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  1. #1

    JCI unt programing

    I am trying to program a loop presure reset in a unt with hvac pro. I am monitoring a hot water loop and want to open a bypass valve when the pressure gets to high. I can make it work using the ai to ao setup and selecting reset schedule and adding all of the other info. But I would like to add some delay into the logic so it is not hunting or changing constantly. So I have tried to select PI control with reset schedule so I can add some deadband or time to the logic. But I cannot get it to output to my OA point and control any thing. If I do just the reset schedule I can make it work fine. Anyone have any ideas on how to add some delay into a reset schedule?
    Thanks
    Wayne

  2. #2
    Join Date
    Jun 2009
    Location
    StL, MO
    Posts
    572
    What you describe is a conventional control loop so a reset schedule is not required. If you set up an AI to AO, PI Control, side loop you'll have what you need.
    UA LU 562

  3. #3
    Join Date
    Mar 2009
    Location
    Several Miles from Sane
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    1,441
    As DaveCR said use a simple PI control loop. The reset will only create problems for you.

    Here are some suggestions for your tuning parameters.

    Let X = Differential setpoint ((10PSI))

    Prop Band will equal 5X ((50PSI))
    Dead Band will equal .05X (5% of X), ((1-2 PSI))
    Integration... start with 200

    Preliminary checkout: get your output to change at a rate of aprox 1-2 % per second for starters. If it changes too fast increase PB by 25%, too slow decrease by 25%.

    Also increase the filter value of the Analog Input to about 3 (roughly 4.5 seconds) to keep the loop from spiking.

    Good Luck, let us know how it works out O.K.

    CCI (Controls Cause Insanity)
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  4. #4
    Thanks for the reply's. I have been gone for a couple of days and got time this mourning to work on this.
    Ok This is my first attempt at a PI loop programming in a unt. So if I understand this correctly with a PI loop X would = what you want to maintain as a setpoint presure. ( What I was trying to do with the reset schedule was if presure = 25psi then bypass valve would be 100 % open and if presure = 10 psi then bypass would be 0% open. ) With the PI sequence it looks like I would use X=20 (for 20psi)
    Does the 50 in your prop band mean it will go up to 50 psi or is that just an intergration number?

    X=20 psi setpoint
    Prop band =50
    dead band=1
    Int=200

    And also it looks like 20 becomes the middle of the swing, below this closes the valve and above it opens it up. ( I have a test bench with a unt on it with pots on the inputs and an anolog guages on the outputs.) so I am able to download and watch what happens.

  5. #5
    Join Date
    Mar 2009
    Location
    Several Miles from Sane
    Posts
    1,441
    I am not going to ask why but...

    AI to AO
    Reset Schedule


    Input 1 10 PSI
    Input 2 25 PSI
    Output 1 0% (0VDC)
    Output 2 100% (10VDC)

    If that doesn't work you messed something up !
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  6. #6
    What my senairio is that I have a DX9100 sitting in a control cabinet now only controling a bypass valve and a pump command . I am wanting to change that Dx out so I can use it elsewhere in my building and put in a unt controler. The reason I was messing with the reset schedule was that I have a couple of loops here using that for tempature reset on hot water loops and I thought I would try and use the same thing for a presure bypass since I have not used any PI loops before. So at this time I am playing with a PI loop and that was why I was asking about the settings. I have been working on the Johnson controls here at the school for about 15 years and like I said this is my first attempt at setting up a PI loop.
    Wayne

  7. #7
    Join Date
    Mar 2009
    Location
    Several Miles from Sane
    Posts
    1,441
    Based on your test results and description you want a simple reset.

    If you want to do A PI control loop refer to previous post and "Play with it" on your test setup.

    Open the Help inside HVAC-Pro, lots of good stuff there.

    And like I said, I didn't want to know why !
    If sense were so common everyone would have it !

    All opinions expressed are my own. Any advice provided is based on personal experience, generally accepted fact or publicly available information. As such, it is worth exactly what you paid for it, not a penny more not a penny less !!

  8. #8
    Join Date
    Dec 2009
    Location
    Canada
    Posts
    39
    If you want the valve open at 100% at 25psi, 0% at 10psi, just use a PI loop, with setpoint of 10, prop band of 15 (10+15 = 25psi full open).

    If you want more accurate control to a specific number (like 15 or 20psi), make the actual number you want your setpoint, triple the prop band (say 50) and add the integration number (200ish would be a good starting point).

    If the valve "hunts" up & down, increase either your prop band or integration. Next step I would probably try would be 75 prop band, 400 integration. Or just one at a time.

    good luck!

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