Dittus, Hansjörg
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Item-typ:Veröffentlichung, Touchdown Dynamics and the Probability of Terrain Related Failure of Planetary Landing Systems : A Contribution to the Landing Safety Assessment Process(2015-11-20); ; ; Landing safety assessment is an integral element of the planning of a landing mission. This thesis contributes to such assessment with the modelling and deduction of the functional limits of a legged landing system and its terrain-related failure probabilities. A mathematical method has been developed to determine these terrain-related failure probabilities. The lander's touchdown dynamics is represented by a high-fidelity numerical multibody simulation which is validated by experimental data from a dedicated test campaign. The analysis of the terrain-related failure probabilities remains incomplete without knowledge about the geotechnical properties of the landing site. This information is obtained from a landing site characterization. An analysis step extracts this information from high resolution digital terrain models under consideration of the specific baselength determined by the landing platform's footprint. A robotic lunar landing mission is used as application case study.Dissertation440 173 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, An analysis of design reuse in the development of small body nanolander and surface science stations : Supporting reuse, modular design and platforming with Model Based Systems Engineering(2020-01-17); ; ; Design reuse, i.e. reuse of design artefacts of space missions is gaining more importance in the frame of cost efficiency improvement i.e. getting more science out of the limited funding in space systems development. This has found application in the Earth orbiting spacecraft sector, but may also be of interest for deep space scientific missons. This thesis contributes to the practice of design reuse with the development and demonstation of a method for a more systematic reuse approach as compared to the ad-hoc reuse widely common. Using a real life reuse case from an ESA phase A mission concept study, the process is described based on a developed general ontology of system design under consideration of the pecularities of a deep space scientific misson and uses Model-Based Systems Engineering methods. A Reuse Matrix has been proposed as a tool to analyse reuse effort a-priori based on individual delta development effort of components and the propagation of changes over the component borders. The latter are shown to be depending on aspects such as the number of interfaces and the number of constraints involved in the change propagation, as well as the extent of change of the latter. Three different case studies are presented: (1) a single reuse scenario, which develops a mission concept for a MASCOT-derived Phobos surface station using the presented method and derives the reuse matrix for this special case; (2) a platform based design-for-reuse analysis, which determines requirements and derives a platform approach for nanolander systems based on the MASCOT lander heritage and (3) a modular lunar infrastructure designed for a special case of reuse of the same system design for different use cases in one mission scenario. The three case studies prove the general applicability of the process and the usability of the Reuse Matrix, which benefits from the existence of a desciptive system model including system requirements, functional breakdown and physical allocation, as well as connected performance models which are fed with the information stored in the system model. The MASCOT lander system design has been demonstrated as very well suited for effective reuse and shows to be a good starting point for a platform-based family of nanolander for a broad variety of small and airless bodies.Dissertation430 289 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Development of Components for Implementation of Free Fall Experiments to Test the Weak Equivalence Principle(2016-10-21); ; ; In this work I describe the development of an electrostatic positioning system (EPS) for improving the starting conditions of the test masses in a free fall experiment to test the weak equivalence principle. The equivalence of intertial and gravitational mass is one of the fundamental principles in physics. One version of this principle is the weak equivalence principle (WEP), also known as Universality of Free Fall (UFF). It says, a test mass with neglectible self gravitation behaves independently of its properties in a gravitational field. In the recent centuries, the WEP has been investigated by using different methods. My literature recherche shows, that surprisingly few free fall experiments using macroscopic test masses have been performed. And their results have not reached the accuracy of other methods jet. At the drop tower Bremen, a free fall experiment using high precision, Supraconducting Quantum Interference Device (SQUID) sensors was developed. In 2001 it tested the WEP by measuring the difference between the accelerations of two test masses falling freely with an accuracy up to $ eta 10 -7 $. By improving the different components of the Experiment and especially the starting conditions of the test masses, an improvement in the accuracy of several orders should be possible. The center of mass of the test masses should be positioned in the same place within 0.3 mircometer und their velocities should be minimized to 0.1 m/s. Therefore, a electrostatic positioning system (EPS) in axial direction was developed und the first results of the characterisation of the protoptype are presented in this work. The experimental set-up and the controller are described in detail. Characteristic parameters of the system are defined and the experiments to determine them are described. By using a high precision weighing balance, the force exerted on the test mass by the EPS was measured. This results help to modell the system. Measurements of different positionings of the test mass were used to determine the position resolution of the EPS and other characteristic parameters. A suprising result of these measurements is the EPS can be used with and without the wire connected to the test mass for electrically grounding it. The parameters of the controller had not been changed between both measurements and only the scaling factor to convert the voltage signals into position signals changes slightly. The results of the characterisation provide new aspects for simplifing and improving the axial EPS and developing an radial EPS in the future. The combination of both EPS will improve the starting conditions of the test masses and therefore the conditions for the SQUID sensors significantly. This will be a main step in improving the accuracy of free fall test of the WEP up to $ eta 10 -13 $.Dissertation373 243 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Funktionstests der Nutzlast für die MICROSCOPE-Mission : Freifalltests am Fallturm Bremen(2019-08-23); ; ; The payload of the french space mission MICROSCOPE containing two differential accelerometers had to be tested in zero-g conditions before launch. By using special drop capsules at the drop tower Bremen at ZARM (University of