Hierarchical Sliding Mode Control for Underactuated Multicopters With Time-Scale Separation Enforcement

This paper is concerned with the attitude and position control of underactuated multirotor aerial vehicles in the presence of matched disturbances and uncertainties, using the hierarchical scheme that nests the attitude control loop inside the position one. It is well-known that the effectiveness of this scheme depends on a proper control tuning for achieving a sufficient time-scale separation (TSS) between the closed-loop (faster) rotational and (slower) translational dynamics. However, a TSS cannot be ensured under an ideal sliding mode position control law since the attitude command, computed from the position control signal, is infinitely fast. The present paper tackles this problem in a way to enforce TSS without losing robustness and using a dull trial-and-error tweak of gains. That is achieved by designing, on the one hand, a new adaptive integral sliding mode attitude control law (AISMAC) that, under a sufficient smooth attitude command, ensures the existence of an attitude sliding mode during all the time, including the adaptation phase, thus allowing an infinitely fast inner loop. On the other hand, the outer loop is equipped with a disturbance-observer-based proportional-derivative position control law that ensures the required smoothness of the attitude command and provides robustness with respect to unknown force terms. The proposed design is extensively evaluated in a realistic simulator and shows to effectively enforce the TSS.