Hierarchical Robust Control for Unmanned Quadrotor Transportation Systems: Exploiting Beneficial State-Coupling

Quadrotor has been widely used because of its high maneuverability and its ability to take off in limited space. It is very challenging to eliminate the swing in the course of its dispatching task because there is no direct control of the payload. The quadrotor transport system has to realize not only the control of the position and load of the outer loop, but also the control of the attitude of the inner loop, which shows the cascaded underactuated–underactuated property. This feature makes the quadrotor precise positioning and effectively restrain the load swing become extremely difficult. This paper presents a nonlinear hierarchical control scheme for quadrotor transport system, which makes full use of the cascading characteristics of the system and separates the inner loop and outer loop controller design to facilitate the design process. To be more specific, in the outer loop subsystem, a virtual control vector is designed based on the proposed energy storage function, and a saturation function is introduced to make the desired attitude unaffected by any singularity. For the inner loop, a geometric attitude tracking controller based on the body coordinate system is designed to drive the quadrotor to achieve the desired attitude. Based on that, a new control method which makes full use of the coupling effect is proposed, which not only achieves the precise positioning of the quadrotor, but also achieves the effective load swing elimination.