Sandboxing Controllers for Stochastic Cyber-Physical Systems

TL;DR

This paper introduces a novel sandboxing approach for unverified controllers in stochastic cyber-physical systems, providing probabilistic safety guarantees in noisy environments, which was not addressed by prior deterministic solutions.

Contribution

It presents the first method to sandbox unverified controllers in stochastic CPS with probabilistic safety guarantees, extending existing deterministic sandboxing techniques.

Findings

Guarantees safety probability in noisy environments.

Rejects unsafe control inputs based on probabilistic thresholds.

Maintains safety using an optimal safety controller when needed.

Abstract

Current cyber-physical systems (CPS) are expected to accomplish complex tasks. To achieve this goal, high performance, but unverified controllers (e.g. deep neural network, black-box controllers from third parties) are applied, which makes it very challenging to keep the overall CPS safe. By sandboxing these controllers, we are not only able to use them but also to enforce safety properties over the controlled physical systems at the same time. However, current available solutions for sandboxing controllers are just applicable to deterministic (a.k.a. non-stochastic) systems, possibly affected by bounded disturbances. In this paper, for the first time we propose a novel solution for sandboxing unverified complex controllers for CPS operating in noisy environments (a.k.a. stochastic CPS). Moreover, we also provide probabilistic guarantees on their safety. Here, the unverified control input is observed at each time instant and checked whether it violates the maximal tolerable probability of reaching the unsafe set. If this probability exceeds a given threshold, the unverified control input will be rejected, and the advisory input provided by the optimal safety controller will be used to maintain the probabilistic safety guarantee. The proposed approach is illustrated empirically and the results indicate that the expected safety probability is guaranteed.