| Thanks to advantages such as vertical takeoff and landing,hovering in any position,ease operation,and simple structure,multi-rotor unmanned aerial vehicles(MRUAVs)have been widely used in military and civil fields to carry out complicated missions such as reconnaissance,monitoring,exploration,and plant protection.Currently,non-planar configurated MRUAVs have gradually moved from theoretical research to practical application.Compared with conventional planar configuration,non-planar one retains the original advantages while enhance the stability of spatial movement in further.Although non-planar MRUAVs have greatly expanded the application scenarios of UAVs,researches on control system design are relatively few and not comprehensive enough.The impacts from persistent winds and gust disturbances have not been considered in most studies,which not only affects the flight performance of UAVs seriously,but also degrades flight stability.To this end,a new robust predictive control approach is proposed in this paper such that the impacts from both wind fields can be reduced simultaneously and the control performance of the UAVs in trajectory tracking applications can be improved significantly.Main works and contributions of this paper are fourfold:(1)Based on the Newton-Euler method,a six-degree-of-freedom motion model for nonplanar MRUAV is constructed.Conventional planar configuration relies on non-zero attitude angles which can generate three axial components of resultant lift in the inertial coordinate system for realization of spatial translational motion.In the six spatial motion degrees of freedom,conventional planar configuration has only four control inputs,namely,resultant lift,roll torque,pitch torque,and yaw torque,making the UAVs have underactuated characteristics.The non-planar MRUAVs designed in this paper has a special configuration in which the lift of the rotor blade is no longer perpendicular to the horizontal plane of the aircraft,making the resultant lift have three axial components in the inertial coordinate system without relying on non-zero attitude angles any more such that fully actuated control can be realized.(2)Based on the six-degree-of-freedom motion model of the non-planar MRUAVs,position and attitude control systems are designed in this paper,respectively.Due to the underactuated characteristic of the conventional planar MRUAVs virtual control inputs companying with resultant lift and three axial torques must be introduced to achieve sixdegree-of-freedom motion control,resulting in strong coupling between the attitude subsystem and position subsystem,increasing the difficulty in control system design.Pitch and roll motions have saturations,which results in poor capability for resisting disturbances for the UAVs.Vertical motion of the UAV is controlled by using the vertical component of the resultant lift to counteract the gravity.In contrast,translational and rotational motions are completely decoupled in the non-planar configuration.Without any aid from the attitude angles,axial translational motion control can be realized,making attitude control and position control independent mutually.Saturations are no longer exist in pitch and roll motions such that system robustness of the MRUAVs against disturbances is improved significantly.Due to the complete decoupling between translational and rotational motions,translational movement can be carried out in any attitude angles.System stability will not be affected if surging of the attitude caused by sudden changes occurs.(3)A disturbance compensation-based predictive control system is designed.Persistent wind fields can affect both the speed and acceleration of the MRUAVs,resulting in existence of both matching and non-matching disturbances in the position subsystem.A linear extended state observer is designed to transform the system affected by the aforementioned disturbances into an equivalent system affected only by matching disturbances,so as to estimate the system state variables.Gusts will cause instant impacts on the MRUAVs such that large system output overshoots and input surging are easy to occur.A predictive controller is designed based upon the equivalent system to reduce system output oscillation and input surging.A disturbance compensator is also designed to improve the robustness of the closed-loop system against disturbances.(4)Superiorities such as smaller trajectory tracking error and stronger robustness of the proposed approach compared with the commonly used approaches are demonstrated through numerical simulations and flight test experiments.In addition,trajectories of the non-planar MRUAVs are smoother;and the output overshoots are successfully avoided.The system only requires a small amount of energy inputs to reduce tracking error and achieve precise control,thus achieving better performance. |
