Quaternion Feedback Based Autonomous Control of a Quadcopter UAV with Thrust Vectoring Rotors

In this paper, we present an autonomous flight controller for a quadcopter\nwith thrust vectoring capabilities. This UAV falls in the category of\nmultirotors with tilt-motion enabled rotors. Since the vehicle considered is\nover-actuated in nature, the dynamics and control allocation have to be\nanalysed carefully. Moreover, the possibility of hovering at large attitude\nmaneuvers of this novel vehicle requires singularity-free attitude control.\nHence, quaternion state feedback is utilized to compute the control commands\nfor the UAV motors while avoiding the gimbal lock condition experienced by\nEuler angle based controllers. The quaternion implementation also reduces the\noverall complexity of state estimation due to absence of trigonometric\nparameters. The quadcopter dynamic model and state space is utilized to design\nthe attitude controller and control allocation for the UAV. The control\nallocation, in particular, is derived by linearizing the system about hover\ncondition. This mathematical method renders the control allocation more\naccurate than existing approaches. Lyapunov stability analysis of the attitude\ncontroller is shown to prove global stability. The quaternion feedback attitude\ncontroller is commanded by an outer position controller loop which generates\nrotor-tilt and desired quaternions commands for the system. The performance of\nthe UAV is evaluated by numerical simulations for tracking attitude step\ncommands and for following a way-point navigation mission.\n