163 research outputs found
Lef kun je leren. Een onderzoek naar de determinanten van lef en een methode voor lef ontwikkeling in organisaties
No full textLef kun je leren. Een onderzoek naar de determinanten van lef en een methode voor lef ontwikkeling in organisaties
No full textContains fulltext :
183455.pdf (Publisher’s version ) (Open Access)Radboud University, 28 februari 2018Promotores : Bos, R. ten, Weggeman, M.D.C.P
Broadband Liquid Dampers to Stabilize Flexible Spacecraft Structures
No full textMass-spring and liquid dampers enable structural vibration control to attenuate single, coupled lateral and torsional vibrations in diverse structures. Out of these, the passively tuned liquid damper (TLD) class is wanted due to its broad applicability, extreme reliability, robustness, long life time and ease of manufacturability. In this PhD thesis, the theory, design, verification and validation of multi-mode TLDs in terrestrial and mainly spacecraft (S/C) applications have been studied. The most challenging TLD design of the type “tube-with-endpots” was the Chinese meteorological FY-2 S/C nutation damper in the 90s. The extreme performance requirements like the 0.5” residual nutation damping angle implied an extended test program which led to refined insights in the recursive calibration method and limiting damping performance. The test analysis results and the involvement in the in-orbit analysis of the Ulysses S/C nutation anomaly in the same period, led the author to the idea of a multi-mode TLD system. The concept was proposed and successfully applied in the Cluster S/C for the effective damping of both nutation and coupled wire boom (antenna) oscillation modes. To come that far, the essentials of spacecraft dynamics and its control required an extension of the liquid flow models and an appropriate TLD design methodology to include multi-mode excitations. The TLD key performance parameters are the dissipation rate, residual damping angle and the resonance frequency which is directly related to the effective damping length. This parameter used to be obtained by an educated guess on basis of test heritage. The existing practical design rules, however, were overruled by new insights which are based on the latest scientific results from fluid mechanics. This knowledge and the extensive analysis of all available TLD damping performance tests resulted in a new refined methodology to estimate the effective damping length properly. The eventual value, however, must still be determined via recursive calibration cycles but better initial estimates reduce the required test times significantly. The residual damping angle is limited by the TLD endpot behavior which is determined by the physics of the liquid meniscus interaction with the endpot wall. Though the TLD design is characterized by a very low residual angle with almost zero dead-band, the very limit is not clear. This issue was investigated using multiple models and experiments whilst the state-of-the-art in the scientific literature from nano-tribology and wetting transitions on biomimetic surfaces was explored. Test refinements are proposed to decrease damping fluctuations and extend the low angular test range. Although, the limiting angle is not known, there is strong evidence that the limits can be extended beyond the 0.1” flight value. The early design phase of the broadband Cluster TLDs in 1991 and the TLD developments up to 2012 were studied. Moreover, the spin-stabilized magneto-spherical S/C Bepi-Colombo, Cluster, RBSP, DICE, Themis and FAST are compared which confirm the applicability of the multi-mode TLD concept. The study of the generic theory of wire boom oscillations, gyroscopically coupled to the S/C hub spin and nutation modes, resulted in a new harmonized parameterization and derived equations. The Cluster TLD system with in addition the internal wire boom damping enable the boom deflection limit and its damping time constant to be design parameters. On basis of this knowledge, a recursive bottom-up TLD design methodology was developed. The stability study including the wire boom composition made clear where the limit of multi-mode modeling is reached and breadboard experiments and practical engineering trade-offs are required. The optimal wire boom deployment strategy using the multi-mode damping principle was analyzed. At small angular deflections, however, material artifacts and anelastic flexure dominate and only dedicated engineering tests can clarify these issues. The current status of the TLD design was investigated by comparing the RBSP S/C [2012] ring TLD and the Cluster S/C [2000] endpot TLD designs. The combination of the Cluster TLD bottom-up design methodology with the 9 degrees of freedom RBSP top-down model completed the model base for the design of multi-mode TLDs in flexible S/C. The RBSP TLD suffers with considerable angular off-sets and inrush time constants which are not accounted for in the RBSP model. The Cluster TLD design, however, lacks these artifacts. RBSP S/C flight validation data, however, are not yet available. The nutation related Cluster flight data validate the TLD model predictions firmly within the requirements. This renders an indirect but incomplete prove of the effectiveness of the TLD system design. It is hard, however, to trace and validate the designed multi-mode performance itself. It is, therefore, of