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PID Control Stopped Working

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  • PID Control Stopped Working

    Good Day,

    I have recently replaced one of my D4-450 CPU's with a new D4-454 CPU. I downloaded the backup program onto it, and it all worked perfect.

    After almost a month, the PID control stopped working. Right now I have to control my valves by placing the controller in manual, and adjusting the output myself. As soon as I put the controller into Auto mode, the output changes to 5 milliamps and stays there. Even if the controller shows 4095 in the output, the milliamp output on the analog output card does not change. Only If I put the controller in Manual, and and adjust the output to 100%, does the analog output card give me 20 milliamps. This has now suddenly happened on all 8 of my loops.

    What happened?

  • #2
    Do the loops still have the same setup information in the V memory locations as the D4-450 did?
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    • #3

      yes, I downloaded the backup program and the loops worked for about a month without problems.... I checked each loop's settings on the PID setup and confirmed that the V-Memory is the same.

      I trend my PV, SP and OUTPUT in the PID setup, and the controller clearly increases the output to 100% (4095). When I test my milliamp output, I only get 5 milliamps. When I put my controller in Manual mode and adjust the output to 100%, then I measure 20 milliamps on my output card.

      It's weird... but now the plant operator has to control these loops in manual, so i'm desperate to find the problem quickly.



      • #4
        I found the problem.....

        The plant has been operating at low feed rates for the past couple of years. The control outputs have almost never gone past 50%, because it was never required. Now that we are running full capacity do we see this error occuring.

        The problem was the scaling on the output card. Each output was scaled between 0 and 1000, where the output from the controller was 0 to 4095. This means the output would reach 1000 and 20 milliamps would be sent to the valve, then the output increases above 1000, and the output card drops the output as a safe mode.

        As soon as I scaled the controllers output to 0 to 1000, everything worked fine again.