In this thesis, the kinematic properties of Single Gimballed Control Moment Gyroscopes (SGCMGs) are investigated. Singularity phenomenon inherent to them is explained. Furthermore, existing steering laws with their derivations are given. A novel steering law is developed that may provide singularity avoidance or may be used for quick transition through a singularity with small torque errors. To avoid singularity angular momentum trajectory of the maneuver is to be simulated in advance for the calculation of singularity free gimbal histories. The steering law, besides accurately generating required torques, also pushes the system to follow trajectories closely if there is a small difference between the planned and the realized momentum histories. Thus, it may be used in a feedback system. Also presented are number of approaches for singularity avoidance or quick transition through a singularity. The application of these ideas to the feedback controlled spacecraft is also presented. Existing steering laws and the proposed method are compared through computer simulations. It is shown that the proposed steering law is very effective in singularity avoidance and quick transition through singularities. Furthermore, the approach is demonstrated to be repeatable even singularity is encountered.

In this thesis, recently developed Blended Inverse (B-inverse) steering law is applied to two different redundant actuator systems. First, repeatability of Binverse is demonstrated on a redundant robotic manipulator. Its singularity avoidance and singularity transition performance is also demonstrated on the same actuator system. It is shown that B-inverse steering law provides singularity avoidance, singularity transition and repeatability. Second, its effectiveness is demonstrated for an Integrated Power and Attitude Control - Control Moment Gyroscope (IPAC-CMG) cluster, which can perform energy management and attitude control functions simultaneously. For this purpose, an IPAC-CMG flywheel is conceptually designed. A control policy is developed for the energy management.

Thesis (MScEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: A new task scheduler for the Attitude Determination and Control System (ADCS) of the Stellenbosch University Satellite (SUNSAT) has been designed and tested on a personal computer. This new scheduler is capable of uploading new control tasks, or changing existing control tasks, on an individual basis. This is an improvement on the current ADCS task scheduler, where the control tasks are hard-coded in the scheduler, requiring the entire software image of the scheduler to be uploaded if a new task is to be added, or an existing task is to be changed. The new scheduler was developed using the Java programming language. The Java ClassLoader class is used to dynamically load tasks to a linked list. The scheduler thread runs through this linked list and schedules all the tasks that have become schedulable. New tasks can be added to the list without stopping the scheduler. The new scheduler has been successfully implemented on a personal computer, laying a good foundation for implementation in an embedded environment based on processors such as the T800 Transputer of the ADCS or the 80386 processor of the secondary onboard computer (OBC2). / AFRIKAANSE OPSOMMING: 'n Nuwe taak skeduleerder vir die orientasie beheerstelsel (Engels: Attitude Determination and Control System, of ADCS) van die Stellenbosch Universiteit Satelliet (SUNSAT) is ontwerp en getoets op 'n persoonlike rekenaar. Hierdie nuwe skeduleerder het die verrnoee om ekstra beheertake op te laai, of bestaande beheertake te wysig, onafhanklik van mekaar. Dit is 'n verbetering op die huidige ADCS taak skeduleerder waar take hard gekodeer is in die skeduleerder en waar vereis word dat die volledige sagteware beeld van die skeduleerder opgelaai moet word indien 'n nuwe taak bygevoeg wil word of 'n bestaande taak gewysig wil word. Die nuwe skeduleerder is ontwikkel met behulp van die Java programmeringstaal. Die Java C/assLoader klas is gebruik om take dinamies te laai en te voeg by 'n skakellys. Die skeduleerder proses stap dan deur hierdie skakellys en skeduleer aile take wat skeduleerbaar geword het. Nuwe take kan by die skakellys gevoeg word sonder om die skeduleerder te stop. Die nuwe skeduleerder is suksesvol ge'lmplementeer op 'n persoonlike rekenaar en Ie 'n goeie grondslag vir implementering in 'n toegewyde stelsel omgewing gebaseer op byvoorbeeld die T800 Transputer van die ADCS of die 80386 verwerker van die sekondere aanboord rekenaar (OBC2).

