Quaternion-based Dual loop Nonlinear Trajectory Control of Quadrotors
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Many multirotor controllers achieve globally stable attitude control through the usage of quaternions, as it does not inherent any singularities, unlike other alternatives such as Euler angles or the Direction Cosine Matrices (DCM). However, globally stable position or velocity control of quadrotors have rarely been achieved; most controllers limit their attitude within a certain range to avoid such singularities and lose stability. This thesis focuses on quadrotors and presents a globally stable quaternion-based dual loop nonlinear control scheme so that the quadrotor can achieve any attitude configuration during position or velocity control without losing stability. The two control loops are structured so that the outer loop controls the translational velocity or position, and the inner loop controls the attitude. The outer control loop uses a Proportional-Integral (PI) feedback structure. The proportional action is in terms of the translational position or velocity of the quadrotor, and the integral action is primarily used to eliminate steady-state error. The inner attitude control loop is a Proportion-Derivative (PD) feedback loop, where the proportional and derivative action is in terms of the vector component of the quaternion of the quadrotor attitude and quadrotor angular velocity, respectively. The two control loops are linked in a manner so that a globally exponentially stability can be achieved. Additional feedback and feedforward, which acts as compensations for the nonlinear dynamics of the quadrotor and gyroscopic torques, were further employed to guarantee the global stability of the quadrotor. The thesis also investigates different singularity and ambiguity issues, more specifically singularities that can arise during attitude control while trying to achieve globally stabilizing position control; these singularities, when undealt with, may cause the controller to lose control of the quadrotor leading to instability at different locations. The results section shows that once all solutions are employed robust and accurate control of the quadrotor can be achieved. The designed controller was both simulated and tested experimentally with a commercial quadrotor.