In this paper a hierarchical position tracking controller for a quadrotor is presented, which consists of an inner attitude controller and an outer position controller. The tracking error dynamics form a nonautonomous cascade built up by a linear attitude error subsystem and a linear position error subsystem, coupled by a nonlinear interconnection term. The stability properties of the cascaded system essentially rely on the characteristics of the interconnection term, which strongly depends on the choice of the thrust. We characterize a class of admissible thrust control laws in terms of closed loop asymptotic stability and demonstrate the significant influence of different laws on the transient behavior of the system.
In this paper we design a continuous, nonlinear state feedback controller that aligns the body-fixed thrust direction of a quadrotor helicopter with a constant desired direction given in an inertial coordinate system. The controller is designed based on an energy shaping approach and is able to saturate the control torques. Almost global asymptotic stability of the desired thrust direction is proven for arbitrary unknown moment of inertia matrices. Performance of the closed loop system is verified by simulations.
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