RSLogix5000 PID PV Scaling Question

Greetings All,

I have a quick question regarding the PV scaling parameters in regards to PID Settings. I searched through all the manuals I could search for at AB as well as the knowledge base, but didn't find any definitive answer to my hopefully easy question.

Here's the particulars:

Processor: 1756-L61 ControlLogix5561 Controller
AI Card: 1756-IF8 Module
Transmitter: 0 - 100" LT

Here's the config settings for Ch2 of the IF8 card via screen shot:




Here I set the Engineering Units for the 0 to 100" that relates to the 4 to 20mA. In the settings of the PID instruction I'm using... specifically the PV settings I'm kind of thrown off regarding the "unscaled" and "scaled" parameters. In other words I'm scaling the value via Ch2's AI config. What needs to go in the PID PV settings? Here's my lame guess:



Would this be correct? Any help or insight would be appreciated.

Regards,
Bobby

You are correct. If the analog input is scaled to engineering units, then the unscaled and engineering unit scale in the PID configuration are identical.

danny.michael said:

You are correct. If the analog input is scaled to engineering units, then the unscaled and engineering unit scale in the PID configuration are identical.

Click to expand...

Hi Danny,

I agree 100% with your answer and would like to expand it and see if you agree:

"...then the unscaled and engineering unit scale in the PID configuration are identical AND EQUAL TO THE ANALOG INPUT RANGE".

Let's say the analog input was scales to 0-600 PSI.

Then .MINI = .MINS = 0 and .MAXI = .MAXS = 600.

If you don't do that (let's say, you leave them both equal to 0-4095), then the calculated error fed to the PID calculation is wrong. Please refer to the top diagram on page 526 of Publication 1756-RM003O-EN-P - November 2012.

The "Convert Units to %" block calculates Error(in EU) × 100/(.MAXS - ,MINS).

If .MAXS and .MINS don't match the scaling range, then the error in % will be wrong.

What do you think?

Thank you

Yes. Your expanded description is correct.

RS5000 PID Blocks

Hey, I was looking for this equation and found on a old RS5000 Manual.
the MAXI , MINI, MAXS and MAXS are shown in the picture below and they are very critical for tunning and for PID performance.

The PID algorithm makes the calculations below

ERROR% = ERROR_EU *(100)/(MAXS - MINS)

PV_EU (shown in the pop up from PID) = (PV_inputfromPID - MINI)*(MAXS - MINS)/(MAXI - MINI) + MINS

In the first screen shot, the analog input setup.... shouldn't the "input range" be 4-20ma rather than 0-20ma?