Robustness, performance analysis and practical improvements of modified uncertainty and disturbance estimator based control

Modified uncertainty and disturbance estimator (MUDE) based control shows great control performance and robustness against unexpected dynamics in the presence of time delay. However, there are still some problems that have not been analyzed theoretically but actually hinder its further applications, such as noise amplification and control signal fluctuations under delay mismatch. On the other hand, the trade-off between robustness and control performance is still a critical issue in MUDE based controller design. However, some doubts arise when extending the single-loop sensitivity function analysis method to MUDE (multi-loop) control systems. To this end, this paper comprehensively analyzes the performance of MUDE, including control performance and robustness, and proposes some practical improvements to address its issues (denoted as IUDE). An ambiguity in analysis of sensitivity functions for multi-loop control systems is first clarified. Then, based on the two-degree-of-freedom equivalent control structure and sensitivity functions, the control performance and robustness of the MUDE based control system are analyzed in detail, including noise sensitivity and control signal response. Finally, some practical improvements are proposed and a quantitative tuning rule is also derived to facilitate its application. Simulation results show that the proposed control method effectively mitigates the problems of noise amplification and control signal fluctuation while almost not degrading the control performance. Water-tank control experiments have also been performed to validate the efficacy of the proposed method, which depicts a promising prospect in control practice.