Help with Omni wheels?

[Armando Gonzalez]

Although mecanum wheel provide excellent mobility and a relatively fast movement speed, the system to make them work weighs too much. Each wheel weighs about 2.5 pounds + 2.5" CIM Motor + transmission = alot of weight simply to make the robot move

[kristenliz_28]

our team was considering using omni wheels, but it would be hard to herd/move the balls out of the way. Of course, I guess the reason to use omni wheels is so you can juts go around quickly

[Andy Baker]

Originally Posted by tawnos23

I would suggest to buy metal omniwheels instead of the plastic ones. Our first year with omniwheels we bought the plastic ones, and they broke to pieces during the course of a regional.

Yes, that first year, I thought that it was a good idea to put a 10% fiberglass fill into the black polycarbonate. While that made the tensile strength higher, it was lower rated on the Izod impact rating. The fiberglass mad the material brittle, and when teams hit the side of the 1 point ramp in 2006, the wheels broke. The result of these breaks were improving these wheels and creating the aluminum omni wheels.

So... last year we had black polycarbonate with no fiberglass filler, and that resulted in zero complaints about breakages.

If I had to pick which omni wheel to use on a FRC robot, I would choose the 6" aluminum omni wheel, and use an Omni Spacer to make them a dualie omni wheel. That is a sweet setup. The polycarbonate wheels are nice, and can handle 2 bearings, but the aluminum wheels take up less space (they are thinner) and they are repairable. The only negative about the aluminum omni wheels is that they only have the ability to have 1 bearing mounted to them, that is unless you add on an extra hub.

Andy B.

[ebarker]

Originally Posted by Andy Baker

So... last year we had black polycarbonate with no fiberglass filler, and that resulted in zero complaints about breakages.

Last year we used the plastic polycarbonate omni wheels and we are a very happy customer. Zero problems. In fact we are ordering more for this year tomorrow.

[Richard Wallace]

Originally Posted by Andy Baker

...

If I had to pick which omni wheel to use on a FRC robot, I would choose the 6" aluminum omni wheel, and use an Omni Spacer to make them a dualie omni wheel. That is a sweet setup. The polycarbonate wheels are nice, and can handle 2 bearings, but the aluminum wheels take up less space (they are thinner) and they are repairable. The only negative about the aluminum omni wheels is that they only have the ability to have 1 bearing mounted to them, that is unless you add on an extra hub.

I agree dual 6" aluminum omniwheels are sweet. In '06 my team spent a lot of effort building our own, so it was nice to have the option of buying them from AndyMark last year.

Our '07 chassis used single 6" aluminum AM omniwheels on the corners of a 6WD kitbot, and kit AM wheels in the middle with one CIM per side -- not a defense-oriented drivetrain, but nice for maneuvering. To get around the single bearing limitation Andy mentioned above, we mounted the inboard bearings in the sprockets that drove each omniwheel. This required boring out the center hole in the sprocket to 1.125", which is a relatively easy lathe operation in a standard 3-jaw chuck if the tooth count is a multiple of three.

[Storcky]

Our team used two omniwheels in the front and two driven traction wheels in the back. The only problem with it was we could not get the omnis to stop turning when we wanted them to. This resulted in the front of the robot continuing to spin after we expected it to. Also, although they were very manuverable, another disadvantage that it was nearly impossible to go straight.

[lynca]

Originally Posted by Storcky

Our team used two omniwheels in the front and two driven traction wheels in the back. The only problem with it was we could not get the omnis to stop turning when we wanted them to. This resulted in the front of the robot continuing to spin after we expected it to. Also, although they were very manuverable, another disadvantage that it was nearly impossible to go straight.

Driving Straight is definitely possible with omnis. Arc turning with two back omni can cause your back two omni wheels to fishtail moderately making the robot more difficult to drive. FRC#494 in 2006 used omnis in the corners instead of two omnis on the back side to help control fishtailing. Read the full report on page 160 of the 2006 FIRST Robotics Book.

[Alan Anderson]

Originally Posted by Storcky

Our team used two omniwheels in the front and two driven traction wheels in the back. The only problem with it was we could not get the omnis to stop turning when we wanted them to. This resulted in the front of the robot continuing to spin after we expected it to. Also, although they were very manuverable, another disadvantage that it was nearly impossible to go straight.

It sounds like you might have the center of gravity of your robot too far forward for easy controllability. Putting it closer to the traction wheels would help things a lot.

When I joined the TechnoKats as a mentor, my first real programming challenge was to compensate in software for exactly this situation. The code detected the rear wheels' rotation (Banner optical sensors pointing at alternating reflective/nonreflective sections of hub) and adjusted the relative power to the left and right side motors in order to keep the wheel speeds consistent for perfectly straight travel.

[sweetie_pi]

thanks for the help that will definately help our plans