Safety Filtering with an Infinite Number of Constraints

TL;DR

This paper extends control barrier function theory to handle infinite safety constraints, providing conditions for invariance, regularity, and practical controller design, addressing limitations of existing backup CBF methods.

Contribution

It introduces a theoretical framework for infinite constraints in CBFs, including regularity conditions and connections to optimal-decay CBFs, enhancing safety controller design.

Findings

Nagumo's Theorem reduces to barrier-like inequalities under certain conditions.

Associated CBF controllers can be at least continuous with proper regularity.

The theory addresses limitations of backup CBFs in safety-critical systems.

Abstract

Control barrier functions (CBFs) provide a rigorous framework for designing controllers enforcing safety constraints. While CBF theory is well-developed for a finite number of safety constraints, certain applications, e.g., backup CBFs, require an infinite number of constraints. Despite the practical success of CBFs, several fundamental questions remain unanswered when safe sets are defined with an infinite numbers of constraints, including: necessary and sufficient conditions for forward set invariance, the actual definition of CBFs associated with these sets, the regularity properties of the resulting controllers, and the ability to reduce a collection of infinite constraints to a finite number. This paper addresses these questions by extending CBF theory to the infinite constraint setting. We identify regularity conditions under which Nagumo's Theorem reduces to barrier-like inequalities and when the associated CBF controllers are at least continuous. We further connect these results to optimal-decay CBFs, bridging theoretical conditions for invariance and practical instantiations of the resulting controller. Finally, we illustrate how the developed theory addresses limitations of backup CBFs.