SA Instrumentation & Control | Volume 40 | September 2024

CONTROL SYSTEMS

Figure 2.

Figure 3.

• A much lower frequency cycling, almost sinusoidal in nature and of higher amplitude than the high frequency fluctuations, could also be seen in various places on the response as the level came down to SP. What was going on? Firstly, random relatively high frequency variations around a mean level are often seen in PV signals. We define this as noise. It can be caused by disturbances in the process itself or by the measuring method, as was probable in this case. Typically noise in flows may be caused by turbulence, and in tank levels by ripples and disturbances on the surface of the liquid in the tank. The high frequency fluctuations seen in the test definitely fall into the noise category. I have written many articles about noise and its effects on processes. Contrary to popular belief, it generally has very few deleterious effects on the actual control, but it can make the PV signal look awful. Unfortunately, with the advent of digital control systems, it is very easy to apply filters so that the noise cannot be seen on the PV signals on the operators’ consoles. However, and most

unfortunately, filtering can itself have adverse effects on the quality of control. The reasons for this are outside the scope of this article. C&I personnel often ruthlessly suppress the noise by applying filters, and these days operators are not used to seeing it. When one removes filters, the operators then generally mistake the noise for control instability, and often become very unhappy. It often needs quite a lot of discussion and persuasion to get them used to controlling with noisy PV signals on their displays. On the other hand, the slow cycles seen in the test were definitely caused by instability. In integrating processes this is normally evidenced by quite low frequency cycling as can be seen here. The reason for the instability in this case was almost certainly due to the poor as-found tuning in the controller. The new tuning was P = 4,0 and I = 38,0 minutes/repeat. It can be seen in the second half of the closed loop test shown in Figure 2. how well the PV responded to a step in SP. Figure 3. shows the loop’s control performance after the optimisation, in a closed

loop test recorded for over 15 hours. The SP was kept constant, but one can see how well the control responded to fairly significant load changes that occurred during this period. This is evidenced by the movements of the PD, which was working quite hard to keep the PV on SP. Also, in this test one can see that there is a small level of noise on the unfiltered PV. This noise appears much bigger on the PD signal, which is because it is amplified by the proportional gain of 4. C&I personnel often mistakenly become worried about seeing this, as they think the valve will be adversely affected. However, one must remember that pneumatically operated valves have a lot of natural damping in them, as there are relatively large air volumes in their actuators, and the noise frequencies on the PD signal are generally far too high to affect the valve. I always advise people who are concerned to go out into the field and put a finger on the actuator stem so they can be assured the valve is not in fact jumping around.

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