PID problem

[Vidmas30]

Good day,

Once again I'm with question about PID.. Yesterday was testing object, where I have 3 PID loops: 1loop for temperature control and 2 loops for pressure. For pressure loops done autotuning, then changed some parameters by hand and it goes just fine and stable. But I got problem with temperature control loop:

there's steam boilers with valves that heating water. I'm controlling valves to hold water temperature (SP). Theres big innertion on system, when valve opens, water temperature starts to rise, when reaches SP, valve starts to close, but water temperature still rises and overshoots about 10-12C (you know what I'm talking about). When I run autotune, it gives me such parameters: P=-0,1%, I=-1s, D=2s. With these parameters PID calculates CV 38% (valve position) and CV doesn't change anymore.. No matter that I tried to autotune several times with different setpoints it gives me same results.

When I tryed to tune PID by hand at one setpoint (70C) it began to control temperature almost stable (+-0.5C) and valve gets at position (stable) and if necessary opens and closes by some %. but when i changed SP by 5C (65C) it reacted fast enough and stabilized again, but with error (about 1,5C) (66.5). Then I schanged SP by 5C again (60C) and again it reacted fast enough but error rised and was about 3C (63C). Then I tryed to change PID parameter to eliminate this statical error, but it gone wrong. PID gives me "integral wind up" or "integral wind down" messages and I couldn't get to previous state anymore. Now PID regulates valve in such way: when temperature is droping and reaches SP (t=sp) it begins to open valve, temperature still falls, valve opens. When temperature stop dropping and begins to rise it begins to close valve. And when temperature reaches setpoint, CV becomes about 0% (but valve is not fast enough) and temperature overshoots about 1.5C. And it repeats over and aver again. I couldnt make valve to stabilize and not to oscilate about SP. Spent half night trying, but nothing helped. Tryed different tuning methods, but no good. PID reacts different ways. When seems to stabilize about one SP, then I change SP and once again: oscilations and not stable system. Maybe someone had such issue? I need slow PID: it apons valve by couple %, waits, then opens more if theres need or closes by couple and so on. Is there anyway to do so?

PV 0..100.0C (SP 60..90C)

CV 0..100%

Thank you!

[Vidmas30]

Hello again,

Yesterday went to the object to play with above mentioned PID. Used PID server and now I understand why Emil likes it.. Great tool! By the way PID server's autotune gave better results than PLC's. After AT it gave P=1000 (100%) I=130s, D=34s. Then after some experiments (the PID was veeery slow) I came to P=90%, I=25s, D=37s. Results in pictures. What do you think? PID not tuned

[nagaraj]

I was going to suggest a high value for P too. I have never used PID server. How is it better than auto-tune or manual tune. I have never been a fan of auto-tune anyways.

[Vidmas30]

Hi,

Like Emil said, during autotune system gathers some essential data about process and gives a good starting point to manual tuning. So, I tried all methods (plc's (vision) autotune, pid server's autotune and end up with manual tuning) and I am quiet happy with the results! It's worth to try just for knowing what it is..

Thank you for your oppinion!