Unified incremental nonlinear controller for the transition control of a hybrid dual-axis tilting rotor quad-plane

TL;DR

This work tackles the control of overactuated hybrid dual-axis tilting rotor quad-planes by developing a Unified Incremental Nonlinear Controller that blends nonlinear control allocation with an SQP solver. The framework integrates AoA protection and a yaw-rate coordination model to maintain stability and lift during transition from hover to forward flight, while allowing full 6 DOF authority at low speeds and adaptive control at higher speeds. Key contributions include a normalized nonlinear optimization that jointly optimizes physical actuator commands and virtual attitude references, a tailored error controller with speed-dependent gains, and real-time implementation on a Raspberry Pi with validated flight tests showing accurate tracking and smooth transitions. The practical impact lies in enabling robust, high-performance transitions for TRUAVs in gusty environments, with potential extensions to fault tolerance and more nuanced actuator prioritization.

Abstract

Overactuated Tilt Rotor Unmanned Aerial Vehicles are renowned for exceptional wind resistance and a broad operational range, which poses complex control challenges due to non-affine dynamics. Traditional solutions employ multi-state switched logic controllers for transitions. Our study introduces a novel unified incremental nonlinear controller for overactuated dual-axis tilting rotor quad-planes, seamlessly managing pitch, roll, and physical actuator commands. The control allocation problem is addressed using a SQP iterative optimization algorithm, well-suited for nonlinear actuator effectiveness in thrust vectoring vehicles. The controller design integrates desired roll and pitch angle inputs. These desired attitude angles are autonomously managed by the controller and then conveyed to the vehicle during slow airspeed phases, when the vehicle maintains its 6 DOF. We incorporate an AoA protection logic to prevent wing stall and a yaw rate reference model for coordinated turns. Flight tests confirm the controller's effectiveness in transitioning from hovering to forward flight, achieving desired vertical and lateral accelerations, and reverting to hovering.

Figures (15)

• Figure 1: A picture of the hybrid dual-axis tilting rotor quad-plane.
• Figure 2: Definition of the rotor elevation angle $b$ and azimuth angle $g$.
• Figure 3: Motor spinning direction, rotor numbering and vehicle geometry with respect to the Center of Gravity (CG).
• Figure 4: Diagram of the Unified Incremental Nonlinear Controller. In the diagram, the dashed lines represent variables estimated through sensor (for the vehicle states and accelerations) or actuator model (for the current actuator state vector). The red blocks are running on the primary flight computer, the blue block runs on the Raspberry pi while the green block represents the evolution of the vehicle dynamics.
• Figure 5: Diagram of the linear Error Controller. The gains value for the dual axis tilt rotor quad-plane are reported in Table \ref{['tab:gains']}.