Omni Wheel/Ball Transfer/Caster?

[lippy]

Hello,

I posted here a while ago asking for direction on a robot I was wanting to build, and based on a reply here I decided to go with a differential drive setup.  I made my first robot using some small motors from Amazon and a 3D printed chassis that I designed, and I'm pretty pleased with it so far (pic attached).  Now that I've learned a (very) little bit about what I'm doing here, I want to scale up my design to the 8x10 robot that I was ultimately hoping to build.

I've got some good sized motors/mounts/wheels from Pololu and a Lexan sheet that I'm going to use for the basic chassis, but I'm stumped trying to source a front wheel.  Pololu's roller wheels are too small for this design, I've read bad things about using casters and I'm not sure about all the options for ball transfers or Omni-wheels.  Are there any resources/write-ups out there about selecting the right wheel here?  I'm going to need about a 45mm offset between the bottom of the chassis and the bottom of the wheel.  I could cut a notch out of the chassis for a large Omni-wheel or 3D print an offset between the chassis and the roller if need be.

[bdeuell]

Your first robot looks great.

If you are looking for heavier duty ball transfers look here:http://www.mcmaster.com/#standard-caster-ball-transfers/=ug5l7x


I can think of a few advantages for both the omni wheel and the ball  transfer:

One advantage of the ball transfer is the wheel radius is the same regardless of which direction i.e. the ball will be able to roll over the same size object regardless of the direction it is traveling.

the omni wheel also has the disadvantage of being an uneven rolling surface. this can cause vibrations in the platform as the contact point of the wheel changes.

An advantage of a omni wheel is the internal bearing surfaces are separate from the ground contact surface. in a ball transfer when the surface of the pall gets damaged (from abrasive surfaces such as concrete) it will degrade the bearing surfaces inside the ball transfer.

The omni wheel is probably more optimized for forward movement given a certain mass. i.e. an omni wheel with the same mass as a ball caster would probably have a larger primary diameter making it able to roll over larger obstacles in forward movement. however the rollers would probably have a smaller diameter the the advantage would be lost during turning.

[Fr0stAngel]

Hi, I would recommend that you read the tutorials on Chassis and Robot Dynamics first.

http://www.societyofrobots.com/mechanics_dynamics.shtml#wheel

http://www.societyofrobots.com/mechanics_chassisconstruction.shtml

You will find some good information on wheel selection, types and torque calculation.

[lippy]

Thanks for the replies.  I decided to go with a ball transfer to start.  I've got the first iteration of the chassis built, my AVR-based board is at fab right now with OSH-Park and I've started the software for it.  This thing is coming together.