Continuous sliding mode disturbance rejection control for flexible joint robots with input saturation

In this paper, a robust disturbance rejection control scheme is proposed for the trajectory tracking of flexible joint robots (FJRs) affected by multi-source disturbances and input saturation. The unknown time-varying disturbances, which are categorized into matched and mismatched, generally encompass uncertainties in internal parameters, coupling effects, unmodeled dynamics, and changing external environments. To address these disturbances, the mismatched disturbances are transformed into matched disturbances through the flatness method, eliminating the computational cost of estimating mismatched disturbances. Meanwhile, a generalized proportional integral observer (GPIO) is adopted to estimate the unmeasurable states and disturbances. In addition, to deal with input saturation, an anti-windup compensation auxiliary system is constructed. Finally, an output feedback control framework based on continuous sliding mode control (CSMC) is further designed. The proposed control approach facilitates asymptotic convergence of the tracking error and mitigates the chattering effect inherent in traditional sliding mode control (SMC), thereby enhancing both control and dynamic performance while satisfying input constraints. A detailed stability analysis of the closed-loop system is also presented. The feasibility and effectiveness of this approach are validated through comparison with a traditional disturbance rejection control scheme.