Fixed-Time Constraint-Aware Sliding-Mode Control for Ride Comfort Enhancement in Active Vehicle Suspensions

Active vehicle suspensions must improve ride comfort while respecting strict suspension-travel and tire-load constraints under uncertain road disturbances. Although sliding-mode control ensures robustness, conventional designs provide only asymptotic convergence and may generate chattering, limiting transient predictability and practical implementation. This paper proposes a continuous, non-singular fixed-time sliding-mode controller (FxT-SMC) with explicit constraint awareness for nonlinear quarter-car active suspensions. A state-dependent sliding map guarantees fixed-time convergence with an initial-condition-independent settling-time bound, while a smooth reciprocal barrier embedded in the sliding surface enforces forward invariance of the admissible travel set without discontinuous projection. Global fixed-time stability and bounded zero dynamics are rigorously established via Lyapunov analysis. Experimental validation over representative road profiles demonstrates predictable sub-2-second transients, strict satisfaction of travel and tire-load limits, and significant improvements in ISO 2631-1 comfort metrics compared with passive suspension, classical SMC, and super-twisting control. The results highlight the effectiveness of fixed-time, constraint-aware sliding-mode control for practical intelligent active suspension systems.