Mixer Speed Control

We just recieved a mixing system driven by a PF 70, and controlled by a PID loop in the (CL 5561) processor. The mixer speed is measured via prox input to a 1756-HSC/A card.

-There are 12 pulses per revolution
-The HSC is in frequency mode, with a sample time of 1 second
-The PID loop update time is 5 msec

Shouldn't the PID update time match the HSC sample time?

Also, based on this, the best resolution I can get is 5 rpm (60 sec/min)/(12 pulses per rev)

I'm trying to improve the speed control if possible, it's not terrible but it does bounce around some and I'm wondering if improving the resolution will help. I'm going to increase the sample time of the HSC, any danger in using a longer sample and loop update time?

Thanks for any advice,

Rob

If the VFD driven motor is directly controlling the mixer speed, I am not sure why a PID would be required. You should be able to get the final driven element under control without a PID.

What is between the motor output shaft and the mixer speed sensor that introduces a variable (hence calls for a PID instruction)?