Custom frame vibrations at high SoC.

ArduCopter
1

Hello ArduPilot community!

I am fairly new to ArduPilot and I am struggling with what looks to me like FCU tunning.

However, since this is a custom frame let me describe it to you so that you know what you are looking at.

For my application I need to fit a fairly large sensor on my quad I have created a wide custom frame. The main plate is large at 140x210mm it is manufactured from 3mm carbon fiber, the arms are 6mm thick and are sandwiched between a small bottom plate and the main plate on which he FCU is mounted, above the FCU there is a top plate which is connected to the main plate with 40mm brass standoffs. At the front of the drone there is a high 130mm electronic housing for now made out of PETG, which is mounted to the main plate and to plate with M3 bolts and a central M6 bolt running tough the whole height of the drone and clamping the radar box from top.

Now, despite the box being quite tall the CoG of the quas sits more or less in the middle of the propeller plane, the front/rear imbalance is equalized by the battery.

From what I feel the frame is stiff and I feel no play, nether the less I know that the screw holes had a bit of play, which after I have clamped them down is not present at all. Ofc the drone could be made mechanically even stiffer, however this is costly and time consuming. So that’s why I want to first consult experts to try to address my issues in software.

The problem is that my drones vibrates or rather rocks back and forth - take a look at the screenshot - it does so most of the time and especially when I try to hover it. However - though I am unsure but it seems that - the vibrations are especially prevalent when my battery is above 50% SOC. When at low SoC it flies fine it handles quite well.

Vibrations
Vibrations1829×1413 430 KB

To make sure that the vibrations are not caused by resonance of the electronics box I have also flown the frame without the box, but this did not resolve the issue. Moreover with smaller props on the drone vibrates more not less so it seems like higher motor RPM only makes it worse.

I tried to adjust tune some parameters, but this had no visible effect.

Changed parameters:
MOT_BAT_VOLT_MAX: 0 → 25.2
MOT_BAT_VOLT_MIN: 0 → 18.0

ATC_RAT_RLL_P: 0.135 → 0.110
ATC_RAT_RLL_I: 0.135 → 0.110
ATC_RAT_RLL_D: 0.0036 → 0.0025

INS_GYRO_FILTER: 20 → 15

I also attach a video of the drones vibration and my log files, hopefully you can help!

Best regards
Mark

1 Like
2

You will need to adjust a lot more than just those parameters.

What method are you using to do the changes in the parameters?
How are you making sure that you change the parameters in the correct order?

Trial-and-error is not your best friend when it comes to expensive, fragile flying vehicles.

3

Hello Lucas,

I am not sure what do you mean by what methods? If you mean how I do that, then I use MissionPlanner. If you mean why those? Then I must say that I though that since my frame is quite wide for a quad then relaxing it around roll might help, but more than that I dont know.

What parameters would you adjust and how would you do that if not trial and error? Do you mean something like notch filtering based on the IMUs logges frequencies?

Also do you think that this wobble can be fine tuned by software or is it more likely a frame rigidity issue?

Thanks for help,

Mark

4

By methods, I mean, do you have a checklist? Do you have a dependency list so that you can see which parameters depend on which other parameters? How do you know what parameters to change first? And yes hardware problems should be solved first before trying to use software solutions. Have you read the entire ArduPilot documentation? Are you sure you know what you are doing?

You could use AMC to avoid trial-and-error and use a known, tested, safe configuration sequence.

Make the frame as rigid as possible. Follow these rules

5

That looks like an oscilation, you don’t need to enable everything in LOG_BITMASK, keep it default or 180221 because current IMU log is unusable. You also need to lower your PIDs further down - twice from default or even more. Starting fresh with AMC is also good option.

6

Well before you can configure a notch filter you would be looking at whether you have output oscillation. And because you are on default PID’s you have plenty:

RCout
RCout3400×1062 375 KB

As suggested the Rate PID’s must be reduced as well as the Vertical Acceleration Controller gains. All in the Wiki and of course the AMC. In fact in the AMC docs you will see an example of output oscillation that looks just like yours…

1 Like
7

Thanks for your help!

Halving the PID values and Vertical Acceleration Controller parameters has resolved this issue completely. Now the flight is smooth.

Once again thanks to you all!