Nonlinear Control of a Tethered UAV: the Taut Cable case ? Marco M. Nicotra a,b , Roberto Naldi b , Emanuele Garone a a Service d’Automatique et d’Analyse des Systèmes, Université Libre de Bruxelles Av. F.D. Roosevelt 50, CP 165/55, 1050 - Bruxelles, Belgium b Center for Research on Complex Automated Systems, Alma Mater Studiorum (University of Bologna) Viale C. Pepoli 3/2 40136 - Bologna, Italy Abstract This paper focuses on the design of a stabilizing control law for an aerial vehicle which is physically connected to a ground station by means of a tether cable. By taking advantage of the tensile force acting along the taut cable, it is shown that the tethered UAV is able to maintain a non-zero attitude while hovering in a constant position. The control objective is to stabilize the desired configuration while simultaneously ensuring that the cable remains taut at all times. This leads to a nonlinear control problem subject to constraints. This paper provides a two-step solution. First, the system is stabilized using a cascade control scheme based on thrust vectoring. Then, constraint satisfaction is guaranteed using a novel Reference Governor scheme. Key words: Unmanned Aerial Vehicles, Stability of Nonlinear Systems, Constrained Control. 1 INTRODUCTION Recent advancements in the field of Unmanned Aerial Vehicles (UAVs) have lead to the availability of inex- pensive aerial robots with a growing range of applica- tions ranging from surveillance [1] to advanced robotic operations including environment interaction [9], grasp- ing [16]-[10] and manipulation [22]. The full potential of these systems, however, is still limited by key factors such as flight time, computing capabilities and airspace safety regulations [3]. A possible solution to these lim- itations is to connect the UAV to a ground station by means of a tether cable able to supply energy, transmit data and/or apply forces. Since the dynamic properties of the UAV are deeply in- fluenced by the cable, the safe deployment of tethered UAVs requires the development of specific control strate- gies. Early works on the subject [19]-[17] studied the stabilization of tethered UAVs using linearized models. Although the primary interest in tethered UAVs is their ? This work is supported by a FRIA scholarship grant and the PF7 European project SHERPA. Email addresses: mnicotra@ulb.ac.be (Marco M. Nicotra), roberto.naldi@unibo.it (Roberto Naldi), egarone@ulb.ac.be (Emanuele Garone). virtually unlimited flight-time [11], recent results have shown the advantage of using the taut cable as an ad- ditional control input. Possible examples include: guid- ing the landing of a helicopter on a ship [15], improving fight stability in the presence of wind [18],[2], and using multiple cables to achieve full actuation [13]. Moreover, it has been shown in [7],[21] that the taut cable config- uration can also be used to measure the position of the UAV. A common feature of these papers is that the ca- ble tension is controlled by an actuated winch, whereas the UAV position is controlled by the UAV itself. This paper investigates an alternative approach where the actuated winch imposes only the cable length whereas the UAV controls its elevation angle while en- suring a minimal cable tension. It is worth noting that the proposed control law can also be applied to the case of a fixed-length cable since it does not require the presence of an actuated winch. To the author’s best knowledge, this approach to the control of a tethered UAV has not been addressed previously. The first contribution of the paper is to show that the tethered UAV is able to achieve a set of equilibrium con- figurations that is different from the untethered case. This set is characterized both analytically and geomet- rically. The main contribution of this paper is the de- Preprint submitted to Automatica 4 October 2016 arXiv:1610.00348v1 [cs.SY] 2 Oct 2016