Hike

Good day sir,

I am using S7-200 PID for the first time and i am facing problem in implementing it. My application is very simple and for testing of PID only i prepared a test program which is attached.

I have a pump with VFD operated. The purpose of pump is to maintain the setpoint pressure in the line. I have following inputs and outputs which i tested perfectly.

AIW0 ----- Feedback signal 0-10V for pressure (0-10 bar)

AQW2 ----- VFD speed signal (0-10V)

Q0.1---- VFD start command

Now before i start the PID i tested the pump with the analog signal AQW2 and it was runing ok. And also in the mean while i was able to read the correct pressure in the line at AIW0.

Then i started the PID.. i noticed that the PID gives running command approximately 40% to the VFD however the setpoint is never achived. Infact it maintains the pressure at around 1.5 or 1.6 bar when setpoint is feed 3.0 bar.

I can't understand this behaviour of PID. And once i put on auto-tune it behaves the same.

Can any one tell me what's wrong in the test program.

Your prompt response is highly appreciated.

Best regards,

Hike

Hello,

As we are blind of your process and situation of your

actuator, I assume that there is no problem from field

side. Such a case may come from improper selected gain

of controller based on process gain.There is a relationship

to find the process gain and also how select the controller

based on that, but I suggest a step wise increase of gain

and monitor the result.I also recommend to change the parameters

when loop is in manual to avoid rapid action on output of

controller.

If this is not your case, please update.

Best regards

Hamid Hosseini

Hike

Good day Mr. hdhosseini,

The process is very simple. it has only one pump and 1 pressure transmitter. I have configured the same system earlier on S7-1200 PLC and working fine.

Even if i have selected wrong gains then the PID should work properly on PID tune control panel.

Can you look at the program and tell me if there is any thing missing in it which is required to run the PID.

Be noted that if setpoint is 3.0 bar the PID brings the pressure to 1.5 bars and when i increase the setpoint to 4.0 bars the PID maintains the pressure at 2.2 bars.