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Different performance of PIDs with same parameters

There are 2 identical pelletizing systems in 2 separate DeltaV systems/manufacturing cells.

Different versions - DV7 and DV13.

In DV7 PID performance is satisfactory with gain=0.5, reset=5, PVfilter=10s :

However in DV13 system, where whole project is copied/converted from DV7 system), performance is much worse:

PID tuning is not identical(gain=0.52, reset=10.5, PVfilter=8s) because otherwise pelletizer interlocks because of pressure/flow overshoot.

However, what mystifies me is not performance.

DV7 PID responds to -100 SP change with roughly -2% change to OUT. And DV13 PID has longer reset(so theoretically should be slower), yet responds to same -100 SP change with -5% OUT.

Why 2 PID blocks with the ~identical parameters/scaling/configuration/controlled devices, same PV, same SP change(and therefore error) - return that much different OUT???

Mindaugas
  • James Beall

    In reply to Mindaugas:

    Mindaugas,
    So when you say the "initial response", you are talking about the response of OUT from the time the SP change is made, until the directrion of the initial OUT change begins to reverse. This portion of the response includes Proportional AND Integral (and Derivative if not 0) and is impacted by the Process response. For example, during the dead time of the process response, the Integral will create a ramp.

    That being said, I can see that the downward action of the OUT is responding to the ARW_HI_LIM setting of 40 (units of OUT_SCALE=%). When the OUT is above the OUT_HI_LIM (40% in this case) and is moving downward, the Integral action is sped up by a factor of 16 (Effective Reset= Reset/16) until the OUT is below the OUT_HI_LIM! Books on line explains this. This is what is causing the increased speed and amount of movement on the downward SP step. I recommend that the ARW_HI_LIM be set to the 100% value of the OUT_SCALE (100% in this case). I see this action in both the V7 and the V13 examples. Apparently the difference in the two processes exacerbates the problem for the V13 case.

    Except in rare cases, the ARW feature in DeltaV is not needed and the ARW_HI_LIM and ARW_LO_LIM should be set to the high and low values of the OUT_SCALE to disable the feature during normal PID control. Note that these limits have the same units as the OUT_SCALE (e.g. if the OUT_SCALE is 0-5000 lb/hr, the OUT_HI_LIM should be 5000 and the OUT_LO_LIM should be 0).

    Please try this and let me know what you find. Don't hesitate to let me know if you want more information on this topic.

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