PID RSLogix 5000

Hi, people!.



Need your wisdom for this doubt I'm having.



I need to program several water loops with a cooling system (0-100% glicol valve).
I've started taking a look at the PID function in RSLogix5000 (V31), but I've realized that this function only has "COntrol Variable" (for the valve control) and "Process Variable" (for the temperature measurement), but there is not a place where I can asign the SetPoint Variable:

Image1


In the PID configuration, there is a SP FIeld but it seems is only gonna work in a manual mode.


Image2



So, I can't refer an HMI tag (i.e.) as a SP for this PID function?.


Thanks for your help.

Hi, and welcome to the forum. I didn't take a lot of time looking at the images, but I think I can help you.

In order to move a setpoint into the PID, you must move the value (from a MOV instruction) into (Tag name of PID).SP. So in my air handler units I've been working on, I would move as an example "Supply Fans VFD SP" (MOV) to "PID_Supply_Fans.SP." The tag name of the PID in this example is: "PID_Supply_Fans".
Then the setpoint will show up in the PID settings you can see.
Don't forget to have a free running timer for the update.
Lastly, you can assign the input values to your setpoint (in my example, "Supply Fans VFD SP" from your HMI keypad. Hope this helps.

That was very helpful, Saultgeorge! Thanks.



I'm programming in structured text, so I've modificated a bit your indications, but assigning the SP directly makes no errors and -until I Simulate it - Im going to think thats correct:



PID.SP = HMI_Tag_SP;

PID(PID_Name, X, X, X, X, X, X);


I usually apply this kind of asignation in the AddOns, but them use to Showme the posible parameters (Instruction Definition).It seem that in PID is not the same.



In the other hand, I thing the PID timer is set internally, in the "configure PID" window. (Right button over the PIDs name).



Then, again. Thankyou.-

You can also take a look at the PIDE instruction which has some extended features.

I know, Dravik! But I need a easy adjustment and I Prefer the PID function that seems more simple to me.



If I get some troubles I will give PIDE an opportunity.



Thx for your comment.

You're welcome!! As far as the timer goes, all the PID logic I have written is in ladder. There is a rung that has an XIO that's a done bit, and the timer. The timer is set at 500ms. So it's self resetting. The rung that has the PID instruction has an XIC done bit from the same timer, which cycles or updates the PID every half second. This is what A-B tech support told me needs to be in there. The update time setting in the PID instruction configuration needs to match the update time preset in the timer. Good luck on your project!!

I generally create a timed task instead of using a timer, but that works.

Dravik--yes, thanks for reminding me of that. I have seen in the instruction help where the PID can be assigned to a periodic task, but I don't have a lot of experience at configuring a periodic task. These timers were in a reference program I used to create logic for all these air handlers we are doing, so I just continued using them. Thanks again

Make sure the set point value assigned to the .SP register is between the engineering unit scaling values .MINS and .MAXS. The PID instruction makes all internal computations on a percent-of-input-range basis, and uses these values to convert the set point to percent.

In the "old days" (i.e., PLC-5) an out-of-range set point would fault the processor. In Logix, it is only a minor fault (code 36), which will be in the processor status fault history if this is the problem.

Note!
Make sure you set the time period IN THE PID SETTINGS TO MATCH THE TIME YOU ARE CALLING IT AT!!!


Sorry to yell, but I've seen lots of people forget this.


And, for the record, for the most part, I prefer the old PID rather then the PIDE instruction; especially if you don't understand PIDE from top to bottom, inside and out, sideways, diagonalways, and through the fourth and fifth dimensions.