Despite increasing computer control, not all loop tuning actually optimizes. A new note from ExperTune’s George Buckbee describes four types of PID control loops which tuning cannot improve, in fact it may make matters worse. First case is when the loop is operating at a limit, if, say, a valve is fully open or closed. Here control is a waste of time. It is preferable to find out why the loop is limited and to get the controller back into a normal range.
Failing instrumentation likewise gives rise to situations where control will fail. Another ‘do not tune’ scenario occurs when a loop is already tuned. Different criteria can define ‘tuned’ and operators may need to study some process fundamentals to answer the question ‘what does good tuning really mean?’
Another common problem is trying to tune a loop when the root cause of the problem is somewhere else. ’ Process plants are complex places, full of dynamic interactions that spread through the plant. You need to find the root cause of process upsets and eliminate them at the source.’ Buckbee’s company has software tools to help in such cases. Cross-correlation of sensor data with historical data using PlantTriage’s process interaction map can pinpoint the root cause of a problem.
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