Patent ID: 8145328
Filing Date: 2012-03-27
Classification: G05B

Abstract:
1. A method for detecting and quantifying stiction in an industrial control loop comprising a control valve, comprising the steps of: (a) obtaining process output (pv) and controller output (op) data from operation of the control valve; (b) determining whether or not the control error signal (sp-pv), or the pv signal, or the op signal, is non-Gaussian and non-linear, by determining a Non-Gaussianity Index (NGI) and a Non-Linearity Index (NLI), where both the NGI and the NLI are greater than zero in accordance with where bîc is the average squared bicoherence and bîc 2 max is the maximum squared bicoherence, σ bic 2 is the standard deviation of the squared bicoherence and bic crit is the statistical threshold/critical value obtained from the central chi-square distribution of squared bicoherence; (c) if the control error signal, or the pv signal, or the op signal is both non-Gaussian and non-linear, obtaining the pv and op relationship; (d) choosing a segment of the pv-op data which has a regular oscillation; (e) determining from the pv-op data segment whether or not the process is suffering from stiction by determining whether a plot of the pv-op data segment is elliptical; and (f) quantifying the amount of apparent stiction by measuring the width of the ellipse of the pv-op data in the direction of op.