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Cascade PID.

Created by: Martins_Eng at: 4/30/2012 4:26 PM (26 Replies)

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4/30/2012 4:26 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Lords. It is possible and if so how and made a PID cascade in the Siemens S7-300 using the FB41 Thank you.

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4/30/2012 4:32 PM	Rate (0)
Aret Posts: 2348 Rating: (265)	Yes, it's possible by connecting 1st regulator output to 2nd regulator input. Please use search feature before posting a request. You can look on for example /tf/WW/en/Posts/71187 /tf/WW/en/Posts/38430
Aret Posts: 2348 Rating: (265)	Last edited by: Aret at: 4/30/2012 4:37 PM
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4/30/2012 4:35 PM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 8/31/2025 Posts: 6874 Rating: (1368)	Martins_Eng Lords. It is possible and if so how and made a PID cascade in the Siemens S7-300 using the FB41 Thank you. Dear Martins_Eng, You will find a great deal of valuable information if you search the forum.Check the following thread for a simple explanation provided by "dchartier". (Cascade_loops.jpg ) Cascading PID loops Best regards, Hamid Hosseini
	Demo Channel on Youtube
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This contribution was helpful to 1 thankful Users Ton Verra

4/30/2012 5:39 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Gentlemen, in my case I control the index of Ph, where Ph I have a meter and a flow meter acid and a control valve for metering the acid. Ie have two Variables and the flow of reading and Ph output and control that had served the valve. I read the documentation but in the PLC ladder and mounted as a logical type of LMN will Fb41 FB41 to Sp 2 and in turn the LMN of the 2nd goes to the valve and the SP is the first being the desired pH value? They can assist in the implementation of how to mount the ladder to the PLC cascade PID i
	Last edited by: Martins_Eng at: 4/30/2012 10:40 PM They can assist in the implementation of how to mount the ladder to the PLC cascade PID i
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5/1/2012 2:58 PM	Rate (0)
Aret Posts: 2348 Rating: (265)	Hello again! Let's examine it thoroughly (seems i miss something) - you have 1 control element - valve, so the only thing you can influence is flow of concentrated acid. Now if your concentration would be constant you would need only 1 loop - input variable = current pH, manipulated = % of valve openness. Next - your acid can have different pH and thats why you want to make cascade?
Aret Posts: 2348 Rating: (265)
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5/1/2012 4:23 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Aret Hello again! Let's examine it thoroughly (seems i miss something) - you have 1 control element - valve, so the only thing you can influence is flow of concentrated acid. Now if your concentration would be constant you would need only 1 loop - input variable = current pH, manipulated = % of valve openness. Next - your acid can have different pH and thats why you want to make cascade? The reason to do cascade, and because the concentration as a constant-water flow, or enter the water and acid also that the pH varies I intended using a pH meter and the external loop flowmeter acid in the loop with domestic output to the valve that will dispense the acid. So the castata, but I was in doubt how to assemble the logic in the PLC using the FB41, the principle of cascade PID I know but I saw the application of the same with FB41

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5/3/2012 7:20 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	Hmmm, could you describe your object detailed? Just how i see it: 1) If you acid flow has always pH much less then pH of your tank (i.e. inflow contains stronger acid) then you could operate variable 'amount of acid' = inflow_pH*Flow. Thus you have simple PID without cascade. 2) If 1) is not true then how your system shall react in situation of inflow_pH > tank_pH (i.e. acid in the tank is stronger) - close valve to 0%? 3) If inflow_pH is slightly less then tank_pH then what's reaction time you have? Do you have some sort of mixer inside or?
Aret Posts: 2348 Rating: (265)
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5/3/2012 1:59 PM	Rate (0)
Lpmspa Advanced Member Joined: 2/8/2007 Last visit: 8/27/2025 Posts: 12 Rating: (10)	i have make a library for a PID cascade look in my web site... blog.ranghetto.com

