Quick PID questions

saultgeorge

Lifetime Supporting Member
Join Date
Jul 2015
Location
Detroit
Posts
535
Hey folks. I am getting ready to light off a large AHU and I have a couple of PID questions that I need answers to right away. I have researched the A-B user manuals but it's easier to ask you, Ha.
First, let's say my process variable is a temperature and I'm going to control that temperature with a control variable that will increase or decrease the percentage that a gas valve is open to run a burner in the AHU. So in the configuration tab, do I set the control action at PV-SP or the other way? What does that action do as far as the operation of the PID? Is that an inverse action/reaction?
Secondly, I'm not using the derivative, so what's the basic get started settings for P and I? I have them at: P 5.0, I .0009. What do I change to either slow down the change in the valve percent or speed it up?
My settings are: Loop update time: 2 sec. DB value 5.0 with no zero crossing enabled, and no output bias. This is a PID in RS 5000 v. 20.04. Thanks for all your knowledge and your willingness to share.
 
Assuming that increasing the analog output (CV) to the valve increases the controlled temperature (PV), then you want (SP - PV). If your PV is too cold, it will be less than SP, thus (SP - PV), the error, will be positive. That will result in the loop increasing the CV, allowing more gas, and heating the air flow.

Since Allen-Bradley always uses proportional gain (not band), increasing the P term will have relatively larger impact on the CV for a given error, making it appear to speed it up.

The I term depends on which form of the equation is selected. If ISA, the units are minutes per repeat, meaning smaller values result in more integral action. Independent gains are different, where I is integral gain in inverse seconds. In this case a larger I value is more integral action.
 

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