Water Presure control system

Test400 · Apr 16, 2008

[font=Arial, Helvetica, sans-serif]Hi Every one.[/font][font=Arial, Helvetica, sans-serif]I'm very new to PLC's, and i'm written small code to control I/O.And i've worked with, Touch screens, integrated via Ethernet, and control VFD's[/font]

[font=Arial, Helvetica, sans-serif]My knoledge is very limited, when it comes to plc programing.I need some Advice regarding PID in ladder logic
I'm workign with the Allen Bradley CompactLogix L32E.[/font]

[font=Arial, Helvetica, sans-serif]I want to program and design a system that will control water presure, given a user input via a touch screen.[/font]

[font=Arial, Helvetica, sans-serif]I'm not very sure how the PID system works. but, what i do know is, i need to use one to get this program up and running.[/font]

[font=Arial, Helvetica, sans-serif]My idea is to, have a user enter a psi value through the touch screen, then,a pump keeps it at that set presure, using a VFD.[/font]

[font=Arial, Helvetica, sans-serif]I guess, usign a preasure tranducer, through an analog card, to measure the preasure would be a start, but, i cant wrap my head around anythign after that.[/font]

[font=Arial, Helvetica, sans-serif]Please help me with this.[/font]

drawson · Apr 16, 2008

I think you're on the right track depending on exactly what you want to do.

Firstly before trying to use PID control you need to at least have a basic understanding of what it is.

You can start here for a basic explanation:

http://en.wikipedia.org/wiki/PID_controller

Second I have some questions for you?

What type of pump do you have?

Are you really trying to control pressure or are you actually trying to control flow using a pressure gage as an indication of the flow?

How accurately do you have to control the pressure or flow?

How quickly does your system need to respond to changes in set point?

Test400 · Apr 17, 2008

Hi Drawson,

Thanks for takign the time, to help.
I did alot of Learining, and know, what a PID is, and What it does.

Set point would be, the X psi value i want,
Proces Variable would be the VFD i want to control
Error would be the diffarancce between the two,

i think i understood that right.

Are there any tutorials on how to set up the PID function in RSlogix5000.

Since the vfd control output is given in a %, how can i tie that in with the vfd?

I'm actually controling, the preasure.
I need the system to respond in a few seconds.(not sure if i answered that correctly)
I'm not sure what you mean by "what type of pump i have"

BR
Test400

Test400 · Apr 17, 2008

Sorry, My mistake,

Process Variable would be the Presure Tranducer readings.

Controle Variable would be the user input of the Preasure , needed.

Set point would be the Max, Preasure allowed?

if that is all what i think it is, how do i control the vfd speed reff, registers, with the "Output %" ?

TWS · Apr 17, 2008

Try reading this: 1756-rm003-en-p.pdf Chapter 13

allscott · Apr 17, 2008

Read this too. Your probably better off to do the PID in the drive, most are capable of this.

http://www.plctalk.net/qanda/showthread.php?t=33263&highlight=barn

Test400 · Apr 17, 2008

I'm working off a Program that one of our contractors did. And i'm trying to make sence out of it, so, i can write the logic for my project.

I attacthed it, if anyone would want to look at it, and help me out.

Test400

rta53 · Apr 17, 2008

Using pressure to control a pump is pretty straightforward. I have done this with mainly proportional control(P) and maybe a small bit of Gain(I). Derivative(D) is not needed. Tuning a PID loop is pretty much a matter of fine tuning the control to achieve the desired result. Controlsoft,Inc has a pocket PID tuning guide that might be helpful to you.

drawson · Apr 17, 2008

Test400 said:
Sorry, My mistake,

Process Variable would be the Presure Tranducer readings.

Controle Variable would be the user input of the Preasure , needed.

Set point would be the Max, Preasure allowed?

if that is all what i think it is, how do i control the vfd speed reff, registers, with the "Output %" ?

You're close...

Process Variable: what you are trying to control - in this case pressure and is measured as you indicated by a transducer.

Control Variable: What the PID loop varies in order to change the process variable - in this case the frequency of the VFD output.

Set Point: The numeric value that you would like to maintain the process variable at - What the Operator will enter on the HMI
_________________________________________________________________

The reason I asked you about the type of pump you have is because it will determine whether what you want to do is possible or if pump damage may occur.

For example you may have a centrifugal pump. If you so you can control pressure but you will have to limit the output speed or you will cause cavitation and destroy the pump impeller.

Test400 · Apr 29, 2008

Hi drawson,

Thanks for your help.I tried to do some programing,But it didnt turn out very sucsessfull.
I attached an image of, my off line ladder logic, ifyou could take a look at it, see, what i'm doing wrong.

Set Point : 800 would be the psi, i want to reach,
P V: Local:1:I.ChoData0 is the Tranducer im reading off of.
P V, reading is 0 , but it's gives me, the Transducer readign when, the prog is online.
CV : is the VFD speed registry that needs to be controled

Rung #7 was just a Test Rung i added.

Just for testing purposes, im runnign this without an HMI, and, i wanted to manually enter the desired psi values.

Could you pls help me with this?

Test400

Test400 · Apr 29, 2008

Test400 · Apr 29, 2008

Test400 · Apr 30, 2008

Anyone?

Eddie Willers · Apr 30, 2008

One of the things you need to do is change the execution timing of the PID instruction. The PID instruction's rung has to be executed at a periodic rate equal to the value you've entered as the Loop Update Time (.UPD).

The simple ladder logic way to do that is to use a repeating timer and condition the PID rung with the .DN bit. Another way is to place the unconditioned PID rung into a routine that is part of a Periodic Task in the CompactLogix.

Water Presure control system

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