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Cascade PID.

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

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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: (153)	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: (264)	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: (264)
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5/6/2012 11:46 PM	Rate (0)
Martins_Eng Experienced Member Joined: 7/17/2010 Last visit: 2/17/2024 Posts: 184 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 (474 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: 9/9/2024 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: (153)	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: 2/17/2024 Posts: 184 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: (153)	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: 2/17/2024 Posts: 184 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 (426 Downloads)

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12/3/2012 9:53 AM	Rate (0)
metwally mustafa Silver Member Joined: 10/10/2010 Last visit: 10/25/2024 Posts: 307 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: 2/17/2024 Posts: 184 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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