Rotor ground effect oscillations and control

[PaulPounds]

Greetings all!

It's been a long time since I've had a chance to check out the forum, so I need to catch up. While I'm going through the archives, here's my question du jour:

I have a rotor that is a drive unit for this four-rotor robot I'm constructing. To design the attitude controller, I have a single motor mounted vertically on a lever arm with a PID controller and an IMU. I've tuned the controller so that it should be stable and stationary, but I keep getting oscillations.

The rotor will occasionally dip up to 5 degrees in a way that doesn't seem related to the control. My current theory is that it might be a result of the rotor being in ground effect. I've seen a few things about rotors losing power in certain ground effect conditions - does this sound like a likely culprit?

Thoughts?

take care,

-Paul

Un

 

[GraviMan]

"I've seen a few things about rotors losing power in certain ground effect conditions"

I gather this is normally a vertical surface compressing the vortex against the tip. The result is that the downwash circulates up to form a partial ring state (ie loss of lift). Scary stuff for rescue pilots!

Assuming that you are confident in your control system (gain and phase margins etc), there is another possible cause. Even "still" air has a suprising amount of currents in it. Any heat source will cause a convection current, this is what causes cumulus clouds. I presume you havn't got a teetering, or gust/velocity correction device.

Are you operating indoors or outdoors? If indoors, you would be suprised how small draughts can be channeled. Rememer Kelvins theorem - once vorticity exists, it stays there. Viscosity will eventually remove it, but your machine could be causing it's own draughts!

Mart