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# PID LOOP DIFFERENT UNITS OF MEASURE

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Hey guys,  Not really new to Allen Bradley per-say but I am very unfamiliar with PID loops and the best way to utilize them.  To be honest I had never even heard of it until a colleague said that it would be the best way to run my program.  Let me know what you think.  Here's the application.

I have an Allen Bradley MircoLogix1400 with IF2OF2 analog card.

Controlling the speed of a motor via VFD.

4-20ma signal coming in from the flow meter of water entering the tank.

4-20ma signal coming from scale letting me know how much of the other product is left.

0-10v signal from PLC to VFD to adjust motor speed.

The customer is going to have a separate system controlling water into the tank, we are going to use the signal from flow meter to measure how many gallons of water are entering per minute.

My system will also be measuring the weight of that product in stock and feed it into the same tank as water at a 10:1 water:product ratio.   The motor will be running an auger that pulls product from stock and dumps it in the tank.  So we will be measuring how much weight is being lost per min and convert that to gallons per minute.  So once i have the rate of water flow in gpm and the rate of flow for product in gpm, i assume thats when the PID comes in?  Can anyone give me some pointers.  I'm positive it's not as difficult as I am making it out to be but again, never used it before so any little bit of info helps!

Thanks

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Basically the first thing you have to do is figure out what is your Process Variable (feedback) and your Control Variable (setpoint).

I'm not an expert on PID's but I used PID's in the past to control temperature/heating. Let's say I want the temperature to be 120 degrees F. Basically the Process Variable is the actual temperature. The Control Variable is the temperature setpoint. Let's say my thermocouple is reading 80 degrees (Process Variable), this is less than 120 degrees F so I want the Control Variable (in my case the PLC analog output controlling the heating) to increase. The PID loop will handle this. Hopefully this example shows you why the PID is useful.

In your case the Control Variable will probably be the 0-10v signal from PLC to VFD to adjust motor speed. I don't completely understand how your system works, but it sounds like your Process Variable will be calculated from the flow meter signal or the signal coming from the scale.

Hope this helps.

Also read the Instruction Help for the PID instruction if you haven't yet, open up your RSLogix software then go to Help, then Instruction Help, then look for "PID".

Edited by AndrewG
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Does anybody know of any training courses available for Allen Bradley PLC's?  I understand ladder logic and i've created quite a few programs on my own but I need to get up to speed with the more technical functions of these controllers.

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For Allen-Bradley PLC training, your options are:  A-B technical classes, A-B "on-the-move" labs (typically free and short), 3rd party classes/labs, online training classes, read the extensive literature.

Refer to Chapter 20 of the MicroLogix 1400 User Manual for background on the PID function.  http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1766-rm001_-en-p.pdf

Allen-Bradley has a manual for the enhanced PIDE function used in Compact/ControlLogix processors.  Not the same function block as the PID (SLC-500 based), but the background fundamentals are the same.

The subject of PID loops is not a quick black and white answer, but is essential for many programming applications and has been around a long time.  Think of a PID control loop like the sloppy steering of your dad's old pickup truck.  Sloppy steering (wide range of motion), your faster response to compensate for the sloppiness (time response), smaller adjustment when you do compensate (amount of gain).

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On 7/28/2017 at 5:39 PM, abaca23 said:

Does anybody know of any training courses available for Allen Bradley PLC's?  I understand ladder logic and i've created quite a few programs on my own but I need to get up to speed with the more technical functions of these controllers.

I found this helpful.  Its got a lot of information, but it is written with the Allen Bradley PLC in mind:

http://www.eng.utoledo.edu/~wevans/chap19_S.pdf