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Unread 17-07-2007, 03:08
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Re: what motor did you use(arm)

Quote:
Originally Posted by ChrisH View Post
Power = force x distance / time
...
As an example let's run the numbers:

We want to move a 0.5N object approximately 3 meters in 4 seconds. That will take 0.375 watts.
...
If we assume a 2m arm, we only actually need 0.5N *2m = 1Nm.
...
I puposely left out the power required to move the arm itself as that can be adjusted by using counter balances or springs.

So it really does not matter too much which motor you use as long as you use it correctly and keep all hardware within its operating limits.
Excellent points, Chris. I'm a physics teacher, so I get all geeked up when this stuff comes up. I see a mistake some teams make is under-estimating the amount of torque required to move not the game piece, but their robot's ARM! True any motor could be used to pick up the 1 lb tube, but what about that 1x1 box tubing and the grabber assembly at the end? 16 W Mabuchi motor?? Well, if the arm weighs a modest 10 lbs with its CM located 2 ft from the pivot = 20 ft-lbs (27 N-m)... That means that mabuchi would need to be geared down at least 1800:1 and moving 0.033 rps or 12 degrees/sec. Yikes or better yet, Yawn!!

I know Chris was simply making a point to look at the physics, specifically mechanical advantage of gearboxes. Hmmm... maybe we could re-write the Archimedes quote ... "if I had a big enough gearbox for my 16 W mabuchi, I could move the world". Another hmmm.. makes me think of an off-season project!!!
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