pic: OMNI Drive

[Ether]

Originally Posted by pribusin

They thought it could give fast straight drive speeds along the two main axes of the wheels

Kinematically, it will be faster along the diagonals.

[pribusin]

Originally Posted by Ether

Kinematically, it will be faster along the diagonals.

How do you figure? Aren't you always losing some forward vector from all four wheels due to all rollers being angled and only part of the rotational vector getting translated into forward motion? I'll give you that you have four wheels propelling rather than two.

[JamesCH95]

Originally Posted by pribusin

How do you figure? Aren't you always losing some forward vector from all four wheels due to all rollers being angled and only part of the rotational vector getting translated into forward motion? I'll give you that you have four wheels propelling rather than two.

You get v1/(2)^1/2 + v2/(2)^1/2=vd, assuming v1=v2, then vd=sqrt(2)*v1

That is to say: the diagonal speed will be ~140% of either pair of wheels' speed.

See:



I imagine that this drive base would be quite tippy given the small base of support. The setup below is more common because the base of support is larger, and the fastest drive speeds are now aligned with the major faces of the chassis.

[Boltman]

Wheres you Fuel intake? hope you are not relying on Hoppers. I suspect many games within first 30 secs will have all fuel on floor...and going to loading station is a slow process.

[Ether]

Originally Posted by pribusin

How do you figure? Aren't you always losing some forward vector from all four wheels due to all rollers being angled and only part of the rotational vector getting translated into forward motion?

Using your argument, how is it possible for a sailboat's speed to exceed the wind speed that is propelling it?

[mr_yes]

Like with a mechanum drive system, one key to this being a viable drive train is lots of practice with it under realistic conditions. Because omniwheel systems drive differently than tank-style, the driver's brain has to be re-trained.

[GeeTwo]

If you keep the wheels in the same locations but rotate the chassis 45 degrees (or vice versa), you get what is sometimes referred to as Killough Drive (edit: pictured in post#7). This eliminates (or at least greatly reduces the "tippiness" at the corners. It is also kinematically the same as mecanum.

[Ether]

Originally Posted by GeeTwo

It is also kinematically the same as mecanum.

Not.

Mec fwd/reverse kinematics is same as standard wheel.

[GeeTwo]

Originally Posted by Ether

Not.

You do have to gear the Killough Mecanum ~40% faster to get the same behavior. I should have said similar (same shape, different scale).

Originally Posted by Ether

Mec fwd/reverse kinematics is same as standard wheel.

I'm not sure what you're saying here. What sort of standard wheel?

[Ether]

omni vs mecanum vs standard wheel chart

[Lil' Lavery]

While each wheel has different kinematics between Omni and mecnanum, as a 4-wheeled system they behave the same. You just swap the "diagonal" behavior in one system for the cardinal directions in the other.

Originally Posted by GeeTwo

You do have to gear the Killough ~40% faster to get the same behavior. I should have said similar (same shape, different scale).

How do you figure.

[jaweth12@gmail]

Check this pdf on page 12
http://www.simbotics.org/files/pdf/drivetraindesign.pdf

[OZ_341]

Team 341 did a variation of this in 2007 and 2008.
However, we placed the wheels at the corners.
Our nickname for this configuration was "Z-drive".

If you do this, you have to be resigned to the fact that everyone can push you around at will.

We were OK playing a "run and gun" game in 2007. It allowed us to always face the goal with the tubes and just drive sideways. Then in 2008 you were not allowed to contact each other, so we did not have to worry about defenders.

We won the Philly Regional with this drive configuration in those two years.
That being said, it has a limited range of uses for most games.

[GeeTwo]

Originally Posted by GeeTwo

You do have to gear the Killough ~40% faster to get the same behavior. I should have said similar (same shape, different scale).

Originally Posted by Lil' Lavery

How do you figure.

Ether answered in the previous post. √2 - 1 =~ 40%.

Originally Posted by Ether

omni vs mecanum vs standard wheel chart

Edit: As I look at Ether's chart, mecanum should be 40% faster, or omni 30% slower.

And thanks for keeping us honest, Ether!

[pmattin5459]

If you haven't built a holonomic drivetrain before, don't design one in the middle of build season. Just use a tank drive. If you're really set on it, then i would suggest mecanums, which can be added to a standard chassis fairly easily, or, if you really want the omnis, put them in the corners diagonally.