Rita Cunha

A nonlinear position and attitude observer on SE(3) using landmark measurements

This paper addresses the problem of position and attitude estimation, based on landmark readings and velocity measurements. A derivation of a nonlinear observer on SE(3) is presented, using a Lyapunov function conveniently expressed as a function of the difference between the estimated and the measured landmark coordinates. The resulting feedback laws are explicit functions of the landmark measurements and velocity readings, exploiting the sensor information directly in the observer. The proposed observer yields almost global asymptotic stabilization of the position and attitude errors and exponential convergence in any closed ball inside the region of attraction. Also, it is shown that the asymptotic convergence of the estimation error trajectories is shaped by the landmark geometry and observer design parameters. The problem of non-ideal velocity readings is also considered, and the observer is augmented to compensate for bias in the angular and linear velocity measurements. The resulting position, attitude, and bias estimation errors are shown to converge exponentially fast to the desired equilibrium points, for bounded initial estimation errors. Simulation results are presented to illustrate the stability and convergence properties of the observer.

Landmark based nonlinear observer for rigid body attitude and position estimation

This work proposes a nonlinear observer for position and attitude estimation on SE(3). Using a Lyapunov function based on the landmark measurement error, almost global exponential stability (GES) of the desired attitude and position equilibrium points is obtained. It is shown that the derived feedback law is an explicit function of the landmark measurements and velocity readings, and that the landmark geometry characterizes the asymptotic convergence of the closed loop system solution. Almost global exponential stabilization in the presence of biased velocity readings is also achieved. Simulation results for trajectories described by time-varying linear and angular velocities and for distinct initial conditions on SE(3) are presented to illustrate the stability and convergence properties of the observer.

Robust Take-Off for a Quadrotor Vehicle

This paper addresses the problem of robust take-off of a quadrotor unmanned aerial vehicle (UAV) in critical scenarios, such as in the presence of sloped terrains and surrounding obstacles. Throughout the maneuver, the vehicle is modeled as a hybrid automaton whose states reflect the different dynamic behaviors exhibited by the UAV. The original take-off problem is then addressed as the problem of tracking suitable reference signals in order to achieve the desired transitions between different hybrid states of the automaton. Reference trajectories and feedback control laws are derived to explicitly account for uncertainties in both the environment and the vehicle dynamics. Simulation results demonstrate the effectiveness of the proposed solution and highlight the advantages with respect to more standard open-loop strategies, especially for cases in which the slope of the terrain renders the take-off maneuver more critical to achieve.

A Globally Stabilizing Path Following Controller for Rotorcraft With Wind Disturbance Rejection

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.

DYNAMIC MODELING AND STABILITY ANALYSIS OF MODEL-SCALE HELICOPTERS WITH BELL-HILLER STABILIZING BAR

This paper presents an accurate self-contained helicopter dynamic model, derived from first-principles, and that is specially tailored for model-scale helicopters. The simulation model includes the rigid body, main rotor flapping, and Bell-Hiller stabilizing bar dynamics. Particular emphasis is placed on the analysis of the stabilizing bar and on the evaluation of its impact on the overall helicopter dynamics. The model is parameterized for the Vario X-Treme acrobatic helicopter, and solutions for a set of trimming trajectories are identified and discussed. Dierent simplifications, needed to derive models for control system design, are presented, compared, and their influence on the resultant dynamics evaluated. The eect of changing the physical parameters of the stabilizing bar is also assessed. An LQ state feedback controller is synthesized to stabilize the vehicle in forward flight. Simulation results obtained with the full nonlinear dynamic model and the forward flight control system are presented and discussed.

A leader-following trajectory generator with application to quadrotor formation flight

This paper presents a strategy for real-time generation of formation trajectories using a leader-follower approach. A trajectory generator prescribes the motion of a group of virtual vehicles, using a Lyapunov-based nonlinear controller that stabilizes the position of the leader in the reference frame of the virtual vehicles at a predefined distance vector. This strategy differs from the standard approach of defining the desired distance vector in an inertial frame and can be used to obtain rich formation trajectories with varying curvatures between vehicles. By imposing adequate constraints on the motion of the virtual vehicles, the generation of valid formation trajectories is naturally guaranteed, bypassing the demanding task of obtaining complete path descriptions. The trajectories are generated online and provided to a trajectory tracking controller specifically designed for quadrotor vehicles. Simulation and experimental flight tests are presented to evaluate the performance of the solution proposed, applied to formation control of quadrotors.

A path-following preview controller for autonomous air vehicles

An operating concept and a laboratory analysis methodology were developed and tested to examine how four-dimensional trajectory analysis methods could support higher levels of automation for separation assurance in the National Airspace System. Real-time simulations were conducted in which a human controller generated conflict resolution trajectories using an automated trial planning resolution function, but only in response to conflicts detected and displayed by an automatic conflict detection function. Objective metrics were developed to compare aircraft separation characteristics and flying time efficiency under automated operations to that of today’s operations using common airspace and traffic scenarios. Simulations were based on recorded air traffic data from Fort Worth and Cleveland Centers and conducted using today’s and nearly two-times today’s traffic levels. Results suggest that a single controller using trajectory-based automation and data link communication of control clearances to aircraft could manage substantially more traffic than they do now with improved route efficiency while maintaining separation. The simulation and analysis capability provides a basis for further analysis of semi-automated, or fully automated, separation assurance concepts.

Landing of a Quadrotor on a Moving Target Using Dynamic Image-Based Visual Servo Control

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.

Cooperative Control of Multiple Marine Vehicles Theoretical Challenges and Practical Issues

Abstract This paper is a brief overview of some of the theoretical and practical issues that arise in the process of developing advanced motion control systems for cooperative multiple autonomous marine vehicles (AMVs). Many of the problems addressed have been formulated in the scope of the EU GREX project, entitled Coordination and Control of Cooperating Heterogeneous Unmanned Systems in Uncertain Environments. The paper offers a concise introduction to the general problem of cooperative motion control that is well rooted in illustrative mission scenarios developed collectively by the GREX partners. This is followed by the description of a general architecture for cooperative autonomous marine vehicle control in the presence of time-varying communication topologies and communication losses. The results of simulations with the NetMar SyS (Networked Marine Systems Simulator) of ISR/IST are presented and show the efficacy of the algorithms developed for cooperative motion control. The paper concludes with a description of representative results obtained during a series of tests at sea in the Azores, in 2008.

Affine parameter-dependent preview control for rotorcraft terrain following flight

This paper presents a terrain-following controller for rotorcraft that takes into account the terrain characteristics ahead of the vehicle as measured by a laser range scanner. The methodology used to solve the terrain-following control problem poses it as a discrete time path following control problem in which a conveniently defined error state-space model of the plant is augmented with terrain preview data. A piecewise affine parameter-dependent model representation is used to accurately describe the linearized error dynamics for a predefined set of operating regions. For each region, the synthesis problem is stated as a state feedback H 2 control problem for affine parameter-dependent systems and solved using linear matrix inequalities. An alternative technique to compute the feedforward preview gain matrix is proposed that avoids solving linear matrix inequalities involving a large number of unknowns. The resulting nonlinear controller is implemented within the scope of gain-scheduled control theory using the D-methodology. Simulation results obtained with the full nonlinear helicopter model are presented and discussed.