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Step 7 PID controller CONT_S, Feed Forward

Created by: mister rbt at: 2/7/2008 11:56 AM (10 Replies)

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2/7/2008 11:56 AM	Rate (0)
mister rbt Posts: 9 Rating: (1)	Hello, I'm want touse aPID controller (CONT_S, FB42) to heat a tank with warm (heated) water. A solenoid valve (open/close) is used to control the waterflow. Now i'm using one PT100 in the return of the water to control my proces. Now somebody advised me to messure also the temp in the supply of warm water and use Feed forward control. My question now: what is the influence of the DISV parameter input with CONT_S (FB42) PID Controller when i connect the new PT100 to it.Setpoint temp is aprox 70 Degr. thanks, Richard
mister rbt Posts: 9 Rating: (1)
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2/8/2008 10:01 AM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Mister rbt, I cannot really imagine how this control work. You say you have a heated water tank. You have one water outlet and you want to control temperature of that water, right? How do you heat tank? I assume you use hot water, but don't understand if the hot water is mixing in the tank, and it comesthrough inlet pipe or what? It would be good if you could attach some kind of sketch of this process control scheme.

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2/8/2008 10:26 AM	Rate (0)
mister rbt Posts: 9 Rating: (1)	Hello, The principal is: A tank is heated (with warm water). There is a water inlet and outlet for water.Inside this tank it's a systemof pipes so that (Closed system with inlet and outlet)there can flow product thrue the pipes and get heated by the warm water. There are two seperate systems. ik hope that this clarifies it greetings Richard
mister rbt Posts: 9 Rating: (1)
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2/8/2008 12:47 PM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Mister rbt, is this a process scheme you're refering to? If I understood correctly process variable you're trying to control is temperature of water that is entering from the left on process scheme(A-B). Water that flows through pipes inside tank receive heat from the tank and in that way it rises its temperature. You have a solenoid valve on hot water supply inlet to the tank and you want to place there PT100 to use it as DISV input to PID block? Attachment Process Scheme.pdf (772 Downloads)

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2/8/2008 2:09 PM	Rate (0)
Aret Posts: 2348 Rating: (264)	As i understood you have simple heat exchanger.(Correctmeifi'mwrong) System 2 separate loops - one with some product (cold water), 2 - closed loop with hot water with solenoid valve (are you sure about this? Guess if valve has only 2 states - it's ordinary valve or?). Sochangingvalve"openess"youchagehotwaterflowthrough2ndloop thuschangingamountofheattransportedtoproduct. PLC You have PID block, which takes temperature on outlet as PV. You want to take also hot water temperature in attention to make precisier PID control - so you insert PT100 somewhere in closed loop. You can try DISV variable - i've never used it. So i'm unsure how it would affect regulation. Actually such system will be good enough without such "feed forward" in my opinion - because it's quite slow.
Aret Posts: 2348 Rating: (264)	Last edited by: Aret at: 08.02.2008 14:11
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2/8/2008 2:26 PM	Rate (0)
mister rbt Posts: 9 Rating: (1)	Hello mister Pandiani, your attachment is almost right but the PT100 to messure the temp is not on produkt temp but on the outlet of the warm water. (see attachment) On the inlet we want to place an extra PT100 for feedforward control. But i don't exactly know what the functionis on CONT_S and if i can use this for that function. the product we want to heat is not water but fertilizer. Thats heated in the pipes by the warm water. greets Richard Attachment antwoord.pdf (602 Downloads)
mister rbt Posts: 9 Rating: (1)
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2/8/2008 6:21 PM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Richard, I don't think it would be good idea just to connect mesurement from added PT100 to DISV input. It will be like you artificially create constant disturbance. Added PT100 can help, but in my opinion you need to take derivation of it i.e. you need to create rate of change that you'll will connect to DISV. In discrete forme derviation can be approximated with ( PVa(n)-PVa(n-1) )/T, where T is sample time. PV(n) is current and PV(n-1) is previous value that second PT 100 show. T is sample time and it corresponds to cyclical time. For OB35 defualt is 100 ms. You can try this calculated signal multiplied with some constant factor you can adjust to connect to DISV. Whole idea is something like this: if there is a sudden increase of temperature of inlet water, calculated derivation will have greater value, this will appear like greater error and whole process loop should act faster (with proper adjustment). Derivation of constant signal is zero, so this additional commponent will be as greater as inlet temperature is changing faster. You can do experiment and try, I don't see a reason why it wouldn't work, althouh I'm not sure good regulation can be achieved with ON/OFF actuators. Best regards
	Last edited by: Pandiani at: 08.02.2008 18:22
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2/10/2008 2:08 PM	Rate (0)
ayu Posts: 1 Rating: (0)	so smart u r
ayu Posts: 1 Rating: (0)
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2/10/2008 3:12 PM	Rate (0)
mister rbt Posts: 9 Rating: (1)	Hello, mister Pandiani, ---------------------------------------------- Whole idea is something like this: if there is a sudden increase of temperature of inlet water, calculated derivation will have greater value, this will appear like greater error and whole process loop should act faster (with proper adjustment). Derivation of constant signal is zero, so this additional commponent will be as greater as inlet temperature is changing faster. ---------------------------------------------- thanks for your comment, what you said about the sudden in/decrease .... isThe reason why we would like to add this extra PT100. I think thatyour comment clarifies a lot. By the way we don't use on/off actuators but is motor controlled valve wich will open more and more the longer you keep 24V on it. Same of course with closing it. It's now a question of experimenting en try and error. Best regards
mister rbt Posts: 9 Rating: (1)
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2/10/2008 5:25 PM	Rate (0)
Pandiani Bronze Member Joined: 10/31/2005 Last visit: 9/4/2024 Posts: 366 Rating: (31)	Dear Richard, I hope you'll be able to tune your control sheme properly. Please keep us informed. We'd like to know final result.

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