Safety filters activate early to prevent robot control failures
Timely Activation of Safety Filters via One-Step Reachability Expansion
Robotics
Summary
Robots need to avoid dangerous states to operate safely. Usually, safety systems step in only when a robot is about to enter these risky zones, but real machines check less often than theory assumes, risking sudden errors. The authors found a way to predict and catch these dangers one step earlier, making safety interventions timely enough to prevent accidents. Their method adjusts the safety boundary so robots can react in time even with discrete control updates.
What this means in practice
- •For robotic control engineers: Design safety filters that activate early enough to prevent robots from entering unsafe states despite discrete control sampling.
- •For autonomous vehicle developers: Use expanded safety boundaries to ensure timely intervention in vehicle control systems that operate with discrete updates.
Authors
Javier Borquez
Abstract
Least-restrictive safety filters based on Hamilton-Jacobi reachability provide strong safety guarantees by overriding a nominal controller only when the system reaches the boundary of the set of unsafe states defined as a Backward Reachable Tube (BRT). These guarantees, however, rely on the continuous-time nature of the underlying formulation. In practice, robotic systems apply control at discrete sampling intervals, which creates a mismatch where the system may jump into the unsafe BRT between updates, allowing failures that are theoretically avoidable. This work introduces a principled solution based on a one-step expanded BRT that predicts all states capable of reaching the true BRT within a single timestep. By using this expanded boundary as the activation condition for the safety filter, safety interventions occur early enough to ensure correctness under discrete-time execution. We formulate this expanded set as a modified reachability problem and compute it using standard continuous-time solvers.