Geometric trajectory tracking with attitude planner for vectored-thrust VTOL UAVs

Abstract

This paper addresses the trajectory tracking problem for VTOL UAVs in which the thrust vector can be delivered only in a fixed direction with respect to the aircraft frame. The proposed control law, based on an inner-outer loop strategy, guarantees robust tracking of a desired position trajectory. The attitude error dynamics, associated to the inner loop, is stabilized by a control torque that does not cancel non-harmful nonlinearities and that tracks at the same time a desired heading direction. This choice results in a non-autonomous feedback interconnection between the attitude and position loops, the stability of which is studied within the framework of differential inclusions. The proposed control force accounts for attitude errors to improve transient performance compared to other designs. The control scheme is tested on a multi-body model of a hexacopter UAV with actuators dynamics and aerodynamics effects, not included in the control design model.

Integral ISS-Based Cascade Stabilization for Vectored-Thrust UAVs

“…This structure is exploited in the hierarchical control strategy presented in the next section. This work focuses on the stabilization of the underactuated model (4)- (6). According to this model, it is either possible to stabilize a desired attitude and altitude or a desired position and rotation about the gravity axis e 3 .…”

“…The goal of this paper is to propose a control design which overcomes the limitations, in terms of transient performance, of modern strategies developed to globally stabilize the position of underactuated UAVs. In particular, the control designs proposed in [4], [5] as well as in our previous works [6], [7] ensure stability of the intrinsic cascade via a reduction approach [8] assessing the necessary property of "boundedness of solutions" by requiring the position stabilizer to possess Input to State Stabiliy (ISS) with respect to small inputs [9]. This small-signal ISS property is typically guaranteed by employing nested saturations-based stabilizers [5]: although very robust, this solution is known to yield poor transient performance [10].…”

“…There is a significant body of literature, well surveyed in [1], dealing with the stabilization problem for vectored-thrust UAVs as formulated in this paper, which is challenging also because it evolves on a nonlinear manifold corresponding to the set of rigid body motions. Early works on the subject date back to [2] and [3], while recent approaches are presented in [4], [5], [6].…”

“…In particular, we suggest using an exponential attitude stabilizer [13] and a Quasi Time-Optimal (QTO) stabilizer [14] and we prove that this choice makes the UAV position dynamics integral ISS. The QTO stabilizer is the prototypical example of aggressive solutions, achieving excellent transient performance, which cannot be included in existing designs, notably [4], [5], [6], since proving (the previously required) small-signal ISS property is hard if not impossible. A simulation example discussed in Section V highlights the advantage of the proposed solution with respect to a nested saturations approach in a scenario characterized by a significant initial configuration error.…”

Adaptive augmentation of the attitude control system for a multirotor unmanned aerial vehicle

“…The problem of attitude control law design for multirotor Unmanned Aerial Vehicles (UAVs) has been studied extensively in the literature. When dealing with nominal operation, fixed-gain linear or nonlinear controllers typically suffice to solve the problem in a satisfactory way 1 . If more challenging questions such as, e.g., actuator degradation and faults, severe external disturbances, parameter uncertainties and time delays, are to be considered, then more advanced approaches are needed.…”