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Unread 03-11-2009, 11:51
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JesseK JesseK is offline
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Re: pic: 955 T-Shirt Bot @ Halftime

Another way to look at the battery life problem: you're driving more than double the weight of a typical FRC robot with only 70% of the torque you think you have. At least theoretically, mecanums give the same torque sideways as they do forward/reverse. The primary torque issue comes on the diagonals. On the strict diagonals you're only running 2 motors, and even then you're only getting 70% of those outputs since the rollers are still 45 degrees offset from the rotation of the wheel. Hence you're moving 275lbs with only 35% of the available torque in the entire drivetrain.

Just to move forward, I estimate your bot draws a constant 45A through 4 motors. On the diagonals, that gets up to 56A through 2 motors -- no biggie since there's minimal impact. Yet the massive current draws come when you are moving laterally, say, 60-80 degrees off of forward where the 2 motors that strictly help move sideways pull ~30A and the diagonal motors pull ~45A -- that's a total of 75A to be able to move in a 360 degree lateral plane.

The quick solution: in programming, disable diagonal movements, even minor/gradient ones. The robot can move strictly forward, strictly sideways, or turn. It's not as impressive, but should keep you from having to purchase additional transmissions for the moment.

As a curiosity, how much distance does your bot need to get up to maximum speed from a dead stop?
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