PID Instructions RSLogix500, help

Dilemma..PID Instructions. I've read through back and forward and read Through again and I still don't quite grasp the setup of the outer face instruction. PID in General, I'm proficient for the most part, but here in the Logic what can be place there and and why is what I trying to tackle. I understand the 23 Blocks needed for the instruction. The Control Block, nope. The Process Variable..Pretty much the Output at the SCP's input. I believe that I get the Control Variable, that will be the rate at what is changing, the input maybe such as an I:x.x I also see that it could be an Integer and if it is and integer as and input..what the hell is controlling it??...what I still don't get at all is the Integer Control Variable. Please explain all. Please take it easy. What I'm really shooting for is to be set one up here in the lab for light training. TY much

Have you read up on this? There's a pretty clear explanation of the PID instruction.

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1747-rm001_-en-p.pdf

Process variable := the system condition you're trying to control, pressure, flow, temperautre, etc.
Control variable:= the system output to the device that you're controlling with, a control valve, pump, heater, etc.

Thank you but again...the reason for my post is per the logic

showshocka said:

Dilemma..PID Instructions. I've read through back and forward and read Through again and I still don't quite grasp the setup of the outer face instruction

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The Control Block, nope. The Process Variable..Pretty much the Output at the SCP's input. I believe that I get the Control Variable, that will be the rate at what is changing,. . .

Click to expand...

No, the PV (Process Variable) is NOT the PID Output, but instead is the process variable to be used for control (such as a temperature reading).
The CV (Control Variable) is the PID Output that is used to control some device that can regulate the parameter to be controlled (such as a CV Output to a heating element to control the temperature).

Use the RSLogix online Help file, and look under "PID". It has complete descriptions of all the front-face variables. Now when you get that front part, go to the interior PID "Setup Screen" to enter the Proportional, Integral, and Derivative factors. That will really confuse you, but it is what tunes the PID instruction to make it work correctly.

Control Block Length - Specify an integer file, for example N7:0. The file length is fixed at 23 words.

Process Variable PV - The element address that stores the process input value. This address can be the location of the analog input word where the value of the input A/D is stored. This value could also be an integer value if you choose to pre-scale your input value to the range 0-16383.

Control Variable CV - The element address that stores the output of the PID instruction. The output value ranges from 0-16383, with 16383 being the 100% ON value. This is normally an integer value, so that you can scale the PID output range to the particular analog range your application requires.

Setup Screen - Double click Setup Screen on the instruction to bring up a display that prompts you for other parameters you must enter to fully program the PID instruction.

Rockwell Software

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SS, l would suggest you search and read the holy grail on this and lots of other stuff written by Ron Beaufort. He covers it better than anyone else can explain. If you struggle after that then it would be time for one on one with him (pay the MONEY (l was going to say time to give up, but that isn't like me, Much))
In re reading your post, when you say you under stand PID, in Rons posts if you dig deeper than just how PID works he covers it all with his HOTROD examples, which is exactly what you would want for your test lab. But it isn't light training. As Peter Nachtwey also gives training on the Maths behind it and as Ron bows to his knowledge, for use mere mortals, it is beyond confusing.

SS, when you say an Integer as a Input, l think you may find that inputs are volts/mA so there is no integers unless/until it has been scaled. So you are looking at an integer that is changing because it has already been scaled from Volts/mA to a suitable engineering unit.

duckman said:

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Will do...and Thanks.
I'm going to ask not a Dumb Question but a Question that sounds Dumb...But in General what are the typical relationships between SCP, SCL and PID instructions when receiving an analog input an giving analog output based on that same analog input. I sure there are many ways of doing it...I am trying to get a Picture while I'm reading through.

SS, l wont try to hard to explain what you want, as RON and SLC Instruction help say it way better than l. But if you have a analogue signal coming into the PLC (V,mA) it needs to be converted to a resolution suitable to the equipment you are using (i.e, 8bit,16 bit, 32 bit) which is why you use the Scaling Blocks (SCL/SCP) then when it has been scaled to something the PLC can monitor, you then point the PID to where the info is and it does it's job.
Read all Ron's posts and you will have a good understanding (I think he must have worn a few keypads out with his in depth, but easy to understand answers to peoples questions).