Derivative Action, Blessing or Curse (Booklet and Software)

To paraphrase the children's story "girl with a curl in the middle of her forehead", when derivative action is good it can be very, very good, and when it is bad, it can be horrid. Of the three controller adjustments typically available, derivative action is the most unforgiving of a guess-and-test approach to tuning. If you don't know what you are doing, you are better off not using it. Indeed, if you do know what you are doing, you will often decide you are better off not using it.

Derivative action must be set relative to certain lags in the loop. To determine these lags it is usually, if not always, necessary to make an open-loop or a closed-loop test. If too little action is used, it might as well not be used at all. If too much is used, it can cause instability.

For more information you may refer to a booklet "Controller Tuning and Control Loop Performance,a Primer" Second Edition, which is available in the US for $17, postpaid if prepaid. It has sold 29,703 copies to date. Control Engineering Magazine has referred to it as "PID Without the Math".

Information is available at:

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from which there is a link to a page about companion software ($11). If you wish, I could e-mail you a brochure on either.

David W. St. Clair Straight-Line Control Co.

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