Schneider Concept - Reverse Action with PID1 Block

Hi,

I'm currently trying to do a simple reverse acting PID control of a pressure control valve in Schneider Concept however am having a few issues.

The control valve is installed between supply and return lines, such that when the valve opens fluid is allowed to bypass the system and return to the source (tank), thus reducing pressure.

The problem I am having is Schneider Concept (Modicon PLC) does not seem to have a proper reverse acting function for PID control.

The help guide states:
"A reversed behavior of the controller can be achieved by reversing the sign on GAIN. A positive value on GAIN causes the increase of the output value, for a positive disturbance value. A negative value on gain causes the decrease of the output value, for a positive disturbance value."

I tried this however it does not achieve the same effect - a gain of 0 will result in the valve being fully closed, where it should be fully open to release all pressure. A small increase in gain should slightly close the valve, however a small gain will only slightly open the valve.

I have also tried swapping SP and PV around and it achieves the desired effect only with gain, however I am not sure what effect it will have on integral and derivative.

Could anyone please advise whether the swapping SP/PV approach is valid or is there a proper way to do this in Concept?

Many thanks,
John

So, did you try this, when gain have negative value?

If you put gain to zero, then you only have I or ID-control without P-term. (direct PID-control)

Another way is to use positive gain and subtract PID-output from maximum control value


valve_control = 100%- PID_output

=>

PID-output 0% => valve 100% open

PID-output 100% => valve closed (0%)


Some control valves have also setting, where you can select if 4mA or 20mA is fully close or open

Thanks for your help Lare.

Thought I would post a quick reply for others.

I ended up using 100-output method and left the valve as 4-20mA 0-100%.

I also had to invert the following variables:
YMax = 100 - Min
YMin = 100 - Max
YMAN = 100 - Manual Setpoint

Gain, TI and TD were not manipulated any further.

Cheers,
John