Quaternion-based nonlinear attitude control of quadrotor formations carrying a slung load

Abstract

Control of a pair of quadrotors carrying a slung load is addressed. A hierarchical control algorithm is implemented on the leader quadrotor. The control system features a linear quadratic tracking controller on its outer loop for velocity command tracking and generates the thrust vector commands for the inner loop. The inner loop uses the to-go quaternion calculated from the thrust vector commands to control the attitude. The second quadrotor, designated as the follower quadrotor, has another hierarchical control structure on it. This control system features a Lyapunov function based nonlinear controller for formation guidance and thrust vector command generation in the outer loop. In the inner loop, the same nonlinear attitude controller is used. The dynamics of the two quadrotor slung load system is modelled and two simulation codes are developed utilizing first rigid, and then flexible load carrying rods. Simulation results are presented to show the efficacy of the control algorithm to keep the two quadrotors flying in a fixed geometrical formation in spite of the unmodelled disturbance dynamics of both the rigid bars, flexible bars and the load.