SLC505 PID loop output

Jeff23spl

Member
Join Date
Jan 2010
Location
Canada
Posts
860
Hello

I would like to know if there is a way to get more resolution from a pid output loop

RS500 and a SLC 5/05

My PID seem to be in % going only from 0 to 100 inside an integer...

I don't understand because, the PV and SP is under a 14bit resolution (0-16384) why the output isn't working this way?
 
By default the CV of a PID will range from 0-16383. The CV% you see in the PID config window is there to make it easier on the brain while you're monitoring.

You can take that CV value and scale it any way you want outside the PID. I don't have access to software and manuals at the moment so forgive me if I have made a mistake.
 
By default the CV of a PID will range from 0-16383. The CV% you see in the PID config window is there to make it easier on the brain while you're monitoring.

You can take that CV value and scale it any way you want outside the PID. I don't have access to software and manuals at the moment so forgive me if I have made a mistake.

Sounds right to me Okie.

Jeff:

You will have to take your CV Output and then Scale it properly to your Analog Output Card. We can't help you with that, since you didn't give us enough information. As OkiePC pointed out, the PID Output to your Control Variable Address will be 0-16383. Each Analog Output and Input Card for that matter, has different Scaling depending on the PLC Family, Type, and Model. Note that some of the newer AB 1746 Analog Output Cards will allow you to choose PID Scaling in their Configuration. So, that may be one way to go.

That is the best I can do with the limited information you posted.

Stu.....
 
ok sorry for info, i'm fine with scaling from module card etc

Acualy i used a PID to controle a process valve for air flow

I have a flow meter as PV scaled 0-16384 to the Word 14 and a flow setpoint also scaled 0-16 384 to the word 2 and i use the output word 16 to position the valve to the right position.

Because it's a very big valve, i may need more resolution than 1% in the 0-40% area

I tested it and works fine but has i said, the word 16 only change from 0-100....i was expecting to get it to go up to 16383 but i don't figure out how....
I checked all other words of the block to see if another one may look like the output as 0-16383 but still no clue....

Is there something blocking the output to 100 ?
 
on the front of the PID instruction itself - what address do you have listed for the CV? ... look in that box and you will see the value changing from 0 to 16383 ...

SO ...

do NOT use word 16 to control the valve ... use the value in the CV location instead ...

OkiePC mentioned this idea back in Post #2 ...

The CV% you see in the PID config window is there to make it easier on the brain while you're monitoring.

specifically, that value is meant to be displayed for an operator's HMI, etc. ... it's NOT intended to be used for controlling the system ... the CV (Control Variable IS intended for control) ...

and a tip: quit saying 0 to 16384 ... it's 0 to 16383 instead ...
 
Last edited:
I knew it was simple but even with the clue in front of me, i was looking everywhere else in the control block without seeing the other parameter....

I knew AB was better than giving that low resolution but it's strange to have the good output out of the control block that is explained in the help file etc...

Many thanks to everyone

and i will update my range to 16383 instead
 

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