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Sep
26

Tips-N-Techniques (TNT) – Reset Has No Sense of Direction

We live in a digital world but don’t stare at digital values of process variables.

If a reactor temperature is below set point as shown in the Good Tuning Pocket Guide Figure 2 for a PID loop faceplate, should the steam or water valve be open? After looking at a faceplate or the digital value for temperature on a graphic display, most people think the steam valve should be open when the temperature is below set point. Reset provides a direction of action that is consistent with this human expectation. However, the proper direction for a change in controller output and the split-ranged control valve depends upon the trajectory of the process variable (PV). If the temperature is sharply increasing or is closing in on the set point, the coolant valve should be opening. Gain and rate action will recognize that a set point is being approached and position the valves correctly to prevent overshoot. In contrast, reset has no sense of direction and sacrifices future results for immediate satisfaction. Reset won’t try to open the coolant valve until the temperature is above set point. In two separate applications in chemical plants, it was reported that the PID controller was seriously malfunctioning because the wrong valve was supposedly open, when in reality it was just gain and rate doing their job to prevent overshoot. View image

The sign of the delta in a PID controller output for the proportional and rate modes depend upon the direction of the change in the process variable (PV). In other words for reverse action, an increase in the PV will cause a decrease in the controller output. However, for reset action from the integral mode, the direction of the change depends upon the sign of the error. Thus, for the reverse action controller, if the PV is below set point, the contribution from the integral mode is an increase in the controller output even if the PV is rising. To see how deltas rule in process control and why an automation engineer would make a great president, check out the September Control Talk column. http://www.controlglobal.com/articles/2008/312.html

Note that in this column, Stan is misquoted. The last sentence of the 3rd dialog by Stan should read “The delta in output from the rate mode also depends on these same deltas.”

All of the faceplates and operator graphic displays I have seen in my 40 year career show digital values or indicating bars that are a snapshot. Operators, technicians, and engineers staring at these operator interfaces will come to the wrong conclusion as to what the PID controller output should do, particularly for slow and integrating loops. Most manual tuning is done based on staring at these interfaces, which is one of the main reasons slow loops have too much reset action. People don’t have the patience or ability to visualize the trajectory. You need a trend chart with the proper PV and time scaling. For temperature, the scale should narrow for the PV due to low noise tight control and wide for the time due to the slow response. The trend chart with proper scaling should pop up whenever a person wants to inspect or tune a loop. Even better would be a trend of the future prediction of the process variable similar to what is provided by the operator interface to model predictive control packages. With new adaptive control software that automatically identifies process dynamics such as DeltaV Insight the trend of the future trajectory could be added based on the model used for tuning. This would be valuable for slow loops and particularly for integrating, and runaway processes.