Nonlinear control performance assessment in the presence of valve stiction

Control performance assessment or CPA is a useful tool to establish the quality of industrial feedback control loops, but in practice its usefulness is hampered by nonlinearities in the feedback loop. The Harris index is a popular and easily automated metric used to quantify performance of the loop. However, computing the Harris index simply ignoring common process nonlinearities such as valve stiction will lead to an over-estimate of the index, and consequently a false sense of security. In this paper, we propose two complimentary strategies for accurately assessing the quality of the control performance for loops suffering from modest valve stiction thus avoiding the bias inherent in the standard CPA calculations.