pic: Dual omni-wheel

[Richard Wallace]

Originally Posted by Stud Man Dan

We ended up milling the slots a bit larger in order to places washers in to keep the rubber wheels firm and sliding easier. ...

Good luck I'd love to see the finished product hopefully you'll post some pictures. I don't have any pictures of our modified wheels or our finished product but omni directional is deffinately a lot of fun.

Thanks for reviving the thread, Stud Man Dan.

I had been meaning to post some pictures. We did complete a holonomic drive train demonstrator, and used it as part of tutorials at St. Louis FRC Regional Fall Training Camp about a month ago. Total weight as driven was about 69 lb. Controllability on linoleum or carpet is excellent. Traction is not great, but may be adequate for a flat surface, offense-oriented game, and it should improve somewhat when weight is added.

I'll post some more pictures tonight. One shows the anti-binding washers (ersatz thrust bearings) that you described above -- we added them after I posted the picture that started this thread.

[Rohan_DHS]

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

[Pat Fairbank]

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?

There's two basic ways in which omni-wheels can be used.

The first involves placing 3 or 4 omni-wheels at angles to each other. With proper motor control, this allows the robot to travel in any direction, regardless of it's orientation - this is referred to as a holonomic drivetrain.

The second method involves a regular 4- or 6-wheel-drive drivetrain, where some of the wheels are replaced with omni-wheels. The robot still maintains almost the same amount of traction and manoeuverability (as opposed to using casters) but turning is much easier since the omni-wheels can slip sideways.

Hope that answers your question.

[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.

[Richard Wallace]

Recently posted images. Top view, bottom view, and first test run.

Next we plan to try other roller materials, aiming for better traction than we obtained using polyurethane. Maybe gum rubber, as team222badbrad recommended earlier in this thread.