Robust Attitude Control With Fixed Exponential Rate of Convergence and Consideration of Motor Dynamics for Tilt Quadrotor Using Quaternions

In the existing literature on the robust control design of UAV systems, the controllers are designed without considering motor dynamics. Hence, if these controller gains are not correctly tuned, the system undergoes oscillation and may even go unstable. We have demonstrated this through an experiment in this work. Here, we propose a novel control strategy that considers actuator parameter uncertainties, including motor dynamics for a tilt quadrotor. This strategy is based on the traditional two-loop control scheme where the inner loop controls the angular velocity, and the outer loop controls the vehicle’s attitude based on quaternions. In the quaternion-based controller, usually, the convergence rate increases when the quaternion starts closer to its equilibrium point, thus making it challenging to design a linear controller for the inner loop. To overcome this, we propose a nonlinear control with a varying gain for the outer loop that ensures the quaternion has a fixed convergence rate. We propose the control design of the inner loop, which consists of a disturbance observer (DOB) and a linear controller. The DOB is optimally designed to minimize external disturbances in the presence of model uncertainties. With the DOB, a linear controller is designed for the inner loop, guaranteeing robust stability and performance against the model and actuator parameter uncertainties. The results of experimental flights are reported in this paper, and the corresponding videos are at https://youtu.be/dS9WUR0yLhA. Note to Practitioners—This paper was motivated by the potential application of the tilt quadrotors in industry, such as surveillance in confined spaces and landing on high-speed moving targets, due to their independent control of position and orientation. We demonstrate that a controller designed without consideration of motor dynamics can result in unwanted oscillatory or unstable behaviour in the case of a UAV. So, the proposed method considers these dynamics during the design. In most of the existing control designs based on quaternion, the convergence rate depends on the initial condition. This paper proposes a nonlinear control with a varying gain to achieve a faster response that ensures the quaternion has a fixed convergence rate. The stability and convergence of this nonlinear controller are analyzed mathematically and validated through experiments. In UAVs, the precise moment-of-inertia and motor parameters are difficult to estimate, and external disturbances such as wind gusts are always present in outdoor environments. Therefore, this paper designs a robust control scheme which guarantees robust stability and performance against inertia and motor parameter uncertainties. Secondly, a disturbance observer (DOB) whose filter coefficient is optimally tuned such that the effect of external disturbance is minimized in the presence of model uncertainties. The proposed approach is validated on a tilt quadrotor in an outdoor environment and has a faster response than the existing ones having same maximum convergence rate. Thus, this proposed approach has potential applications, like landing a tilt quadrotor on a ship while the ship is in motion under the influence of waves and winds.