Closed-loop step response identification for improving on-line autotuning of load disturbance rejection

Based on a closed-loop step response test, a low-order model identification method is proposed to facilitate on-line controller tuning for load disturbance rejection in industrial engineering practices. By introducing a damping factor to the closed-loop step response for realizing the Laplace transform, a frequency response estimation algorithm is given in terms of the closed-loop system configuration for operation. Correspondingly, a model identification algorithm is analytically derived for obtaining a low-order process model of first-order-plus-dead-time (FOPDT). Another identification algorithm is developed for improving model fitting accuracy over a specified frequency range interested to controller tuning, specifically detailed for obtaining the most widely used FOPDT and second-order-plus-dead-time (SOPDT) models, based on a weighted least-squares fitting of the process frequency response points estimated in the specified frequency range. Using the identified models, a model-based controller design and tuning method is proposed for improving load disturbance rejection. Illustrative examples from recent references are used to demonstrate the effectiveness and merits of the proposed identification algorithms and controller tuning method.