Differential GPS for small UAS using consumer-grade single-frequency receivers

Consumer-grade single-frequency GPS receivers with their known limitations are the predominant means of localization for small Unmanned Aircraft Systems (UAS). More intricate maneuvers such as automatic landings require a level of accuracy this class of receivers does not provide. As a contribution to improve the positioning accuracy without sacrificing the low-cost approach of this class of vehicles, a Local Area Augmentation System (LAAS) has been developed, based on consumer-grade single-frequency miniature GPS receivers both for the base station and airborne positioning. On the part of the airborne receiver, the conventional approach of carrier phase smoothing has been extended by incorporating Doppler measurements to propagate the position during carrier phase signal outages or in the event of cycle slips. Pseudoranges and the augmented carrier phase observations are merged by means of an indirect linear Kalman filter in the position domain. The characteristics of the error state allow for some simplifications that reduce the computing effort of the filter. To evaluate the system’s performance under dynamic conditions, raw GPS data have been collected on a ground based moving platform and processed with Simulink. The results show a significantly improved 3D position accuracy compared to the standalone receiver solution

An open benchmark for distributed formation flight control of Fixed-Wing Unmanned Aircraft Systems

The capability of autonomous formation flight has the potential to significantly enhance the utility and efficiency of small low-cost Unmanned Aerial Systems (UAS). Formations of small, inexpensive fixed-wing UAS allow for the sharing of remote sensing functionality, mission-level redundancy and range enhancements due to aerodynamic interactions widely exploited by migratory birds. This article presents a benchmark problem for scalable distributed flight control of formations of UAS with only local relative state information, one of the open problems in this field as of today. The benchmark is openly available and comprises a nonlinear six degrees of freedom dynamics model of an electric glider UAS. In this article we furthermore introduce a nominal guidance frame that does not require state information of other UAS and point out a fundamental issue related to wake vortex tracking during formation maneuvers. A set of LQ baseline controllers that are part of the benchmark is presented along with simulation results

Collision-Free Rendezvous Maneuvers for Formations of Unmanned Aerial Vehicles

This article discusses the rendezvous maneuver for a fleet of small fixed-wing Unmanned Aerial Vehicles (UAVs). Trajectories have to be generated on-line while avoiding collision with static and dynamic obstacles and minimizing rendezvous time. An approach based on Model Predictive Control (MPC) is investigated which assures that the dynamic constraints of the UAVs are satisfied at every time step. By introducing binary variables, a Mixed Integer Linear Programming (MILP) problem is formulated. Computation time is limited by incorporating the receding horizon technique. A shorter planning horizon strongly reduces computation time, but delays detection of obstacles which can lead to an infeasible path. The result is a robust path planning algorithm that satisfies the imposed constraints. However, further relaxation of the constraints and fine-tuning is necessary to limit complexity

Contributions au vol en formation serrée de petits drones

Electrically driven mini drones are likely to have lower endurance than larger drones, mainly due to the limited aerodynamic efficiency of small, low elongation wings. The limited storage capacity of the on-board batteries on the mini-drones also reduces total endurance. The exploitation of aerodynamic interactions, inspired by migratory birds, as well as in-flight refuelling, are promising approaches to improve the endurance of mini-drones while allowing payload distribution.In the context of conventional aircraft formation flight, it was observed that a significant reduction in the following aircraft's energy consumption could be achieved by placing it in the wake vortices of the predecessor. In our context, this implies very precise displacements of the following mini-drone whose relative position (lateral and vertical) must remain at a fraction of the size of the predecessor mini-drone. In addition, the docking procedure used for exchanging batteries between UAVs in flight involves similar or even stricter guidance performance requirements. These strong performance constraints are also accompanied by particularly high robustness requirements on the control laws due to poorly known disturbances caused by wake turbulence.The work proposed in this thesis provides theoretical contributions aimed at enabling precise trajectory control thanks to advanced techniques based on a discrete-time predictive sliding-mode guidance scheme. On the other hand, we are also interested in advanced localization algorithms in order to feed collision avoidance strategies.Les mini-drones à propulsion électrique sont susceptibles d’avoir une endurance inférieure à celle de drones plus grands, en raison, principalement, de l’efficacité aérodynamique limitée des petites ailes de faible allongement. La capacité de stockage limitée des batteries embarquables sur les mini-drones réduit également l’endurance totale. L’exploitation des interactions aérodynamiques, inspirée par les oiseaux migratoires, ainsi que le ravitaillement en vol , sont des approches prometteuses pour améliorer l’endurance des mini-drones tout en permettant une distribution de la charge utile.Dans le contexte du vol en formation classique des avions, on a observé qu’une réduction significative de la consommation d’énergie de l’avion suiveur peut être obtenue dès lors qu’il se place dans les tourbillons de sillage du prédécesseur. Dans notre contexte, cela implique des déplacements très précis du mini-drone suiveur dont la position relative (latérale et verticale) doit rester à une fraction d’envergure du mini-drone prédécesseur. Par ailleurs, la procédure d’amarrage utilisée dans l’échange de batteries entre drones en vol implique des exigences de performance de guidage similaires, voire plus strictes. Ces fortes contraintes de performance s’accompagnent en outre d’exigences de robustesse particulièrement élevées sur les lois de commande en raison des perturbations mal connues induites par les turbulences de sillage.Les travaux proposés dans cette thèse apportent des contributions théoriques visant à permettre un contrôle précis ds trajectoires grâce à des techniques avancées fondées sur un schéma de guidage prédictif discret par modes glissants. On s'intéresse d'autre part à des algorithmes avancés de localisation permettant de caractériser des régions de confiance garanties de la position des membres d'une formation afin d'alimenter des stratégies d'évitement de collisions

