CLX PID

[Betico 0]

Hello I'm trying to tune a PID in ControlLogix I'm doing the timer option to trigger the PID, my IO is 1771 IFE & 1771 OFE on Controlnet, my update time from the input module is around 270 ms my PID has an update time of 300 ms, I'm using Dependent mode, I'm trying to control flow with a modulating valve. My problem is that the PID as the PV reach the SP the output of the PID jumps back (but the PV is not close to the SP) no matter the number that I put so far. Does anybody has an explanation for that. I'm not using the derivative term of the PID. What number can I put on the Proportional and Reset (Ti) to start playing without this jump back. I first try with the same number that I have in the PLC5 (using PD configuration file) but it does not work, is there any way to convert those number that I have in PLC5 to ControlLogix. Thanks in Advance.

[Guest]

I assume (gosh, I hate that word) that you had a PLC-5 system that worked ok and now you’re trying to use a ControlLogix system instead ... if you are using the same equation (Dependent) in the ControlLogix that you used in the PLC-5, then the tuning numbers SHOULD be the same ... if the same tuning numbers won’t work, then you PROBABLY have something wrong with either: (1) the PID’s Loop Update time and/or the PID triggering method ... or ... (2) the scaling for your inputs and/or outputs ... note: there are other things that could cause problems but these two are the main culprits ... as for the first condition: ... you said that your PID has a Loop Update time of 300 ms ... please confirm that your trigger timer has a preset of 300 ms also ... and that you have the timer’s done bit as the only condition on the PID rung ... important: if the time interval between the PID trigger events does not MATCH the Loop Update Time as set up in the PID configuration, then the Integral action will be inconsistent with the tuning numbers that you’ve entered ... as for the second condition: ... the 1771-IFE and 1771-OFE modules usually have signals which range from 0 to 4095 ... these can be rescaled to other ranges but that is NOT recommended ... so please tell us how your scaling is set up for your IFE and OFE modules and for your new PID ... the more details the better ... an even better idea: zip up your entire ControlLogix program file and post it ... we’ll be glad to take a look at it for you ... also ... it would be a good idea to do a screen dump of a Trend to show us how your PID is reacting ... you can post that too ... here’s an example that someone posted awhile back to give you the idea ... but please try to use a better color combination ... the dark blue on black is hard to see ... best regards, Ron

[Betico 0]

First of all thanks for the response. Regarding the scaling that I'm using in the IFE and OFE is 0-4095. One thing that I forgot to mention was that I'm doing staggering message in order to be able to read data from multiple IFE's module's with the same message (type Block xfer read), but my update time is 270 ms for the data coming from the IFE's cards. I'm attaching a copy of the PID logic in the Rslogix5K V13. Also the Timer is the only condition to trigger the PID and the Update time of the PID and the Preset of the timer is the same in this case .5 sec. At this point I'm unable to post the trend for the PV and the CV since I'm not at the plant for the moment. But is a good Idea to trend the PV and CV an see maybe something that I'm not seeing right now. The number that I had in plc 5 that almost work for everything were Proportional: (KC)=0.3 to 0.8 and in the Reset Time (Ti)=6 to 15 those were good number to start for PLC 5. But I never experice before something like that, the output jumping back without reaching the SP. Thanks again Edited 23 Aug 2004 by Betico

[Guest]

sorry but I wasn't able to open the file you posted ... maybe you could try again? ... I see that it has been downloaded about 13 times so far ... was anyone else able to read it? ... best regards, Ron Edited 23 Aug 2004 by Ron Beaufort

[Betico 0]

Okay, here are some screen shots with the logic and configuration of the PID. Thanks again. Edited 7 Sep 2004 by Betico