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Unread 15-07-2015, 20:56
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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: pic: Four Speed Three CIM Gearbox

Quote:
Originally Posted by AdamHeard View Post
The CIMs aren't very efficient actually, there are more efficient motors in the kit by a good margin (lower power though).
All of the motors that are significantly more efficient than the CIM have their peak efficiency at a much higher speed (11-17k rpm vs 4500 rpm). This means that another level of gearing will be required to reduce these high-speed inputs down to a mechanically useful speed, offsetting most of the increase in motor efficiency. And you'd need more motor controllers, too. At least you could use a single encoder to know the speed of your power train.

Quote:
Originally Posted by AdamHeard View Post
Also, it's simpler for teams just to add more motors and gearing independently than it is to centralize PTO it all. It just doesn't make sense for FRC.
No argument. Everything that is currently an independent motor would have to be replaced with a clutch, CVT, or similar mechanism, which brings us back to Kevin's original suggestion to use the shifters on a PTO rather than to make a 4 speed single function transmission.

I often find that at least thinking about (and sometimes even building) the extreme cases leads to ideas that prove useful later. IIRC, the OP tossed this up as a bit of a "blue sky" thread, and I've been treating it as such. Checking back, OP included "probably weighs a thousand pounds" and "I do not actually intend to build this thing." As a result of this thread, I now know to keep an eye out for high power jobs that don't require simultaneous wheel drive. 2013 and 2015 both provided this sort of challenge (pyramid climb and can burglar respectively), so it can't be that rare. It wasn't in my mental toolbox six months ago.

Another thing that makes me really like the PTO concept is the addition of a flywheel. It is possible to tap more power from a PTO with a flywheel for a short period than from motors, especially given the new motor brownout situation. I like having a defined brownout over an undefined one, but it will probably be several years before the FRC community learns to take the best advantage of the new opportunity/challenge. At which time we'll switch to yet another control system ;-P>

Thinking a bit more, 2014 (Aerial Assault) also provided a PTO possibility with the ball thrower. We had six CIMs on our 2014 robot, two for a "kicker" and four for drive. (We used pneumatics and an AM gearmotor for the ball pickup.) We nearly always threw the ball while stationary, so sharing all six CIMs between drive and a more elegant and powerful launcher might have been an improvement on both fronts, especially as most of our "accelerator" designs would have thrown the ball from higher off the carpet than the kicker did.
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Last edited by GeeTwo : 15-07-2015 at 21:41. Reason: added flywheel paragraph, then AA paragraph
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