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C-more button for pump speed on/off on PID

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  • C-more button for pump speed on/off on PID

    Please excuse my ignorance as I am very new at the use of PLCs and ladder logic. I have a P2K equipped with analog voltage in and analog voltage out cards. In a simplified explanation, I have flowmeters that send a 0-5VDC signal to indicate flow rate and pumps which use a 0-5VDC signal to adjust pump speed.
    The 0-5VDC pump speed signal is controlled using a PID loop, but I want the user to be able to switch the pump on/off from the C-more panel. Turning off the first rung of the PID loop (PID enable) does not stop the pump (i.e. 0VDC signal out), it just continues running at the same speed. What would the easiest approach be to set the signal to 0 when the C-more button is in the "off" state and then start back up in control of the PID loop when the button is in the "ON" state?
    Thanks!


  • #2
    You will need to write additional PLC code, that when the pushbutton in the C-more is set to "off," that you force your analog output to zero and disconnect the output from your PID loop.
    If you've done the very best you can, worrying won't make it any better - Walt Disney

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    • #3
      I'm curious why you are trying to stop a pump with a speed reference rather than just removing the start or run signal to the VFD?

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      • #4
        Using a VFD command for 0 HZ is going to supply current to the motor all the time. I hope you have a motor that can handle the heat generated. It will require additional cooling methods.

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        • #5
          Thanks everyone for your input. I have resolved this issue with a COPY command. Cow these are small 12VDC pumps that are not being controlled by an external VFD. If there is no voltage supplied on the pump speed lead, the pump does not operate at all.

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          • #6
            Are these small gear dosing pumps? What brand?

            It sounds like you have figured out how to do this, but if the Cmore is turning on/off the bit PID_Enable, you would have a NOE for the PID function block. YOu can then use a to copy a 0 to the ouput. You may also want to zero out the bias as well so that when you enable he pid again the command does not make a large jump.

            PID_Enable COPY
            --------------[\]---------------------------------------[ 0 PID_Output]

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