Contributions to Tight Formation Flight Control of Small UAS

Les mini-drones à propulsion électrique sont susceptibles d’avoir une endurance inférieure à celle de drones plus grands.L’exploitation des interactions aérodynamiques, inspirée par les oiseaux migratoires, ainsi que le ravitaillement en vol , sont des approches prometteuses pour améliorer l’endurance des mini-drones. La commande par modes glissants d’ordre supérieur en temps continu (CTHOSM) a été considérée comme un candidat prometteur à ce problème ouvert difficile et a été appliquée avec succès à des modèles cinématiques simples. Dans nos travaux, nous étudions les implications de la présence de la dynamique de la boucle interne et de l’implémentation en temps discret à des taux d’échantillonnage modérés et constatons alors que l’application de la commande CTHOSM devient impossible. Nous proposons donc un schéma de guidage prédictif discret par modes glissants pour approximer les performances de la commande CTHOSM pour une dynamique réaliste du drone. On propose également un problème de référence accessible pour d'autres chercheurs. Les algorithmes de localisation probabilistes existants ne permettent pas la caractérisation de régions de confiance garanties de la position des autres membres de la formation. Dans ce contexte, nous proposons un nouveau filtre ensembliste caractérisant de telles régions de confiance sous forme ellipsoïdale. Nos premières évaluations ont montré que les efforts de calcul induits par cette mise en œuvre restent parfaitement compatibles avec les contraintes des systèmes avioniques des petits drones.Small, electrically driven unmanned aircraft are likely to suffer from inferior endurance compared to their larger counterparts. Upwash exploitation by tight formation flight, as well as aerial recharging are the most promising control-driven approaches to mitigate this disadvantage. Continuous time higher order sliding mode control (CTHOSM) has been considered as a candidate for this challenging open problem and was successfully applied to simple kinematic models in simulation, where excellent relative position tracking performance can be demonstrated. In this work we study the implications of the presence of inner loop dynamics and discrete implementation at moderate sampling rates and we find that it precludes the application of CTHOSM control to fixed-wing UAS. We propose a predictive discrete sliding mode guidance scheme to approximate the performance of CTHOSM control assuming realistic fixed-wing UAS dynamics. We show that the proposed guidance scheme in combination with inner load factor tracking loops and a disturbance observer allows for relative position tracking performance compatible with the requirements of upwash exploitation. We propose as well an openly accessible benchmark problem. Existing probabilistic localization algorithms cannot provide guaranteed confidence regions of the relative position between UAS. We present a set membership filter that provides ellipsoidal regions guaranteed to contain the relative positions of the other UAS. It is compatible with the hardware constraints of small low-cost UAS. Simulations suggest computational efforts compatible with the computational resources typically available onboard small UAS

Structured control law design and robustness assessment for the automatic launch of small UAVs

Automatic launch is an important capability towards the truly autonomous flight of Unmanned Aerial Vehicles (UAVs) that does not require the presence of an expert pilot, as it is often the case today. In this work a complete approach to the design and robustness assessment of a set of control laws for the automatic launch of fixed-wing UAVs is presented. The proposed control system consists of an airspeed tracking loop and a nested lateral guidance loop. Important nonlinearities such as actuator saturations and signal delays are taken into account for controller synthesis and robustness evaluation. Due to the high risk inherent to flight testing the launch phase, extensive Monte Carlo simulations over the space of model uncertainties and initial launch conditions have been performed on the nonlinear model of a flying-wing type UAV, including atmospheric turbulence. Time consuming Monte Carlo simulations are complemented by testing for robust stability and identifying worst-case performance configurations using Structured Singular Value mu-analysis methods