Omni Wheel Configuration

[Madison]

Originally Posted by thursam

Designing our own chassis.
Live axles.
8" diameter.
Lateral maneuverability and a new experience (for learning and advancement with our engineering concepts.)
We are only using omni wheels. Why is it unusual?

It should be pretty straightforward to use omniwheels in that arrangement. Both VexPro and AndyMark sell 8" diameter omniwheels. Both of them also offer a variety of hubs that bolt onto the wheels and will give you a hex or keyed bore. You need one hub per wheel.


Driving omniwheels on a live axle is no different than driving traction wheels.

[thursam]

Is that everything, or is there anything else that we would need? We found a diagram for a basic drive system, but we don't need some of it.

[Madison]

Originally Posted by thursam

Is that everything, or is there anything else that we would need? We found a diagram for a basic drive system, but we don't need some of it.

Okay, generally --

You need a gearbox with appropriate gearing for 8" wheels. You probably want something in the 12:1 range.

You'll probably need form of power transmission between the gearbox output and your wheels. This is usually chain and sprockets or belt and pulleys. It could also be gears. The power transmission component that turns your wheel's axle will have to be bored appropriately.

In FRC, a live axle is either a hex shaft or a keyed round shaft in nearly all cases. If you're using a hex shaft, you just need a hex bore in your sprocket or pulley and a hex hub attached to your wheel. If you're using a keyed shaft, you'll need a keyed bore on the sprocket and a keyed hub for the wheel. You'll also need a machine key -- a square piece of steel that sits between the shaft and the hub and transmits the torque between the two of them.

You'll probably need shaft collars or some form of retaining rings to stop the wheel and sprocket/pulley/gear from sliding around on the shaft axially. You want to use spacers for this as well.

Folks don't usually use a drive made entirely from omniwheels because it offers almost no resistance to be pushed sideways by a defender. If your strategy relies upon constant mobility around the field, being susceptible to defense may not be a big tradeoff. If any function of your robot requires you to stop in place (to aim, to score, to collect, to catch), an omniwheel drive may be a large liability.

The upside, of course, is that you can swap in a pair of traction wheels for a pair of omniwheels pretty easily and that'll give you some additional resistance to being pushed, spun in place, etc.

[Ether]

Originally Posted by thursam

Is that everything, or is there anything else that we would need? We found a diagram for a basic drive system, but we don't need some of it.

On the off chance you don't already know this, if you want to strafe with those omni wheels, you'll need to mount them at 45 degrees.

[thursam]

Thank you both, very much. You have definitely helped a lot.

[yash101]

OmniWhels are a great piece of hardware. They have so many uses, that it would be useless to name all the possible configurations!
If you want to use it as a holonomic drive, just put a wheel on every of the four sides on the bot. If you want to use OmniWheels to pivot easier and more efficiently, just do a tankdrive, with omni wheels as the far-edge wheels. If you want to be able to easily drift, just make a tankdrive, with nothing but omniwheels!

[kevin.li.rit]

Originally Posted by Ether

On the off chance you don't already know this, if you want to strafe with those omni wheels, you'll need to mount them at 45 degrees.

Or at 120 degree intervals!

http://www.ludep.com/working-out-the...he-kiwi-drive/

[SoftwareBug2.0]

Originally Posted by kevin.li.rit

Or at 120 degree intervals!

http://www.ludep.com/working-out-the...he-kiwi-drive/

You're both wrong. Just make them not all point the same way.

[Nate Laverdure]

Originally Posted by SoftwareBug2.0

You're both wrong. Just make them not all point the same way.

Note that this description isn't sufficient for describing a holonomic drivetrain based on omni wheels. For example, the attached design is not holonomic (source image).

[Ether]

Originally Posted by Nate Laverdure

Note that this description isn't sufficient

It's neither sufficient nor necessary.

[gpetilli]

a couple of files from previous CD posts (credits are in the files) that I think you will find useful in your trade-off analysis.

[kevin.li.rit]

Originally Posted by Nate Laverdure

Note that this description isn't sufficient for describing a holonomic drivetrain based on omni wheels. For example, the attached design is not holonomic (source image).

Wikipedia seems to disagree.

http://en.m.wikipedia.org/wiki/Holonomic_(robotics)

[Nate Laverdure]

Originally Posted by kevin.li.rit

Wikipedia seems to disagree.

There's no disagreement. With the design I posted, you'd be able to control only 2 of 3 DOFs.

[kevin.li.rit]

I see. The attached differs from the source.

[JamesCH95]

A pragmatic suggestion: seriously consider how reasonable it would be for your team to successfully execute a holonomic drivetrain, for what sounds like the first time ever, starting half-way through Week 3 of build. Your drivetrain is the single most important sub-system on your robot, thus I strongly encourage you to use a low-risk approach.