# Holonomic tri wheel drive

I am aware of the fact that 3 onmi-directional wheels in a equilateral triangle will make a good holonomic drive, but what happens when you use a right triangle. Is there any noticeable change in performance, handling or speed? My theory is it will make little diffrence. I am addind the wheels to the bases instead of the corners.

“Yes”, it can be done.

The lack of symmetry will almost certainly complicate the software you use to control the motors/wheels.

Blake

it probably will not function the same as a typical “kiwi” drive 3 wheel robot. i dont know if the programming will be different, but i am sure that it will not move as easily as if it had the usual 120 degree offset.

I think it would be much like the standard holonomic 4 wheel drive but with less power. The back wheel might serve for little more than turning. I’ve never played with the holonomic drives myself, but it would be a good experiment.

How would you angle the wheels from each other. Would you have one on each side of the right triangle or are you going to put them on the corners somehow.

And then of course, Why?

I will mount them on the sides

Ok now the next question why not add another side in and make all the programming easier?

That wouldn’t be as cool though. good if you’re looking for a challenge of course. I don’t think a right triangle would steer as well as an equilateral triangle though. The main thing I see other than programming issues is that the omni wheels would most likely work best at the same angle to each other. Otherwise the robot would not move as well in some directions as more friction would be generated from the omnis trying to spin in an odd way.

Yes, it will work.

If you have 3 omni-wheels perfectly spaced they will always be providing the same power in the direction of motion, regardless of the direction of motion.

In the situation you described, some directions of motion will have more power applied in the direction of motion than others.

Does this matter? No, not really. It can definitely mess with your programming a little bit though.

I’ve built both the three wheel and the four wheel versions. The four wheel version has more “power” when going in a direction where two of the wheels are driving and there is smaller drag on the sideways wheels (the rollers).

On the three wheel version there is not a condition where you are getting the best traction from the omni’s.

I had an equilateral triangle, not a right triangle, but I doubt that would make that much difference in the drive forces. My wheels were attached to the sides, not the points.

And I agree with John, the programming on right triangle version will be more complicated as you change the direction of motion.

Were you able to test your theory? How did it come out?

3w holo (aka kiwi) takes 3 motors and 3 channels
4w holo or Mechanum drive takes 4 motors and 4 channels
2w or 4w drive (squarebot) typically takes 2 or 4 motors and 2 channels
It can spin in place, but not slide sideways.

One of the students on our team came up with a “Ortho XY” drive system,
(aka “5th wheel”) to allow side sliding.
Take a square 4wd design (all wheels pointing forward/backward)
and add a 5th wheel sideways in the center of the square.
Using 5 omni wheels, this plan allows X,Y motion.
One stick does std 4w arcade drive, another control slides left or right.
For extra performance, use a servo to lift the 5th wheel off the ground when not side-sliding, and press it down to lift the other wheels when side sliding.

How well does that 5 wheel system work? When you mention the center of the square, are you talking about the very center or jut the center of a back (or front) edge?
I am wondering how pushable that design would be, with all omni wheels…

This 5 Omniwheel system was used by a few of the top ranked Chinese teams at the VRC World Championship in Los Angeles this past spring. To put it mildly, it was very effective.

The robot I saw had the 5th wheel at the center of the square, leaving the front edge free for a ramp mech, and the back edge free for a pushplate.
If you put a 5th wheel far from the center of drag, then it tilts as it slidles.
If you put a 5th wheel at both front and back edges without lifters, with a common control, then they drag somewhat during normal turns.
If you put them at both front and back edges with independent controls, then you have 4w+2w holo drive with a prefered forward/back double wheel drive. That might be a nice tradeoff, if you have motors and wheels to spare.

Regarding Pushability: Many good bots I saw at QQ finals used 4wdrive all omniwheels to give good manueverability in turns. They are somewhat pushable sideways due to the rollers. At least 5th wheel drive lets you counteract that push (or manuever around it) more than without the 5th wheel.

I think the 4 wheel makes it faster. With the tri wheel, the wheels are tilted away and towards each other, which is a large caster angle, which isn’t very efficient for speed but makes good for turning in normal cars. With the 4 wheel, the wheels are pitted at 90 degrees from the other, so 2 wheels get traction and 2 spin the mini rollers. This setup is better for speed. I’m not sure of the handling though…i don’t have omni’s to test it