2/7/2011 2:17 PM  
Joined: 1/7/2011 Last visit: 3/28/2024 Posts: 2 Rating: (0) 
Dear All, I've got a question concerning the intergral calculation of the PID control loops. I'm using a PID controller (S7300 & FB41) to control a very slow T° regulation. It takes more than8 hours for the system to go from ambiant t° (22°C) to the setpoint (37°C) I'm using a PID controler so, when the PV is lower than the SP, the value of integral calculation grows and become to be huge=> it leads to . Here are my parameters: Gain = 0.5 Intergal time = 18000 s (5 Hours) (value based on a step response) Derivative time = 600 s My question is : Is it possible to limit the integral calculation at a specific period in the past? For example, if my PID is on since 10 hours, I'd like that the integral calculation only "remember" the last 6 hours for example and "forgot" what happens before. Is this parameter existing in S7 PID's?? I don't see anything in the documentation and thus I assume that the integral calculation is based on the value since the start up of the PID. Now i'm just emptying the integral "memory" by resetting the PID when reaching the SP. Thanks for your answers and/or comments. Bastien 
2/7/2011 4:00 PM  
Joined: 1/28/2009 Last visit: 9/7/2024 Posts: 6846 Rating: (1365) 
For Derivative mode you need to set values for 2 parameters: TD and TM_LAG Check the STEP7 online help for this function to how select these 2 values.
Check this link for integral properties: As illustrated below, integration value is like accumulation of some values over a period of time.The most important property of integration which used mostly in Calculus is Additivity.So every an hour save LMN_I which is saved in the corresponding DB of the function block (DB41.DBD84) in my program.So I think you need some memory area to save result of integration. If you want to get rid of old integration value, subtract an old value with a new one and save in I_ITLVAL , whenever you want , you can insert this value to active integration value by I_ITL_ON.
BR 
Last edited by: hdhosseini at: 2/7/2011 5:22 PM 

2/7/2011 4:48 PM  
Joined: 1/7/2011 Last visit: 3/28/2024 Posts: 2 Rating: (0) 
Dear hdhosseini, Thank you for this tip, I'll try this asap. best regards Bastien 
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2/8/2011 7:14 PM  
Joined: 1/28/2009 Last visit: 9/7/2024 Posts: 6846 Rating: (1365) 
Dear bma@artelis.be Also check this link for Step response to PI controller and also to get a sense of how integration time effects the LMN value. PID CONT_C FB41And also check the attachment for step response of casual diff function.BR 
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