On 4/29/14, 9:08 PM, Al Kozakiewicz wrote:
But if I hook up my 10 MHz clock and count the time between pulses, my
accuracy is +1/-0 count out of 1,250,000 counts. That's a whole lot
more accurate.
This is the key thing that misses the entire point. It's not the accuracy of the time
measurement that's the issue. It is the uncertainty of when position transitions are
crossed. With either electromagnetic or optical sensors, that uncertainty is 1/8th of a
revolution with an 8 ppr transducer. The fact that you can measure the time between
"pulses" accurately is not disputed. The issue is that the rotational distance
(hence velocity) 8 pulses represents is known only to +/- 12.5%.
Not really. Most position sensors used in anemometers have a fairly
well defined transition point that is fairly consistent from pulse to
pulse. For instance, if there's 8 magnets and a reed switch, there's an
absolute uncertainty of when the magnet is close enough to close/open
the switch, but it's the same offset for all pulses.
It's not like the pulse occurs randomly within a 45 degree sector.
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