  The problem of dynamic locomotion over rough terrain requires both accurate
foot placement together with an emphasis on dynamic stability. Existing
approaches to this problem prioritize immediate safe foot placement over longer
term dynamic stability considerations, or relegate the coordination of foot
placement and dynamic stability to heuristic methods. We propose a
multi-layered locomotion framework that unifies Control Barrier Functions
(CBFs) with Model Predictive Control (MPC) to simultaneously achieve safe foot
placement and dynamic stability. Our approach incorporates CBF based safety
constraints both in a low frequency kino-dynamic MPC formulation and a high
frequency inverse dynamics tracking controller. This ensures that
safety-critical execution is considered when optimizing locomotion over a
longer horizon. We validate the proposed method in a 3D stepping-stone scenario
in simulation and experimentally on the ANYmal quadruped platform.
