Uniform Feasibility For Smoothed Backup Control Barrier Functions

TL;DR

This paper introduces a smoothing technique for Control Barrier Functions to guarantee safety constraint feasibility in backup control methods, applicable to both bounded and unbounded safe sets.

Contribution

It provides explicit bounds and conditions under which smoothed CBFs ensure safety constraints are feasible, improving backup control safety guarantees.

Findings

Smooth approximation of nonsmooth safe sets becomes a CBF under certain conditions.

Explicit lower bounds on the smoothing parameter guarantee feasibility for compact safe sets.

Tail conditions ensure uniform CBF properties for unbounded safe sets.

Abstract

We study feasibility guarantees for safety filters developed using Control Barrier Functions (CBFs) when a safe set is defined using the pointwise minimum of continuously differentiable functions, a construction that is common for the backup CBF (BCBF) method and typically nonsmooth. We replace the minimum by its log-sum-exp (soft-min) smoothing and show that, under a strict safety condition, the smooth function becomes a CBF (or extended CBF) for a range of the smoothing parameter. For compact safe sets, we derive an explicit lower bound on the smoothing parameter that makes the smooth function a CBF and hence renders the corresponding safety constraint feasible. For unbounded sets, we introduce tail conditions under which the smooth function satisfies an extended CBF condition uniformly. Finally, we apply these results to BCBFs. We show that safety of a compact (terminal) backup set under a backup controller, together with a condition ensuring safety of the backup trajectories on the relevant boundary of the safe set, is sufficient for constraint feasibility for BCBFs. These results provide a recipe for a priori feasibility guarantees for smooth inner approximations of nonsmooth safe sets without the need for additional online certification.