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This paper presents a control solution for the problem of slung load transportation, with simultaneous damping of oscillations, using an underactuated autonomous quadrotor vehicle in the presence of external disturbances. The strategy put forward is divided into two parts: (i) resorting to an error definition that encompasses the pendular motion of the suspended load, a controller inspired in sliding‐mode theory is developed to compute a vectored thrust actuation that actively damps out the system's oscillations; (ii) two different approaches are illustrated for the stage of attitude tracking: one stemming from the conventional backstepping technique, which controls the vehicle in angular velocity commands; and another, also based on sliding‐mode theory, and built intrinsically in SO(3)$$ \mathsf{SO}(3) $$, that produces torque commands. The overall control scheme is proved to be robust and stable, with the load oscillations shown to be damped out in the process. Simulation and experimental results are presented in a trajectory tracking scenario to validate and assess the robustness and efficiency of the proposed technique. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of trajectory tracking for a quadcopter subject to parametric uncertainties and disturbances. Using the backstepping technique and ignoring parametric uncertainties and disturbances, a nominal controller is proposed for the quadcopter. In order to ensure that the controller is adaptive to parametric uncertainties (i.e. unknown but constant mass and tensor of inertia) and external disturbances, an exponentially stable estimator is designed based on the Kalman–Bucy filter. The closed-loop system resulting from the interconnection of the plant, the controller, and the estimator is such that the backstepping error is uniformly ultimately bounded. Furthermore, simulation and experimental results are presented, validating the stability and robustness properties of the proposed control strategy. [ABSTRACT FROM AUTHOR]

In this article, we propose a solution to the problem of path following for a quadcopter aircraft with unknown vehicle parameters (mass and moment of inertia) and external disturbances. By employing the backstepping technique, the proposed adaptive control strategy guarantees the following: the quadcopter is globally steered toward, and kept within, an arbitrarily small neighborhood of a desired smooth path, achieving global uniformly ultimately boundedness; compared to trajectory tracking, a smoother convergence is obtained as the control actuation signals (thrust force and torque) are bounded with respect to the position error, and the designed timing law ensures that the desired path starts to move only when the vehicle gets close to the desired path; and a single adaptive control law can be used for accurate motion control of aerial vehicles with a wide range of inertial properties, without the need for retuning control gains or other parameters. Moreover, the controller is also made robust to external constant and slowly time-varying disturbances through the design of disturbance estimators. To demonstrate the effectiveness and performance of the proposed control strategies, simulation and experimental results are presented and analyzed. [ABSTRACT FROM AUTHOR]

In this article, we address the task of cooperative path following control of multiple autonomous quadcopters in the presence of unknown external disturbances. Under the assumption that the communications among the vehicles are bidirectional and continuous, a synchronized path following strategy is proposed that regulates the speed of each vehicle along its path to reach consensus in relative position. Moreover, collision-free transient paths from the vehicle initial positions to a group of suitable selected positions along the desired paths are designed. Building on the backstepping technique, the proposed path following controller for each individual vehicle drives the quadcopter toward, and to stay within an arbitrarily small neighborhood of its corresponding desired path, achieving global uniformly ultimately boundedness. In addition, the devised controller guarantees that the actuations are bounded with respect to the position error. The controller is also made robust to external constant or slowly time-varying disturbances by the introduction of dynamic estimators for the disturbances. A projection operator is used to ensure that the estimates remain within the prescribed bounds and are sufficiently smooth to be backstepped. In order to validate the effectiveness and performance of the proposed methodology, we present and analyze both simulation and experimental results. [ABSTRACT FROM AUTHOR]

A real-time 3-D trajectory planner for the leader-following formation control of autonomous vehicles is proposed. The planner relies on the definition of a trailer body whose hinge point is rigidly attached to the leader and assigns each follower to a distinct point of such a trailer. Convergence results for the trailer body reference frame are presented based on Lyapunov analysis, which guarantee that the planning can be independently implemented by n followers, with a common leader, as they asymptotically behave as n points of a unique trailer body. As such, the need for communication among followers is dispensable. The proposed strategy is divided into trajectory planning and trajectory tracking, the first problem being vehicle-independent. Experimental results obtained with quadrotor vehicles are presented, which demonstrate the richness and suitability of the planned trajectories. [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear backstepping controller designed for an underactuated quadrotor-slung load system that drives the load to a predefined trajectory. The quadrotor and the load are modeled as a rigid body and a point mass, respectively. The controller is designed through Lyapunov and backstepping techniques, and the control laws for thrust and angular velocity are presented, which guarantee the closed-loop system to be asymptotically stable. Experimental results are provided to assess the validity and performance of the proposed controller. [ABSTRACT FROM AUTHOR]

In this paper, we address the tasks of trajectory tracking (TT) and path following (PF) for a hovercraft vehicle in the presence of uncertain parameters and unknown external disturbances. Building on the backstepping technique, the proposed TT (PF) control laws are able to drive a hovercraft toward and stay within a neighborhood of a reference trajectory (path), achieving global practical stability. Moreover, the devised control strategies also guarantee that the actuations remain bounded with respect to the position error. The controller is made robust to constant external disturbances and uncertain model parameters by the introduction of dynamic estimators for the disturbance and friction coefficients. A projection operator is used to ensure the estimates remain within prescribed bounds and are sufficiently smooth to be backstepped. In order to validate the efficacy and performance of the proposed controllers, we present and analyze both simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents an integrated visual servoing solution to the problem of quadrotor attitude estimation and stabilization based solely on image feedback from a pan and tilt camera and biased rate gyros. The solution comprises a control law for the quadrotor position and attitude, a control law for the pan and tilt camera platform, and an estimator for the vehicle attitude, whose interconnection stabilizes the quadrotor in hover above certain terrain landmarks. Lateral-longitudinal stabilization is achieved with a nested saturation control law by feedback of the image measurements, estimated attitude, and corrected angular velocity measurements and the vehicle is stabilized vertically by means of image feedback. Resorting to the input-to-state stability properties of the controllers, the quadrotor’s position and attitude are shown to converge to the desired equilibrium point and, in the end, the interconnected system’s closed loop is proven robust to observer estimation errors. Additionally, the pan and tilt camera is actively actuated to keep the relevant features visible in the image field of view for most operating conditions. The robustness and performance of the proposed control and estimation architecture is illustrated through both simulation and experimental results. [ABSTRACT FROM AUTHOR]

