Question about high speed counter

hi, im using cj1m cpu22 and I plan to use the built in high speed counter to count the pulses from a rotary encoder. We are measuring the rpm of a screw which will either speed up or slow down. I know that the high speed counter will be able to read real fast pulses but will it be able to read slow pulses as well? I asked this question because someone from our engineering dept told me that theyve already tested that and high speed counters are not accurate when it comes to reading slow pulses, is that true?? if so, any alternatives i can use? thanks.

What is the highest and lowest pulse frequency you wish to measure?
I would use either the PRV or PRV2 instructions. These can be set for differing sampling times of 10mS, 100mSec or 1S depending on your application requirements.

Your engineer probably didn't explain himself very well. In terms of actually counting pulses the high speed counter will be just as accurate with high speed or low speed pulses. The difference is how you use the pulses when you make calculations. If you have a 'set time' and count how many pulses within the 'set time' then if you don't have many pulses your calculations can't be that accurate.

To calculate RPM from pulses you can:
How many pulses in a set time - accuracy is lost at slow pulse count.
How long from one pulse to next - accuracy is lost at high pulse speed.
How long from one count of pulses to the next and vary the number that you count.

Have a look at this thread:
http://www.plctalk.net/qanda/showthread.php?t=52823&highlight=time+pulses

Bryan

thanks a lot bitsnbytes and bryang, I could use the information you have given. It did help a lot especially the thread that you posted.