great scientific value to obtain Attitude Determination and Control System flight data. A successful TLD development requires risk mitigation as an essential part of systems engineering (SE). An inventory of boundary conditions was made thinking ahead for production and project cost escalations. In the high-tech industry, however, there is little focus on a scientifically based bottom-up SE approach though such effort does pay off. It was one of the quests of this thesis to prove the added value of such an investment. As a result, the developed methodologies do contribute to a profound SE approach in the development of multi-mode TLDs. The space qualified broadband TLD design with endpots is an excellent choice for use in future spin-stabilized S/C with wire boom configurations. The results of the PhD thesis enable the extreme refinement of the given damper concept. Market research and the allocation of dedicated solutions are a way towards valorization. Terrestrial spin-offs in the engineering fields of refined (ultra) centrifuges, pulsating industrial piping systems, windmills, earthquake control of building structures, shipbuilding and bridge stabilization offer the best valorization opportunities in short terms.Earth Observation and Space SystemsAerospace Engineerin
A Reconfigurable GPS/Galileo Receiver Front-end for Space Applications
No full textThe trend of space technology developments is moving from high power consuming, bulky and costly systems towards low-power, small, low-cost and flexible systems. Thus, the spacecraft can benefit from multi-purpose and flexible systems which can be low-power and low-cost by employing new technology. One example of such system is a GNSS receiver capable of adapting the number of used frequency bands depending on its power constraint and the required accuracy. In the past, a Global Navigation Satellite System (GNSS) receiver has primarily been used for navigation purposes. However, as the number of available GNSS systems has increased, the potential of introducing new applications using these systems has also increased. Such applications become more robust and their performance can be improved if the GNSS receiver can operate with more than one GNSS system. On the other hand, extensive research and developments in state-of-the-art integrated circuit (IC) technology facilitates the integration of complex systems in a very compact and efficient manner. This miniaturization can be spun into space applications which are very complex systems by itself. Using this potential leads to new approaches in spacecraft design as well as potentially new space applications which may require short time-to-market. The research objective of this thesis is to develop a space-capable, flexible, multipurpose, low-power and low-cost GNSS receiver front-end. This front-end shall be able to process GNSS signals from different GNSS systems and different frequency bands. In chapter 1, an overview of navigation systems along with applications of GNSS systems is presented. This chapter also provides and introduction to state-of-the art IC technology and its advantages and disadvantages for using it in space. Thesis objectives and research questions conclude this chapter. Chapter 2 covers the fundamentals of satellite positioning system and provides a detailed explanation of GPS and Galileo signal structures followed by investigating a generic GNSS receiver architecture. This chapter is concluded with explanation of requirements for designing a space capable GNSS receiver. In chapter 3, existing radio receiver front-end architectures are reviewed and compared. The most suitable architecture, i.e. zero-IF, is selected. Finally, an innovative and flexible architecture for GNSS receiver front-end based on zero-IF is proposed. Chapter 4 begins with technology selection and calculations of link budget of the proposed receiver front-end. It is followed by reviewing various implementations of building blocks of the front-end and their comparison. In this chapter an innovative mixer architecture for zero-IF architecture is proposed which overcomes its two main problems, the DC offset and flicker noise. The chapter concludes by selecting the most suitable circuits for mixer, quadrature oscillator and analog to digital converter (ADC) for this receiver front-end. In chapter 5, the circuits of the mixer, quadrature oscillator, amplifier and ADC are developed in transistor level followed by verification simulations. The results of the simulations verify the expected behavior of the proposed mixer as well as the quadrature oscillator, amplifier and ADC. The thesis is concluded in chapter 6 with summary of the results, recommendations and future outlook.Earth Observation and Space SystemsAerospace Engineerin
Autonomous Relative Navigation for Small Spacecraft
No full textThe thesis deals with the relative navigation between two small formation flying spacecraft. The inter-satellite distance is measured using locally generated radiofrequency ranging signals. Design considerations for the spacecraft and the relative navigation system are discussed as well as the estimation of the relative state. The influence of the number of antennas on each spacecraft, the antenna baseline, the ranging accuracy, the inter-satellite distance, and the relative motion on the relative navigation results and the observability of the system is extensively studied.Space EngineeringAerospace Engineerin
