32767
RCOU, 742898, 1241, 1241, 1241, 1241, 1241, 1241, 32767, 32767
RCOU, 742799, 1697, 1603, 1542, 1756, 1571, 1728, 32767, 32767
RCOU, 742898, 1241, 1241, 1241, 1241, 1241, 1241, 32767, 32767
This time the motors are all 1241 - interestingly different from 1231, but still very close, not nearly enough for flight and all equal in all motors.
In case you wonder if there is something with the drone, both bugs have been reproduced in two different drones several times: the loiter crash three times and the RTL crash twice.
In case it helps, in addition to the dataflash logs I have telemetry logs from mission planner as well as videos, but I assume that the dataflash logs are sufficient, as they had enabled practically everything relevant (I think) - certainly they had ATT, CTUN, NTUN, IMU, RCOU, GPS, and RCIN.
If this is a known bug, please drop a line here, if I can help in tracking it down, let me know.
Best,
Mihai
Replies
I think this just might explain one of the crashes I had with my X8.
Mihai, can you share some details of your multirotor? It sounds like you're still having some instability problems, and I wonder if there's something wrong with the setup. Is this thing really big?
Sure,
it's a bit of a pig - AWU is 2kg (4.4lb). It's a hex X frame, using Turnigy Talon 2.0 carbon fiber frame (super-sturdy BTW - heavy but very strong), with a 3S 5000mAh nanotechnology battery, 800kV motors, 20A Turnigy Plush ESC.
The motors:
http://www.hobbyking.com/hobbyking/store/__43369__NTM_Prop_Drive_28...
It swings 12"x4.5" propellers.
Tuning was a bear, but once it was done I got higher P (0.20 - 0.22 vs the default 0.15) and much higher D (0.014 vs 0.004 if I recall exactly). With this parameters it is ROCK STABLE! When it has good satellite reception, it stays put even in 30mph gusts!
Oh, and I don't have 1 of them, I have 6-7 (eventually will get to 15 - still building them).
Let me know if you have any questions or concerns.
Thanks,
Mihai
hex.jpg
Thought I responded to this yesterday?
The APM displacement from CG, I'm not sure that's a problem. I have the same situation on most of my helicopters. In the past I have had some issues where I wondered if this was the cause, but I'm not ready to blame it on that yet.
Thank you Rainer. As far as I can tell the hardware is working fine - my problems are related to giving a command. Is having the APM centred important (and if yes, why)?
Thanks,
Mihai
I'm not sure how close to center of gravity the APM should be but lets say it is mounted rearward of the the CG. When the copter pitches forward the accelerometers will sense an upwards and forward movement in addition to the rotation. When the APM is mounted on the CG only the rotation will be sensed and not the false indication of vertical and forward movement. It is possible to mount autopilots away from the CG by telling it where it sits in relation to the CG. It can use that information to calculate the actual movement of the airframe. I dont think APM supports that. I would imagine it would hog CPU power to make those calculations. Pixhawk might have the power to handle it.
Makes sense. I'll move them.
Thanks,
M.
Is having the APM centred important? I thought a bit about it and I thought that at least the accelerometers ought to show about the same values, and if not calibration would take care of it. I have to say that stability is usually just fine. The hover is usually around 50%, so there ought to be enough thrust for stability. Flying it in 30mph gusty wind also convinced me that stability is great. I was honestly impressed by it.
M.
Ah, thanks - I'm not that far from CG - probably 10-15cm. And the GPS is exactly on top of the APM (as far up as the cable allows it).
Thanks,
Mihai
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