ExoCube is the latest National Science Foundation (NSF) funded space weather CubeSat and is a collaboration between PolySat, Scientific Solutions Inc. (SSI), the University of Wisconsin, NASA Goddard and SRI International. The 3U will carry a mass spectrometer sensor suite, EXOS, in to low earth orbit (LEO) to measure neutral and ionized particles in the exosphere and thermosphere. Measurements of neutral and ion particles are directly impacted by the angle at which they enter EXOS and which leads to pointing requirements. A combination of a gravity gradient system with a momentum bias wheel is proposed to meet pointing requirements while reducing power requirements and overall system complexity. A MATLAB simulation of dynamic and kinematic behavior of the system in orbit is implemented to guide system design and verify that the pointing requirements will be met. The problem of achieving the required three-axis pointing is broken into four phases: detumbling, initial attitude acquisition, wheel spin-up, and attitude maintenance. Ultimately, this configuration for attitude control in a CubeSat could be applied to many future missions with the simulation serving as a design tool for CubeSat developers.

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 thesis deals with the scheduling problem for a constellation of Earth observation satellites, focusing on modelling the attitude dynamics to assess the tasking capabilities. A target selection algorithm is developed considering the time dependent manoeuvres between targets and the time-dependent value of the observed targets. Further, a closed-loop dynamics simulation is carried out to assess the agility of the 6U platform and verify the results of the algorithm. The work does not intend to present definitive numerical results, rather the goal is to develop a holistic framework that allows appraising the performance of a platform and the fulfilment of the mission objectives, aiming to maximise the collective value of the observed targets. Given the inputs in terms of platform, sensor, orbit and list of targets, this work serves to simulate the target selection and imaging at an arbitrary day and time for a chosen observation window. / Denna studie behandlar problemet med schemaläggning för en konstellation av jordobservationssatelliter och fokuserar på att modellera attityddynamiken för autonomt utförda uppgifter med beaktande av satellitens kapacitet. En målvalsalgoritm utvecklades med hänsyn till både tidsberoende manövrar mellan målen och tidsberoende värden för de observerade målen. Dessutom utfördes en simulering av styrdynamik i ett slutet system för en 6U-plattform för att bedöma och verifiera målvalsalgoritmen. Arbetet avser inte att presentera definitiva numeriska resultat, utan syftet var att utveckla ett helhetsramverk för möjlig bedömning av plattformens prestanda och att studera plattformens förmåga att välja mål som maximerar det samlade värdet av de observerade målen. Med givna ingångsvärden i form av plattform, sensor, omloppsbana samt lista over mål, ger detta arbete möjlighet att simulera satellitens val av mål i en avbildning vid en godtycklig dag och tid för ett valt observationsfönster.

The purpose of the study is to evaluate the possibility of using gridded ion thrusters as a means of attitude control for a solar sail as a part of the sunshade project, which aims to place 10^8 solar sail sunshade spacecraft, each with an area of about 10 000 m^2, at the Sun-Earth Lagrangian point L1 in order to reduce Earth's global temperature. Two types of solar sail sunshade spacecraft were studied. The first type, referred to as the sunshade demonstrator, had an area of 100 m^2 and a mass of 10 kg, and the second type, referred to as the full-sized sunshade, had an area of 10 000 m^2 and a mass of 90 kg. To determine the significance of using ion thrusters for the attitude control system, the mass of the required fuel, as well as the total mass that had to be added to the spacecraft to implement the attitude control system, was calculated. Two types of journeys were studied for each spacecraft type: starting from Low Earth Orbit (LEO) to L1 and from Geostationary Orbit (GEO) to L1, respectively. The results showed that the duration of the journey of the full-sized spacecraft was about 570 days from LEO to L1 and 370 from GEO to L1, respectively. The required amounts of fuel for the respective journeys were 580 g and 15 g, respectively, and resulted in a total additional mass of 7.8 kg and 7.2 kg, respectively.

No description available.

