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Thread: PID Loops

  1. #1
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    Aug 2004
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    Question

    Looking for information on how to setup PID loops. Starting at the basics any info would be great.

  2. #2
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    http://www.google.com

    search word: PID Loop Tuning

  3. #3
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    I found the article linked to in this thread helpful:

    http://hvac-talk.com/vbb/showthread.php?threadid=93655

    The bare essential aspect of PID loops is that it is a control process to maintain a setpoint as the process variable changes. IOW, say you have an AHU with a chilled water coil and the AHU is designed to maintain a discharge air temperature of 55 degrees. As heat load over the coil or chilled water temperature through the coil varies, the discharge air temperature will vary unless there's some way to compensate for these changes. Enter the PID loop; Proportional, Integral, Derivative. In this case, the chilled water coil is modulated by a final control device, specifically a chilled water valve, via the PID logic in order to return discharge air temperature to setpoint whenever a deviation from setpoint occurs.

    If the heat load increases, more chilled water is added to return discharge air temperature to setpoint. If heat load falls off, chilled water flow is reduced to keep setpoint. The PID loop works to achieve this end without making the valve "hunt" or cycle, which would result in actual discharge air temperature over and undershooting setpoint endlessly unless corrected.

    Your eyes, brain, and right foot are a PID loop when you drive. You look at your speedometer and choose 65 mph as your setpoint. You press on the gas pedal until you get to setpoint. Then a big hill comes up and unless you add more gas, you'll sag below setpoint. After you crest that hill you hit a downgrade. You let off the gas to maintain 65 mph. Next, you hit a level patch. You find yourself needing to vary the gas pedal position very little to stay at 65 mph.
    The "process variable" is the speed of your car. "Setpoint" is the speed you want to maintain regardless of road changes (unless you hit heavy traffic, of course!). The "final control device" is the gas pedal. The PID loop is you. How quickly you return to setpoint when driving conditions change is a result of how well you respond to the changes without going too fast or slow. This is what we're after with a PID loop.

    Proportional:

    Controller will read any error between process variable and setpoint and output a change to the final control device based on proportional band. Proportional only control will never achieve setpoint, it will only respond to variance from setpoint and attempt to bring the process variable closer to setpoint in accordance with proportional band.

    Integral:

    Integral output adds or subtracts from the proportional output to eliminate error between actual process variable measurement (such as temperature) and setpoint.

    Derivative:

    Rarely used for HVAC, it is essentially a logic sequence that attempts to predict the future of the integral value; it helps achieve setpoint faster and reduces over and undershoot.

    How to tune a PID loop will vary with controller manufacturer. The basic thing to know is make any changes to the PID parameters in small increments, and be patient as the loop adapts to any changes. Also, once the loop stabilizes, it's good to introduce an "upset" (basically change the setpoint a few notches) to see how well the loop responds and if it can re-establish setpoint in a reasonable amount of time.

    And...don't tune a loop on FRIDAY!

    [Edited by shophound on 10-05-2006 at 10:19 AM]
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
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    Aug 2004
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    PID

    Thanks Shophound...For the info.

  5. #5
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    proportional- how far from setpoint am I (deviation)
    integral- how long have I been away from setpoint(time)
    derivitive- typically not used.
    In a nut shell.

  6. #6
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    Practically Canadian ehh.
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    Trane151 do you work for Trane? If so then you should have CNT-APG002-EN. For those that do not you can find it here.

  7. #7
    My version.

    P = 1/gain. As gain increases control can be tighter or more prone to oscilation

    I = Makes control occur at the setpoint instead of below the setpoint. A constant multiplied by the integral of the error term. Direct Digital Controls (DDC) need methods to prevent reset windup (adding forever).

    D = Prevents overshoot of temperature and usually used in fast responding systems. A constant applied to the derivative of the error term.


  8. #8
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    PID control logic seems to be slightly different between different manufacturers. PID values don't always mean the same thing.

  9. #9
    In theory, they should be the same. You need to watch out for the units. A popular unit for I is repeats/minute.

    What can cause some problems is when a digital controller replaces an analog one. In an analog one, the maximum value of reset is limited by the power supply rails and the value of the PID equation. In a digital one, lets hope the designer limited it to 100%. They could limit the output to 100% or limit the error to 100%.

    The idea is to prevent reset wind up is that you stop integrating when the output reaches 100% or the integral of the error reaches 100%.

    A popular controller, but not for HVAC, uses the term Max power, but it's really the maximum output of the error amplifier. This variable can alter the values of PID.

    P = 1/G: All of the terms are supposed to have G in them, I believe. It's been a decade or so, since I had to do a PID loop from scratch (FORTRAN). If it's omotted, then that can severely change P, I and D.

    A direct quote from here: http://www.expertune.com/tutor.html

    "Depending on the manufacturer, integral or reset action is set in either time/repeat or repeat/time. One is just the reciprocal of the other. Note that manufacturers are not consistent and often use reset in units of time/repeat or integral in units of repeats/time. Derivative and rate are the same."

  10. #10
    Join Date
    Aug 2003
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    225
    Originally posted by roadgear16
    proportional- how far from setpoint am I (deviation)
    integral- how long have I been away from setpoint(time)
    derivitive- typically not used.
    In a nut shell.


    I,m going to agree with this = KISS Keep it simple stu "pid"

    IN Hvac The "D" Portion is Rarely used!

    But think space as(P) Time (I)and (D) as Distance

  11. #11
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    Alaska
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    I would read the manufacturers recommendations for settings. These rules of thumb give you an excellent starting point.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

  12. #12
    Join Date
    Mar 2005
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    Alberta, Canada
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    Originally posted by roadgear16
    proportional- how far from setpoint am I (deviation)
    integral- how long have I been away from setpoint(time)
    derivitive- typically not used.
    In a nut shell.
    "derivitive- typically not used." Not true. Its what makes PID actually effective. otherwise its just a PI loop and the control aspect acts totally different. Some applications dont need PID but some wont run without it. It all depends.
    "I'm really easy to get along with once you people learn to see it my way."

    "I'll try being nicer if you'll try being smarter."

  13. #13
    Join Date
    Feb 2006
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    250
    Originally posted by bryan l
    Originally posted by roadgear16
    proportional- how far from setpoint am I (deviation)
    integral- how long have I been away from setpoint(time)
    derivitive- typically not used.
    In a nut shell.
    "derivitive- typically not used." Not true. Its what makes PID actually effective. otherwise its just a PI loop and the control aspect acts totally different. Some applications dont need PID but some wont run without it. It all depends.
    My mistake, I should have said that the D value is left at zero 99.99% of the time instead of "typically not used"

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