Quote:
Originally Posted by A_Reed
Ah! good point. I do have all other power calculations assumed to be using 1/4 of the robot weigh on one wheel (4 wheels evenly bearing the weight). My mistake is that I assumed the same one wheel for acceleration in that it would be accelerating the entire mass of the robot by itself when it would only be accelerating 1/4th the mass as well, right?
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Quite honestly, I'm not sure about the 1/4th mass thing. I've only ever analyzed it as an overall robot system of wheels to the floor so weight transfer during acceleration is somewhat negligible; I'd never thought about it that way.
Attached is what I come up with for a SuperShifter with a 42:34 chain connection to 4" wheels @148lbs total, (our drive train this year). Min time to 27ft is 2.6s; your #'s give me 3.3s or so. I think I use 2.43Nm as stall, whereas you use 1.78 @ the 40A breaker; I honestly don't know which is correct. In the end, the times on field roughly match either calculation since there are a number of other factors such as driver control & software tuning.
The only really important thing to know is how any proposed design changes (4"-6" wheel size, 17:48 gearing vs 20:45 on the output of the SS, etc) effect things relative each other so decisions can be made and bad situations (unable to turn, constantly-popping breakers, etc) are avoided.
As a quick aside -- upping the torque allotted for my spreadsheet in order to account for a properly-matched RS-775 saves ~0.5 seconds for getting to 27feet. Yet it also gives you a narrow sliver of extra power curve to push through a road block during your acceleration. I say "narrow" because eventually (as Jared eludes to) you pull too much current for your 120A breaker.