Predefined‐Time Disturbance Observer Based Command Filtering Control of Free‐Flying Flexible‐Joint Space Robots

ABSTRACT This paper addresses the predefined‐time control issue of free‐flying flexible‐joint space robots (FFSR) under the influence of system uncertainties, external disturbances and input saturation. A novel predefined‐time stability theorem is proposed for a class of nonlinear systems, with convergence time bounded by a user‐specified constant. Building upon this, a predefined‐time disturbance observer is developed to approximate the lumped disturbance, including system parameter perturbations, unmodeled dynamics and unknown disturbances. Additionally, To address the challenges related to “computational explosion” and singularity issue, the theorem is also applied in the design of command filter to estimate the derivative of the virtual control law. Consequently, a predefined‐time command filtering (PTCF) control scheme is further developed, incorporating a novel anti‐saturation auxiliary algorithm and a nonsingular filter error compensation mechanism to compensate for the adverse influence caused by actuator saturation and filter error simultaneously. Lyapunov stability theory and numerical simulations demonstrate that, under the proposed command filtering controller, the FFSR system remains predefined‐time stable, and the system states converge within the user‐specified time.