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Unread 03-05-2007, 19:49
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Teaching Teachers to Teach Tech
AKA: Jason Brett
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Re: mecanum vs. omni

Quote:
Originally Posted by JesseK View Post
This man gets it. It's not about the robots.

I think the point he makes is that even if it sacrifices time and competition success, you have to get your hands dirty or you'll never know for sure.

.....

Another technical piece that I'd like to know is what kind of power transfer systems do you put on each drive wheel? In Atlanta I saw robots with straight 1:1 drives, 1:5 ratios (or similar) and there was one team that had the 3-speed drill transmissions attached to each mechanum motor, and the Jester Drive CAD shows something like 1:3 connected to 1:3 again.

Also, generally I've seen large-diameter mechanum wheels (are the AM's 6"?), but what is gained or sacrificed in the power and control realm by going to smaller wheels?
Thank you for the kind comment. I think there are a lot of people who "get it"! And thank goodness... that's what makes FIRST so cool. As for the drive train, I don't really think there is much point in putting a lot of torque to the wheels in either a mecanum or omni system... after all, if you are in a pushing match with one of these drive systems, you are probably losing. So I expect you will see them geared higher... and, as there are four gearboxes, rather than two, you will be less likely to see them have (or need) multi-speed gearboxes. We used 12:1 banebots with a direct drive system. For the first year in four years we NEVER lost a chain! The Andymark mecanums are 8" diameter, so we're moving pretty fast at high speed, and don't have much poop to push with... but when you build omni-drive, you're building a wide receiver, not a linebacker. If you are pushing... you're losing!

As for control, on board feedback and programming is much more crucial, simply because you have an extra degree of freedom. We went with a PID speed control loop set by encoders on each axle... but I know teams have also had success with a gyro. So long as all the wheels are on the ground (almost all of the time on a carpeted floor) we have good pretty good control of what we are doing. So so long as you have good feedback on-board, and an intutive control system, and time to practice... you really don't give much up in the way of control.

There are a few examples on youtube... just search up mecanum or meccanum or mechanum. Wikipedia also has a link to some video from a company that builds mecanum forklifts... which are really rather cool.

Jason