pic: Dual omni-wheel

[Tristan Lall]

Originally Posted by Rohan_DHS

i gotta pretty newb-ish question...what is the advantage of an omni wheel over a regular wheel...is it traction?

I hope that you also meant to ask about the disadvantages...at least for holonomic systems.
Mainly, those revolve around the force components going off in strange directions, and cancelling each other out, in order to produce a desired motion. For example, a robot with omniwheels at the corners, each at 45° to the principal driving direction of the robot, can be modelled as having force components in the direction of travel, and the perpendicular direction. Due to the 45° angle, and the fact that the wheels are 90° apart, if you drive the robot forward, the motion will be provided by the forward components, and the sideways components will act against each other (left vs. right) and do no useful work.

ΣF_y = 2 F sin(45°) + 2 F sin(135°) (Force in y-dir)
ΣF_x = 2 F cos(45°) + 2 F cos(135°) (Force in x-dir)
η_y = ΣF_y / 4 F (Efficiency in y-dir)
η_x = ΣF_x / 4 F (Effiiciency in x-dir)

For the simple case of driving forward, you get an efficiency of around 70% in the driving direction, and (of course) 0% in the perpendicular direction. As the output to each wheel varies in a holonomic drive system, the components move, but in general, either the efficiency suffers, or the total power does (as in, two wheels fully forward, and two fully perpendicular—50% power, but 100% efficiency). The same goes for mecanum wheels, in a conventional configuration.