APPLYING D-ACTION TIPS AND TRICKS
WHEN DO YOU APPLY D-ACTION? TIPS AND TRICKS FROM YAZZOOM
Would this PID control loop benefit from D-action or not? Every process control operator or engineer has probably asked himself this question several times during his career. Here are some tips and tricks that will assist everyone who has to deal with this question.
Within a PID controller, the derivative action determines the level of anticipation based on the past history. This means that derivative action reacts on large changes in either the process variable or the control error, depending on the PID equation that is used. Using derivative action can be beneficial when the process has long dead times, especially when the ratio of (dead time / time constant) > 1.5. Applying such a D-action results in three important changes for the control loop in such circumstances: the error magnitude will reduce and the oscillation period will decrease. These results are also clearly visible in the figure.
Furthermore, it can be useful to low-pass filter the process variable in order to avoid excessive movement of the control output and hence of the control valve. However, another reason to use D-action can be in case the control valve suffers from stiction but cannot be repaired or replaced yet: in this case the more dynamic control output as a result of the D-action will help to keep the valve responsive, and in this case you will rather want to limit the amount of filtering on the signal.
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