RSLogix500 PID Scaled Error Reading

bro08170

Member
Join Date
Aug 2006
Location
Utah
Posts
15
I am tuning a PID loop on an SLC5/05 processor. The problem is- when viewing the PID setup window (while running), the scaled error doesn't compute correctly (i.e. when E=SP-PV, SP=10, PV=5, Scaled Error SE=10???). To simplify I have Proportional Gain set to 1.0, and Integral and Derivative Gains set to zero. This setup is using a VFD controlled pump and I need to set the scaled flow rate using a flow meter for the feedback input(PV).
 
I saw the same thing on my 504. What I did to correct this is scale my input (SCP) from 0 to 10 to 0 to 16383 and use that result for your PV.
 
we'll eventually be able to nail down your problem ... but ... it would be a LOT quicker if you can post your .RSS file ...

my "best guess" so far: you don't have the analog input signal properly scaled BEFORE it is presented to the PID's PV (process Variable) location ... for the PID instruction (in an SLC system) to give optimum results, the PV location MUST range in value from 0 to 16383 ...

possible cause for confusion: when some (many?) people hear the word "scaled" they immediately think of values expressed in "engineering units" such as "gallons per minute" or "degrees F" ... this gets confusing when you're reading some of the Allen-Bradley manuals, because they mention that the input signal must be properly "scaled" before it reaches the PID ... (note that I used the word "scaled" in exactly the same way in the last paragraph) ... so to some people, the book instructions mean "convert the input signal into engineering units before it reaches the PID" ...

that's WRONG ... in simplest terms, the range "0 to 16383" is SCALED - and SCALED properly for the PID's PV input ... if this "scaled" terminology is what's confusing you, you are NOT the first to fall into the trap ...

if this doesn't help, try posting your .RSS file ...

PS EDIT ... it looks like my distinguished colleague Mark Buskell has come up with the same "best guess" while I was typing ...
 
Last edited:
Scaled for PID!!#$%^&

Ron B.- You're both right: I fell into the "scaling" trap and was trying to scale to engineering units before the PID loop. So I'll try using 0 to 16383 and see how it goes. Thanks!

P.S.- If I am using a class III analog input card (that is configurable for "scaled for PID 0 to 16383), then hopefully I won't need to use the SCP command? Wish me Luck.
Brent
 

Similar Topics

I have two questions: When you open the settings for the PID you have the option of doing manual and auto does manual use the values entered and...
Replies
2
Views
1,740
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...
Replies
7
Views
2,828
Setting up a PID instruction to control the temperature of oil in a small tank. First time using PID instruction. The heaters are 480VAC...
Replies
4
Views
2,515
I have been troubleshooting a system containing PID's that doesn't perform correctly (it's nothing dangerous) the setup screen shows a setpoint...
Replies
3
Views
3,088
Greetings everyone! I'm designing the automation for a Flame Control process, but I;ve been having some trouble on the programming and even more...
Replies
21
Views
4,575
Back
Top Bottom