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Aug
28

Tips-N-Techniques (TNT) – Effect of Sample Delay on Tuning and Loop Performance

If you are interested in how sample time delay affects PID tuning and performance, you might want to check out Advanced Application Note 005. AdvancedApplicationNote005 This note provides quantitative guidelines as to what point additional sample time delay causes a noticeable deterioration in performance and excessive oscillations for a given controller tuning. Alternatively, the note predicts new tuning settings for smooth control. Not emphasized in the note is that while the control may be smooth after retuning a PID for an additional delay time, the integrated absolute error (IAE) per Equation 3-1 in the note will be accordingly larger. Also, if the controller is tuned for maximum load rejection, the peak and integrated errors are proportional to the total loop dead time and dead time squared, respectively per references 2 through 4 in the note. The total loop dead time is the summation of all time delays in the loop including the control module execution time.

This leads to some rules of thumb that the additional delay from sampling should be less than 1/10 of the total loop dead time and process time constant. For more normal tuning, the sample delay should be less than 1/5 of the loop dead time and process time constant. For liquid pressure control, most of the loop dead time is the module execution time. The measurement sample time should be less than 0.04 seconds for a module execution time of 0.2 seconds. For flow control, the controller tuning can often be slowed down (Lambda increased to 4 or more) to accommodate the additional delay and sample times of several seconds are OK. For most level loops, the integrating process gain is so small (Lambda is so large) that measurement sample delays are inconsequential. For furnace and incinerator pressure control with set points in inches of water column, the response is so fast – the integrating process gains are so large (Lambda is so small), that the sample time requirement is similar to liquid pressure control. For gas pressure control of large header or vessel volumes with set points in psi, the allowable sample time can be substantially larger (e.g. seconds) because the process time constant or integrating process gain is usually slow. For temperature and composition control of vessels, quite a large sample time (e.g. minutes) is tolerated because the process time constant or integrating process gain are so slow. However, for runaway reactors, sample times approaching the runaway time constant are disastrous. For bioreactor and distillation column composition control, sample times in hours may be acceptable.