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5/3/2012 3:44 PM	Rate (0)
Aret Posts: 2348 Rating: (265)	RangO, i guess Martin has problems on choosing control loops, not on implementation itself. Although i can be wrong
Aret Posts: 2348 Rating: (265)
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5/3/2012 10:17 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Aret RangO, i guess Martin has problems on choosing control loops, not on implementation itself. Although i can be wrong Lords. My doubts were not about the execution of the PID but the assembly of the waterfall, but I have to use two Fb41 to make the mesh internal and external to the PLC My logic and the following: SP process Ph PV main and the water pH. Once you have a flow meter for the acid, So my process control valve The logic is being: SP1 = PH process (Operator) PV1 = PH Transmitter (4-20mA) LMN1 = SP2 PV2 = Flowmeter (4-20mA) LMN2 = Valve control (4-20mA)

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5/4/2012 9:05 AM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	SP1 = PH process (Operator) PV1 = PH Transmitter (4-20mA) LMN1 = SP2 PV2 = Flowmeter (4-20mA) LMN2 = Valve control (4-20mA) Yes this is a cascade loop implementation, the bad news is that it will never work and is it not clear what you want to regulate or maybe you're wrong about using cascade loop. I guess you might need a ratio loop, not the cascade. You can't mix apples and potatoes,it's not clear what you regulate. If your valve dispenses acid, then this is wrong setup. Ifvalve controls flow,thenyou dont needa cascade.

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5/4/2012 6:59 PM	Rate (0)
Aret Posts: 2348 Rating: (265)	Martins, could you please post here screenshot of your WinCC project (if any exists) to let us see the object - whats on inlet, whats on outlet, where sensors are installed and what their typical ranges. This info is REQUIRED to make adequate advise, otherwise we can just mislead you. Only 1 more thought - maybe you can control pH of your acid in another part of plant - for example if you make it out of water and dry acid powder....
Aret Posts: 2348 Rating: (265)
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5/6/2012 11:46 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Aret Martins, could you please post here screenshot of your WinCC project (if any exists) to let us see the object - whats on inlet, whats on outlet, where sensors are installed and what their typical ranges. This info is REQUIRED to make adequate advise, otherwise we can just mislead you. Only 1 more thought - maybe you can control pH of your acid in another part of plant - for example if you make it out of water and dry acid powder.... Gentlemen, look at the basic design of my application Attachment Cicle.JPG (623 Downloads)
	Last edited by: Martins_Eng at: 5/6/2012 11:49 PM
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5/7/2012 11:31 AM	Rate (0)
Lpmspa Advanced Member Joined: 2/8/2007 Last visit: 8/27/2025 Posts: 12 Rating: (10)	Hello Martin, I have seen the block diagram and I believe correct. My idea of implementation is: the first loop reads the pH and determines the flow the second loop regulates the flow. add a feed forward control type to the first loop by inserting a noise value. using FB41: first loop IN: PV = pH measurament LNM_HI = Max flow ( for ex in l/h) LMN_LO = minimum Flow (or 0) SP = the pH you need DISV = the flow your system needed calcolated in theory or 0 OUT LMN= The flow (for ex in l/h) second loop IN: PV = Flow from flowmeters LMN_HI = 100% LMN_LO = 0% SP = from first loop OUT LMN = to the valve i think the good cycle_time first loop is approx. 10-20s second loop is approx. 2-5s

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5/9/2012 10:42 AM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	OK, now I see. You will regulate the flow of acid, my only thought is that the pH loop can have a large dead time, because the water must recirculate trough 1st tank to 2nd before acid reaches pH sensor. IMO would be better to move pH sensor on 1st tank, so you will get a faster response that means larger loop gain is possible, means faster regulation and less trouble caused by dead time. Forget the PID if you have dead time.

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5/9/2012 2:37 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Hello I have some dead time. but I can not change the sensor due to the process itself which Which have to ensure a difference in pH between the boxes and the first and second neutralization of the acid and process control. However, the dead time and relatively small.

