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Unread 09-02-2017, 07:25 PM
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GeeTwo GeeTwo is offline
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AKA: Gus Michel II
FRC #3946 (Tiger Robotics)
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Re: Drive system calculations

If you're more interested in theory than a canned answer:
  • Top speed is affected by load, but not as much as I'd have thought before getting in to robots. The various FRC calculators, based on a number of measurements and observations, estimate a robot's top speed based on a percentage of the "free" (no load) motor speed, most typically 80% to 85%.
  • The load will more significantly affect acceleration, especially at startup from a stationary position and in a "pushing match" where the robot is not moving (or only slowly) but applying a force against another robot, field element, or other resistant object. The calculators will take you through this, but approximately:
    The motors each draw their "free current" with no benefit other than overcoming friction. Above that point, torque (and therefore applied force on the carpet) is proportional to the current, after multiplicative increases for gearing down and decreases for the wheel radius. Note that a 40A breaker can actually pass 50A for about an FRC match, and rather higher currents for short time periods. Also check out the various curves for your motors - the BAG and CIM style motors can draw stall current for a significant number of seconds before giving up the smoke - but may require cooling between matches, whereas air-cooled motors such as the 775pro cannot draw anywhere near stall for more than a couple of seconds, but can draw current up to the point that they can vent it essentially forever.

    Vex has great measurements online for how much current is required to shut down each of the common FRC motors. (click on each motor for the info)
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