Talking about talking

Columnists Greg McMillan and Stan Weiner, PE, bring their wits and more than 70 years of process control experience to bear on your questions, comments and problems in this month’s installment of Control Talk.

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On average, we can say the upset arrives in the middle of the interval, so the average dead time is 1/2 the time interval. For unsynchronized digital devices, the worst-case dead time could be the summation of the time intervals. If the output is done at the end of the time interval, the dead time varies from one to two time intervals for an upset that arrives just before and after the input, respectively. This is the case for chromatographs and other analyzers where the sample is processed, and the analysis is ready at the end of the cycle time. Here the average is 1.5 times the time interval (cycle time).

Stan: In practice, the scan time of DCS inputs is set to reduce jitter and aliasing. But exception reporting and data compression can cause a distorted view of the data and can make any trend look flat, particularly large plot scales or short time frames.
 

    

This Month's Puzzler

What Time’s the Execution?
In a simulation test on the effect of module-execution time, what is the additional dead time from the module for a disturbance added to the measurement input to a PID block in the module with the same execution time as the module?
Send an e-mail with your answer, questions, or comments to The Puzzler.

Greg: Even when dead time is introduced, it has little effect on performance for detuned controllers, since the integrated absolute error for the upset depends on the controller tuning settings. In last month’s column, we discussed how an increase in digital time intervals did not have an effect on a controller tuned with a Lambda factor of one until the total dead time exceeded half the process time constant. Thus, tests on the effect of intervals and cycle times should use different relative timings of unmeasured disturbance and various tuning settings.
 
Stan: And speaking of communication, we offer the following Top Ten List.

Top Ten Signs Talk Radio’s Not for You

 10. You have taken to using one-word sentences and one-syllable words like “yep.”
 9. You think John Wayne was too verbose.
 8. You feel life is just a series of subtitles.
 7. Friends take to using sign language around you.
 6. Strangers think you have laryngitis.
 5. Your parents keep turning up their hearing aids.
 4. You run off to become a “roadie” for the Blue Man Group.
 3. Your favorite entertainment is watching mimes.
 2. You let your slides do the talking.
 1. You are mistaken for a statue.


  About the Authors
Greg McMillan and Stan Weiner, PE, bring their wits and more than 66 years of process control experience to bear on your questions, comments and problems. Write to them at controltalk@putman.net.
 
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