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Re: Mecanum vs. Tank Drive?
Since our team has used mecanums a couple of years, I'll chime in.
First, exactly centered weight distribution isn't 100% necessary. Things will go wrong if your CG is 4 times closer to one pair of wheels than another, but a CG that close to the edge is terrible on any drivetrain. Follow normal CG location principles and you'll be fine.
Flat floors matter somewhat more. We used mecanums* in 2011, and the undercarpet plywood support for the towers would often often skew us when we hit it crooked. Closed loop yaw rate control will help a lot here. (*Technically half octocanum. Only 2 sticky wheel modules because weight.)
You're not going to get pushed entirely across the field by every robot, but you're definitely going to lose to heavyweight well designed tank drives. You do need to account for this in your drivetrain evaluation. Expect it to add extra point-to-point seconds to your cycle relative to a tank drive. On our 2011 bot, even just the two extra sticky wheels helped. The driver would drop them to stop a sideways shove and power out of it. They didn't get fast and good at this till Champs.
Since you're probably pitting a 4-CIM mecanum vs a 6-CIM tank drive, the mecanum is going to have less push and acceleration even before tracion comes into play.
Mecanums are not actually complicated to program. There are labview, C++ and Java blocks/classes for open-loop mecanum control. I know there's also C++ and Java code for closed loop yaw control and field centric control as well.
All that said, I'd still go with a 6-CIM or 4-CIM + 2 miniCIM tank drive. It's pushier and has less moving parts and breakables than mecanums. It will inevitably get you where you want to go faster. If you need a system to quickly line up a gear without backing up, add something to slide the gear side-to-side on your robot. Moving the gear side-to-side for alignment is the goal. You don't have to move the entire robot to do so.
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Lone Star Regional Troubleshooter
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