pic: Omnibot

[Tazlikesrobots]

We used a design very similar to yours this year and it worked very well for us! Our chassis was square shaped and the wheels were placed as close to the edge as possible.

[XXShadowXX]

You will most likely be pushed around alot with your current design (this is from my personal experiance). But omni's mounted to and prependicular to the direction of movement could spell trouble.
Also you don't need to mount your wheel in a perfect square, if you mount what I'm guessing is your forward and rear omni futher back you will increase stabbility. Just make sure your wheel that are found on the same axis (left and right, forward, and, rear) are mounted so that they lie on the same circle. If your really good you should be able to make both of your circles have the same center point (and center point of robot), but good luck...

[gimpnub]

why don't you use a three-wheel setup instead of a four-wheel setup?

correct me if i'm wrong but i think it would give you more stability and reduce your weight at the same time

your only difficulty might be programming it with 0, 60, and 120 degrees instead of 0, 90, 180, and 270 degrees, but there are also less wheels to program

i don't know how speed would be affected by the 3-wheel setup

if i'm wrong in any areas please forgive me and ignore my comments

[Joe G.]

The main advantage of four-wheel omni that I can think of is power. Even with vectors doing crazy things to the power output, 4 motors is stronger than 3.

[Protronie]

Very interesting... is this something that going to be built or just a design exercise ?

-p

[kajeevan]

Right now its just a design but who knows I may just get to build one.

[EricH]

Originally Posted by gimpnub

why don't you use a three-wheel setup instead of a four-wheel setup?

correct me if i'm wrong but i think it would give you more stability and reduce your weight at the same time

your only difficulty might be programming it with 0, 60, and 120 degrees instead of 0, 90, 180, and 270 degrees, but there are also less wheels to program

i don't know how speed would be affected by the 3-wheel setup

if i'm wrong in any areas please forgive me and ignore my comments

Reduce weight, yes, but 3-wheel setups have to be carefully thought out. I can only think of 4 off the top of my head: 330 (2002) had a triangular robot and no omnis; 67 (2005) had a peculiar triangular setup that I don't quite remember, but it involved flopping down; 16 (2006) had a 3-wheel swerve with pads to keep the frame from hitting the carpet and digging in, and 148 (2008) had a 3-wheel swerve patterned off of 118's V6 and a much smaller robot than the other three. The only one of those that involved omnis was 67's, IIRC. I remember hearing that 67's drive code took up an awful lot of room on their controller...

More stability is debatable. If you've got it fully in a square or circle, possibly. But with a frame like this, I don't think so. You've got a big risk that one corner will go down and dig into the carpet.

Speed might not be affected. I'd have to do the vectors to figure that out, and I'm not in a position to do that right now...

[Aren_Hill]

Originally Posted by EricH

The only one of those that involved omnis was 67's, IIRC. I remember hearing that 67's drive code took up an awful lot of room on their controller...

Im almost completely positive 67 in 2005 was a 3 wheel swerve with no omni's involved.

you can see the front two wheels here : http://www.chiefdelphi.com/media/photos/20772

[EricH]

Originally Posted by Aren_Hill

Im almost completely positive 67 in 2005 was a 3 wheel swerve with no omni's involved.

you can see the front two wheels here : http://www.chiefdelphi.com/media/photos/20772

I'm pretty sure it was a kiwi-type system, like below. I tried to search, but no luck yet.

[AdamHeard]

Originally Posted by EricH

I'm pretty sure it was a kiwi-type system, like below. I tried to search, but no luck yet.

Nope, sorry Eric, but 67 definitely had a 3 wheeled crab in 2005.

[EricH]

Originally Posted by AdamHeard

Nope, sorry Eric, but 67 definitely had a 3 wheeled crab in 2005.

Actually, I found a picture... of 67 2005... in a match... on two wheels, showing the third. NOT a true swerve. Swerve, yes. Three wheeled robot, yes. But not a three-wheel swerve. One wheel steered, three drove. Zoom in on the picture. One wheel is turned; the others appear not to be able to.

[AdamHeard]

Originally Posted by EricH

Actually, I found a picture... of 67 2005... in a match... on two wheels, showing the third. NOT a true swerve. Swerve, yes. Three wheeled robot, yes. But not a three-wheel swerve. One wheel steered, three drove. Zoom in on the picture. One wheel is turned; the others appear not to be able to.

Actually, I found a picture... of 67 in 2005...during build... on the ground, showing all three wheels. A true swerve, with all three wheels able to rotate.

No hard feelings eric

[seanwitte]

If you move the wheels to opposite corners the math will work out exactly the same as it would for the configuration shown.

There are two advantages that a holonomic base built using this configuration has over a swerve drive:

1) The modules are lighter and less complex.
2) The robot can translate and rotate at the same time (I've seen this called "frisbee motion")

The programming is more complex, but there are plenty of examples available to get you started.

[EricH]

Originally Posted by AdamHeard

Actually, I found a picture... of 67 in 2005...during build... on the ground, showing all three wheels. A true swerve, with all three wheels able to rotate.

No hard feelings eric

And rotate independently... No wonder the code took so much space in the controller.

Back on topic: If you're going to do 3WD, plan it out first. Carefully. As noted, only one robot (to our knowledge) has had a 3WD omni system; most of the other 3WDs were crab/swerve systems.

[Andrew Schreiber]

Originally Posted by EricH

And rotate independently... No wonder the code took so much space in the controller.

Back on topic: If you're going to do 3WD, plan it out first. Carefully. As noted, only one robot (to our knowledge) has had a 3WD omni system; most of the other 3WDs were crab/swerve systems.

Also, the one robot that did a 3 wheeled omni (857) didn't use code to control their wheels, they used hardware (http://www.chiefdelphi.com/media/photos/16410) Dont forget that sometimes that solution works better than complex code. No reason to make the code more complex than it needs to be. Could it be done in code? Yes, is complex code always better? That is up to you.