Disturbance Observer-Based Singularity-Free Predefined-Time Attitude Tracking Control of Quadrotors: Theory and Experiments

In this article, a disturbance observer-based singularity-free attitude tracking control scheme is presented for quadrotors. First, a predefined-time disturbance observer is constructed to estimate and compensate for external disturbances, enabling the fast convergence of disturbance estimation. Then, a singularity-free predefined-time sliding mode surface is devised with the directly nonsingular property, and an auxiliary function is introduced in the controller design process to avoid the potential singularity problem. On this basis, a predefined-time controller without singularity is proposed to drive the attitude tracking errors to the origin within an expected convergence time. Unlike the majority of existing predefined-time control schemes, this work circumvents the singularity problem without relying on continuous piecewise functions or quadratic-fraction functions, resulting in a more concise and clear stability analysis. Finally, three different control schemes are implemented for experimental comparison, and the results confirm the effectiveness of the proposed control scheme.