PID in ControLogix

[Guest Sporty]

Hello, Anyone have experience setting up/tuning PID in ControLogix? What I have is a setup controlling a modulating gas valve in a direct fired HVAC unit. I have i tuned fairly close, but not to what I want. My setup is as follows: Logix5550 to ControlNet to 1734-ACNR to 4-20mA Output to Gas Valve. I've looked a 3rd party tuning software, but it is too expensive. Should I use just one PID or a cascade? Not on any serious time table, but would like to get it done before it gets warm again. Thanks, Sporty.

[Chris Elston 79]

Another engineer friend I know says: " I am by no means an expert on PID tuning, but I have had good luck by starting with all constants at zero and then slowly increasing P until the system just starts oscillating and then backing it down a little. Then turn up D until the response gets twitchy and then backing it off until the response smoothes out. The I term is probably the least critical. As long as it isn't real big it could be set about anywhere. This sequence may help get to a reasonable starting point."

[Contr_Conn 4]

I am not a PID expert, but: I don't think this is correct because Integral gain is resopnsible for holding PV at setpoint. I think in your comment you need to swap "I" and "D". RSlogix has Autotune option (Extra $) for PIDE Function Block (FB is Extra also) Works really well if you correctly select process type. Look at these excellent posts at another friendly forum by Ron Beaufort http://plctalk.net/qanda/showthread.php?po...72417#post72417 http://plctalk.net/qanda/showthread.php?po...73609#post73609 http://plctalk.net/qanda/showthread.php?s=...&threadid=11242 http://plctalk.net/qanda/showthread.php?s=...&threadid=11969 Edited 19 Feb 2005 by PLC_Cables

[Sporty 0]

Thanks for the links PLC_Cables. Those helped a lot. I came in today and did some more tuning and think I've got it dialed in pretty good. I did get a quote on Expertune software, but it is too expensive for what we need. It is nice though. The only thing left I want to try is setting up the PID in a Periodic Routine to see if it works any better. Thanks again for the help. Sporty

[Contr_Conn 4]

It is highly recomended to use ControlLogix PID with periodic task. Make sure periodic task rate is exactly equal to the Loop Update time. For PIDE using periodic task is a must as PIDE takes loop update rate from the periodic task rate automatically. I think Ron explained why it is very important in his posts.