Quote:
Originally Posted by Oblarg
So, I've been doing a bit of back-of-the-envelop calculations, and it seems that for any drive geared for a top speed of under ~12 feet-per-second, a dead pushing match (i.e. comparable to pushing against a wall) is going to be traction-limited rather than motor-limited.
If this is the case, why is the spread of gear ratios offered on most FRC shifters so big? If you're already traction-limited in a pushing match, you're not going to be getting much utility out of gearing your robot to push harder. A 2.56:1 shifter spread will result in a drive that's set for 16 ft/s in high-gear having a low-gear which seems wastefully slow - does it actually result in better performance than, say, 2:1, or perhaps even less?
I feel that I'm missing something here. Note that for all my calculations I've been assuming a robot weight of ~150 lbs, and a wheel COF of ~1 - I am aware that the former is a bit on the heavy side, and the latter is a bit lower than what you'll actually get with plaction tread.
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You're missing current draw. For a drive train with 4 CIMs and 12 fps gearing, the CIMs will be pulling somewhere in the ball park of 80 amps a piece in a pushing match (highly dependent on wheel CoF and efficiency). According to the 40 A breaker spec. sheet, this will hold for 1.5-3.9 seconds before tripping breakers. (Tripping breakers means you stop moving, which is usually not so good in a match)
http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf
If you want a 4 CIM drive train to pull 40 Amps per motor in a pushing match (which should hold indefinitely), the gearing will usually correspond to around 5-7 fps. If you want a top speed in high gear in the 13-18 fps range, then you're looking at a 2.56:1 spread.
Also, voltage drops due to wiring and other components become more significant as current increases. V = IR. Less current means more juice for your motors.