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Ongoing changes in spaceflight – continuing miniaturization, declining costs of rocket launches and satellite components, and improved satellite computing and control capabilities – are advancing Satellite Formation Flying (SFF) as a research and application area. SFF enables new applications that cannot be realized (or cannot be realized at a reasonable cost) with conventional single-satellite missions. In particular, distributed Earth observation applications such as photogrammetry and tomography or distributed space telescopes require precisely placed and controlled satellites in orbit.
Several enabling technologies are required for SFF, such as inter-satellite communication, precise attitude control, and in-orbit maneuverability. However, one of the most important requirements is a reliable distributed Guidance, Navigation and Control (GNC) strategy. This work addresses the issue of distributed GNC for SFF in 3D with a focus on Continuous Low-Thrust (CLT) propulsion satellites (e.g., with electric thrusters) and concentrates on circular low Earth orbits. However, the focus of this work is not only on control theory, but control is considered as part of the system engineering process of typical small satellite missions. Thus, common sensor and actuator systems are analyzed to derive their characteristics and their impacts on formation control. This serves as the basis for the design, implementation, and evaluation of the following control approaches: First, a Model Predictive Control (MPC) method with specific adaptations to SFF and its requirements and constraints; second, a distributed robust controller that combines consensus methods for distributed system control and $H_{\infty}$ robust control; and finally, a controller that uses plant inversion for control and combines it with a reference governor to steer the controller to the target on an optimal trajectory considering several constraints. The developed controllers are validated and compared based on extensive software simulations. Realistic 3D formation flight scenarios were taken from the Networked Pico-Satellite Distributed System Control (NetSat) cubesat formation flight mission. The three compared methods show different advantages and disadvantages in the different application scenarios. The distributed robust consensus-based controller for example lacks the ability to limit the maximum thrust, so it is not suitable for satellites with CLT. But both the MPC-based approach and the plant inversionbased controller are suitable for CLT SFF applications, while showing again distinct advantages and disadvantages in different scenarios.
The scientific contribution of this work may be summarized as the creation of novel and specific control approaches for the class of CLT SFF applications, which is still lacking methods withstanding the application in real space missions, as well as the scientific evaluation and comparison of the developed methods.
Dielektrische Elastomersensoren sind aus Elastomermaterialien aufgebaute Sensoren mit einem kapazitiven Messprinzip. In ihrer einfachsten Form bestehen sie aus einer dehnbaren Elastomerfolie als Dielektrikum, die beidseitig mit leitfähigen und ebenfalls dehnbaren Schichten als Elektroden bedeckt ist.
Damit entsteht ein mechanisch verformbarer elektrischer Kondensator, dessen Kapazität mit der Dehnung der Elastomerfolie stetig ansteigt. Neben solchen Dehnungssensoren lassen sich mit einem geeigneten geometrischen Aufbau auch dielektrische Elastomersensoren realisieren, bei denen eine elektrische Kapazität mit einem angelegten Druck bzw. einer Kraft auf die Oberfläche, mit einer Scherkraft oder mit der Annäherung eines elektrisch leitfähigen oder polarisierbaren Körpers wie z. B. der menschlichen Hand messbar ansteigt.
Durch ihre vielfältige Funktion, intrinsische Verformbarkeit und flächige Ausgestaltung weisen Dielektrische Elastomersensoren erhebliches Potential in der Schaffung smarter, sensitiver Oberflächen auf. Dabei sind weitgehende und individuelle Adaptionen auf den jeweiligen Anwendungszweck durch Abstimmung geometrischer, mechanischer und elektrischer Eigenschaften möglich. Die bisherige Forschung beschränkt sich jedoch auf die Analyse und Optimierung einzelner Aspekte ohne das Potential einer übergreifenden systemischen Perspektive zu nutzen.
Diese Arbeit widmet sich daher der Betrachtung der Sensorik als Gesamtsystem, sowohl horizontal - von abstrakten Modellen bis zur Fertigung und prototypischen Anwendung - als auch vertikal über die Komponenten Material, Struktur und Elektronik.
Hierbei wurden in mehreren Teilgebieten eigenständige neue Erkenntnisse und Verbesserungen erzielt, die anschließend in die übergreifende Betrachtung des Gesamtsystems integriert wurden. So wurden in den theoretischen Vorarbeiten neue Konzepte zur ortsaufgelösten Erfassung mehrerer physikalischer Größen und zur elektrischen und mechanischen Modellierung entwickelt. Die abgeleiteten Materialanforderungen wurden in eine tiefgehende Charakterisierung der verwendeten Elastomer-Kompositwerkstoffe überführt, in der neuartige analytische Methoden in Form von dynamischer elektromechanischer Testung und nanoskaliger Computertomographie zur Aufklärung der inneren Wechselwirkungen zum Einsatz kamen.
Im Bereich der automatisierten Prozessierung wurde ein für die komplexen mehrschichtigen Elektrodenstrukturen geeigneter neuer lasergestützer substraktiver Fertigungprozess etabliert, der zudem die Brücke zu elastischer Elektronik schlägt.
