Robust Safety Filtering for Input-Constrained Underactuated Linear Systems

Abstract

We develop a robust safety filter for input-constrained underactuated linear systems subject to unknown bounded-rate disturbances and structured model uncertainty entering through known distribution channels. A disturbance observer provides an online disturbance estimate and a dynamic estimation-error radius that define the certified uncertainty envelope used in the robust high-order control barrier function constraints. For general polytopic actuator sets, the resulting robust safety-admissible input set is polyhedral, while the shared scalar-input specialization yields an exact interval, a necessary-and-sufficient pointwise feasibility condition, a signed feasibility reserve, and a closed-form safety projection. A domain-wise certificate relates robust HOCBF control demand to available actuator authority, while a finite-horizon energy identity quantifies deviation from the unconstrained $H_\infty$ reference without claiming preservation of its original attenuation level. Across all $16$ structured-uncertainty corners in the linear-model actuator-stress test, the filter remains feasible and safe under $\tau_{\max}=1.18$ N$\cdot$m, with $\max\abs{p}=0.0740$ m, $\max\abs{\theta}=0.205$ rad, $\max\abs{\tau}=1.173$ N$\cdot$m, and $\min\mu_S=7.27\times10^{-3}$ N$\cdot$m.