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Sintra, Portugal

Rocha B.,University of Lisbon | Silva C.,Portuguese Air Force Academy | Suleman A.,University of Lisbon
Shock and Vibration

The paper presents a structural health monitoring system based on propagation of tuned Lamb waves and their interference with discontinuities. The dispersion curves are studied to determine the appropriate type and dimension of transducers and to select the optimum scanning frequencies and relevant propagation modes. A piezoelectric sensor network was implemented in an aluminum plate in order to generate and to sense the wave propagation and associated reflections. The algorithm developed for damage detection relies on the comparison of undamaged and damaged responses of the structure. Combinations of filters and statistical methods were applied to detect differences in the sensor signals acquired for the two different states (damaged and undamaged), corresponding to damage reflections. In order to eliminate the false positives due to noise, a probability analysis is performed to obtain the final damage position. The software designed for the current application allows the automatic calculation of dispersion curves, it executes the scans, performs data processing, executes the detection algorithm and presents the probable damages and their positions in a graphical form. Experiments were performed with the introduction of cumulative damages in the plate such as surface and through-the-thickness holes and cuts, ranging from 7 mm to 1 mm in diameter. Additionally, a stringer was attached to the plate by a single rivet line to simulate an aircraft skin structure. Cuts originating from rivet holes and connecting adjacent rivets, as well as loosened rivets were detected by the system. The introduction of the stringer resulted in a loss of precision in the determination of the radial position of the damages near it. Also, the network revealed significant difficulties in the detection of damages beyond the stringer. © 2010 - IOS Press and the authors. Source

Oliveira T.,Portuguese Air Force Academy | Encarnacao P.,Catholic University of Portugal
Journal of Intelligent and Robotic Systems: Theory and Applications

The work here presented contributes to the development of ground target tracking control systems for fixed wing unmanned aerial vehicles (UAVs). The control laws are derived at the kinematic level, relying on a commercial inner loop controller onboard that accepts commands in indicated air speed and bank, and appropriately sets the control surface deflections and thrust in order to follow those references in the presence of unknown wind. Position and velocity of the target on the ground is assumed to be known. The algorithm proposed derives from a path following control law that enables the UAV to converge to a circumference centered at the target and moving with it, thus keeping the UAV in the vicinity of the target even if the target moves at a velocity lower than the UAV stall speed. If the target speed is close to the UAV speed, the control law behaves similar to a controller that tracks a particular point on the circumference centered at the target position. Real flight tests results show the good performance of the control scheme presented. © Springer Science+Business Media Dordrecht 2012. Source

Gomes A.,Portuguese Air Force Academy | Gomes A.,Instituto Universitario Of Lisbon Iscte Iul | Dias J.G.,Instituto Universitario Of Lisbon Iscte Iul
International Journal of Aviation Psychology

Latent growth mixture modeling is a statistical approach that models longitudinal data, grouping individuals who share similar longitudinal data patterns into latent classes. We evaluated the application of this method in a sample of ab initio pilot applicants (N = 297), using longitudinal data collected from a military flight-screening program (where the applicants flew seven required flights), resulting in a final pass–fail outcome. Results showed the existence of a two-class solution (Cluster 1 presented an initially higher performance and contained 75% of the Pass candidates) and the psychomotor coordination and general adaptability showed a significant effect. © 2015 Taylor & Francis Group, LLC. Source

Cruz G.C.S.,Portuguese Air Force Academy | Encarnacao P.M.M.,Catholic University of Portugal
Journal of Intelligent and Robotic Systems: Theory and Applications

This work is framed within the PITVANT project and aims to contribute to the development of obstacle avoidance techniques for unmanned aerial vehicles (UAVs). The paper describes the design, implementation and experimental evaluation of a potential field obstacle avoidance algorithm based on the fluid mechanics panel methods. Obstacles and the UAV goal position are modeled by harmonic functions thus avoiding the presence of local minima. Adaptations are made to apply the method to the automatic control of a fixed wing aircraft, relying only on a local map of the environment that is updated with information from sensors onboard the aircraft. Hardware-In-Loop simulations show the good performance of the proposed algorithm in the envisioned mission scenarios for the PITVANT vehicles. © 2011 Springer Science+Business Media B.V. Source

Boavida J.B.M.,Institute Telecomunicaes | Morgado J.A.P.,Institute Telecomunicaes | Morgado J.A.P.,Portuguese Air Force Academy | Fernandes C.A.F.,Institute Telecomunicaes
Optics and Laser Technology

Optimized HR-AR-DFB laser structures are proposed foreseeing their use in directly modulated optical communication systems. They present, simultaneously, high single-longitudinal-mode yield, stable single-longitudinal-mode operation, high power efficiency and stable emitted wavelength. Lasers harder to manufacture were previously proposed elsewhere and did not succeed in achieving such high performances. A detailed assessment of the proposed lasers is presented and discussed. © 2010 Elsevier Ltd. All rights reserved. Source

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