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PID tuning

Created by: Clau at: 10/10/2007 10:37 AM (15 Replies)

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10/10/2007 10:37 AM	Rate (0)
Clau Posts: 28 Rating: (0)	Hello, Can anyone tell me how to adjust the parameters (gain, time constants, lag time,...)of a PID controller for a very slow process. In my process Ineed to control a temperature and for this reason I have to act on a valve. The problem is when the temperature is high, the PID controller acts on the valve, trying to open it, but the process is very slowly and even if the PID tries to open the valve in order to decrease the temperature, the temperature is still increasingfor some time and a certain limit is exceeded, crushing the entire process. Can anyone, please, give me an advice? Thanks Clau
Clau Posts: 28 Rating: (0)
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10/10/2007 12:29 PM	Rate (0)
Liftaren Posts: 21 Rating: (2)	If you want the PID to act faster, you can adjust the values for P (gain) and integration time (I). The deadband (D) I never use, so there I am lost if you are using it. Don't know how much you know about regulators, but think of it like a car: The gain is the accelerator and the integration is how fast you are pressing harder on the accelerator. So by increasing the gain you put more power into the regulator and by decreasing the integrationtime you give the regulator more and more power faster. A word of warning though! Do not increase the gain too much (normally you can keep it lower than 1) since this will cause the regulator to start working like a clock pendulum.
Liftaren Posts: 21 Rating: (2)
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10/10/2007 1:09 PM	Rate (0)
Clau Posts: 28 Rating: (0)	Hello, The problem with my PID controller is that it never stabilizes at a certain value. I always have some oscillations +/- X%. For example if I put a set point of 100^oC, the temperature will oscillate between 94^oC and 105^oC and this is not goodfor the process. Clau
Clau Posts: 28 Rating: (0)
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10/10/2007 5:39 PM	Rate (0)
Denilson Pegaia Diamond Member Joined: 9/23/2005 Last visit: 12/31/2025 Posts: 4435 Rating: (1487)	Hi I worte a FAQ about the top, but it was not jet published. Follow there is a perliminary version. Attachment PID tunning.zip (3634 Downloads)
	Denilson Pegaia Siemens Industry Sector Technical Support
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This contribution was helpful to 2 thankful Users m@nju AutoGen_376035

10/10/2007 5:45 PM	Rate (0)
Denilson Pegaia Diamond Member Joined: 9/23/2005 Last visit: 12/31/2025 Posts: 4435 Rating: (1487)	Hi, one TIP: with only P control,PV should doesn't exced the SP.
	Last edited by: Pegaia at: 10.10.2007 17:45 Denilson Pegaia Siemens Industry Sector Technical Support
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10/11/2007 10:48 AM	Rate (0)
hambar Posts: 12 Rating: (1)	Hello, My coleg is home now and I rested on site. I read your text. I will use it today. The problem is that my process have a large time delay (dead time) and it is not the same if the process is movig one direction or the other. One othe issue I considered is to use a Smith algoritm.(acordinf to theory this is the best solution) As regarding the use of the proportional signal alone, I belive I can use itas last instance because I have to create a function K= f(SP) and also, I am afraid that disurbances can be amplifiedif using it alone.
hambar Posts: 12 Rating: (1)
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10/11/2007 4:59 PM	Rate (0)
Denilson Pegaia Diamond Member Joined: 9/23/2005 Last visit: 12/31/2025 Posts: 4435 Rating: (1487)	Hi, maybe you can use a FB for temperature (FB58, for example) and set a control zone to aprimorate the regulation.
	Last edited by: Pegaia at: 11.10.2007 17:04 Denilson Pegaia Siemens Industry Sector Technical Support
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10/15/2007 7:30 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	As far as i remember big lag time (deadtime at which PV doesn't react on output change) is one of the bigest problems with PID regulators. If it has same order as process time it's very hard to get satifying regulator....Soit'saproblemofideaofPIDregulation,notSiemensimplementationofPIDandnot yourGain,TiandTdsettings... PS. I may be wrong, but try to decrease Gain value(leavingTdandTi) - think it should be small for lagged process...
Aret Posts: 2348 Rating: (265)
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10/15/2007 3:46 PM	Rate (0)
hambar Posts: 12 Rating: (1)	Hi, After 4 days the results are the folowing: - using a Smith predictor is a must in such systems. And it realy works. The bad thing is not geting the rigth rising time, or dead time(I just hoped that what I got was rigth) - the parameters of PID must be adaptive.
hambar Posts: 12 Rating: (1)
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10/16/2007 5:52 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	With which method did you get adaptive PID parameters?
Aret Posts: 2348 Rating: (265)
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