Detection of Oscillating Control Loops

Abstract

Oscillations in process control loops are a very common problem. Oscillations often indicate a more severe problem than irregular variability. The presence of oscillations in a control loop increases the deviations from the setpoint of the process variables, thus causing inferior products, larger rejection rates, increased energy consumption and reduced average throughput. There are several reasons for oscillations in control loops. They may be caused by excessively high controller gains, oscillating disturbances or interactions, but a very common reason for oscillations is friction in control valves. Detection and diagnosis of oscillatory behaviour in a production process is of importance because process variability has an impact on profit. In the context of performance assessment and troubleshooting, the objectives of oscillation diagnostics of control loops can be stated as to i) identify sustained oscillations in a control loop, and ii) distinctly detect and quantify the root-cause(s) once the loop is confirmed to be oscillatory. In this chapter, the methods that are present in the literature to identify oscillations in process variables are discussed in detail. Their applicability to industrial control-loop diagnosis is critically evaluated on various simulation and industrial case studies.