Peter Wallers Robot Ramblings

[engr]

From the video it looks like the blades are flexing outwards due to the top being to flexible?

[peterwaller]

Made a little progress built new stronger arm and rebuilt robot.
Robot was OK until it went over a hole in the arena then proceeded to self distruct again.
This time the arm remained complete and I actually got it on video.
http://youtu.be/Pj_NleCYeq8

[haz]

unlucky Peter, thats a real shame to have that happen to it after all the effort thats gone into it.

i think you should call it lemming as it keeps killing itself

[peterwaller]

Just been looking at the video in my not too good video editor and have had an attempt to isolate the frames around the self destruct.

Frame 1 it is Ok.
Frame 2 start to lift as suction fails.
Frame 3 lifts further and chassis starts to turn.
Frame 4 still rising and chassis spinning.
Frame 5 disappering.
Basically it seems to lift into the air where the chassis starts to spin in the opposite direction to the blade and because it isn't balanced spin eratically.
It looks like it is the centrifugal forces that throw out the components not the blade hitting them although that does occure as they pass through the arc of the blade.
This is probably what was happening on my previous non suction attempt at a full body spinner which would leap into the air if I got near about half throttle.
Looks like this may be a problem that I can't overcome as the arena is always going to suffer damage at some stage in the event so loss of suction is always going to be a problem.

[peterwaller]

As a complete change just hooked up two of the tiny motorgearboxes with the receiver, battery and switch out of also run and they weigh in at 6.8gms

[mark_m]

Great robot! I think I've mentioned this before in relation to your previous full body spinner, but I think this robot is even more sensitive to the Dzhanibekov effect.
Basically, bodies spun around some axes are unstable.
video: http://www.youtube.com/watch?v=L2o9eBl_Gzw
mathematical explanation: http://en.wikipedia.org/wiki/Tennis_racket_theorem / http://mathoverflow.net/questions/81960 ... -mathemati

Your spinning arms are being rotated about an axis that is unstable. One solution is to add 'fly-bar' like features (http://en.wikipedia.org/wiki/Helicopter ... zer_bar.29, though note they would need to be bigger and heavier, unless your arm can be flexible like a helicopters blade) Alternatively the bar needs to be made into a disc, or kept as a straight bar, with no downward offset blades.

[peterwaller]

That may well be the problem Mark but those equations make my brain hurt.
In the mean time while I am deciding where to go from here I have built a new Alsorun.
Here are the insides using DT receiver with built in H bridge, tiny gearmotors and a single cell lipo.

Had to fit thicker tyres to improve ground clearence which brought the all up weight to 14gms

They also improved the speed
http://youtu.be/trClZVlkI8M
Yes Dave it is held together with selotape.

[mark_m]

Will you be at Brighton Maker Faire? I'd like to talk to you about your robots in general, and we could chat about your spinner instability then if you're interested.

[peterwaller]

Yes I shall be there not officially on the stand but helping out and bringing my robots.
Always up for a chat about robots look forward to seeing you there.

[Remote-Controlled Dave]

Woo! Cellotape!