Having an in-depth knowledge on the performance characteristics of space mechanisms in flight operation, with special attention to nominal vs. anomalous performance, is vital for mission success. On many unmanned spacecraft for Earth observation missions, reaction wheel assemblies are used in combination with magnetic torquers for their attitude control. Understanding the magnitude of potential electromagnetic interactions between both types of attitude control actuators is of particular interest for large spacecraft as they are usually equipped with strong magnetic torquers. In this frame, experimental investigations have been performed on simplified test set-ups with flight representative reaction wheel assemblies operated in external homogeneous magnetic fields as well as in close vicinity of magnetic torquers which create inhomogeneous magnetic fields. The test results have been successfully correlated with computer-based simulation output obtained from models with different levels of complexity. The impact of critical parameters like the location of magnetic torquers relative to reaction wheels and their material properties such as electrical conductivity and magnetic permeability have been particularly studied. It has been found that magnetic torquers pointing orthogonal to the reaction wheel spin axis cause the highest influence on the reaction wheel's performance characteristics. The material choice for the flywheel rotor, being either ferromagnetic or paramagnetic, has a strong influence when exposing the reaction wheel assembly to external magnetic fields. In general, the increase of loss torque noticed with all reaction wheels tested has been caused by eddy current effects. In this frame, the impact of using ferromagnetic materials has been surprisingly strong. Specifically, the local distortions and guidance of the magnetic field due to ferromagnetism has a highly amplifying effect on eddy currents. However, interestingly it has also been found that the impact of material choice is much more severe when considering homogeneous magnetic fields and strong magnetic torquers while being less important with relatively small magnetic torquers. The main reasons for this finding have been compensating effects of ferromagnetic vs. highly conductive materials. / Une connaissance approfondie des caractéristiques de performance des mécanismes spatiaux en vol, et plus particulièrement des performances nominales comparées aux performances anormales, est d’importance vitale pour la réussite d’une mission. Pour les missions d’observation de la Terre, la plupart des engins spatiaux non habités sont équipés d’ensembles de roues de réaction ainsi que de magnéto-coupleurs pour le contrôle d’attitude et la stabilisation. Comprendre l'ampleur des interactions électromagnétiques potentielles entre les deux types de capteurs de contrôle d'attitude est particulièrement pertinent pour les engins spatiaux de grande taille car ceux-ci sont généralement équipés de puissants magnéto-coupleurs. Dans ce cadre, des études expérimentales ont été réalisées sur des bancs d'essais simplifiés avec des ensembles de roues à réaction représentatifs du vol fonctionnant dans des champs magnétiques externes homogènes ainsi qu'à proximité immédiate de coupleurs magnétiques (champs magnétiques hétérogènes). Les résultats des tests ont été corrélés avec succès grâce à des simulations informatiques sur des modèles présentant différents niveaux de complexité. L'influence de paramètres critiques comme l'emplacement des magnéto-coupleurs par rapport aux roues de réaction et leurs propriétés matérielles telles que la conductivité électrique et la perméabilité relative ont été particulièrement étudiés. Il a été établi que les couples magnétiques pointant orthogonalement à l'axe de rotation de la roue de réaction ont le plus d'influence sur les caractéristiques de performance des roues de réaction. Le choix du matériau pour le rotor de volant, c’est à dire ferromagnétique ou paramagnétique, a une forte influence si l'ensemble de roue de réaction est exposé à des champs magnétiques externes. En général, l'augmentation de la perte de transfert de couple constatée avec toutes les roues de réaction testées a été causée par les effets de courants de Foucault.Dans ce cadre, l'influence des matériaux ferromagnétiques a été étonnamment forte. En effet, les distorsions qui en résultent et le guidage du champ magnétique amplifient fortement les courants de Foucault. Cependant, il a été constaté que l'effet du choix du matériau est beaucoup plus important si l'on considère des champs magnétiques homogènes et des grands coupleurs magnétiques. Toutefois, cet effet est moins important avec des petits coupleurs magnétiques.

This dissertation develops a general framework for designing stabilizing feedback controllers and observers for dynamics with state/time dependent gains on the control signals and measured outputs. These gains have potential singularity periods but satisfy a technically non-trivial condition referred to as persistence of excitation. A persistence filter design constitutes the primary theoretical innovation of this work around which the controller and observer development is centered. Application areas of singular gain systems considered in this study include robotics, biomechanics, intelligent structures and spacecrafts. Several representative problems involving singular, time-dependent gains are addressed. The specific contributions of this dissertation are outlined as follows: (i) a stabilizing feedback for linear, single-input systems with time-varying, singular control scaling is designed that allows arbitrary exponential convergence rate for the closed-loop dynamics. An adaptive control generalization of this result allows asymptotic convergence in presence of unknown plant parameters. An extension to a special, single-input nonlinear system in the controller canonical form is also proposed. It is proven that this control design results in bounded tracking error signals for a trajectory tracking objective; (ii) observer design for linear, single-output systems with time-varying, singular measurement gains is considered. A persistence filter similar in structure to the control counterpart aids an observer design that guarantees exponential state reconstruction with arbitrary convergence rates; (iii) the observer and controller designs are combined to obtain an exponentially stabilizing output feedback controller for linear, single-input, single-output dynamics with singular gains on both the control and measurements. A novel separation property is established as a consequence. The construction motivates applications to stabilization with reversible transducers which can switch between sensor and actuator modes. The results are verified on two illustrative applications, vibration control using piezoelectric devices and inverted pendulum stabilization with a DC motor. The linear result is further generalized to include state dependent gains; (iv) application of the persistence filter theory to spacecraft attitude stabilization using intermittent actuation is explored. The intermittence is characterized by a time-varying, periodically singular control gain. A nonlinear persistence filter allows construction of an exponentially stabilizing controller and simulations verify convergence with intermittent actuation where conventional proportional-derivative control fails; (v) a stabilization result for a special multi-input, linear system with time-varying matrix control gains is presented. The matrix gain is assumed to be diagonal but allows fewer controls than states subject to a controllability assumption in absence of the singular gain matrix. The single-input adaptive control results are shown to extend to the multi-input case. An application to angular velocity stabilization of an underactuated rigid spacecraft is considered. / text