Calculating Distance Using Velocity Command

[marshg]

I have a hypothetical question. Lets say that I have a project where I have a motor drive that sends a 0-10V signal to an Allen-Bradley PLC that represents the motor's speed (0 - 4000 RPMs). I'm not planning on using an encoder or a high speed proximity sensor w/ a high speed input card (unless there is no other way).
My plan:
1. Scale the 0-10V signal to figure out motor RPMs
2. Divide RPMs by 60 to figure RPS (Revolutions/Second)
3. Calculate the time for 1 motor revolution
4. Move this calculation into a Timer Preset
5. Use the Timer Done bit to increment a counter

Now, this looks like it may work untill you take the PLC's scan time into account. If the motor is going 4000RPM, the time it takes for 1 revolution to occur is 15ms. Is there anyway I would be able to accurately keep track of the number of motor revolutions using this method?

 

[jacekd]

Could you define your perception of "accurate" ?
If your scan time would be 5 ms then you have ca. 30% error (in best case) without taking other factors into account.

Regards,

Jacek

Do it right or don't do it at all.