Dead Time from Discrete
Devices and Analyzers
Rate of Change
Module
The use of a rate of change as the controlled variable is described for PID control of an exothermic reactor in my book,
A Funny Thing Happened on the Way to the Control Room, and for model predictive control of a bioreactor in
New Directions in Bioprocess Modeling and Control.
Stan: In the old days, some project managers suggested using thermocouple DCS input cards instead of individual thermocouple transmitters to save money. The resolution (e.g., 11 bits), large-scale ranges (e.g., 632º) and fast scan rates (e.g., 0.25 sec) of these older DCS cards caused A/D chatter that was greater than the true change in temperature (e.g., A/D chatter of 0.30 versus a true temperature change of 0.001º). This A/D noise meant you could not use rate action or hardly any gain action even though it was permitted and even desirable per the process dynamics. The best solution is to use a smart transmitter and a newer DCS with a 16-bit A/D, but even so, a fast control execution interval may just be increasing loading rather than information and performance. A similar consideration holds for level control because the true level change during a time interval may be 0.0001% or less.
Greg: Whether we are talking about analyzers or any sort of digital communication, control and processing, a dead time is created for unmeasured disturbances from the time interval. The actual dead time to detecting and reacting to an upset depends upon the relative timing of the read (input), write (output) and the upset. If the output is done right after the input, the dead time varies from nearly zero to one time interval for an upset that arrives just before and after the input, respectively.
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 Times 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 months 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 Radios 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.
Greg McMillanandStan 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 [email protected].