A Run Time Assurance Approach for Safe Control of a Quadrotor

Abstract This paper presents a comprehensive development and testing of Run-Time Assurance (RTA) methods based on Control Barrier Functions (CBFs) for a quadrotor drone testbed in both 1 Degree-of Freedom (1-DoF) and 2-DoF configurations, for which safety constraints are enforced on the pitch angle and longitudinal position, respectively. The primary controllers used in this work are quadratic optimal controllers, designed to make the drone follow reference signals in the form of square waves, and are implemented in both continuous and discrete-time. We examine RTA filters embedding a CBF including some extensions such as High Order CBF (HOCBF) and discrete-time CBF (DCBF). The approaches are validated in simulations and demonstrated on a real-world quadrotor platform at the University of Houston, TX, USA. In the experiments, the reference trajectory was purposefully designed to exceed the boundaries of the prescribed safe sets at times, leading the primary controller to command potentially unsafe control actions. The RTA filters, then, adjusted the primary controller input when necessary to ensure that safety constraints were met at all times.