An Online Approach to Active Set Invariance

This paper presents an online approach to safety critical control. The common approach for enforcing safety of a system requires the offline computation of a viable set, which is either hard and time consuming or very restrictive in terms of operational freedom for the system. The first part of this work shows how one can constrain a system to stay within reach of an appropriately chosen backup set in a minimally invasive way by performing online sensitivity analysis around a backup trajectory. For linear systems, we show how to use an optimal backup strategy in the form of a Model Predictive Controller (MPC) to maximize the operational freedom of the system. The second part of this work shows how to leverage this capability and factor in state constraints to enforce set invariance only based on online computations of sensitivities. For linear systems, the optimal strategy is again considered and we show how one can perform the sensitivity analysis based on a measure of feasibility of a state constrained MPC. This approach is illustrated in simulation on a linear inverted pendulum.