Quantification of valve stiction and dead band in control loops based on harmonic balance method

The presence of stiction in control valves often causes oscillations in control loops, with negative effects on quality and cost of goods. Step that follows diagnosis is to quantify this stiction, in order to decide about maintenance or to implement compensators that can improve control loop performance until next plant stop. The describing function method is a well known scheme to predict the period and amplitude of limit cycles in control loops, requiring the knowledge of linear and nonlinear parameters of system model. Here, a method is proposed to estimate these nonlinear parameters using the parameters of linear model and amplitude and period of limit cycle produced by nonlinearity. The result is a simple and efficient algorithm that can be easily extended to other nonlinearities. The conditions for existence and uniqueness of solution for dead band and stiction estimations are obtained. Also, the error from estimations is related to total harmonic distortion of control signal. The usefulness of the proposed method is demonstrated through its application to two examples.