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Problem with tuning PID_S, PV remains 30-35 degrees above SP which parameters to change
Created by: Oreca at: 1/9/2012 11:37 AM (3 Replies)
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| 1/9/2012 11:37 AM | |
Joined: 3/14/2008 Last visit: 11/18/2024 Posts: 2132 Rating:
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Hello, I have a PID_S control with temperature as PV_IN and a slow-acting actuator that controls an air valve to let air into a burner. The gasflow/pressure is controlled by an air/gas actuator. The problem i'm having is that the PID is unstable. If I have SP=200, the temperature goes at least 40 degrees above SP and when it reaches PV, then the temperature gets too low, also a large overshoot. I now managed somehow to have a steady signal, but now the SP remains 30-35 degrees above setpoint. If I increase the SP, then the overshoot begins again. According to the PID tuner, I have a TI of 10460s, a gain of -1530 (I changed it now to 2, because I don't think that was a good value) and I have a Td of 5s, with a lag time of 0.1s (both are needed to enable D-logic I believe). Can somebody tell me how which parameters I have to change so that the PV goes to SP? |
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If I could help you, you can use the RATE button. Thank you. |
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| 1/9/2012 7:42 PM | |
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Posts: 2661 Rating:
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Hi Oreca, Have you tried FB 59 “TCONT_S”? You’ll find the manual “PID Temperature Control” in the PCS7 documentation.
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| 1/10/2012 9:00 AM | |
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Joined: 3/14/2008 Last visit: 11/18/2024 Posts: 2132 Rating:
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Hello Phantom, I don't want to use it, as I want to control it in HMI (using pcs7 7.1) like all the rest of the PID controllers. But I think I found the solution... I set the Ti to 100s instead of 10460s. Just a guess. When I changed the Ti, I found that the PV was going down to SP level. That is what I needed. Not sure why the I-action is responsible for the PV to stay 30 degrees above SP. I also had a problem when going from lets say 100 to 200 degrees, the PV always overshoots more than 50 degrees. This because the actuator is slow. What I did was changing the setpoint ramp limits from default (100 degrees/s) to 0.36 degrees/s. The value is the time it takes too increase from setpoint A to setpoint B while opening the valve to 100%. The PV doesn't overshoot anymore and now I have a very stable controller with 5 degrees offset maximum. One question: Why do you wanted me to use the TCONT_S block? PID_S is the same? |
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If I could help you, you can use the RATE button. Thank you. |
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