Safe Beyond the Horizon: Efficient Sampling-based MPC with Neural Control Barrier Functions

TL;DR

This paper introduces Neural Shield-VIMPC, a sampling-based MPC method that uses learned control barrier functions to ensure safety beyond the prediction horizon efficiently, suitable for real-time applications with nonlinear dynamics.

Contribution

It proposes a novel approach combining learned control barrier functions with variational inference MPC, improving safety and efficiency in nonlinear systems.

Findings

Substantial safety improvements over existing sampling-based MPC methods.

Enhanced sample efficiency enabling real-time planning on CPU.

Validated effectiveness through simulation and hardware experiments.

Abstract

A common problem when using model predictive control (MPC) in practice is the satisfaction of safety specifications beyond the prediction horizon. While theoretical works have shown that safety can be guaranteed by enforcing a suitable terminal set constraint or a sufficiently long prediction horizon, these techniques are difficult to apply and thus are rarely used by practitioners, especially in the case of general nonlinear dynamics. To solve this problem, we impose a tradeoff between exact recursive feasibility, computational tractability, and applicability to ``black-box'' dynamics by learning an approximate discrete-time control barrier function and incorporating it into a variational inference MPC (VIMPC), a sampling-based MPC paradigm. To handle the resulting state constraints, we further propose a new sampling strategy that greatly reduces the variance of the estimated optimal control, improving the sample efficiency, and enabling real-time planning on a CPU. The resulting Neural Shield-VIMPC (NS-VIMPC) controller yields substantial safety improvements compared to existing sampling-based MPC controllers, even under badly designed cost functions. We validate our approach in both simulation and real-world hardware experiments. Project website: https://mit-realm.github.io/ns-vimpc/.