Thread: Team Update #18
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Unread 17-03-2011, 10:10
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Re: Team Update #18

Quote:
Originally Posted by Ether View Post
Could you be a bit more specific? What "implied assumption" are you referring to?

The only assumptions used in the calculation were to ignore friction (which was explicit, not implied) and to ignore additional stored kinetic energy (rotational). These tend to cancel each other out. Over very short distances (such as 1/4"), the stored rotational KE easily overcomes friction, so it's actually a conservative assumption. You can demonstrate this for yourself, if you like: with the wheels raised, power your minibot with 12V and then remove the power. Observe how much the wheels turn after the power is removed.
Alright, I did say implied and you were explicit about the friction. Yet it was still ignored in the conclusion without any explanation in the original posts. Ok, so there's the explanation, which I still don't think allows for a valid conclusion based upon the calculation. Your latest statement makes the implied assumption that the unloaded motors act identical to the real system during impact. Impact forces induce shock into the gearing which momentarily increases the friction in the gearing/bushings with a positive correlation to the amount of force applied to the plate. Impact forces induce friction in the bolts on the triggering mechanism, which may be bind on the plate if the impact causes a torque moement. Thus it may take 474 newtons at a specific instance in time to slow a minibot down without taking friction into account -- yet until we characterize the friction in the system we're making a dangerous* conclusion that "more than enough force is applied to moving the plate".

*dangerous because the process of coming to the conclusions would then be acceptable to use elsewhere.

I can't characterize the friction with numbers or equations, which I will concede. Interestingly, that's the very specific reason we [industry] do shock testing for our real world complex systems. The equations simply don't match the results of the real implementation.
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Last edited by JesseK : 17-03-2011 at 11:01. Reason: clarity
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