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  1. #1
    Join Date
    Feb 2011
    Posts
    43

    Johnson PCT Manually tuning PID's

    I need some help manually tuning these loops. I have a forwarded email from a johnson rep detailing the steps for tuning out the loops. The problem is if I set my integral to anything less than 120 the PID goes unreliable from an input that is out of range. Is this some how linked to one of the other settings? etc Interval or process dead time? Does anybody have basic tactics for setting up there loops? Are you just messing with prob band and integral or are you going all out with process range, dead time and time constant as is suggested in the email I have received.

  2. #2
    Join Date
    Feb 2011
    Posts
    43
    Also for some reason the loop keeps going up a bit and then just goes right back down to zero and its no where near setpoint. It doesn't do that if the integral is at zero which strangely is the only thing you can set it below 120 with out it going unreliable...

  3. #3
    Join Date
    Dec 2007
    Location
    Connecticut
    Posts
    924
    Hello dhoyt,

    Shoot me an email. My address is in my profile. I have a solution for you.

  4. #4
    Join Date
    Jul 2008
    Posts
    1,479
    Ziegler-nicholls
    120s is 0.5 repeats per minute...way way too fast ... You have what is called "integral windup". Double yr proportional band and Try something in the order of 600 for integral time.
    What process are you trying to control?
    What is yr proportional band or p-constant?

  5. #5
    Join Date
    Mar 2007
    Posts
    379
    Quote Originally Posted by dhoyt View Post
    I need some help manually tuning these loops. I have a forwarded email from a johnson rep detailing the steps for tuning out the loops. The problem is if I set my integral to anything less than 120 the PID goes unreliable from an input that is out of range. Is this some how linked to one of the other settings? etc Interval or process dead time? Does anybody have basic tactics for setting up there loops? Are you just messing with prob band and integral or are you going all out with process range, dead time and time constant as is suggested in the email I have received.
    Just out of curiosity, what are you trying to control?

  6. #6
    Join Date
    Jan 2008
    Location
    Near Philly
    Posts
    524
    Quote Originally Posted by noskilltech View Post
    Just out of curiosity, what are you trying to control?
    Just take a look at the AI input and the PID processor input and make sure they are setup for the same control function. For example, if one is setup for GPM and the the other setup for PSI, it will cause complications. Didn't really mean to quote noskilltech, just a general response.

  7. #7
    Join Date
    Feb 2011
    Posts
    43
    Im trying to control steam Hx's and steam coils on AHU's. The inputs for the pre processor and loop are good. Im going to try increasing my integral and prop as matrix suggested and see how that goes.

  8. #8
    Join Date
    Feb 2011
    Posts
    43
    The logic view is an absolute necessity when you write the logic yourself. I think he was referring to when you do the Q&A (which is the worst thing you could possibly attempt) and then look in the logic of those modules... **** show... If you cant write the programs yourself you need more training or you should think of another career path.

  9. #9
    Join Date
    Mar 2007
    Posts
    379
    Have you tried just setting it to autotune and leaving it alone for a day? There have been some instances that I've had to manually tune my PID's in CCT or PCT (steam to dom hw hx's), but for the most part if you use system selection tool in CCT or PCT, the loops for preheat coils will usually tune themselves out pretty tight after about 6-8 hours. This may not be the case for everyone, but I've had fairly good luck with them in the past.

  10. #10
    Join Date
    Feb 2011
    Posts
    43
    They where set up for auto tuning and would work great in till you changed the setpoint then they would freak out and go out of control. This seems to be the common issue with auto tune from what I have read. I had the Hx's running at 140 deg F for weeks then they where having some issues and they wanted me to bump them up to 180 deg F. The loops never quite came under control and when I set them back down to 140 deg F at the end of the day that was it. They couldnt recover and the valves would open close open close... It ended up over heating and I had to have a guy cycle power to the control as I wasnt onsite anymore. I have been dealing with crazy logic to limit the outputs of the PID's and resetting there tuning based off of this and that and honestly its ridiculous... At this point I would just rather tune them out myself like every other control software I deal with and be done with it... Unfortunately these loops are not cooperating... I can get them close with my prop band but as soon as I try to add any integral at all they go out of wack. I had the temp almost right at setpoint so I figure perfect a little integral will get me right there. I put in 600 as preciously suggested in an above post and the valves go closed and still have opened 15 mins later and Im 15 deg F below setpoint... I dont understand its like the dont work anything like any other loop I have tuned in the 5 other software's im familiar in... What am I doing wrong here?????

  11. #11
    Join Date
    Feb 2011
    Posts
    43
    So Im tring to control steam Hx's for a building Hw Loop. I have a loop and a pre processor. Adaptive tuning is off in the PID and manual tuning is set true in the pre processor.

    process range: 60
    Time constant: 300
    Process Dead Time: 300
    Prop Band: 25
    Integral Time: 600
    Interval: 120

  12. #12
    Join Date
    Jul 2008
    Posts
    1,479
    Quote Originally Posted by dhoyt View Post
    process range: 60
    Time constant: 300
    Process Dead Time: 300
    Prop Band: 25
    Integral Time: 600
    Interval: 120
    http://cgproducts.johnsoncontrols.co...F/12011147.pdf

    Interval: Sets the Eff Period. This value is the time period between normal PID algorithm executions -- 120 is TOO BIG

    ...do you want this thing controlled in near-real-time ... or only evaluated every 2 minutes?
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  13. #13
    Join Date
    Jan 2008
    Location
    Near Philly
    Posts
    524
    [QUOTE=dhoyt;14263051]They where set up for auto tuning and would work great in till you changed the setpoint then they would freak out and go out of control. This seems to be the common issue with auto tune from what I have read. I had the Hx's running at 140 deg F for weeks then they where having some issues and they wanted me to bump them up to 180 deg F. The loops never quite came under control and when I set them back down to 140 deg F at the end of the day that was it. They couldnt recover and the valves would open close open close... It ended up over heating and I had to have a guy cycle power to the control as I wasnt onsite anymore. I have been dealing with crazy logic to limit the outputs of the PID's and resetting there tuning based off of this and that and honestly its ridiculous... At this point I would just rather tune them out myself like every other control software I deal with and be done with it... Unfortunately these loops are not cooperating... I can get them close with my prop band but as soon as I try to add any integral at all they go out of wack. I had the temp almost right at setpoint so I figure perfect a little integral will get me right there. I put in 600 as preciously suggested in an above post and the valves go closed and still have opened 15 mins later and Im 15 deg F below setpoint... I dont understand its like the dont work anything like any other loop I have tuned in the 5 other software's im familiar in... What am I doing wrong here.

    I've been using these controllers with great respect to the auto-tune feature. To be honest I'm always within the JCI realm, meaning DAT control or duct pressure control etc. etc. I'm really not a big fan of it but so far so good. I can see where it maybe suspect or fragile. What you are trying to control doesn't seem to out of the normal, maybe it just doesn't fit in the JCI box. I know eventually I'd would have to cross this bridge, you just beat me to it. I'd love to see your program file, please, please, please forward it my way. It's only a matter of time before I'm in same boat as you. Either way, please let me be included with your resolve.

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