Thesis (MScEng)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: One of the main subsystems of SUNSAT is the Attitude Determination and Control System (ADCS), responsible for the orientation and positioning of the satellite. Due to the integrated architecture of the system, several shortcomings were identified. A possible solution to the problems is the implementation of a distributed system. The design of a communication protocol for a distributed system is the focus of this thesis. An investigation on different network topologies and communication protocols as well as error control techniques is carried out to establish a combination that meets the requirements of the ADCS. Based on defined protocol specifications a detailed protocol design is proposed. Then the protocol is implemented using a layered software structure that emanates from the OSI layering model, to provide well defined software structures and interfaces. A series of measurements have shown that the protocol meets the functional requirements of the ADCS and further provides reliable data transfer on the network. / AFRIKAANSE OPSOMMING: Een van die vernaamste dele van SUNSAT is die "Attitude Determination and Control System" (ADCS) wat verantwoordelik is vir die orientasie en posisionering van die satelliet. Verskeie tekortkomminge as gevolg van die geYntegreerde argitektuurvan die stelsel, is geYdentifiseer. 'n Moontlike oplossing vir die probleme is die implementering van 'n verspreide stelsel. Die ontwerp van 'n kommunikasie protokol vir 'n verspreide stelsel is die fokus van die tesis. 'n Ondersoek na verskeie netwerk topoloqie en kommukasie protokolle, asook foutbeheer tegnieke is uitgevoer om vas te stel watter kombinasie die ADCS se vereistes sal bevredig. 'n Gedetaileerde protokol ontwerp is voorgestel gebaseer op gedefineerde protokol spesifikasies. Hierdie protokol is toe geYmplementeer deur gebruik te maak van vlak gestruktureerde sagteware wat afkomstig is van die OSI model, met die oog op goed gedefineerde sagteware strukture en koppelvlakke. 'n Reeks meetings het aangedui dat die protokol die funksionele vereistes van die ADCS bevredig en dat dit verder betroubare data verplasing oor die netwerk verskaf.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: CubeSats are small, lightweight satellites which are often used by academic institutions due to their application potential and low cost. Because of their size and weight, less powerful attitude controllers, such as solar sails, can be used. In 2010, the Japanese satellite, Ikaros, was launched to illustrate the usage of solar sails as a propulsion system. Similarly, by exploiting the solar radiation pressure, it is possible to use a solar sail, together with three magnetorquers, to achieve 3-axis attitude control of a 3-unit CubeSat. Simulations are required to demonstrate the attitude control of a sun-synchronous, low Earth orbit CubeSat using a solar sail. To allow the adjustment of the solar sail, and its resulting torque, a mechanical structure is required which can be used to position the sail within two orthogonal axes. Although the magnetorquers and solar sail are sufficient to achieve 3-axis attitude control, the addition of a reaction wheel can be implemented in an attempt to improve this control. / AFRIKAANSE OPSOMMING: CubeSats is klein, ligte satelliete wat dikwels deur universiteite gebruik word weens hul lae koste en groot toepassings potensiaal. As gevolg van hulle gewig en grootte, kan minder kragtige posisie beheerders, soos byvoorbeeld sonseile, gebruik word. Die Japannese satelliet, Ikaros, was in 2010 gelanseer om die gebruik van ’n sonseil as aandrywingstelsel te illustreer. Net so is dit moontlik om die bestraling van die son te gebruik, met behulp van ’n sonseil, en drie magneetstange om 3-as posisiebeheer op ’n 3-eenheid CubeSat te bekom. Simulasies word benodig om die posisie beheer van ’n sonsinkrone, lae-aard wentelbaan CubeSat met ’n sonseil te demonstreer. ’n Meganiese struktuur word benodig vir die posisionering van die sonseil in twee ortogonale asse sodat die sonseil, en dus die geassosieerde draaimoment, verskuif kan word. Alhoewel die magneetstange en sonseil voldoende is om 3-as posisiebeheer te bekom, kan ’n reaksiewiel bygevoeg word om hierdie beheer te probeer verbeter.