Why Does Control Loop Performance Change Over Time? What is Time Variant Behavior?

You visit a doctor annually for a physical examination.  Your car gets an oil change every 3,000 miles.  Your company assesses profit versus loss every 3 months.  The list of routine “check ups” goes on and on.  Why?  Because such things are important and they change over time.  It’s valuable both to appraise their relative health and to determine what adjustments may be necessary.  As the foundation for regulatory control at most production facilities, PID control loops are no different.

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