New Robots

leo-rcc · Post by **leo-rcc** » Mon Mar 08, 2010 10:22 am

Cavecrusher wrote:
i say it won't, you remember the last incarnation of MBY? Just because the point of contact was below the center of rotating mass MBY would flip over when hit, very annoying.

One other problem is that a bar spinner doesn't have the nice gyro effect of a big disk, so not so stable.

The selfrighting was beautiful to see though.

peterwaller · Post by **peterwaller** » Mon Mar 08, 2010 7:16 pm

I have now built up the complete chassis and fitted the electronics and new spinner arm.
http://i109.photobucket.com/albums/n66/ ... MG0965.jpg
http://i109.photobucket.com/albums/n66/ ... MG0964.jpg
It seemed to run well and spin up OK so I went for a couple of 13gms steel blocks on the end of the arms to form the weapon heads.
http://i109.photobucket.com/albums/n66/ ... MG0967.jpg
This turned out to be completly useless as when the spinner gets to about 25% speed there is a suddend thump and the robot jumps into the air and lands on its back.
I can see no evidence of the spinner catching the ground or any part of the robot but it has happend all three times I have tried it.
This seems to be the same as happens on my previous FBS but on that as the body was held on by the motor magnetic field only the body and motor case flew off.
Looks like I shall have to settle for a much lighter cutting blade.

Cavecrusher · Post by **Cavecrusher** » Tue Mar 09, 2010 9:20 am

peterwaller wrote:This turned out to be completly useless as when the spinner gets to about 25% speed there is a suddend thump and the robot jumps into the air and lands on its back.

told you so

peterwaller · Post by **peterwaller** » Wed Mar 10, 2010 11:26 am

It looks as though the problem is the large mass I was trying to spin and the balance. I have now replaced the steel blocks with a couple of Titanium blades and it is much better. Start to get a bit unstable at about 75% speed but it hit a sheet of 5mm polycarb about 10 times and only once flipped.
http://i109.photobucket.com/albums/n66/ ... MG0969.jpg
http://i109.photobucket.com/albums/n66/ ... MG0972.jpg
Still got to shorten the screws and try and improve the balance but it is starting to look like it might work. It is now 15 gms under weight so I might have a go at improving the stiffness of the arm as long as it doesn't ruin the stability again.

thebeardedone · Post by **thebeardedone** » Wed Mar 10, 2010 2:00 pm

Peter, you are just an excellent robot builder. Do you mind me asking what you use to machine your parts? The quality is excellent.

peterwaller · Post by **peterwaller** » Wed Mar 10, 2010 3:02 pm

I have a small lathe and manual mini mill and a second mini mill converted to CNC.
http://i109.photobucket.com/albums/n66/ ... MG0505.jpg
It can just about machine aluminium although using the small 2mm end mills it takes hours but it is really superb on Polycarb, Delrin, fibreglass and carbon fibre. I also have a couple of bandsaws a pillar drill a small bench shear and a couple of benders.

mark_m · Post by **mark_m** » Wed Mar 10, 2010 10:44 pm

Warning [MATHS ALERT]

I think I just figured out why some bar spinners go mental when they hit stuff, particularly those with curved bars.

I think it's all to do with moments of inertia. not that they have too much, but they have too little in some directions. The math's is complicated but if you spin anything about its second principal axis, it will be an unstable motion.
EXAMPLE: take a book/mobile phone/TV remote/VHS tape/pack of cards/any thing rectangular in shape. throw it upwards with a spin around its biggest, flat face. it's spin should be stable. Now spin it about its smallest face. again its spin should be stable. but if you spin it around its middle face, it should tumble.
I think curved bar spinners are trying to rotate their weapon around the second axis, so they are unstable and will try to spin around one of their other axis, flipping the robot.
[MATHS OFF]
The fix for this instability is to add weight in the other direction like a flybar is added to a helicopter rotor. In theory. Can you try it Peter?

peterwaller · Post by **peterwaller** » Thu Mar 11, 2010 8:40 am

First Mark welcome to the forum.
I am not sure I followed all your argument but as part of my attempts to reduce the flexing in the arm I have designed but not yet made a new one which is basically a 96mm disc with two small projections where the brackets and cutters go.
http://i109.photobucket.com/albums/n66/ ... Newarm.jpg
I did this to try and increase the gyro effect which should fight against any flipping forces but it should also help if your theory is correct as I understand it.

bitternboy · Post by **bitternboy** » Thu Mar 11, 2010 8:53 am

@Mark: I think you've firmly introduced yourself to the forum as a person who knows their maths'n'physics.

Hi by the way.

Team Terror · Post by **Team Terror** » Thu Mar 11, 2010 3:52 pm

Hi Mark, welcome to the forum