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MAR-APR 2018

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AUTOMATION BASICS INTECH MARCH/APRIL 2018 43 An external source of oscillation could affect a control loop through the process or via the SP (if the latter is generated by another controller). This determination can be made by looking at a trend of the SP. It should be easy to see if the SP oscillates. If the SP is oscillating, the control loop generating the set point should be checked, as described later. If the oscillation is coming from the process, a bit more detective work is needed. Trends of all the possible interacting process variables need to be inspected. With highly interactive processes, several control loops may be oscillating at the same frequency. The root cause of multiple os cillations can normally be found by looking at the trends to see which one of them peaks first. This can also be combined with looking at which Figure 1. Troubleshooting the root cause of poor control loop performance Figure 2. Oscillations (top trend) versus random deviations (bottom trend) Figure 3. The triangular wave (middle trend) leads the other oscillations and has the least distortion. signal has the least distortion (the crispest or cleanest oscillation, such as the triangular wave in figure 3). The oscillation leading the others and having the least distortion is most likely the cause of all the other oscillations. One can then troubleshoot the source of oscillation further, as described below. Once the exact loop causing the oscillation has been identified, the root cause can be determined through a few simple steps. The oscillation can have one of two causes: incor - rect tuning or a control valve problem. If the control loop is oscillating because of tuning, trends of the PV and CO are normally smooth sine waves (except if the CO runs into upper or lower limits). If a control valve problem is causing the oscillation, a trend of the CO more Poor control loop performance Set point Filtering Process Valve deadband Valve problem Sluggish tuning Aggressive tuning Fast disturbances Oscillations External origin Cyclical interaction Noise Disturbance Random deviations Internal origin

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