Aug. 10, 2006
Control Talk columnists McMillan and Weiner describe the period of time between when you first take a drink and when you first recognize the effect and bypass the next round for coffee as dead time.
By Greg McMillan and Stan Weiner, PE

Stan: Have you ever been asked a question on process control that leaves you at a loss for words? Here is your chance to amaze friends and relatives (or at least baffle them) by learning some control concepts and analogies. If you understand the ideas and their significance conveyed in the answers to the following devastating questions from students in Gregs class during his brief but illustrious career as a professor, you might just become famous by Friday. The answers never made it into a university text for obvious reasons.

Student: What is loop dead time?

Greg: Dead time is that period of time from the start of a disturbance until the controller makes a correction that arrives at the same point in the loop that the disturbance entered. The controller needs to see the upset, react to it, and send the correction to the right place. To appreciate dead time, consider when you go to a party and start drinking. The period of time between when you first take a drink and when you first recognize the effect and bypass the next round for coffee is dead time. As the dead time approaches zero, the portion of the open-loop error that appears as the closed-loop error approaches zero, and nonlinearities and high process gain (high proof and small body) become unimportant.

Student: What is the open-loop error?

Greg: Well, if you continued drinking, your state of intoxication would exponentially increase and reach a steady state with you passed out on the floor. The open-loop error is the alcohol concentration at this point. It is the result of having the controller (your mind) in manual or disconnecting it from the loop (your body). A smart party animal knows when to say "when," according to a famous seer.

Student: What is the closed-loop error?

Greg: The peak excursion of the alcohol concentration until the coffee first enters the bloodstream is the closed-loop error. For the well-tuned PI and PID controllers, the best you can do is limit the closed-loop error to the maximum deviation from set point that occurs at 150% and 110%, respectively, of the loop dead time. Often, the party mind does not behave like a well tuned controller, and the closed-loop error reaches an unacceptable level for driving, especially if there is a non-self-regulating process.

Student: What is a non-self-regulating process?

Greg: If your liver break down and remove alcohol at a fixed rate, the alcohol concentration ramps, which is characteristic of an integrating process. If an increase in intoxication causes a decrease in your restraint, you might start drinking faster. This is positive feedback, and the acceleration of the concentration corresponds to a runaway process. People with such non-self-regulating processes are easy to spot at parties; they are the ones with the lampshades on their heads.

Student: How can you reduce loop dead time?

Student: What if you can't change the plant design?

Greg: Then your best bet is to use feedforward control for disturbances that can be measured when the dead time in the disturbance path is smaller than the dead time in the correction path to a common point in the process; use cascade control for disturbances that can be isolated by an inner loop when the inner loop dead time is several times smaller than the outer loop dead time. With the help of Mark Sowell, we conclude with a top ten list, which doesnt have much to do with dead time but is still timely on the road to fame and fortune . well maybe just fame.

### Top Ten Signs You Have Too Many Alarms

(10) The console is more exciting than a pin ball machine.
(9) The only time there are no alarms is when the console is unplugged.
(8) Operators are deaf to the high pitch tones.
(7) The operator can play along with his favorite rap song on the acknowledge button without missing a beat.
(6) A woodpecker is being trained to keep pecking the acknowledge button.
(5) The alarm password is going for \$50 on the Plants black market.
(4) Visiting the control room for a doughnut is not worth the noise.
(1) Because they are free is the basis for your alarm design strategy.

Greg McMillanandStan Weiner, PEbring their wits and more than 68 years of process control experience to bear on your questions, comments, and problems. Write to them at [email protected].

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