3 Axis Gimbal w/Radians Extreame vibration!!! ????

Hi Kyle:
There are several factors that might be causing this so we'll need to eliminate them.

Do you see the vibration when the C8 is airborne and the Radians are off or in Stabilized mode (with no camera person slew)?

Could you post a short clip of the video showing the vibrations?
What I'm looking for is whether the vibrations are continuous or episodic?

Could also confirm that, at the midpoint between the pan servo pulley and the large pulley how much deflection there is when you push laterally with your fingertip with a force of a pound or so? It should be about 2mm if the belt is tight.

People have been experimenting with two things:

1. Tying a cable tie in a horizontal loop around the O-rings to reduce the amount of lateral movement (and I'm presuming the vibrations you're seeing are indeed lateral).

2. Putting grommets (or shock absorbing foam) around the vibration isolators hex standoffs -- again to reduce lateral movement.

Hope this helps.
Andy.

 

You are not alone. The problem comes because of that challenge of (a) isolating the gimbal from the vibrations of the frameset (booms/motors/hub) and (b) in the process, turning the gimbal into, in effect, a pendulum with a definite natural frequency at which it wants to oscillate.

Add to that you've got four sets of counter-rotating motors and props which continually change speed, and the Radian sensors which are trying to remove motion by counteracting it and you can see that it has to be a very finely balanced system.

I've got one C8 frameset that likes to shimmy every few seconds and another that's completely smooth -- and that's when I move the same gimbal on to each of them. I suspect the factors at work include:

1. Motor bearings, motor balance, prop balance.
2. Beat frequencies caused by differences in motor speeds.
3. Possible vibration "leaking" into the flight control board via motor cables -- I really need to reposition the motor wires so nothing touches the FC board.

Add to that the fact that the vibration isolators cannot be perfect -- they work well in the vertical axis, but evidence that others have gathered suggests you need to make them more resilient in the horizontal axis with such techniques as tying cable ties around the O-rings in a horizontal loop, or putting grommets around the aluminum standoffs in the isolators so they cannot display laterally as easily.

There is no "silver bullet," I'm afraid. It's a case of tightening and finely tuning things methodically.

Andy.

 

Hi Kyle:
I see what you mean. I presume the Radians were in Stabilized Slew for this?
Did you get a chance to see if the vibration changes when you turn the Radians off?

Is your sense that the frameset (motors/booms/hub) is vibrating, or is it the gimbal?
We'll probably have to work through a diagnostic "tree" to try and figure out what is/are the cause(s).
Andy.

 

Kyle:
Let me know the results of your Radian's-off testing. And when you get decent weather, please send it over to Portland, Oregon.

In my experience, the props and motors can create vibrations that can be transmitted through to the gimbal. If you think of it this way, the props and motors typically generate relatively high frequency vibrations, but the FC board is tweaking the motor speeds constantly, and I suspect (but don't yet know for sure) that the mechanical "system" above the vibration isolators is capable of generating lower frequency vibration because of beat frequencies between the motors/props.

The vibration isolators are supposed to act as a very low frequency band pass filter -- blocking the high frequencies, but (obviously) communicating the very low frequencies of movement of the frameset (motors/booms/hub) so that the gimbal stays with the aircraft! The characteristics of the vibration isolators change with load and temperature -- which is why the stiffness of the O-rings and the number of O-rings that you use in the vibration isolators changes with temperature.

You're evidently flying in cold temperatures, so the O-rings will be more rigid than a warm summer's day, so more frameset vibrations will be communicated.

The key question is therefore: is what we're seeing caused by frameset vibrations being reduced to low frequencies and transmitted to the gimbal by the vibration isolators, or is it the Radian's themselves that are in incipient oscillation that are causing the oscillation and it's nothing to do with the frameset -- or is it a combination of both the frameset and the gimbals?

Once we know the answer to your Radian's-off test, we can move down the correct branch of the diagnostic tree of decisions....at least I hope we can. These compound mechanical systems are non-trivial when it comes to analysis....and, as Tabb puts it: if it flies, it vibrates.

Thiago is on the forum, so you can start a conversation with him by finding a posting and clicking on the name above the icon on the left.

Hope this helps.
Andy.

 

Hi Kyle:
You're right -- if it got worse with the Radians off, then that tends to suggest that the Radians were doing their best to correct the situation, but not being able to.

I think you're diagnosis is likely to be correct too -- you should see the motor mount screws spinning in place. Let us know what you find after you replace the props and re-align the motors.

Andy.

 

Hi Kyle:
Firstly, staying at a Holiday Inn Express should have worked. Please return to your room at once.

That's an interesting theory. Is there a chance you could post a video on Vimeo so we can see what's going on. Your comment about the picture becoming more focussed does indeed imply high frequency oscillation. If your theory is correct, then if you can isolate the axis that is oscillating and just experimentally back the gain down, that should at least identify the cause.

Let me know what you find, please.
Andy.

 

Same for me. Video is private.

Andy.