Speed measurement precision

[naturephoenix]

If we are measuring speed of rotating machine by measuring time between rising edges, precision is at certain frequency equal to fm/system clock frequency(fm is current frequency) . So smaller frequency better accuracy. In a case more impulses/ teets per revolution accuracy is decreasing with this method. But using method calculating number of impulses in certain time second mettod is more accurate with more teets, but how to calculate accuracy with second method?

 

[Peter Nachtwey]

one count per sample time.

 

[naturephoenix]

Look:

Method 1:


EXP 1:

Measuring speed by measuring time interval between two rising edges. Let's say at frequency fm = 50Hz we are using fc = 16MHz system clock .



Accuracy is defined by Acc = (fm/fc).


EXP 2:

In a case we have 1000 rpm (50Hz) and 60 teeths, that's:
Acc = 1000Hz/16MHz


So Acc is reduced in example 2.




Method 2:
Measuring speed by counting number of impulses in fixed period of time.



EXP : 1 -> Same like in Method 1.
Acc = ??


EXP : 2 -> Same like in Method 1


Acc = ??


Acc in Exp 2 should be higher than in Exp 1 with method 2, but I don't know how to calculate it.

 

[Peter Nachtwey]

The resolution is the distance represented by one pulse divided by the same interval.