How would I setup PID for a NO valve, but have it start closed?

[adamruck2]

I have a control valve(spring loaded, air controlled) that is normally open (air to close). My PID tuning is such that there isn't much Kp at all, and that the Ki is doing most of the work.

It works great, except that at process startup the valve starts from wide open and slowly closes because of the Ki. I want the opposite, I want the valve to start closed and slowly open.

Ideally there would be a normally closed valve installed, but I'd like to see if I can address this with programming. Has anyone dealt with something similar, and if so could you point me in the right direction?

Thanks,

Adam

 

[adamruck2]

I think I have it figured out.

When the process starts I just tun on the software manual mode bit (.swm) and set the .SO to 100, and after a few seconds go back to auto mode.

It's not very elegant, but it seems to work.

 

[Mickey]

It's not very elegant, but it seems to work.

Not sure how you would define "elegant" but if it works use it.

Another method would be to manipulate the set-point so it closes the valve and then ramp the set-point to it's desired setting after the system starts.

 

[rupej]

That's how I've always done it. I agree, it seems like there must be a cleaner way but I've never figured one out.

 

[ASF]

You could always invert the output of the PID (so 0-100% gets output to the valve as 100-0%), and then reverse the action of the PID?

 

[Paullys50]

I think I have it figured out.

When the process starts I just tun on the software manual mode bit (.swm) and set the .SO to 100, and after a few seconds go back to auto mode.

It's not very elegant, but it seems to work.

You could always invert the output of the PID (so 0-100% gets output to the valve as 100-0%), and then reverse the action of the PID?

I've done a combo of this.

For fail open control valves I inverse the raw analog output signal, Estop by-passes this. I also manually set the output for 1 PLC scan then back to auto if I want to skip the ramping of a PID loop. Example, filling an empty tank to a level of 75%. Since the tank is empty, no need to wait for the PID loop to ramp the control valve to 100%. Just set it to 90% manually then dump to automatic control and let the PID loop do the rest. Can save some time especially when first starting up some systems.