motor stall current vs Victor ratings

[Joel J]

What I usually see the robot designers do is to select a traction material for their wheels, then calculate the maximum torque they can get with those wheels, given the robots weight, as a first step. So lets say there is a 6 inch tire, a 150lb robot, and the coefficient of friction between the wheel and the floor is 1.3 (IFI roughtop tread). The gearbox designers would calculate the max torque at the wheel to be: 150*1.3*3(tire radius) = 585 in-lbs. Convert that to the oz-in units that's given on the CIM motor chart, and you get 9360 oz-in's at the wheel before it starts slipping. I've typically seen FIRST drivetrains use the 4 identical CIM motors, two per side, meaning that the load would be evenly shared across these motors. So each motor needs to provide 9360/4 oz-in's of torque, or 2340 oz-in. The designer would then determine the amount of torque they would get when pulling 40 amps from the CIM motor, as opposed to the 133 amps at stall (let's say its 105.43 oz-in's, as opposed to the stall torque of 343.4 oz-in). Divide the torque needed from the individual motor by the max load you want it to see, and you get the reduction needed between the motor and the wheel. In this case 2340/105.43 gives a reduction of 22.1948 minimum. When you reduce the free speed of the CIM motor (5310 RPM) 22:1, you get a final RPM of 239.245, which translates to a ground speed of 6.2634 fps. I didn't factor in losses (4.5253 fps, with losses, on most one speeds.. 4.1fps on a typical 2 speed). At this reduction, the idea is that the wheels would start spinning before the motors begin to pull more than 40 amps, preventing stall. I guess the idea is similar to having a low traction surface to play on. If the final speed you get after doing these calculations is too low, then you just lower the "grippiness" of the wheels you are using. If you want to be a strong pusher, you can only lower the grippiness so much before you start losing traction advantage. You'll start getting into the whole tradeoff between speed/torque/current pull that I just can't bother with; I'll stick to programming.

This was a bit rushed. If you want a detailed description of this idea, you can search around the forums for topics on drivetrain design, or gearbox design, or something..