PID Loop Tuning

This is a closed-loop test, based on the fact the output signal responds dynamically to the changing process variable, as well as to the step-change in setpoint.

This is a reverse-acting controller: the output steps up when the setpoint steps up (implying the output would step down if the process variable stepped up).

There do not appear to be any field instrumentation problems revealed in this trend. A manual-mode (open-loop) test would be more informative in that regard, but it appears as though the process is very quick to respond with no discernable dead time or other lags.

The controller tuning is clearly inappropriate for this process. Note the large offset between PV and SP (i.e. how the process variable never settles at the setpoint value, even though it’s clearly a fast-responding process). This tells us the controller is configured only for proportional action, and this process needs integral! We can also tell this from the 180$^{o}$ phase shift between PV and output during the oscillations: this is the classic response of a reverse-acting proportional-only controller with excessive gain.

Aggressive integral action with a minimum of proportional gain should work very well in this process, which is probably a liquid flow-control process.

This is a closed-loop test, based on the fact the output signal responds dynamically to the changing process variable, as well as to the step-change in setpoint.

This is a reverse-acting controller: the output steps up when the setpoint steps up (implying the output would step down if the process variable stepped up).

There do not appear to be any field instrumentation problems revealed in this trend. A manual-mode (open-loop) test would be more informative in that regard, but it appears as though the process is very quick to respond with no discernable dead time or other lags.

The controller tuning is too heavy on proportional action. We can tell this from the phase shift between PV and output during the oscillations, which is nearly 180$^{o}$. Excessive integral action would shift the phase of the output wave further to the right (i.e. so that each peak of the output waveform coincided with the zero-crossing of the PV waveform, or very nearly). The fact that the inverse peaks of the PV and output waves are very nearly aligned tells us that excessive gain (proportional action) is the culprit here. Another clue is the magnification of noise we see in the output trend compared to the PV trend—only proportional action or derivative action can cause this, and since we see no sign of excessive derivative action (e.g. output wave leading the PV wave), we can safely say the problem is too much gain.

The process response time (dead time, lag time) seems to be very short, which is a good thing for process control. We can tell, however, that this is an {\it integrating} process by the way it was able to achieve a new SP value with the old output value. This means it will exhibit some overshoot with SP changes if there is any integral action. We may have to do most of the control through proportional action (albeit much less gain than we are using now!), with just enough integral action to handle load changes.

This is a closed-loop test, based on the fact the output signal responds dynamically to the changing process variable, as well as to the step-change in setpoint.

This is a reverse-acting controller: the output steps up when the setpoint steps up (implying the output would step down if the process variable stepped up).

This process does exhibit some dead time as well as lag time, which explains the setpoint overshoot. A field check of the control element (valve) might be good to do, so see that it is not sticking and causing dead time.

The controller tuning actually looks pretty good here. The only problem is the slight overshoot of setpoint, which may or may not be significant depending on the specific process and the needs of operations personnel. If this overshoot is deemed excessive, we might wish to turn down the proportional action (gain), based on the fact the PV and output waves seem to hit their respective peaks at nearly the same time (characteristic of proportional-dominant action).

Given the existence of dead time and lag time together, we must be careful not to use too much proportional action lest the loop oscillate. Derivative action could be very useful in taming the effects of lag time.