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The use of electrodynamic tethers on-board satellites is an exciting scientific prospect. These conductive tethers provide the means for satellites to generate power and to do propulsion by electrodynamic interaction with the geomagnetic field. Although well researched in theory, the concept has not enjoyed much success in practice. This study aims to utilise low-cost CubeSats as experimental tool to verify many of the theoretical principles that govern the behaviour of conductive tethers in orbit. The study provides a theoretical background of the concept by evaluating past tether missions and analysing existing theory. A feasible application of an electrodynamic tether within the size and weight limitations of a Nano-satellite is formulated. Existing theoretical work is adapted to model the dynamics and electrodynamics of specifically Nano-satellites. Using these mathematical models, control and estimation algorithms are designed which would provide stable deployment of a tethered CubeSat pair and stable control of the orientation of the tethered system. To be able to implement these algorithms on a satellite mission, a prototype of a sensor capable of measuring the angle of the tether using a CMOS camera is designed and built. A hardware platform is built to test the deployment of the tether using an electric motor. Electronics are designed to control the operation of the camera, to do motor control, and to run control and estimation algorithms. Using the results obtained from the practical tests done on the hardware, and using the theoretical models and control algorithms designed, a full orbital simulation of the deployment was done. This simulation includes the performance of the deployment system, the electrodynamic performance of the tether in earth‟s plasmasphere, and the estimation and control algorithms to control the system. Different deployment strategies are analysed and their performance are compared. / AFRIKAANSE OPSOMMING: Die gebruik van elektrodinamiese toue aanboord satelliete is 'n opwindende wetenskaplike vooruitsig. Hierdie geleidende toue verleen aan die satelliete die vermoë om krag op te kan wek en propulsie deur elektriese interaksie met die geomagnetiese veld te kan doen. Alhoewel dit goed nagevors is in teorie, het die konsep nog nie veel sukses in die praktyk geniet nie. Hierdie studie het dit ten doel om lae-koste CubeSats aan te wend as 'n eksperimentele instrument om baie van die teoretiese beginsels wat geld vir die gedrag van geleidende toue in wentelbane te verifieer. Die studie bied 'n teoretiese agtergrond van die konsep deur die evaluering van vorige tou-missies sowel as die analise van bestaande teorie. 'n Uitvoerbare toepassing van 'n elektrodinamiese tou binne die grootte- en gewigsbeperkinge van 'n Nano-satelliet is geformuleer. Bestaande teoretiese werk is aangepas om die dinamika en elektrodinamika spesifiek van toepassing op Nano-satelliete, te modelleer. Deur hierdie wiskundige modelle te gebruik, is beheer- en afskattingsalgoritmes ontwerp wat stabiele ontplooiing van 'n verbinde CubeSat-paar en stabiele beheer van die oriëntasie van die verbinde stelsel sal verseker. Om hierdie algoritmes te implementeer op 'n satelliet-sending, is 'n prototipe van 'n sensor wat in staat is om die hoek van die tou met behulp van 'n CMOS kamera te meet, ontwerp en gebou. 'n Hardeware platform is gebou om die ontplooiing van die tou met behulp van 'n elektriese motor te toets. Elektronika is ontwerp om die kamera te beheer, motor beheer te doen asook om beheer- en afskattingsalgoritmes uit te voer. Deur gebruik te maak van die resultate wat verkry is tydens die praktiese toetse wat gedoen is op die hardeware, en deur gebruik te maak van die teoretiese modelle en beheeralgoritmes wat ontwerp is, is 'n volle wentelbaan-simulasie van die ontplooiing gedoen. Hierdie simulasie sluit die gedrag van die ontplooiingstelsel, die elektriese gedrag van die geleidende tou in die aarde se plasmasfeer, en die afskatting- en beheeralgoritmes om die stelsel te beheer in. Verskillende ontplooiingstrategieë word ontleed en hul gedrag word vergelyk.

