7783 Non-singular fast terminal sliding mode control with fixed-time convergence for 6-UCU parallel mechanism seismic simulator based on nonlinear observer

Abstract

Seismic simulators can accurately reproduce multi-dimensional seismic wave motions. However, its control system faces challenges such as strong nonlinearity, intense coupling, and unknown external disturbances, making precise control compensation difficult to achieve. This paper presents a hybrid control strategy based on a nonlinear observer and fixed-time nonsingular fast terminal sliding mode control. First, the dynamics of a 6-UCU Stewart-platform-based parallel seismic simulator are modeled using the virtual work principle. Next, by integrating fixed-time theory with a nonsingular fast terminal sliding mode surface, rapid convergence of the tracking error within a fixed time is ensured, enhancing the system’s real-time performance while effectively avoiding singularities. To further improve the response speed, a double-exponential sliding mode reaching law is employed. A nonlinear observer is utilized for feedforward disturbance compensation and high-frequency chatter suppression. Finally, MATLAB/ADAMS co-simulation and experimental validation demonstrate that the proposed controller outperforms PID and sliding mode controllers (SMC). Results indicate a 78% reduction in oscillation energy density and achieve steady-state accuracy within ±0.02 mm, validating the efficacy of the proposed approach for high-precision seismic simulation.