Flow control valve PID tuning

[ELake20]

Hi guys,

Need some help.

Goal:

Closed loop flow control of valve positioner to a flow value.

Parts:

I've got a K series Rotork valve with SIPART PS2 valve positioner (I/P) that accepts a 4-20mA command.

I can send the valve a command and it responds so no problems there.

My problem is when I attempt to enter closed loop. When PV = SP the output is reduced to 4mA so the valve then closes. Once the deviation is sensed the process repeats itself.

I am using the base PID instruction for AB CLX.

Am I missing something really simple here or is this not the correct application of this?

I have considered using the PIDE instruction but I don't see what I would gain.

I have poured over the manuals and cannot seem to find any resolution. I'm sure this has come up before?

Thanks in advance

 

[widelto]

Check your PID equation must be : E = SP - PV.

 

[Peter Nachtwey]

You don't seem to be using the integrator.

The integrator term should keep the valve open to supply flow when the PV=SP.

 

[kamenges]

Your proportional gain may need a look too. Assuming you have some continuous flow requirement I wouldn't expect you to be able to get to the point of PV=SP without an integral term unless you proportional gain is so high that you are oscillating between (almost) full open and closed.

Keith

 

[ELake20]

Check your PID equation must be : E = SP - PV.

I changed the configuration to be a derivative of Error rather than PV

edit: read the manual and will probably want to stick with Derivative of PV instead of error...

Control action
Select either E=PV-SP or E=SP-PV for the control action (.CA).

 

[widelto]

Please show us P,I and D values.

 

[MrAnonEMoose]

Does the valve fail-open, or fail-closed? In other words, on loss of air, does the valve open, or close?

 

[ELake20]

Loss of power valve defaults closed
4mA = fully closed
20mA = fully open

The 'I' term Peter suggested using did the trick. The CV no longer drops to zero so that is great.

I think the loop is still a long ways off.

The actuator is very slow, probably 30 seconds open/close. The guy who commissioned that set it up like this probably to eliminate wear/tear.

I'm thinking I want to scale the SP to something bigger. The operators are able to input 0-5, they like million gallons per day (water plant) but behind the scenes I may change it to GPM so the error amount has a little more resolution.

 

[ndzied1]

ELake20, did you ever settle in on a good system? I'm trying to help one our our guys in the field with a rotork on a butterfly valve and having a hard time of it.

Did you use any deadband in your system?

Sounds like our valve is much smaller than yours. We only have a 2" valve.

Thanks,