Thesis (MEng)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: The next generation of CubeSats will require accurate attitude knowledge throughout orbit for advanced science payloads and high gain antennas. A star tracker can provide the required performance, but star trackers have traditionally been too large, expensive and power hungry to be included on a CubeSat. The aim of this project is to develop and demonstrate a CubeSat compatible star tracker. Subsystems from two other CubeSat components, CubeSense and CubeComputer, were combined with a sensitive, commercial image sensor and low-light lens to produce one of the smallest star trackers in existence. Algorithms for star detection, matching and attitude determination were investigated and implemented on the embedded system. The resultant star tracker, named CubeStar, can operate fully autonomously, outputting attitude estimates at a rate of 1 Hz. An engineering model was completed and demonstrated an accuracy of better than 0.01 degrees during night sky tests. / AFRIKAANSE OPSOMMING: Die volgende generasie van CubeSats sal akkurate orientasie kennis vereis gedurende 'n volle omwentelling van die aarde. 'n Sterkamera kan die vereiste prestasie verskaf, maar sterkameras is tradisioneel te groot, duur en krag intensief om ingesluit te word aanboord 'n CubeSat. Die doel van hierdie projek is om 'n CubeSat sterkamera te ontwikkel en te demonstreer. Substelsels van twee ander CubeSat komponente, CubeSense en CubeComputer, was gekombineer met 'n sensitiewe kommersiële beeldsensor en 'n lae-lig lens om een van die kleinste sterkameras op die mark te produseer. Algoritmes vir die ster opsporing, identi kasie en orientasie bepaling is ondersoek en geïmplementeer op die ingebedde stelsel. Die gevolglike sterkamera, genaamd CubeStar, kan ten volle outonoom orientasie afskattings lewer teen 'n tempo van 1 Hz. 'n Ingenieursmodel is voltooi en 'n akkuraatheid van beter as 0.01 grade is gedemonstreer.

Reliability and safety of complex software-intensive systems are proved to be a crucial matter since most of these systems fulfil tasks, where a failure could lead to catastrophic consequences. For example, in space systems such as satellites, a failure could result in the loss of the satellite. Therefore, a certain level of reliability and safety must be assured for such systems to trust the services they provide. Standards set this level and put requirements for the analysis and assurance of these properties using documented evidence. In particular, European Cooperation for Space Standardization (ECSS) standards for space systems require Fault Tree Analysis(FTA) for identifying the causes of system failure and consequently safety hazards, as well as fault trees as evidence for the assurance of reliability and safety. In this thesis, we present a tool supported model-based approach to generate fault tree automatically from an existing system modelling and analysis toolset. CHESS is a system and dependability modelling toolset and integrates Concerto-FLA to enable the support of failure logic analysis. We proposed a model-based transformation from Concerto-FLA to fault tree model and implemented it as an Eclipse plugin in CHESS toolset. A case study is performed in the aerospace domain; more specifically we modelled Attitude Control System (ACS) and automatically generated IEC-61025-compliant fault trees. / AMASS project

Cette thèse s'inscrit dans le cadre d'un projet R&T CNES. Elle concerne l'étude d'un actionneur appelé roue d'inertie, qui fait partie intégrante de l'ensemble SCAO (Système de Contrôle d'Attitude et d'Orbite). Les nouvelles roues proposées, dites Magnétohydrodynamique (MHD) à Conduction, présentent un volant d'inertie fluidique sous forme d'un canal torique, dans lequel un métal liquide conducteur à fort potentiel inertiel est mis en mouvement sous l'effet d'un champ électromagnétique. Contrairement aux roues actuelles, elles n'ont pas de roulements ni d'arbre mécanique ce qui permet un gain en espace, un éloignement idéal de la masse inertielle, et une durée de vie théoriquement illimitée. Aussi, de par la viscosité naturelle du fluide, elles ne présentent pas de non-linéarité autour de la vitesse nulle ce qui évite une perte de précision sur le contrôle du couple de réaction, et donc du pointage du satellite. Le travail réalisé pendant la thèse porte sur l'appréhension des phénomènes MHD consistant en un couplage entre les lois