Quote:
Originally Posted by Ether
Lots of good math showing considerable differences in current draw
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This is really good to see, and demonstrates the importance of gearing properly for the designed load.
The loads on an FRC drivetrain can be to accelerate the robot, to maintain speed by overcoming drag, or to push with some force at a constant speed (perhaps close to zero).
In the third case, the load will be continuous, so you should definitely heed the current figures Ether's shown. You don't want to be pulling 80A per motor continuous, or you
will pop some breakers (a
40A breaker in about a second, or for 6 six motors for a total of
480A, the
main breaker in a few seconds as well).
In the second case, the load will be continuous as well, but much much lower.
In the first case, the acceleration is only transient. 6 CIMs geared 5:1 on 4" wheels get to 90% of top speed (about 18 ft/s with no loss) in about half a second. They are stalled briefly for the beginning of this, but then quickly approach free speed, so there aren't too many amps in too few seconds. In comparison, 6 CIMs geared 13:1 on 4" wheels get to half the high gear's top speed in about a quarter of a second, so while you are putting less load on the system (getting to 90% of low gear's top speed in just 0.1 seconds), the performance improvement due to starting in low gear and shifting at the right time is hardly noticable at all. Even at a distance of 5 ft, the low gear takes 50% longer to complete the traversal than the high gear, and of course it's even more in favor of high gear for longer distances. For this reason, we often remain in high gear unless we see that we're applying load continuously (pushing).
These numbers are all pretty approximate (and also coincidentally almost match our 2014 drive), but I think they show why high gear is important, too. Note that the ability for this high gear to still accelerate as it does is in part due to the 2 extra CIMs in the drive. For a 4 CIM drive, you would not get the same performance over short distances at these reductions, and low gear would be more favored.