Dynamics, Distributed Control and Autonomous Cluster Operations of Fractionated Spacecraft
No full textFractionated spacecraft deploy satellites' functionalities, such as computation, communication, data storage, payload operations and even power generation, onboard several modules that share those functionalities through a wireless network. With the advent of such an innovative space architecture, non-traditional attributes such as flexibility, robustness and responsiveness, in addition to cost and mass, are introduced to the implementation of space systems, and the equilibrium in the design may shift considerably. In order to enable those non-traditional attributes and thus create huge momentum for fractionated spacecraft, this thesis researches on the autonomous operations of fractionated spacecraft with a focus on cluster reconfiguration. In particular, three aspects have been studied thoroughly to lay the foundation for its implementation. First, functional, physical and organizational architectures of a fractionated infrastructure for long-term Earth observation missions have been proposed, which defines the scenario for our research hereinafter. In the scenario, four fractionated modules are considered with a reference orbit of 800km altitude and the fractionated cluster is regarded as a multi-agent system. Second, the relative motion is studied to provide the knowledge of the modules' long-term flight behavior within the passive cluster. This thesis presents closed-form solutions for the problem of long-term satellite relative motion in the presence of J2 perturbations, and introduces a design methodology for long-term passive distance-bounded relative motion. Last but not least, centralized and distributed approaches to the problem of autonomous cluster reconfiguration are, respectively, developed, both for energy-optimal and time-optimal reconfigurations. In the reconfiguration planning, the non-convex collision avoidance constraints as well as the non-convex final configuration constraints have been taken into account. Theoretical results regarding to the optimality and convergence of developed algorithms have been obtained. All above research areas are devoted to studying, exploiting and enabling the non-traditional attributes of fractionated spacecraft. New results have been contributed to the body of knowledge in all three research aspects. For the development of fractionated space systems, our research will shed light on the cluster design, the autonomous organization of modules within the cluster, and the design of distributed energy- or time-optimal reconfiguration. Even though this thesis is focused on the future-oriented enabling technologies for fractionated spacecraft, the developed methodologies are applicable to other distributed space systems such as formation flying. Therefore, our research can be regarded as a step stone for the implementation of future autonomous distributed space systems.Chair of Space Systems Engineering, Department of Space EngineeringAerospace Engineerin
Development of a Robotics-based Satellites Docking Simulator
No full textThe European Proximity Operation Simulator (EPOS) is a hardware-in-the-loop (HIL) system aiming, among other objectives, at emulating on-orbit docking of spacecraft for verification and validation of the docking phase. This HIL docking simulator set-up essentially consists of docking interfaces, simulating the servicing satellite called chaser satellite, the serviced satellite called target satellite, a sensor of the forces and torques during contact, and two industrial robots that hold the docking interfaces, and control satellites motion relative position and attitude. Furthermore, the EPOS includes a real-time controller interface linked to a computer-based numerical simulator of satellites orbital and attitude dynamics. A key feature of this set-up is the feedback loop that is closed on the real force sensed at the docking interfaces during contact. That feedback force is used as driving input to satellites dynamics numerical simulation. This HIL docking simulation concept has the unique advantage of using the measured contact forces and torques, but it presents significant challenges. The high stiffness of the industrial robots and the docking interfaces yields a high bandwidth contact dynamics at impact and, thus, very short contact time durations. These times might be shorter than the inherent time delay of the robot controllers. This leads to physical inconsistency in the docking dynamics and measured variables. This also causes a stability issue in the force feedback HIL system during contact and may cause catastrophic damages to the robots. Additional problems that need to be addressed are the characterization of the stability domain of operation, the compensation of the non-contact forces and torques, such as the measured forces and torques due to gravity effect. Finally, this thesis addresses the task of identifying the dynamic behavior of the robot end-effectors. This thesis addresses the above mentioned challenges and problems and presents solutions towards a stable and safe docking simulation operation of the EPOS facility. First, in order to mitigate the high stiffness and time delay problem, the thesis introduces a novel idea of simulating contact