mecanum wheels on frame

[7-11number1]

This is team 1711 and our robot is having "minor" technical difficulties. Were using 8'' diameter mecanum wheels and it has issues when it comes to driving around. The robot handles forward, backwards, and strafing side to side perfectly well. But the wheels lock up when attempting a turn or circling around on its center of axis.
The wheels are position with the left and right wheels 20'' apart and the front and back being 19''. I know a perfect square is recommended for mecanums but would a small difference like an inch cause the wheels to lock up when attempting turns? We need an experts advice or some links that provide good information on how to properly mount and place mecanum wheels.

http://www.flickr.com/photos/23220170@N07/
(ignore pic with tread and mecanum wheels)

[Vikesrock]

I've never used mecanums and I'm far from an expert, but I believe I remember reading this results from configuring the wheels incorrectly. I can't see what ways the rollers point in the pictures, but I believe the desired configuration is an X and a configuration in the shape of an O causes the behavior you are describing.

I'm sure someone that has used mecanums will be around to either confirm or deny this shortly.

[CCCP]

cool robot
hope you do well at competition
you should post more pictures

[Steve_Alaniz]

Originally Posted by 7-11number1

This is team 1711 and our robot is having "minor" technical difficulties. Were using 8'' diameter mecanum wheels and it has issues when it comes to driving around. The robot handles forward, backwards, and strafing side to side perfectly well. But the wheels lock up when attempting a turn or circling around on its center of axis.
The wheels are position with the left and right wheels 20'' apart and the front and back being 19''. I know a perfect square is recommended for mecanums but would a small difference like an inch cause the wheels to lock up when attempting turns? We need an experts advice or some links that provide good information on how to properly mount and place mecanum wheels.

http://www.flickr.com/photos/23220170@N07/
(ignore pic with tread and mecanum wheels)

Vikesrock is correct. I looked at the picture of your bot and the wheels are the problem. you need to swap them.
Why... ok if you think about it, the bottoms are opposite the top. This will help picture what's happening.
When you rotate, you are gripping into the center and rotating out away, this doesn't do you any good. You want the gripping to go outward and any rotation to cancel anything going into the center.
I know this because last year we used mechanums and had them backwards... and guess what? It would go forwards, backwards and strafe but wouldn't rotate without jumping, vibrating... what ever you want to call it.

Steve

[Alan Anderson]

Having the wheels swapped like that will keep the robot from being able to turn itself, but it makes it amusingly easy to turn under outside influence. I assume that's exactly the opposite of what you want.

[Steve_Alaniz]

Originally Posted by Alan Anderson

Having the wheels swapped like that will keep the robot from being able to turn itself, but it makes it amusingly easy to turn under outside influence. I assume that's exactly the opposite of what you want.

No... I don't think so Alan...I mean all the posts I've read that you've put up are spot on and I'll be happy to admit I'm wrong if you've actually tried it. I have a bot in my garage and I'm pretty sure the mechanums do turn that way. It's actually a 4 wheeld mechanum so I can't address the two traction wheeled in the back but the front are the wheels in question.
Perhaps you mean the loss of torque from the configuration itself, and that's absolutely right, but that's the nature of mechanums.
I still think swapping them will solve the rotational problem and still allow forward, backwards and strafe.

Steve

[EricH]

Originally Posted by Steve_Alaniz

It's actually a 4 wheeld mechanum so I can't address the two traction wheeled in the back but the front are the wheels in question.

They did say to ignore that picture. And it should work (turning) even with traction wheels in the back.

[Vikesrock]

Originally Posted by Steve_Alaniz

No... I don't think so Alan...I mean all the posts I've read that you've put up are spot on and I'll be happy to admit I'm wrong if you've actually tried it. I have a bot in my garage and I'm pretty sure the mechanums do turn that way. It's actually a 4 wheeld mechanum so I can't address the two traction wheeled in the back but the front are the wheels in question.
Perhaps you mean the loss of torque from the configuration itself, and that's absolutely right, but that's the nature of mechanums.
I still think swapping them will solve the rotational problem and still allow forward, backwards and strafe.

Steve

Again, my mecanum knowledge is limited. I believe Alan is referring to the robot's difficulty in rotating itself with the O configuration due to the way the vector math works. However, you can likely see just by looking at the wheels in this configuration that it is quite easy to rotate it with an outside force (i.e another robot spinning you).

[Steve_Alaniz]

Originally Posted by EricH

They did say to ignore that picture. And it should work (turning) even with traction wheels in the back.

Ah! You are right Eric. My mistake. OK in that case I've had my say and will drop out of the discussion.

Steve

Wait a minute... 7-11number1.... are you using 4 wheel mechanums now? If that week 3 photo is what you are using you definatly need to swap front and back wheels to have the correct configuration... FROM THE TOP... lines from the BOLTS should make an "X" .

Steve