This brief addresses the problem of trajectory tracking control of autonomous rotorcraft in operation scenarios where only relative position measurements obtained from light detection and ranging (LiDAR) sensors are possible. The proposed approach defines an alternative kinematic model, directly based on LiDAR measurements, and uses a trajectory-dependent error space to express the dynamic model of the vehicle. A linear parameter varying representation with piecewise affine dependence on the parameters is adopted to describe the error dynamics over a set of predefined operating regions, and a continuous-time $\mathcal {H}_{2}$ control problem is solved using linear matrix inequalities and implemented within the scope of gain-scheduling control theory. The performance of the proposed control method is validated with comprehensive simulation and experimental results, using a mock-up scenario of a pier inspected by a quadrotor. [ABSTRACT FROM AUTHOR]

This paper presents an integrated parameter estimator and trajectory tracking controller for a hovercraft. A generic parameter estimator for time-varying systems linear in the parameters is derived and then particularized for the dynamic model of the vehicle at hand. A trajectory tracking controller is proposed for the nonholonomic hovercraft, which renders the tracking error system exponentially stable and its zero dynamics stable. The interconnection of the estimator and the controller is proven to be locally asymptotically stable. Experimental results attesting the performance and robustness of the controller and its interconnection with the estimator are presented. [ABSTRACT FROM AUTHOR]

This paper addresses the landing problem of a vertical take-off and landing vehicle, exemplified by a quadrotor, on a moving platform using image-based visual servo control. Observable features on a flat and textured target plane are exploited to derive a suitable control law. The target plane may be moving with bounded linear acceleration in any direction. For control purposes, the image of the centroid for a collection of landmarks is used as position measurement, whereas the translational optical flow is used as velocity measurement. The proposed control law guarantees convergence to the desired landing spot on the target plane, without estimating any parameter related to the unknown height, which is also guaranteed to remain strictly positive. Moreover, convergence is guaranteed even in the presence of bounded and possibly time-varying disturbances, resulting, for example, from the motion of the target plane, measurement errors, or wind-induced force disturbances. To improve performance, an estimator for unknown constant force disturbances is also included in the control law. Simulation and experimental results are provided to illustrate and assess the performance of the proposed controller. [ABSTRACT FROM AUTHOR]

This paper addresses the design and experimental evaluation of a robust controller for a quadrotor landing maneuver comprising the approach to a landing slope and sliding on that slope, before coming to a complete halt. During the critical landing flight phase, the dynamics of the vehicle change with the type of contact with the ground, and a hybrid automaton, whose states reflect the several dynamic behaviors of the quadrotor, is employed to model the vehicle throughout the complete maneuver. The quadrotor landing problem is broken down as separate maneuver generation and robust trajectory tracking problems, which are combined to achieve a successful maneuver that is robust to possible uncertainties. The experimental results are provided to attest to the feasibility of the proposed landing procedure. [ABSTRACT FROM PUBLISHER]

This brief addresses the design and experimental evaluation of a global controller to steer a quadrotor vehicle along a predefined path in the presence of constant wind disturbances. The proposed solution consists of a nonlinear adaptive state feedback controller for thrust and torque actuation that: 1) guarantees global convergence of the closed-loop path following error to zero in the presence of constant wind disturbances and 2) ensures that the actuation can be bounded as a function of the position and velocity errors without imposing a maximum for that bound, allowing for high performance control action. A prototyping and testing architecture, developed to streamline the implementation and tuning of the controller, is also described. Simulation results and experimental results, which include a hovering flight in the slipstream of a mechanical fan, are presented to assess the performance and robustness of the proposed controller. [ABSTRACT FROM PUBLISHER]

This paper considers the problem of controlling a quadrotor to go through a window and land on a planar target, the landing pad, using an Image-Based Visual Servo (IBVS) controller that relies on sensing information from two on-board cameras and an IMU. The maneuver is divided into two stages: crossing the window and landing on the pad. For the first stage, a control law is proposed that guarantees that the vehicle will not collide with the wall containing the window and will go through the window with non-zero velocity along the direction orthogonal to the window, keeping at all times a safety distance with respect to the window edges. For the landing stage, the proposed control law ensures that the vehicle achieves a smooth touchdown, keeping at all time a positive height above the plane containing the landing pad. For control purposes, the centroid vectors provided by the combination of the spherical image measurements of a collection of landmarks (corners) for both the window and the landing pad are used as position measurement. The translational optical flow relative to the wall, window edges, and landing plane is used as velocity cue. To achieve the proposed objective, no direct measurements nor explicit estimate of position or velocity are required. Simulation and experimental results are provided to illustrate the performance of the presented controller. • Automatic landing of quadrotors in a cluttered environment using minimum onboard sensors suite. • A new dynamic image visual servo control for vertical take-off and landing UAVs. • Rigorous stability and robustness analyses. • Experimental validation of the control scheme in a real scenario. [ABSTRACT FROM AUTHOR]