de la Mécanique des Fluides et celles de l’Electromagnétisme, au travers de la loi d’Ohm généralisée. A partir d'hypothèses axisymétriques, et dans le cadre des milieux incompressibles et d’un écoulement laminaire, un modèle générale 3D a pu être établie. Puis une formulation 1D cylindrique a permis une résolution analytique, et une autre en 2D axisymétrique, par résolution numérique en différences fines, a permis l'amélioration de la précision des résultats. Ce modèle a permis de comprendre que deux approches étaient possibles pour la conception et plus particulièrement la commande de l'actionneur. Cette résolution, faisant l'objet du développement d'un code numérique, a d'abord porté sur les équations en régime permanent, puis en temporel, afin de caractériser l'actionneur du point de vue de ses deux modes de fonctionnement. La réalisation d’un prototype a permis de quantifier la validité de la modélisation d’un point de vue dynamique. / This thesis is part of a CNES R&T project. It's related to the study of an actuator called flywheel, which is part of AOCS (Attitude an Orbit Control System). The proposed new wheels, said Magnetohydrodynamic (MHD), are constituted by a fluid flywheel in the form of a ring channel, in which a conductive and high inertial potential liquid metal is driven through an electromagnetic field (Lorentz's force). Unlike current wheels, among others types DC brushless motor, MHD wheels, whose rotor is the fluid, have neither bearings nor mechanical shaft. This allows space saving, an ideal distance of the inertial mass, and a theoretically unlimited lifespan. Moreover, thanks to the natural viscosity of the fluid, they do not present a non-linearity around the zero speed which avoids a loss of precision in the reaction torque's control, and therefore the satellite pointing. The work for the thesis focuses on the apprehension of MHD phenomena. Indeed, MHD is a coupling between fluid mechanics' laws (Navier-Stokes, etc.) and Maxwell's equations, through the Lorentz force. From a number of assumptions, and as part of incompressible environment, a genral 3D model has been established. Then a 1D cylindrical formulation allowed an analytical resolution and another 2D axisymmetric one, by finite differences resolution, helped to improve results. This model allow us to understand that both approaches were possible for the design and especially the actuator control. The resolution, which is subjected to the development of a numerical code, first focused on the equations in steady state, then in dynamic, to characterize the actuator in terms of its two operating modes. The realization of a prototype has quantified the validity of the model from a dynamic point of view.

O sistema de controle por alocação dos pólos com filtro foi utilizado para controlar a atitude de um modelo de veículo lançador de satélites. Com este intuito, foram confeccionados um modelo e uma base de fixação que permite a movimentação nos três graus de liberdade. Utilizando a resposta à entrada degrau em conjunto com um sistema de controle PID obtido de forma empírica para estabilizar o sistema, as características da planta foram identificadas e então o sistema de controle por alocação de pólos foi projetado. Este sistema apresentou uma oscilação em torno da referência com amplitude menor do que 0,5° e tempo de pico para a entrada degrau na ordem de 2,17 segundos. Um segundo controlador PID foi projetado de forma analítica para se obter uma referência, porém apresentou resposta com características inferiores ao controlador por alocação de pólos. Os dois sistemas de controle projetados conseguem manter o modelo estável mesmo quando um dos motores é desligado. / Pole placement control system with filter was implemented to control the attitude of a satellite launch vehicle model. With this purpose, a model and a fixing base with three degrees of freedom was made. Utilizing the system response to step input with PID controller empirically designed to stabilize the system, the model characteristics were identified and the pole placement control system was designed. This system oscillated around the reference with amplitude smaller than 0.5° and peak time around 2.17 seconds. Another PID controller was designed analytically for reference, however the pole placement controller had better response characteristics than the PID controller. Both controllers can stabilize the system even when one engine is shut off.