When tuning noisy loops, we recommend in our courses that one should eliminate the noise by editing it out, so the tuning will be done only on the true process response, free of any noise. The controller is controlling the process, and is not controlling the noise.
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Which is the weakest link in a control loop? The answer, without any doubt is that, in most cases, the final control element is the weakest link.
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Reminiscences from Michael Brown on a long and rich journey in the world of automation together with SAIMC.
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The first law of process control could be stated as saying that you cannot control if your measurement is incorrect. We all know this law. However do we all remember it in the heat of the moment?
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The widely held belief in many plants that tuning will solve all base layer control problems is completely fallacious. Bad tuning is generally not the main reason for loops to perform badly. It is important when performing optimisation that all elements in a loop are considered, in addition to the control strategy, before even thinking of tuning.
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It is generally better to live with noise rather than filtering it out, provided it does not cause the final control element to jump around excessively.
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Many process engineers do not appreciate the importance of flow loops in their unit controls. A senior process engineer once told me that flow loops need not be tuned well, as they generally have little effect on the more important and much slower loops like temperatures and pressures.
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This Loop Signature series of articles would not be complete without sections on the problems of noise and filtering, which are two of the most misunderstood areas in process control.
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When analysing a control loop, one of the important things that one must do is to determine the dynamics of the process.
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The integral or I term in a controller is a brilliant thing. It is an extremely elegant and simple solution for eliminating offset in control. However, like everything else in this world, virtually everything good has its bad side as well.
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It is vital, when optimising a control loop, to establish at the outset if the process is self-regulating or integrating, as not only do these two process types behave completely differently, but also they are tuned very differently.
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People often have little realisation as to how badly a faulty valve can affect the performance of the control of the loop. So how can one make a real valve with its problems into a perfect valve? The answer is simple. One makes use of a technique called cascade control, whereby a second controller is used to control the flow of fluid through the valve.
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