PID consideration

[yfcarer]

Hi, everybody.

I am now using CompactLogix platform (1769-L35E) to develop a regulation program. There are several PID loops in this application.
I want to decrease the amount of the PID loops used in the program. So I write a separated sub program containing one PID regulator(which I call it as COMMON PID). When I need to use PID regulation in main program somewhere, I use JSR command to translate the PID parameter PV,SP,P,I,D to the COMMON PID. After running the sub program one time, the output of the COMMON PID will return and the execution return to the transfer place. Then, if the main program need the PID regulation somewhere else, the same procedure occurs: transfer parameter, run PID one time and then return the PID output.

After I got this idea, I always feel there will be something wrong, but I cannot tell where the wrong will be.

Would somebody tell me if I can use the PID like this? If not, where is my mistake? Or what will happen if I use PID like this?

Thanks in advance!
Hope everybody has a nice day!!!

 

[kamenges]

From what I can tell your method will work. However, I don't see where it helps you. What you have done is replaced the PID instructions in your program with JSR and MOV instructions. In the process you have effectively increased your plc processing load.

You can think of the PID instruction loosely as an application specific JSR. You fill in the parameters you need and the plc runs the PID software with those numbers. Putting the PID instruction into a subroutine and calling that just adds additional processing load.

Keith

 

[yfcarer]

Hi, Kamenges.
Thanks very much for your answer!

From your words, I can see that you regards the PID as a "SOFTWARE". So what you tell me is that: no matter how I arrange my PID regulations, the PLC only refers to the same "SOFTWARE PID"? To use PID instruction in several places is, in general, just like to call a sub program.

Is that what you mean?

 

[coolwolf]

Did you mean that? In 1769CPU,PID is software PID. It is one part of programme in the PLC.Both using 5 PID and 10 PID, we use the same programme.

 

[kamenges]

Yes, that is what I meant.


Keith

 

[yfcarer]

Thanks Keith.

You help me a lot!!