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PID Loops

Created by: SPV at: 6/18/2009 2:52 PM (9 Replies)

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6/18/2009 2:52 PM	Rate (0)
SPV Posts: 61 Rating: (4)	Dear All, Can u please specify is there any data sheet where in i can find the number of PID loops that as supported by different PLC'sactually the reason is i have heard that there is a limitation on the number of Loops that can be programmed in the cpu henforth i wanted a clarification on this.
SPV Posts: 61 Rating: (4)
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6/18/2009 3:02 PM	Rate (0)
IBN-Service Posts: 8946 Rating: (1000)	SPV Dear All, Can u please specify is there any data sheet where in i can find the number of PID loops that as supported by different PLC'sactually the reason is i have heard that there is a limitation on the number of Loops that can be programmed in the cpu henforth i wanted a clarification on this. The limit is depending from the size of the PLC-Memory and also from the cycle time, that is allowed for the prozess. Also, any call need a DB, so that the maximum amount of DB is also a possible limit. Anyway, there should be a lot of possible PID - calls inside a CPU300, mutch more inside a CPU400.
IBN-Service Posts: 8946 Rating: (1000)
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6/18/2009 3:35 PM	Rate (0)
AMAZINGAHMED Posts: 719 Rating: (88)	IBN-Service Anyway, there should be a lot of possible PID - calls inside a CPU300, mutch more inside a CPU400. How do you know?
AMAZINGAHMED Posts: 719 Rating: (88)
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6/18/2009 3:41 PM	Rate (0)
IBN-Service Posts: 8946 Rating: (1000)	AMAZINGAHMED IBN-Service wrote: Anyway, there should be a lot of possible PID - calls inside a CPU300, mutch more inside a CPU400. How do you know? Usually, the 400 has larger memory, is faster and can handle more DB.
IBN-Service Posts: 8946 Rating: (1000)
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6/18/2009 4:19 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 12/24/2025 Posts: 12472 Rating: (2722)	Hello SPV; Thesmallest Profibus-DP master available on the S7-300 family, the S7-313C-2DP, has integrated work memory of 32 kB. We managed to setup control for a plastic web machine, including feeder control, tension control and 25 heating zones all controlled with SFC 58 (PID temperature control). It did require some program optimization techniques such as STL programming, multiinstance DBs, indirect addressingand such, but last I heard the little CPU was still working well after 6 years. So yes, the number of loops available will depend mostly on the available work memory and how you set it up. Hope this helps, Daniel Chartier

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6/19/2009 10:48 AM	Rate (0)
Enigma Posts: 49 Rating: (18)	Hi SPV, PID LOOPS... hmmmm... i dont know whether its the limitation, but this is definitely one of the factor for deciding the AS system for the application. I searched some manuals and found this data for you, butthis data is released considering that AS shall be used for PCS7 only. There is no such document for AS system which shall be used with Step7.... But its true as Dchartier has rightly mentioned, it is completely dependant on the type of application and engineering methods you shall opt for. would you like to share your application with us??? regards, Enigma
Enigma Posts: 49 Rating: (18)
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6/19/2009 11:41 AM	Rate (0)
mumbles Posts: 160 Rating: (16)	SPV Dear All, Can u please specify is there any data sheet where in i can find the number of PID loops that as supported by different PLC'sactually the reason is i have heard that there is a limitation on the number of Loops that can be programmed in the cpu henforth i wanted a clarification on this. Hi SPV If you have access to a great book from Siemens titled "Controlling With Simatic" this would help you answer your question. It goes through the steps required to calculating your processing requirements. Speak to your local Siemens rep and perhaps they can provide one to you. Hope this helps you.
mumbles Posts: 160 Rating: (16)
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6/19/2009 11:59 AM	Rate (0)
Navnag Platinum Member Joined: 11/21/2007 Last visit: 12/8/2025 Posts: 2326 Rating: (309)	Hi I had this chart for Old version S7-400 CPUs, It can be updated with the latest CPUs Data. This gives some indication of CPU Cycle time and Memory Consumption etc, based on which you can take your decisions for No. of PID Loops. Regards Navnag Attachment S7400_PID.zip (742 Downloads)
	There is no such thing as the last bug in a program There are no stubborn bugs, There are only stubborn Debuggers.
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6/29/2009 1:45 PM	Rate (0)
Enigma Posts: 49 Rating: (18)	Enigma i dont know whether its the limitation, but this is definitely one of the factor for deciding the AS system for the application. I searched some manuals and found this data for you, butthis data is released considering that AS shall be used for PCS7 only. There is no such document for AS system which shall be used with Step7.... Dear SPV, I feel very shameful but I must tell you that.................I forgot to attach the file in first post .....but regarding this mentioned data please find attached file...... Hope this data would help you!! Regards, Enigma Attachment AS Comparisons .pdf (792 Downloads)
Enigma Posts: 49 Rating: (18)
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This contribution was helpful to 2 thankful Users dchartier Lucky123

7/8/2009 11:04 PM	Rate (0)
Pandiani Silver Member Joined: 10/31/2005 Last visit: 4/22/2025 Posts: 366 Rating: (31)	What is also important is so called dominant tyme constant of your process that you want to control. In some system time constants are lagre, while in others, time constants are small (less than 1 sec). Goal is to have uniformly loaded cycle time which can be obtained by carefully planning PID loops. If process' time constant is large (say 10 or 20 sec), then there is not much sense to call PID in OB35 block which is executed every 100 ms by default. If you put a lot of PIDs in frequently called OB then there is a possibility that normal cyclic program can hardly be processed any further. So answer to your question is that this really depends on time constants of the processes you want to control. Cheers!

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