In der abschließenden Anwendungsevaluierung wurden mehrere ortsaufgelöste und multimodale Gesamtsysteme aufgebaut und geeignete Messelektronik und Software entwickelt. Abschließend wurden die Systeme mit einem eigens entwickelten robotischen Testsystem charakterisiert und zudem das Potential der Auswertung mittels maschinellem Lernen aufgezeigt.
Miniaturized satellites on a nanosatellite scale below 10kg of total mass contribute most to the number of launched satellites into Low Earth Orbit today. This results from the potential to design, integrate and launch these space missions within months at very low costs. In the past decade, the reliability in the fields of system design, communication, and attitude control have matured to allow for competitive applications in Earth observation, communication services, and science missions. The capability of orbit control is an important next step in this development, enabling operators to adjust orbits according to current mission needs and small satellite formation flight, which promotes new measurements in various fields of space science. Moreover, this ability makes missions with altitudes above the ISS comply with planned regulations regarding collision avoidance maneuvering.
This dissertation presents the successful implementation of orbit control capabilities on the pico-satellite class for the first time. This pioneering achievement is demonstrated on the 1U CubeSat UWE–4. A focus is on the integration and operation of an electric propulsion system on miniaturized satellites. Besides limitations in size, mass, and power of a pico-satellite, the choice of a suitable electric propulsion system was driven by electromagnetic cleanliness and the use as a combined attitude and orbit control system. Moreover, the integration of the propulsion system leaves the valuable space at the outer faces of the CubeSat structure unoccupied for future use by payloads. The used NanoFEEP propulsion system consists of four thruster heads, two neutralizers and two Power Processing Units (PPUs).
The thrusters can be used continuously for 50 minutes per orbit after the liquefaction of the propellant by dedicated heaters. The power consumption of a PPU with one activated thruster, its heater and a neutralizer at emitter current levels of 30-60μA or thrust levels of 2.6-5.5μN, respectively, is in the range of 430-1050mW. Two thruster heads were activated within the scope of in-orbit experiments. The thrust direction was determined using a novel algorithm within 15.7° and 13.2° of the mounting direction. Despite limited controllability of the remaining thrusters, thrust vector pointing was achieved using the magnetic actuators of the Attitude and Orbit Control System.
In mid 2020, several orbit control maneuvers changed the altitude of UWE–4, a first for pico-satellites. During the orbit lowering scenario with a duration of ten days, a single thruster head was activated in 78 orbits for 5:40 minutes per orbit. This resulted in a reduction of the orbit altitude by about 98.3m and applied a Delta v of 5.4cm/s to UWE–4. The same thruster was activated in another experiment during 44 orbits within five days for an average duration of 7:00 minutes per orbit. The altitude of UWE–4 was increased by about 81.2m and a Delta v of 4.4cm/s was applied. Additionally, a collision avoidance maneuver was executed in July 2020, which increased the distance of closest approach to the object by more than 5000m.
The attitude and orbit control system of pico- and nano-satellites to date is one of the bottle necks for future scientific and commercial applications. A performance increase while keeping with the satellites’ restrictions will enable new space missions especially for the smallest of the CubeSat classes. This work addresses methods to measure and improve the satellite’s attitude pointing and orbit control performance based on advanced sensor data analysis and optimized on-board software concepts. These methods are applied to spaceborne satellites and future CubeSat missions to demonstrate their validity. An in-orbit calibration procedure for a typical CubeSat attitude sensor suite is developed and applied to the UWE-3 satellite in space. Subsequently, a method to estimate the attitude determination accuracy without the help of an external reference sensor is developed. Using this method, it is shown that the UWE-3 satellite achieves an in-orbit attitude determination accuracy of about 2°.
An advanced data analysis of the attitude motion of a miniature satellite is used in order to estimate the main attitude disturbance torque in orbit. It is shown, that the magnetic disturbance is by far the most significant contribution for miniature satellites and a method to estimate the residual magnetic dipole moment of a satellite is developed. Its application to three CubeSats currently in orbit reveals that magnetic disturbances are a common issue for this class of satellites. The dipole moments measured are between 23.1mAm² and 137.2mAm². In order to autonomously estimate and counteract this disturbance in future missions an on-board magnetic dipole estimation algorithm is developed.
The autonomous neutralization of such disturbance torques together with the simplification of attitude control for the satellite operator is the focus of a novel on-board attitude control software architecture. It incorporates disturbance torques acting on the satellite and automatically optimizes the control output. Its application is demonstrated in space on board of the UWE-3 satellite through various attitude control experiments of which the results are presented here.
The integration of a miniaturized electric propulsion system will enable CubeSats to perform orbit control and, thus, open up new application scenarios. The in-orbit characterization, however, poses the problem of precisely measuring very low thrust levels in the order of µN. A method to measure this thrust based on the attitude dynamics of the satellite is developed and evaluated in simulation. It is shown, that the demonstrator mission UWE-4 will be able to measure these thrust levels with a high accuracy of 1% for thrust levels higher than 1µN.
The orbit control capabilities of UWE-4 using its electric propulsion system are evaluated and a hybrid attitude control system making use of the satellite’s magnetorquers and the electric propulsion system is developed. It is based on the flexible attitude control architecture mentioned before and thrust vector pointing accuracies of better than 2° can be achieved. This results in a thrust delivery of more than 99% of the desired acceleration in the target direction.