mecanum vs. omni

[dtengineering]

We built a mecanum this year... during the six week build period with only a VEX omnidrive holonomic as a practice chassis. It used the banebots encoders to do PID speed control to each wheel. It worked pretty well (thanks to copious amounts of advice available through CD and the FIRST workshop presentation archives), although by the time we had the drive worked out and the robot built there were approximately five minutes of practice for the drivers before we had to drop it into the shipping crate. A typical driver will need much more practice time to maximize the benefits of an omni-drive vs. the benefits of a tank drive. (It is far easier to figure out how to get INTO a pushing battle than to get AROUND one. If you've got an omni drive and you are in a pushing battle... you're probably losing.)

We would have probably won more matches with a tank drive and practice time than with the mecanum... but the mecanums provided a programming challenge for our programming team... the best programmers we have had... or are likely to have... for a few years. They also have inspired the "wow" factor amongst people who view the robot, and generated some interest in the community... and this is where I'll weigh in with two advantages of mecanum over omni...

Omni wheels are cool... but mecanum wheels are COOL!! They just look technologically menacing, and really make people stop to think about how they work. Mecanums are the FIRST equivalent of "spinners" or whatever the flashy shiny rim of the month is in cars these days.

The Andy Mark wheels have the ability to adjust tension in the rollers by tightening the roller screws... in Portland we almost lost an encoder when we fell off a ramp (mecanums can climb, but not like a tank). If we had lost the feedback we could have driven omni in open loop, OR simply tightened down the rollers on a pair of wheels so that they would not roll at all, which would have basically given us the equivalent of a tank drive... we could have easily reset the programming and been back on the field in 30-45 minutes. It would have cost us our omni maneuverability, but we would have still had control. Had EVERYTHING gone screwy with the mecanums, we could have dropped traction wheels on the front and omnis on the back and been a tank drive a few weeks later in Toronto. That is one unmentioned (so far) benefit of having your wheels aligned in a conventional manner. I think the mecanums offer a bit more redundancy and options in the event of a serious competition-season "oops."

Either choice will be a good one to pursue. Keep in mind that (IMHO) it is more difficult to build a competition-winning omni-drive robot (mecanum or omni wheels) than to build a winning tank drive robot. You will end up devoting time and resources to the drive that could otherwise go to programming, actuators, and practicing. That said, when you see your team finally get the drive figured out and go drifting past a defender and slam home a tube... or when your grade 12's get to explain the vector mechanics of the mecanum to an experienced adult engineer who has "never seen a wheel like that before"... that is something you won't get from a tank drive.

Jason