You don't give us much to go on but I doubt you need a fast response.
Try
Gain 0.25 (400% Proportional band)
Reset (integral) 0.5 Repeats / minute (2 minutes / repeat)
No derivative
Is it stable at these settings?
Google Ziegler & Nichols or PID loop tuning, you will find lots of information.
Well right now i have these settings in the controller:
P= -350
I= .3
D= .010
but i'm not reaching my flow setpoint of 21000CFM is staying at 19000CFM aprox. I will try with your numbers and i will get back to you.
Jigonzal,
Please describe your application for us VFD driving a fan, fixed speed fan with a damper or what?
If it's a damper what type of drive changes the damper position?
What control system are you using for the PID? (Brand and model)
Do you know if the I value is in Repeats per Minute or is it Minutes per Repeat?
What is the range of your flow transmitter? (it should be the same as the input module range)
What is the PID controller output at at 19,000? (is it possible that a high limit is set?)
Do you need the flow rate to change quickly when you make a set-point change or is it OK if it takes a minute or two to reach the new setting?
Ditch the D value as Keith suggests you shouldn't need it, only very difficult loops require Derivative.
Try putting the loop in manual and check that you can actually get 21,000 as Keith is inferring.
Does the flow still oscillate when in manual?
In Manual
Raise and lower the output by a few % and let us know what the flow is at each output setting,
e.g. 60% = xxxx CFM, 70% = yyyyy CFM, that will give us a better idea what your gain setting should be.
When you make a step change in the output, how long does it take to reach the new flow rate, that will give us a better idea what the integral setting should be.
Flow control loops tend to be the easiest to tune because they respond so fast to a change.
Good Luck
Roy
