1769 PID Output % . . . % of WHAT?

TheWaterboy

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TheWaterboy
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I am looking at a compactlogix PID rung that shows its output at 19.1% and the value going to the output card (as viewed in teh tags database) is 1173.
The output card scaling is "Percent range" so it should be 0-10000

What on earth is the PID's "Output %" a percent of? 19.1 % of 10000 is not 1173.
 
The very definition of "Per Cent" (according to the old 5th grade mathmatics that is not taught anymore) is "one one-hundredth part", in other words, how many parts out of 100 (not 10000). 19.1% = 19.1 out of 100 or 19.1/100 = 0.191

To find the digital value going to the Output card, you have to first know the Output card Max and Min digital values, then scale accordig to the incoming Per Cent of 0.191.
 
Last edited:
Not sure if you are trying to be clever or not, I'm having a busy day with other stuff so my temperament is kinda short, but as I initially said...

The output card is scaled "Percent Range" which in this modules case is 0-10000. 1173 is whats in the cards output register and is not 19.1% of 10000 (or .191 x 10000)

For more info I measured the output with various inputs:
10000 = 20mA,
4000 = 10.4 mA,
1000 = 5.6mA,
0 = 4 mA
So this verifies that the output module is scaled 0 to 10000 = 4-20mA.
But 19% does not match any of this.
 
The output card is scaled "Percent Range" which in this modules case is 0-10000.
Click to expand...
What can I say? There are PLC developers out there that don't know English or math either. Look Per Cent up in your dictionary. The meaning doesn't change because a few people use it incorrectly.
 
Post your program so forum members can see how the scaling is done.

Note: I am not a RSLogix5000 user.
 
The 19.1% doesn't seem to square with any other of the numbers. It will probably take posting the code around the PID as well as the card setup. Does the PID loop have any min or max constraints on the output?
 
I will post more tomorrow
The Input Card channel config is Percent range (Cant set a specific range in Compactlogix). The help defines "percent range" as 0 - 10000

The Output card channel is also set to percent range.
(why is it called percent range when its 0-10000???)
data format.jpg

The PID has a PV range of 0-16383 (yea, thats odd too)
and a CV range of 0-10000.

none of these numbers give 19%
 
OK I get that. I wish that reasoning was documented better but no matter. The original problem of percentage remains. I'm suspecting it will be on the same level as the EU resolution, but I can't see a relationship there yet.
 
TheWaterboy said:
OK I get that. I wish that reasoning was documented better but no matter. The original problem of percentage remains. I'm suspecting it will be on the same level as the EU resolution, but I can't see a relationship there yet.
Click to expand...

In the PID configuration under the scaling tab you set the 0% and 100% scaling for the CV.
scaling.PNG
 
In the PID configuration under the scaling tab you set the 0% and 100% scaling for the CV.
Click to expand...
Yes, 100% of a quantity is always ALL of it, there is no more. 10000% is an oxymoron, an idea that only has theoretical meaning.
 

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