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Passivity Analysis of Quadrotor Aircraft for Physical Interactions

The broad dissemination of unmanned aerial vehicles (UAV s), specifically quadrotor aircraft, has accelerated their successful use in a wide range of industrial, military, and agricultural applications. Research in the growing field of aerial manipulation (AM) faces many challenges but may enable the next generation of UAV applications. The physical contact required to perform AM tasks results in dynamic coupling with the environment, which may lead to instability with devastating consequences for a UAV in flight. Considering these concerns, this work seeks to determine whether off-the-shelf flight controllers for quadrotor UAV s are suitable for AM applications by investigating the passivity and coupled-stability of quad rotors using generic cascaded position-attitude (CPA) and PX4 flight controllers. Using a planar 3-degree of freedom (DOF) linearized state-space model and two high fidelity 6-DOF models with the CPA and PX4 closed-loop flight controllers, passivity is analyzed during free flight, and stability is analyzed when the UAV is coupled to environments with varying degrees of stiffness. This analysis indicates that quadrotors using the CPA and PX4 flight controllers are non-passive (except for the PX4 controller in the vertical direction with certain vehicle parameters) and may become unstable when the UAV is coupled with environments of certain stiffnesses. Similarities between the results from the linearized 3-DOF model and nonlinear 6-DOF models in the passivity analysis suggest that using an analytical, linear approach is sufficient and potentially useful for vehicle geometry and controller design to improve stability for AM applications.