Bremen) it was possible to test the payload successfully and to demonstrate the correct function of the system. The MICROSCOPE mission aims at testing the Weak Equivalence Principle (WEP) with a new level of accuracy. The correct function of the payload was an important requirement to achieve the mission goal. Another part of the thesis is the development of a new technique for measuring the accelerometer bias by using the catapult mode of the drop tower.Dissertation589 449 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Konzeption, Entwicklung und Umsetzung von atomoptischen Fallturmexperimenten für den Einsatz unter Schwerelosigkeit am Fallturm Bremen(2010-01-29); ; ; Since the possibility of trapping and cooling neutral atoms, ultracold quantum gases have shifted boundaries in a growing field of modern physics based on the first observation of Bose-Einstein condensates in 1995 and appreciated by the Nobel Prizes in 1997 and 2001. The current developments in the domain of atom optics lead to an utilization of ultracold quantum matter techniques in unique practical applications as high-precision atomic clocks, atom interferometer technologies and inertial sensing instruments for gravity field mapping, underground structure detection, autonomous navigation, as well as precision measurements in fundamental physics. The expectations of even higher precision measurements can be performed by arbitrarily extending the time of unperturbed evolution of those quantum systems. In respect thereof weightlessness provides an outstanding basis for such applications and measurements. Motivated by these prospects, many national and international groups have initialized research programs aiming for compact, transportable and ruggedly designed atom-optical experiments, which might be launched in parabolic flights and space applications. Thanks to an easy access to low gravity on earth, realization of quantum degenerated gases in excellent microgravity conditions at the Drop Tower Bremen opened a new kind of perspectives on atom-optical experiments, e.g., to currently achieve longest expansion times of Bose-Einstein condensates within the QUANTUS pilot project (up to one second). Thus, ultracold quantum matter in an environment of weightlessness represents an emerging area of science in quantum engineering with an impressive potential for a future technology and multidisciplinary applications.Dissertation460 207 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Artificial Potential Fields for Autonomous Cluster Keeping(2020-07-23); ; In recent years the concepts of distributed systems and fractionation spread considerably, as they allow the replacement of a single monolithic spacecraft with multiple smaller ones. The desire for such a design transformation stems from the many limitations that are associated with the traditional monolithic option and can be instead overcome through the use of a fractionated architecture. A cluster of spacecraft working together could enhance the mission performances in many ways, e.g., by augmenting flexibility and redundancy, by reducing costs and risks, by overcoming physical limitations. On the other hand these benefits come at a price, since the cluster brings a new series of challenges concerning, for example, the sharing of data, the communication, and the relative motion between the objects. In the field of formation flight, much has been already done in these areas but not everything is directly applicable in a cluster scenario. For what concerns the relative motion, for instance, it is clear that this should always be safe to ensure the success of the mission, but while a formation requires the modules to remain in a precise relative configuration, a cluster only requires satisfaction of minimum and maximum distance constraints to ensure neither that modules drift away, nor that they collide. This peculiar type of requirements on the one side kept boosting the research in the field of the relative motion models, with a particular push in the direction of long-term passive distance-bounded relative orbits, on the other side promoted the development of cluster keeping algorithms, which are desired to be scalable, autonomous and responsive. In this thesis, at first several types of relative configurations are investigated and compared to observe how they influence the evolution of the relative motion and which advantages they bring into a cluster flight scenario. By considering the number of deployable objects and the v budget required for station keeping as main performance indexes, one specific configuration is selected and further explored with the application of the artificial potential field method. The approach of artificial potentials represents a simple and effective path planner, that can influence the motion of the considered objects, for example to steer them towards goal positions while ensuring obstacle avoidance, by using proper attractive and repulsive behaviours. On top of that the method deals well with the desired requirements of autonomy, responsiveness and scalability, and that is why it has been selected to be applied in the cluster keeping problem. The artificial potentials are widely studied and applied in the field of robotics, but the complexity of the equations of motion in the orbital environment significantly limited their spread in the space domain. Space research involving this method is nowadays restricted to small-sized relative motion problems, in which the simplified Hill-Clohessy-Wiltshire model could be used. In this thesis, on the other hand, both small- and large-sized clusters are considered, leading to the need for dropping distance-related simplified assumptions and developing a general cluster keeping approach. Two different artificial-potentials-based architectures are presented, one exploiting the use of virtual reference states that the spacecraft of the cluster track to (indirectly) satisfy minimum and maximum distance constraints, and one dealing directly with the relative distance between the spacecraft to alter their motion and prevent violations of the distance boundaries. By discussing the results of the extensive simulations that have been performed to study the two architectures, the strengths and limitations of these can be highlighted, and eventually a framework employing them both is proposed, showing under which conditions they can be successfully coupled. Conclusions and recommendations for future work finally wrap up the conducted research and close the thesis.Dissertation376 1056 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Autonome monokulare optische Identifikation von Mondkratern aus unbekannter Kameraposition und -Lage(2016-10-28); ; ; Present and future robotic landing missions to the moon with high landing accuracy requirements need a capability of autonomous position and attitude determination. Optical sensors provide a cheap and robust means of generating image measurements, and lunar craters provide persistent landmarks. This dissertation describes a method that allows inferring a full camera pose from a single image of the lunar surface under a wide variety of illumination conditions and for a previously unkown position and attitude of the camera.Dissertation379 112