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  • PID On/Off Temp

    Need to maintain water temp on current project(20 gallons at 140*) and was planning on using PID but with two heating elements run off SSRs. Is there a proper way of accomplishing this? My thoughts were simply comparing the PID output to turn on each element at a certain level of output. (ex. output>10,000 element 1 is on, output>30,000 element 2 is on) Finding the right time to turn each on may take some tuning id guess?

    Or should I skip PID and just write everything out for on/off control with hysteresis? I would like my accuracy to be +/-2*

    Thanks.
    Last edited by durallymax; 02-06-2018, 12:17 PM.


  • #2
    Do-More has a I-Box for that.Anyway I assume that's what its for. Else if you are already committed to Productivity you can imitate that with your PxK code.See the Do-more help file as they have gone to great lengths to explain the operation of the commands and they provide the formulas for it.

    You would still use PID in conjuction with this I-Box as the I-box takes the PID output and decides if the output should be on or off."I assume that is how it works????"
    Attached Files
    Last edited by pbw; 02-06-2018, 12:43 PM.

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    • #3
      If you are using the Productivity platform refer to help file topic P167 or search for Time-Proportioning Control in the help file for an example of how to do this with a PID.

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      • #4
        I've done temperature control with great success with a P2K. I am about to do it with a P1K. As the PSeries-Eng mentioned, look up help topic P167, and scroll down until you see the image of an air balloon.
        If you've done the very best you can, worrying won't make it any better - Walt Disney

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        • #5
          pbw Yes committed to P2K at this point. If I was in Do-More their PWM output would likely be useful in this application?

          I guess I needed to read further, I read everything until the tanks just before the balloon a few days ago. Have to say thanks to the Productivity folks as the info within that help topic finally clicked with me and made the PID process easy to understand. A lot of the stuff I've read elsewhere prior did not actually explain the underlying math, they just tried to make analogies (I'm having Core Math flashbacks), I have a hard time understanding something without knowing the reason behind it and that topic explained it well.

          Earlier today I wrote a simple task with CMP contacts for set points and TMC coils for hysteresis just to try it. I'll re-write it with the PID and time proportioning control.

          Thanks again.

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          • #6
            If you have storage, you probably don't need PID, unless there's a high degree of inflow and outflow. Maybe do on/off with a touch of proportional where you run two heaters if 2 degrees or more below setpoint, and 1 if within. Plus probably a min on-time so as not to chatter the output relays. 30 sec or something.

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            • #7
              I've had similar applications where I just used on/off control. As long as your output has to work reasonably hard (over 50%), on/off control is probably fine. I have a system that controls heat trace on process piping that works well this way. I used Debounce coils to control the outputs to keep the logic simple.

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              • #8
                Make sure you somehow stir the water in the tank. If it's being pumping in and out, that works pretty well. It's surprising how much thermal stratification you will see if not mixed well. This is especially true of air.

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                • #9
                  Set up the PID to output 0-100%, then do a time base output with this % to run both heaters together. Use the PID loop, dont do an on/off. You have the p2k, use it.

                  Time base output to SSR heater relay

                  Click image for larger version  Name:	Capture3.JPG Views:	1 Size:	48.2 KB ID:	110443

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