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5/9/2012 11:19 PM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	You didn't mention about a neutralization before, so you need to explain further. Maybe you will need a third loop and extra pH meter if you want a stable loop. The gold rule of designing cascade loops is to add a regulator for each section as soon as possible. If a third process takes place like a neutralization, then add a new pH meter inorder to control pH in 1st box as quick as possible.

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5/11/2012 6:34 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Yes and a good idea to put the control neutralization will apply this theory, but before hand follows the assembly program as an example of the cascade, see if this coherent extruded. Attachment pH_PID.zip (573 Downloads)

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12/3/2012 9:53 AM	Rate (0)
metwally mustafa Silver Member Joined: 10/10/2010 Last visit: 7/5/2025 Posts: 309 Rating: (15)	thank you experts for this compeletly helpful support

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12/3/2012 3:49 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Colleague what you want to adjust the boiler. It may be necessary to control other I think the feedforward control is more appropriate. But more comment about what you want to calculate

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12/3/2012 3:50 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 7/21/2025 Posts: 188 Rating: (0)	Marko Bursic You didn't mention about a neutralization before, so you need to explain further. Maybe you will need a third loop and extra pH meter if you want a stable loop. The gold rule of designing cascade loops is to add a regulator for each section as soon as possible. If a third process takes place like a neutralization, then add a new pH meter inorder to control pH in 1st box as quick as possible. Colleague what you want to adjust the boiler. It may be necessary to control other I think the feedforward control is more appropriate. But more comment about what you want to calculate

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12/4/2012 10:57 AM	Rate (0)
Marko Bursic Platinum Member Joined: 9/8/2009 Last visit: 7/17/2023 Posts: 1410 Rating: (154)	Could you explain what kind of feedforward did you use? What was the final version of your regulation loop?

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12/5/2012 2:50 AM	Rate (0)
OnoMas Posts: 43 Rating: (0)	RangO i have make a library for a PID cascade look in my web site... blog.ranghetto.com Mr. RangO.. before I read your post I have opened your blog.. but for HMI I cannot opened due to my winCC 2008 flex SP2..... I think you made it in SP3, right?. Could you convert it all your HMI form in SP2... I am still downloading it need 2 days to do so..... Thank you regard
OnoMas Posts: 43 Rating: (0)	Last edited by: O_Moderator at: 12/6/2012 10:44 AM contact data deleted
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7/10/2013 6:13 PM	Rate (0)
pk2_kdh Posts: 54 Rating: (3)	IS IT only only possible to control one single output variable through two no of PID controls.
pk2_kdh Posts: 54 Rating: (3)
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7/10/2013 6:17 PM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 8/31/2025 Posts: 6874 Rating: (1368)	pk2_kdh IS IT only only possible to control one single output variable through two no of PID controls. Hello pk2_kdh, if you mean what is the basic structure of cascade controller, it contains: 2 PID controllers blocks 2 feedback data from process One setpoint from panel because the other is generted internally One final element to regulate also check: Need help in Cascade control method... And final note:While you regulate only one valve, processes will internally interact. Hope that helps Hamid Hosseini
	Last edited by: hdhosseini at: 7/10/2013 6:21 PM Last edited by: hdhosseini at: 7/10/2013 6:19 PM Demo Channel on Youtube
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7/10/2013 6:34 PM	Rate (0)
pk2_kdh Posts: 54 Rating: (3)	can u provide me a sample program back up of cascade pid control?
pk2_kdh Posts: 54 Rating: (3)
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7/10/2013 8:29 PM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 8/31/2025 Posts: 6874 Rating: (1368)	pk2_kdh can u provide me a sample program back up of cascade pid control? Hello again, Of course that is possible: Attachment:Cascade.zip (132 Downloads) and the snapshots: Attachment:result.zip (96 Downloads) Provide what kind of process you want to control, to see if this structure is helping. Best regards Hamid Hosseini
	Demo Channel on Youtube
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This contribution was helpful to 1 thankful Users metwally mustafa

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