Big Quad becomes unstable at high speeds

Hi,
we are currently testing a quad with 20" Propellers and roughly 10kg TOW
We tuned the copter and it flies quite well but when reaching around 20 - 25m/s vertical velocity it becomes unstable and starts to overract
We had this issue mutiple times and so far were not able to narrow it down - it happens both when flying waypoints, or when flying manually in Loiter Mode

It doesnt seem to be a performance issue, there is still more then enough headroom for the ESCs

Does it also happen in Alt Hold? It looks like Loiter/Auto mode produces PIOs.

It seems like the copter is also oscillating vertically, I wonder if PSC_ACCZ_I and P are set correctly?

Safety:

BATT_FS_CRT_ACT,1
BATT_FS_LOW_ACT,2
FENCE_ENABLE,1   <- highly recommended for big copters

You have high vibrations with clipping in IMU2 - this could easily be affecting altitude control.
Once you fix the vibrations you can set ATC_THR_MIX_MAN,0.5

The yaw tuning is wrong, the pitch and roll P and I values look way too low too.
Motor thrust expo is wrong.

Use these:

INS_GYRO_FILTER,23
MOT_THST_EXPO,0.71
PSC_ACCZ_I,0.25
PSC_ACCZ_P,0.125

What ESCs do you have?

Harmonic notch filter is not working because of this:
INS_HNTCH_REF,0 ← set to 1
and INS_HNTCH_BW probably needs to be about 20 at most, not 30.

Set these again to confirm the filter is actually working as planned
INS_LOG_BAT_MASK,1
INS_LOG_BAT_OPT,4

It is probably worth going back to Initial Parameters in MissionPlanner and start tuning over again, or better → use the Methodic Configurator, and redo all the tuning.

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There is a 9 Kg vehicle template in ArduPilot methodic configurator. Use that as staring point and do the changes that @xfacta said when the respective parameters appear on the sequence.

Thanks for the Feedback,
we spent quite some time on the tuning of the system and in manul flight it seems very stable, but obviously we missed something.

Ill double check the values - the Motor thrust expo was determined on Thrust - Teststand so this value should be correct - we currently use Hobbywing DroneCan ESCs

The notch filter is a good input, thanks totaly missed that

something to note is that the system is quite powerful (Hover at ~0.03 Ctun.Tho) , which is why i assume the standard parameters we started with didnt work quite that well

I just double checked in the newest Version i only see one 7kg Template ?

From my limited experience, if copter behaves well in manual (alt hold) mode, but less stable with Loiter, that was due to too low PI, because Loiter reacts much faster than human and tries to overcorrect. If look at the graph you have several instances where the desired pitch oscillates.

I meant to type a 7Kg template.

1 Like

Thanks for the feedback -

I just checked why we ended up with such low values - it just comes from the recomneded Setting in the tuning manual:

Our CTUN_THO is at around 0.035, which is why the learning of MOT_THST_HOVER doesnt work since its lower then the allowed range

Well defenetly try the higher values and see what happens

Your hover throttle at a roughly constant altitude is about 0.08 or 0.07
Set these:

MOT_HOVER_LEARN,0
MOT_THST_HOVER,0.07

This will start at about the correct level, then check and adjust the hover throttle value over a few flights.

You should probably add a minimum payload to bring the hover throttle up a bit, with the output PWM around 1500, which may give improved control and then Autotune would have a better chance of working properly - after all the other fixes (vibrations for example)