Swivel castor or omnidirectional wheel?

[Somchaya]

So.. I'm wondering whether I should use a single omnidirectional wheel or a usual swivel castor for my differential drive robot.

I like the idea of using the omni wheel, but I'm not very sure on how to mount it properly as a castor. Using a normal swivel castor helps in that aspect, since it's easier to mount, but I'm wondering if it'll affect the robot's performance in general.

Any suggestions?

[annoyin_kid]

i dont think it will make much difference which wheel you use. the advantage with castor is that it is less $$$ to fork out (and you could always pull it off some furniture and try to see if it works) and if it screws up you can upgrade to the onmi wheel knowing that the castor wont do any good.

[JesseWelling]

well mechanically speaking the omni has less adverse effects on the drive motors than a castor wheel.
Because the axel of the castor wheel is offset from the vertical axis of rotation there is often a some
weird issues when first moving in a different direction. You will notice this effect in office chairs when you roll
forward and back as the castors swap directions. I would go with an omni wheel for this reason, especially if you
are going to operate the robot on carpet or any thing that will significantly grip the castor wheel.

[Steve Joblin]

You may also want to consider a "ball caster"... McMaster-Carr has a very nice selection... goto http://www.mcmaster.com/ , then type in "ball transfers"

[Admin]

i have a ball caster that i used to use . . . but i dont like them because they are heavy and cant handle rough terrain

for non-rough terrain, they are probably the lowest friction in the bunch

i own and have used all three caster types for robots, but i prefer omni-wheels the most

[Somchaya]

So, I'd like to use the omniwheels, but I'm not really sure how to mount them properly. The idea I have so far is to put the wheel right at the back (or front) sticking out, and the axle basically running on the bottom of the robot for around 2".

Erm, I don't have a diagram of what I mean yet, but I'm experimenting with Goggle Sketchup so I should have a picture in the next day or so.

If there's an easy way to mount them, I'd be really happy to learn 

[JesseWelling]

Plastic.....

[Somchaya]

What about plastic? 

The robot base is going to be built using HDPE. I've never used it before, so this is going to be a learning experience, hehe.

[Admin]

here is the basic idea on how to mount the caster:

get two squares of HDPE (greater than 1/4" thick), align them together, and drill a .249" hole through both at the same time.

get a .25" aluminum beam, and stick it through the omniwheel. pressfit/jam the beam into the holes of both HDPE peices.

then screw mount the squares into your robot (by drilling more holes into the squares).

[Hal9000]

Omni wheels have a few problems associated with getting enough friction, don't they? Like, unless you mount them absolutely smack bang where they are supposed to be.

[Admin]

Omni wheels have a few problems associated with getting enough friction, don't they?

Yea, they often dont get enough friction with the ground - but for a caster, thats a good thing

It also depends a lot on what the caster material is made out of (plastic=low friction, rubber=high friction)

[Somchaya]

Haha this is amusing/creepy.. I was searching on google for "omni wheel castor" and I found this exact thread in one of the top hits!

Pity I didn't answer my question here

[Hal9000]

Omni wheels have a few problems associated with getting enough friction, don't they?

Yea, they often dont get enough friction with the ground - but for a caster, thats a good thing

It also depends a lot on what the caster material is made out of (plastic=low friction, rubber=high friction)

Omni wheels seemed like a great idea, although when this team I was competing against used them, theirs weren't rubber, which would explain why their robot didn't move.

I will try get some rubber ones. Thanks for info.