Adaptive predefined-time sliding mode control for two-dimensional tracking in suspended gravity offloading systems

Ground-based simulation of low-/micro-gravity environments is an important enabling technology for space exploration, and suspended gravity offloading (SGO) systems have been widely employed because of their practicality and scalability. However, the two-dimensional tracking subsystem of an SGO system is subject to strong dynamic coupling, underactuated characteristics, external disturbances, and model uncertainties, which makes high-precision tracking control difficult to achieve. To address these challenges, this paper proposes an adaptive predefined-time disturbance observer-based sliding mode control framework for the two-dimensional tracking subsystem of SGO systems. First, an adaptive predefined-time disturbance observer is developed to estimate the lumped disturbance caused by external disturbances and model uncertainties. Then, a nonsingular predefined-time sliding mode controller is designed to improve tracking accuracy and robustness while avoiding the singularity problem commonly encountered in terminal sliding-mode designs. Compared with conventional finite-time and fixed-time control methods, the proposed method allows the upper bound of the convergence time to be explicitly prescribed by a user-defined time parameter and is independent of the initial conditions. Finally, comprehensive numerical simulations are conducted to verify the tracking performance, disturbance rejection capability, and robustness of the proposed control strategy.