In alt hold mode octo became very unstable

ArduCopter Copter 4.0
1

Hello,

In alt hold mode my X8 (4x2) became very unstable and crash was avoided changing back to STAB mode.

For altitude measurement a Garmin Lidar was installed.

Flight log are available here:

https://drive.google.com/file/d/1W4qn_orrSoHRISP7DH2a6vZTEHkkir8E/view?usp=sharing

I want to mention that a static notch filter was configured for frequency of 84Hz and BW of 30Hz (these determinations was made using FFT analysis).

Please give me an advice.

Thanks a lot,
Joan

2

I forgot to mention - is about a heavy drone (35kg take off weight) with 31inch propellers

3

With such a big heavy drone it is crucial that you follow the tuning instructions on the wiki without skipping any step. Have you done that?

4

Hi Amlilcarlucas,

Yes. Step by step

5

From the logs, I can say that it’s badly tuned.

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6

If I increase PIDs variables it oscillate hard. At actual wight drone is overpowered

7

Because it not tuned right yet, and it seems underpowered, once pitch and roll deviated too far the motors werent able to produce the required thrust at the right times.
Motor 1 and 7 seemed the worst affected, and motor 8 going to minimum to try and compensate.

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You might be able to tune it better, but this could also be a problem with the ESC/Motor combination. Or if the ESCs are right out on the arms near the motors you could need more capacitors on the power wires right at the ESCs.

Voltage drops accordingly. Current looks like it is reaching a peak/limit at other times during flight too so that might become a problem over time.

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There’s no static or harmonic notch set up in that log. Set this:
INS_LOG_BAT_MASK,1

Also something very important, connect to MissionPlanner, press Alt A and set your correct battery failsafe levels via that dialog, and DEFINITELY set these:
BATT_FS_CRT_ACT,1
BATT_FS_LOW_ACT,2

2 Likes
8

You missed the 1st parameters (step) mentioned in the Initial Tuning guide:
MOT_BAT_VOLT_MAX,0
MOT_BAT_VOLT_MIN,0

1 Like
9

Thanks a lot for your analysis.

A make today another flight in stab mode only and removing PX4 dampers for vibration analysis

this is the log: https://drive.google.com/file/d/1T4wCLV5wAOXbn7qwyhiZOhvTdspNuX0X/view?usp=sharing

10

Like I said , people often do not follow it step by step and wonder why it does not have the top performance that is expected from ArduCopter.

Please read the tuning instructions page again and do not skip any step