Pixhawk 2.4.8 - unstable hexacopter

Hardware Pixhawk family
1

So, I am a newbie and I just built a hexa with 6s and 360 kv motors.
While it flies well, in stable mode I constantly have to give inputs for it to not crash and to hover at a place, it feels like I am flying a heli with a gyro instead of a stable hexacopter.

I am attaching the log BIN here: Filebin | nmm963c3htg0nza9

if any of you can help me understand what is going on or if you can tell me what I should change to make it super stable, it would be very helpful!
Also, I would like to make my copter as stable as a DJI, is this possible at all?

Thanks!

2

That’s what Stabilize mode does. Stabilize Mode If you want to hold position and altitude use Loiter Mode. Read the Wiki for that also as there are parameters you can set.
Tuning is poor, the next step is to configure the Dynamic Notch filter then run Auto Tune. There are comprehensive Wiki’s for both. With that Flight Controller and no ESC telemetry use the Throttle based Notch filter.

Just work thru the the tuning process.

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3

Stabilise is definitely a handful on some multirotors, but it is the go-to mode if something goes wrong - you have control.
AltHold plays much nicer with the throttle being a “fly higher / fly lower” action rather than bare-bones throttle.
Loiter is the sanitised GPS assisted mode that you would normally spend most of you time with. You can let go of the sticks and hold position against wind, or move precisely.
On your transmitter set a 3 position switch for Stabalise, AltHold, Loiter

I find it handy to set a channel and switch for killing the motors → RC13_OPTION,31 (channel 13 in my case)

For the next flights use AltHold
Try to have the hex hover for a while and just do some gentle pitch/roll/yaw movements.

Set these mostly to gather data, but it should help to smooth things out a bit too
INS_ACCEL_FILTER,10 (this should be the default value here, it will help)
INS_LOG_BAT_MASK,1 (gather data for the Harmonic Notch Filter)
INS_HNTCH_ENABLE,1 ← set this then refresh params to see the rest
INS_HNTCH_MODE,1
INS_HNTCH_REF,0.15 (a guess)
INS_HNTCH_FREQ,60 (a guess)
INS_HNTCH_BW,30
INS_HNTCH_ATT,40
INS_HNTCH_FM_RAT,0.7

There’s some vibrations which will affect flight and tuning. It’s not to critical levels, but not the best either.
First check nothing is touching the flight controller or rubbing against it, wires are not too loose or pulled too tight. Secure everything to the frame.
I guess you have the Pixhawk mounted on the 4 foam blocks it came with? Maybe try to add some weight to it, I stick a flattened piece of lead to the underside. Otherwise you have to find softer mounting foam (or gel pads) or change the mount entirely to something like this:
Vibration Damping — Copter documentation

You can see in the graph that vibrations go up quite a bit at some stages. High vibrations, particularly Z axis, will cause control problems and potentially a fly-away.

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If you can tame the vibrations a bit and put in those other settings, let’s see the new log file and you can progress hopefully to Autotune.

EDIT:
During those test flights, are you meaning to operate a channel for a gripper? That’s OK if it’s intended, just curious

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4

@dkemxr @xfacta Thank you for your inputs!

Okay, So I did the changes and learned about auto-tuning, was getting ready to do some yaw flights, but after driving 20 miles out of the city I realized that I had connected the motors in the wrong direction ( I was repositioning my ESCs to make the copter look good :person_facepalming: ) and I did not have any tools with me to change them!
Will test fly tomorrow and update!

@xfacta expecting a crash the last time, I had removed the foam blocks and just used a double-sided tape and forgot about it at all!
I just put the sponge back and hopefully, my vibrations will be lower on the next flight!
About the Gripper - I have powered the servo rails with 5v and attached two servos without any device, hopefully, will be able to put them to use soon… during the flights, uhmm,.I must have flipped them once or twice lol.

I did notice that two servos don’t work well at the same time, the second servo always seems to lag (quite a bit), would you suggest that I keep only the signal wire in the FC and power the servos separately using a battery instead?

5

I think it is better to separate it, even if it is not related to the problem. Avoid passing high currents thru the servo rail.

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So, I did a very peaceful 22mins flight in 8 to 10 miles wind!
the copter was mucchhh more stable than the other day and needed fewer inputs even in slight winds… I could not get a GPS lock and since the light was failing and was about to rain I could not wait anymore and flew without GPS! (But also I could not try out other modes)

Anyways, with the limited knowledge that I have gained, I think there are much fewer vibrations now! The next step would be to autotune the copter and play with different modes…

The LOG: Filebin | y1c0xq5k2r7k0x7r

1 Like