This thesis presents the design, modeling, and control of a fully-actuated multi-rotor unmanned aerial vehicle (UAV). Unlike conventional multi-rotors, which suffer from two degrees of underactuation in their propeller plane, the choice of an unconventional propeller configuration in the new drone leads to an even distribution of actuation across the entire force-torque space. This allows the vehicle to produce any arbitrary combination of forces and torques within a bounded magnitude and hence execute motion trajectories unattainable with conventional multi-rotor designs. This system, referred to as the \omninospace, decouples the position and attitude controllers, simplifying the motion control problem. Position control is achieved using a PID feedback loop with gravity compensation, while attitude control uses a cascade architecture where the inner loop follows an angular rate command set by the outer attitude control loop. A novel model is developed to capture the disturbance effects among interacting actuator airflows of the \omninospace. Given a desired actuator thrust, the model computes the required motor command using the current battery voltage and thrusts of disturbing actuators. A system identification is performed to justify the use of a linear approximation for parameters in the model to reduce its computational footprint in real-time implementation. The \omni benefits from two degrees of actuation redundancy resulting in a control allocation problem where feasible force-torques may be produced through an infinite number of actuator thrust combinations. A novel control allocation approach is formulated as a convex optimization to minimize the \omnis energy consumption subject to the propeller thrust limits. In addition to energy savings, this optimization provides fault tolerance in the scenario of a failed actuator. A functioning prototype of the \omni is built and instrumented. Experiments carried out with this prototype demonstrate the capabilities of the new drone and its control system in following various translational and rotational trajectories, some of which would not be possible with conventional multi-rotors. The proposed optimization-based control allocation helps reduce power consumption by as much as 6\%, while being able to operate the drone in the event of a propeller failure. / Thesis / Master of Applied Science (MASc)

This dissertation investigates attitude transition maneuvers of a gyrosat-satellite between relative equilibria. The primary challenge in transitioning between relative equilibria is the proper adjustment of the system angular momentum so that upon completing the transition maneuver the gyrostat-satellite will satisfy all the requirements for a relative equilibrium. The system angular momentum is a function of the attitude trajectory taken during the transition maneuver. A new concept, the constant orbital momentum equilibrium trajectory or COMET, is introduced as a means to a straight-forward solution to a subset of the possible transitions between relative equilbria. COMETs are a class of paths in SO(3) that a gyrostat-satellite may travel along that maintain a constant system angular momentum. The primary contributions of this dissertation are the introduction and analysis of COMETs and their application to the problem of transitioning a gyrostat-satellite between two relative equilibria. The current work introduces, defines, and analyzes COMETs in detail. The requirements for a path in SO(3) to be a COMET are defined. It is shown via example that COMETs are closed-curves in SO(3). Visualizations of families of COMETs are presented and discussed in detail. A subset of COMETs are shown to contain critical points that represent isolated relative equilibrium attitudes or furcations of the COMET. The problem of transitioning between two relative equilibria is split into the sub-problems of transitioning between relative equilibria on the same COMET and transitioning between relative equilibria on different COMETs. For transitions between relative equilibria on the same COMET, an open-loop control law is developed that drives a gyrostat-satellite along the COMET until the target relative equilibrium is reached. For transitions between relative equilibria on different COMETs, an open-loop control law is developed that transfers a gyrostat-satellite from the initial relative equilibrium to a relative equilibrium that resides on the same COMET as the target relative equilbrium. Acquisition of the target relative equilibrium is then accomplished via the application of the open-loop control law for transitions between relative equilibria on the same COMET. The results of numeric simulations of gyrostat-satellites executing these transitions are presented. / Ph. D.

Numerical schemes for the solution of two-point boundary value problems arising from the application of optimal control theory to mathematical models of dynamic systems, are discussed. Optimal control problems are formulated for rotational maneuvers of multiple rigid body, asymmetric spacecraft configurations with both external torques and/or internal torques. Necessary conditions for optimality are derived through Pontryagin’s principle; solutions to the problems are obtained numerically. Comparison studies using competing numerical methods and various choices of performance indices are reported. / Ph. D.

Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This research focuses on the development of new efficient algorithms for multi-path planning and multi-rigid body constrained attitude control. The work is motivated by current and future applications of these algorithms in: intelligent control of multiple autonomous aircraft and spacecraft systems; control of multiple mobile and industrial robot systems; control of intelligent highway vehicles and traffic; and air and sea traffic control. We shall collectively refer to the class of mobile autonomous systems as “agents”. One of the challenges in developing and applying such algorithms is that of complexity resulting from the nontrivial agent dynamics as agents interact with other agents, and their environment. In this work, some of the current approaches are studied with the intent of exposing the complexity issues associated them, and new algorithms with reduced computational complexity are developed, which can cope with interaction constraints and yet maintain stability and efficiency. To this end, this thesis contributes the following new developments to the field of multipath planning and multi-body constrained attitude control: • The introduction of a new LMI-based approach to collision avoidance in 2D and 3D spaces. • The introduction of a consensus theory of quaternions by applying quaternions directly with the consensus protocol for the first time. • A consensus and optimization based path planning algorithm for multiple autonomous vehicle systems navigating in 2D and 3D spaces. • A proof of the consensus protocol as a dynamic system with a stochastic plant matrix. • A consensus and optimization based algorithm for constrained attitude synchronization of multiple rigid bodies. • A consensus and optimization based algorithm for collective motion on a sphere. / AFRIKAANSE OPSOMMING: Hierdie navorsing fokus op die ontwikkeling van nuwe koste-effektiewe algoritmes, vir multipad-beplanning en veelvuldige starre-liggaam beperkte standbeheer. Die werk is gemotiveer deur huidige en toekomstige toepassing van hierdie algoritmes in: intelligente beheer van veelvuldige outonome vliegtuig- en ruimtevaartuigstelsels; beheer van veelvuldige mobiele en industrile robotstelsels; beheer van intelligente hoofwegvoertuie en verkeer; en in lug- en see-verkeersbeheer. Ons sal hier “agente” gebruik om gesamentlik te verwys na die klas van mobiele outonome stelsels. Een van die uitdagings in die ontwikkeling en toepassing van sulke algoritmes is die kompleksiteit wat spruit uit die nie-triviale agentdinamika as gevolg van die interaksie tussen agente onderling, en tussen agente en hul omgewing. In hierdie werk word sommige huidige benaderings bestudeer met die doel om die kompleksiteitskwessies wat met hulle geassosieer word, bloot te l^e. Verder word nuwe algoritmes met verminderde berekeningskompleksiteit ontwikkel. Hierdie algoritmes kan interaksie-beperkings hanteer, en tog stabiliteit en doeltreffendheid behou. Vir hierdie doel dra die proefskrif die volgende nuwe ontwikkelings by tot die gebied van multipad-beplanning van multi-liggaam beperkte standbeheer: • Die voorstel van ’n nuwe LMI-gebasseerde benadering tot botsingsvermyding in 2D en 3D ruimtes. • Die voorstel van ’n konsensus-teorie van “quaternions” deur “quaternions” vir die eerste keer met die konsensusprotokol toe te pas. • ’n Konsensus- en optimeringsgebaseerde padbeplanningsalgoritme vir veelvoudige outonome voertuigstelsels wat in 2D en 3D ruimtes navigeer. • Die bewys van ’n konsensusprotokol as ’n dinamiese stelsel met ’n stochastiese aanlegmatriks. • ’n Konsensus- en optimeringsgebaseerde algoritme vir beperkte stand sinchronisasie van veelvoudige starre liggame. • ’n Konsensus- en optimeringsgebaseerde algoritme vir kollektiewe beweging op ’n sfeer.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / The aim of this project was to design and test a partial attitude determination and control system for a nanosatellite. The reaction wheel system was designed and tested as an actuator for the nanosatellite. This reaction wheel system consists of four reaction wheels mounted in a tetrahedral formation. A rate sensor system was also designed and its viability for this space application was examined. The rate sensor system consists of 3 orthogonally mounted planes, each with three rate sensors mounted on it. Hardware-inthe- loop tests were used along with an air bearing experimentational setup, which created near frictionless circumstances, to prove the effectiveness of the designed reaction wheel setup. The results following from this project were the following: The reaction wheel system proved to be an adequate actuator for this nanosatellite application and the rate sensor systemwhich was analysed proved to be inadequate for a nanosatellite application.