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PRESTON
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I did a quick search for threads as I know for a fact they exist, in which this has been discussed in quite some depth. Unfortunately it would appear that all the goods ones are in the Picogroup area. Got $650.00? If not, you're just going to have to make sense of my attempt at explaining it.
Take the number of teeth on the crank reluctor that should be there, not how many are there. By that I mean if it's a 60 - 2 reluctor, where 2 teeth are missing, it's really a 60 tooth reluctor.
Frequency is cycles per second, and since you state you are trying to acquire true RPM from this signal not simply frequency, we need to convert cycles per second to cycles per minute ( which is essentially revolutions per minute ).
We also need to specify a point at which the signal will be sampled for the equation, in some cases. If this signal crosses 0, much like a non-floating ground AC producing sensor, then we leave it alone, but if not, we need to pick a point. This is done by adding a -x to the equation, where x is a numerical value. On some digital signals that are really noisy at low voltages, but clean near the top, you may want to play around with that number to move the sample point higher and higher up the signal so that the resulting math channel come out clean.
Here is an example that could be used for an AC producing signal in which the average of the signals amplitude is zero, remember, we don't need to modify the sample point for this one:
(freq A) / 36 * 60 = RPM of a 36 tooth reluctor
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