Aggressive nonlinear stabilisation for VTOL vehicles

The class of systems know as VTOL vehicles encapsulates a wide variety of real world systems of interest including helicopters, jets and unmanned aerial vehicles. These VTOL vehicles exhibit heavily nonlinear dynamics. Consequently, nonlinear control techniques are required to achieve global stabilisation. Often, VTOL vehicles are stabilised using nonlinear cascade techniques. However, if care is not taken, such controllers will contain a singularity that may be encountered during aggressive stabilisation maneuvers. This problem is often overcome through the use of embedded saturation in the controller design such that the singularity is avoided. However, this saturation has the potential to limit controller performance when very aggressive stabilisation is required. In this paper, we propose an alternative design technique for the cascade control of VTOL vehicles. Here we embed additional dynamics into the controller such that we guarantee the singularity is never encountered for all closed loop trajectories. In this manner singularity issues are avoided, while the controller remains capable of executing highly aggressive stabilisation maneuvers.