Quote:
Originally Posted by compwiztobe
A linear extrapolation gives me 4900 rpm (using excel's questionable fit, not sure if its lsq or what), but we guessed it should be following a quadratic trend (since we are increasing the voltage, but keeping the load the same), so that put us (forcing an intercept of 0) at 5200. The fits would be better if we included the origin as a data point (since it does, in fact, not spin, with no voltage applied...). In any case, the numbers are close enough for our purposes - we are happy to be within 20% right now (if this were for my lab, however, it would be a different story).
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Not to annoy you, but I think this is worth discussing because there are some things to be learned here.
I was trying to help you understand the mystery behind your earlier comment:
Quote:
Originally Posted by compwiztobe
These numbers are lower than we had previously been estimating, but hey that's why we test.
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If you
plot the 3 points for which you actually have data, two things are clear:
1) the graph's 2nd derivative is negative (it's
concave down), and
2) the X intercept is
not trending toward zero.
These two observations comport well with what would be expected based on the system that's being modeled: it takes some non-zero voltage to move the belt, and the losses are greater at greater speeds.
So to get a good extrapolation you need to take the above two things into account.
Forcing a zero-intercept and fitting a quadratic gives a trendline which clearly violates those two observations, and gives a misleading answer.
If you fit a quadratic to your actual data (
without forcing it to go through zero), the quadratic will be concave down (as expected) and will extrapolate nicely to 4400 rpm at 12 volts.
If you run 4400 through your calculations, you'll see it agrees much more closely with your observed frisbee speed.