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Unread 25-10-2010, 09:41
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Re: Chainless Mecanum Drive

Quote:
Originally Posted by JesseK View Post
Many teams get away with 1 CIM per side for a 10fps (or less) drive train and do just fine. Calculating the motor load based upon robot weight and gearing inefficiency puts the motor efficiency within a nominal amount of its maximum efficiency while just driving. If sound judgment is used on the drive train with respect to turning (wheel base for traction, or use of omnis, 6WD drop-center, etc), almost the same results would apply. Thus, as long as the team doesn't try to push another traction robot they'll be ok.
I disagree, really. I can't think of any teams that enjoyed competitive success this year with a 2 CIM drivetrain, so I would hardly say "many teams" did. Who are some examples?

Even without pushing, running just 2 CIMs in a drivetrain even geared at 9 fps has many negative drawbacks. For one thing, at 9 FPS your CIMs will stall against a wall. Unlike a 4 CIM drivetrain, which just dips deep past the circuit breaker limits to get the wheels to slip, a 2 CIM drivetrain can't ever slip the wheels, so pushing other robots becomes basically not even an option. Turning is also pretty slow since skid steering relies on wheel slipping. Acceleration is also hampered a bit: In terms of actual distance traveled from a dead stop with 0.5 second of acceleration, a 2 CIM drive goes only 2.6 feet, while a 4 CIM drive goes 3.1 feet. (This effect is a lot more noticeable / dramatic in reality than "on paper").

If pushing is of zero concern for whatever reason (though, in my opinion there has not been a single FIRST game other than 2001 where at least being able to resist pushing was vital), it would make more sense to use 2 CIMs with ball casters supporting some of the robot's weight. This lets you draw less current while driving the same speeds, since your wheel force is lowered.
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