OkiePC
Lifetime Supporting Member
I am working on a system that uses 7 PID blocks in a PLC-5 and converting it to run on a L81E.
The conversion is done with the exception of going through the PID loops with a fine toothed comb. The original logic did not properly trigger the PID instructions (except for one). They were on unconditional rungs and have the loop update time set at 0.2 seconds. The PLC-5 scan time ranged from 17 to 22 ms, so I have set up my new MAIN task to be a periodic with a 20ms periodic rate.
That *should* mean that the PID behavior in the new controller will be as similar as possible but I would really like to fix these loops so that they are programmed "by the book".
I am not sure if I should move the PID loops to a separate periodic task set for 200ms to match the update rate in their instructions, add some trigger timers set for 200ms, change the PID update rate to 20ms, or something else. I am sure that if I do any of those things, the gains will need to be recalculated.
I need to err on the side of caution, so if the safest thing to do is leave them programmed incorrectly, then that is what I'll do, then during commissioning, we'll bring the machinery up very slowly and cautiously and do as much testing and tuning as possible.
Any advice on this?
Thanks.
The conversion is done with the exception of going through the PID loops with a fine toothed comb. The original logic did not properly trigger the PID instructions (except for one). They were on unconditional rungs and have the loop update time set at 0.2 seconds. The PLC-5 scan time ranged from 17 to 22 ms, so I have set up my new MAIN task to be a periodic with a 20ms periodic rate.
That *should* mean that the PID behavior in the new controller will be as similar as possible but I would really like to fix these loops so that they are programmed "by the book".
I am not sure if I should move the PID loops to a separate periodic task set for 200ms to match the update rate in their instructions, add some trigger timers set for 200ms, change the PID update rate to 20ms, or something else. I am sure that if I do any of those things, the gains will need to be recalculated.
I need to err on the side of caution, so if the safest thing to do is leave them programmed incorrectly, then that is what I'll do, then during commissioning, we'll bring the machinery up very slowly and cautiously and do as much testing and tuning as possible.
Any advice on this?
Thanks.