View Single Post
  #9   Spotlight this post!  
Unread 04-05-2007, 11:58
Alan Anderson's Avatar
Alan Anderson Alan Anderson is offline
Software Architect
FRC #0045 (TechnoKats)
Team Role: Mentor
 
Join Date: Feb 2004
Rookie Year: 2004
Location: Kokomo, Indiana
Posts: 9,113
Alan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond reputeAlan Anderson has a reputation beyond repute
Re: mecanum vs. omni

Quote:
Originally Posted by Alex.Norton View Post
...A kiwi drive is easier because of control and the physics of a holonomic but a kiwi inherently unstable because it is a tripod (how many three wheel cars do you see? They tend to be too tippy).
Tippiness is just a matter of where the center of gravity ends up. A Killough ("kiwi") base is actually a very good holonomic platform because it's a tripod. All three omniwheels are always contacting the ground, so it won't go off in odd directions because of losing traction at one corner like a four-wheeled platform can.
Quote:
The reason that side forces aren't a problem with omni is that in an ideal omni wheel you can't have side force since there are rollers in that direction. People use them in tank drives to turn the robot easier because they don't create any lateral force in almost every situation. They only case that creates side force on the wheel is when the side of the wheel contacts something without contacting the roller. I could use 1/16th in acrylic to captivate my wheel because it didn't create side force. That is what I like about them, I only needed one strong piece for mounting my wheels (hub).
Ah, I think I understand now. When you say "side forces", you're apparently talking about thrust forces on the axle. Agreed, an ideal omniwheel won't try to push its axle in or out. And since you just now mentioned tank drive, I think we might have been talking past each other. With a pair of omniwheels instead of traction wheels on a tank-drive platform, the in/out force on the axle is indeed a side force on the robot. I've been focusing on holonomic drives, and I was tripped up by your mention of extra forces on the frame. Since a traditional four-omni drivebase has the axles at 45-degree angles, it puts just as much side force on the frame as a traditional mecanum drivebase.