We consider the problem of controlling an aerial robot connected to theground by a passive cable or a passive rigid link. We provide a thoroughcharacterization of this nonlinear dynamical robotic system in terms offundamental properties such as differential flatness, controllability, andobservability. We prove that the robotic system is differentially flat withrespect to two output pairs: elevation of the link and attitude of the vehicle;elevation of the link and longitudinal link force (e.g., cable tension, or barcompression). We show the design of an almost globally convergent nonlinearobserver of the full state that resorts only to an onboard accelerometer and agyroscope. We also design two almost globally convergent nonlinear controllersto track any sufficiently smooth time-varying trajectory of the two outputpairs. Finally we numerically test the robustness of the proposed method inseveral far-from-nominal conditions: nonlinear cross-coupling effects,parameter deviations, measurements noise and non ideal actuators.
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