Z-Axis Vibration Issues

Hi Nick @Nick_DeGroote,
If after all the suggestions you are still facing vibrations from an unknown source, you might want to try also dampening your motors by placing some silicon sheet (like those used in baking trays) under each motor, something like this.
HTH
Gal

@Nick_DeGroote have you tried hard mounting the cube to the top plate? By mounting the cube on moongel you may have inadvertently excited one (or more) of the natural frequencies of the cube’s vibration isolation.

-Brian

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@BRiskas I have not tried to hard mount the Cube by screwing it directly to the top plate. I realize that 2 of the 3 IMUs are internally damped and that adding another vibration damping device will affect the performance of the internal damping. I ran some tests with different vibration isolation methods with this type of flight controller in the past on a different vehicle (Coaxial Octocopter with 18 inch propellers) and had the best results with the Moongel when comparing it to some of the other commonly used vibration damping materials. So this was the reasoning for going with the Moongel off the bat for this vehicle.

I know I’ve read several posts debating whether or not the Pixhawk 2 should in fact use additional vibration damping. I don’t believe I’ve seen a consensus reached on this issue yet. I assume it will vary depending on the specific vehicle and the corresponding forcing frequency. It could be that one of these frequencies is excited on this vehicle. I’ll have to determine this conclusively with some more testing.

I ran an FFT for IMU0 on the next flight: https://drive.google.com/file/d/1f-hI2vqwX5wiO7y6tnl_QQ7NBGchSSdV/view?usp=sharing

The large spike at ~103 Hz is from the propellers (based on 2x our estimated RPM). There appears to be a harmonic above that in the low 200 Hz range. The other, smaller spike at 52 Hz concerns me a bit more though because I don’t know what’s causing that. I suppose it could be induced from the wires vibrating, or from another frequency being excited… (Correct me if I’m wrong) I’ve read the post here discussing the natural frequency of the Pixhawk 2 here: Dealing with Vibrations and harmonics... a competition

Thanks for the suggestion!

https://drive.google.com/file/d/1f-hI2vqwX5wiO7y6tnl_QQ7NBGchSSdV/view?usp=sharing

Hi @gnitzan, I’ll give this a try if all else fails. I’m curious if you have had success with this yourself and what type of vehicle were you flying? I’ll admit that it concerns me a bit that the rigidity of the platform may be decreased slightly by placing rubber or silicon under the motors. Thanks!

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I wouldn’t use Moongel in any case. On a hot day that stuff turns to goo. If you are going to use a gel pad use Zeal. I’m not suggesting Zeal here just as an alternative to Moongel in general. Leave that product for the drummers.

Have a read of this as it shows that the cube black doesn’t have great vibration damping characteristics below 120Hz, the range in while larger multis operate in. I’ve had similar issues to yours where no amount of mechanical tweaking could fix the issue, so I put on one of those dampeners which consists of 2 plates and 4 grommets sandwiched in between and that made a massive difference for me. Went from average vibrations of approx 30 down to 4.

Why is this? I don’t have any Cube’s but one would think the use case would determine whether isolation other than internal would be helpful. I understand how a too compliant mount is not good either as I have mounted FC’s on such a mount. But experienced builders can deal with that.

Your Google Drive links are access controlled.

Sorry. The links should now be accessible

Nick @Nick_DeGroote,
Actually I had great success with this method on two separate vehicles 680mm which is smaller than your vehicle but it is one more option in the builder toolset.

3m tape is all you need. the Cube has internal damping.

FCs like the Zero and 2.1.777 can be hard mounting with just screws or 3M tape. I guess the best way to look at a real off road shock if to much bounce and high rebound the shock will add more distortion.

I need to move the usb wire off the FC. but that because the wire need to be longer to fit the rear slot.

I’m definitely willing to try the hard mounting the flight controller, but I still want to test some others as well. Not to nitpick wording but a shock absorber is typically different than vibration isolation, as the shock absorber does not need to deal with a harmonic force. Also, this amplification will only occur if the source vibration is relatively close to the natural frequency. I could see some scenarios, albeit not all, where changing this natural frequency could be beneficial, especially if the Pixhawk 2 is less effective for frequencies below approximately 120 Hz.

@Yasiinm I’m pretty familiar with most types of flight controller vibration mounts, but I’m not quite sure what you were referring to in your post. Could you possibly provide an image of the mount or a link to a place where it is described further? I would like to check it out. Thanks.

Referring to a mount like this: https://www.dronetrest.com/t/how-to-build-the-pixhawk-apm-vibration-damping-mount/1016

Ok cool. I would have referred to that as a 3D Printed Anti-Vibration Platform. Thanks for the link.

It’s not 3d printed, it consists of 2 carbon fibre plates.

This is very interesting and actually makes a lot of sense! I see it’s also mentioned in the documentation:
https://ardupilot.org/copter/docs/common-vibration-damping.html#advice-for-reducing-vibrations

Looking at the photos, your props do pass pretty close to the arms, have you considered this as a possible culprit?

The only thing is Philip Rowse does not like the idea. However it’s a case by case issue. I get rock solid video with the Tarot. but my frames have low flex and only a few bolts. I chimed in as my z was a little higher than normal on my Zero 10 prop build I made a few days ago. top image is the 16" prop and bottom is the 10" the high z was with a 10" frame that could be the wires or battery chatter.

I had not really considered this before posting here. I’m sure it’s worth investigating though. I suppose the easiest way to get some more clearance between the props and the arms would be to add standoffs underneath the motors. On the other hand, I don’t feel like the props are really any closer to the arms than some other vehicles, even in this size range. At this point I’m definitely not ruling anything out though.