Open Loop Tuning Method

DCS
1

  1. Choose largest step change in controller output that is safe.

  2. Add a PV filter to keep the controller output fluctuations from noise within the valve deadband.

  3. Make a change in controller output in manual.

  4. Note the time it take for the process variable to get out of the noise band as the loop dead time.

  5. Estimate the process time constant as the time to reach 63% of the final value.

  6. Estimate the process gain as final change in the process variable (%) after it reaches a steady state divided by change in the controller output (%).

  7. To use reaction curve tuning, set the controller gain equal to ½ the process time constant divided by the product of the process gain and dead time.

  8. If the process lag is much larger than the loop dead time, set the reset time setting equal to 4x the dead time and set the rate time setting equal to the dead time. If process lag is much smaller than the loop dead time, set the reset time to 0.5x the loop dead time and the rate time to zero.

  9. If a high controller gain is used (e.g. > 10) use setpoint rate of change (velocity) limits if a big kick in the controller output for setpoint changes is disruptive to operations for PD on error (enable external reset feedback).

  10. Make setpoint changes across the range of operation to make sure an operating point with a higher controller gain or larger process dead time does not cause oscillations.

Monitor the loop closely over several days of operation.

source : ISA

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