Mecanum Wheels & Torque

[ksafin]

Hey everyone,

I was wondering if the CD community could offer answers to a few questions.

Some users pointed out that we had a torque-issue on our Mecanum wheels because we were using CIMPLe boxes. I saw it as a real problem but it was too late to correct for some reasons.

Moving forward, I wanted to see if there was a simple way for us to rectify it by asking these questions:
(we're using 8" mecanums from vexpro)

- Which Mecanum wheels require less torque, the VexPro ones or the AndyMark ones (and which?). VexPro's have .9 coeff. of friction but AndyMarks have .6 (moving sideways) so I'm assuming AndyMark's would require less torque since they have slippery polycarbonate rollers.
- What's the optimal torque needed for 8" mecanums on a standard 150 lb robot to strafe, and to turn and move normally? (using CIM motor)
- What's the lowest torque necessary to do the above? (in order to maximize speed)
- Is there an easy way to make it work given our current setup? (CIMPLe box with 1 CIM motor, a KOP 39 tooth pulley, and KOP 42 tooth pulley on the wheel)

Thanks!

[PAR_WIG1350]

There are two possible ways to rectify the situation, reduce the torque required or increase the torque available.

Using 6" wheels instead of 8" is really the only way to reduce the required torque. Changing the COF probably won't do much since it still takes a set amount of force to accelerate a 150 lb robot the way you want it to accelerate. A lower COF would just reduce how large that force could be.

If 6" wheels aren't an option you could also increase the available torque by either changing the reduction or adding more motors. Adding motors will add a lot of weight and could be difficult to deal with in code if you don't have experience with similar configurations. Also, the best motor to use would be the miniCIM and if you have used even one miniCIM or BAG motor elsewhere on your bot, that option could become a hassle.

A change in your reduction might be the best option. It adds minimal weight and no complexity. The way I suggest doing this is by replacing your belts and pulleys with chains and sprockets of a greater reduction. 35 chain is the easiest to retrofit since it can tolerate imperfect tension better than 25 chain.

[StevenB]

The easiest thing to do is change your sprocket ratio. See the attached document for a quick run through of the math you should follow to figure out what is the right ratio.

[GDG 2337]

Direct driving 8” Mecanum wheels with a Cimple Box is defiantly a bad idea, way too fast and not enough torque. We’ve run 8” Mecanum wheels 3 times, in ’08 and ‘11 targeted speed was 12fps and 10fps for ’10. Keeping the Bot tracking in a straight line is much harder the faster you go.