Racetrack maneuver of aerial refueling by L₁ adaptive controller

In this paper we present a solution for racetrack maneuver of Autonomous Aerial Refueling via the L₁ adaptive controller. The L₁ adaptive controller, employed for this particular maneuver, has a time-varying reference system, defined via interpolation of two baseline gain-scheduled linear time-invariant controllers. This interpolation leads to a closed-loop linear time-varying reference system, which consequently becomes an important element in the adaptive augmentation. The objective of the adaptive augmentation is twofold. First, it aims to recover the nominal performance of the gain-scheduled controlled system at different operating points, without overruling the performance of the baseline controller. Second, the design of the adaptive augmentation should be pursued in a way so that in the absence of uncertainties the output of it is zero. Thus, the adaptive augmentation must include the dynamics of the closed-loop system, controlled by the baseline gain-scheduled controller, as a part of its feedback. The dynamics of the gain-scheduled controller change dependent upon the scheduling variables, leading to a time-varying closed-loop reference system. To match the dynamics of the closed-loop baseline gain-scheduled controlled system, the reference system of the adaptive controller needs to be time-varying too. In this paper, we demonstrate the stability characteristics and the performance of the L ₁ adaptive controller for a time-varying reference system. We further illustrate how the theory can be used for adaptive augmentation of a baseline gain-scheduled controller by simulating racetrack maneuver of autonomous aerial refueling.