Fixed-time sliding mode composite control with prescribed performance for uncertain Stewart parallel mechanism tracking in task space

To improve the tracking control performance in task space of the Stewart parallel mechanism (SPM) with disturbances and uncertainties, a fixed-time sliding mode composite control with prescribed performance (FxT-SMCC-PP) method is developed. First, by constructing a sliding mode term adaptive to the changes of the lumped uncertainty, and embedding fixed-time performance parameters, a fixed-time adaptive sliding mode disturbance observer (FxTASMDO) is designed. Second, the SPM's tracking error is constrained by an asymmetric hyperbolic cosecant prescribed performance function and transformed into a steady equivalent error by modifying the constraint transformation function. Based on the equivalent error, a fixed-time sliding mode control with prescribed performance is designed and combined with FxTASMDO to form the FxT-SMCC-PP, which can achieve the SPM's fast and non-overshoot transient response, high steady-state tracking accuracy, and reduction in control energy consumption. Finally, the effectiveness of FxT-SMCC-PP is validated through simulation and prototype experiments.