Nonlinear Extended Two-State Observer-Based Elastic Prescribed Performance Control for Turntable Servo System With Time-Varying Parameters

This article investigates the tracking control problem of turntable servo mechanisms subject to unknown time-varying parameters, abrupt external disturbances, and input saturation. To address these issues, a nonlinear extended two-state observer (NETSO)-based elastic prescribed performance control (EPPC) scheme is proposed. Inspired by the congelation of variables idea, the unknown control gain is decomposed into a central item and a variation item, which are estimated by an adaptive law and an extended state, respectively. To improve estimation performance and alleviate the peaking phenomenon of fixed high-gain observers, an error-dependent NETSO is developed to estimate both the lumped disturbance and the gain variation. Then, an elastic prescribed performance mechanism is introduced to adaptively adjust the performance boundaries when input saturation or abrupt external disturbances occur. Furthermore, a lumped error incorporating both the estimation error and the tracking error is constructed, based on which an NETSO-based EPPC controller is developed to ensure that the lumped error always remains within the prescribed performance bounds. Meanwhile, rigorous stability analysis is carried out to guarantee the stability of the closed-loop system. Finally, simulation and experimental results on a turntable servo system show that the proposed controller can achieve accurate estimation of the control gain and ensure satisfactory tracking performance.