Modeling and Incremental Nonlinear Dynamic Inversion Control of a Novel Unmanned Tiltrotor

Abstract

A novel, tiltrotor, unmanned aerial vehicle configuration has been designed, and a preliminary dynamic model has been created for control design purposes based on a component buildup approach. This method has been preferred over a conventional stability derivative approach because it models the nonlinear aircraft dynamics in the entire flight envelope, ranging from hover to forward-flight conditions at any airflow angles. The flight control system has then been synthesized based on an incremental nonlinear dynamic inversion technique. The incremental strategy was adopted to solve the problem of the nonlinear dynamic model not being control affine due to effector redundancy, dramatic nonlinearities, and cross-coupling effects introduced by the use of thrust vectoring during hover and transition phases. Local control derivatives were calculated online from the dynamic model. Effector redundancy was managed, developing a control allocation module for distributing the control effort according to a daisy-chaining logic and according to the actuator availability and effectiveness. The timescale separation principle was applied, dividing control laws into two loops: an outer loop controlling slower dynamics, and outputting virtual controls for an inner loop controlling faster dynamics. Different piloting logics were identified for hover and forward-flight conditions, although the largest possible degree of commonality was sought. A command blending strategy was devised to control the aircraft during transition phases. Simulations confirmed the effectiveness of the proposed solutions, showing satisfactory tracking of reference inputs with moderate control effort.