based on a concept called hybrid contact dynamics model. The method, developed in this thesis, is based on a combination of a passive compliance control introduced at the end-effector of the robot and a virtual contact model. The virtual contact model allows the operator to vary the contact parameters which can also be used as a control gain. The method also allows to solve the stability problem coming from the combination of time delay of the robot controller and high stiffness of the robot end-effector. For the passive compliance control, a new device is designed that has fairly known stiffness properties which are softer than the robot and docking interface stiffness. Second, the thesis presents a stability analysis of the proposed method via the adaptation of the pole location method to dead-time systems. The analysis is based on a linearized design model of the dynamics; linearization is performed around the docking geometrical configuration. This work first presents an analysis for the single dimensional case, which is then extended to two dimensions. The highlight of the stability analysis is the development of physically intuitive state-space model that easily unveil the modes of the contact dynamics. The application of the pole location method to the resulting second-order characteristics polynomial is straight forward. The contribution of this analysis is a closed-form relationship, and associated plots, among the system's parameter, i.e., the satellite's masses, the stiffness and damping coefficient of the contact parameters, the delay, and the geometry. In addition, the stability analysis is supported using the passivity method which is valid for three dimensions. Third, a model of the force-torque sensor is presented, and the classical weighted least-squares estimation technique is suggested for the identification and compensation of the non-contact forces and torques from the contact force and the torque measurement. Finally, it is proposed to utilize a LEICA laser tracker, a positioning measurement system, in order to identify the robot end-effectors dynamics behaviors such as the natural frequency and damping ratio. This hybrid contact dynamics model and the accompanying analysis is envisioned as a tool for safe and flexible EPOS operations. This tool shall allow emulation of the desired impact dynamics for any stiffness and damping characteristics within the stability region without recurring to a modification of the hardware. The experimental results of the robotics based hybrid docking simulator comply with experimental data from an air-bearing testbed that was independently performed by this author at the Space Robotics Laboratory of Tohoku University. It demonstrates the validity of the novel EPOS concept of operations and increases the confidence of using this approach for future on-orbit docking/contact algorithm validation, at the EPOS facility.Space Systems EngineeringAerospace Engineerin
Tethers in Space: A propellantless propulsion in-orbit demonstration
No full textSpace tethers are cables that connect satellites or other endmasses in orbit. The emptiness of space and the near-weightlessness there make it possible to deploy very long and thin tethers. By exploiting basic principles of physics, tethers can provide propellantless propulsion and enable unique applications such as the provision of comfortable artificial gravity or the removal of space debris. Nevertheless there are still no tether applications in use today - there appears to be a "gap of scepticism". A safe tether and deployer system has therefore been designed and verified with the help of simulation and innovative ground testing equipment. Through a hands-on educational approach, the YES and YES2 low-cost space tether experiments have been launched into orbit. In September 2007, all 32 km of the YES2 tether are deployed in orbit. With the help of this tether, a student-built re-entry capsule is deorbited over Kazakhstan. This work reports this design and analysis effort, with the aim to raise confidence in the use of space tethers.ASSET/SSEAerospace Engineerin
Collision analysis and mitigation for distributed space systems
No full textCollision analysis and mitigation performed for the QB50 mission. The aim of this thesis report is to identify which mitigation strategies are most suitable for a network of uncontrollable satellites. Furthermore, the aim is to set-out a method to determine the collision probability for a network of uncontrollable satellites, and identify the parameters that influence the collision probability. These methods are applied to the QB50 mission; to find a scenario where the collision probability is lowest. An alternative method is developed by the author to calculate the Gaussian probability, which is applicable for small satellites. As the size of the satellites decreases relative to the error ellipsoid, the probability at a certain moment in time becomes more equal to the probability at the center of the combined sphere (assuming spherical satellites). Now, instead of dealing with a cumbersome volume integral through the combined error ellipsoid, the collision probability can be approached by a line-integral times the area of the combined satellites’ bodies. Four ideal deployment angles for the QB50 mission were found located in a plane of zero pitch and at yaw angles of 34?, 146?, 248?, 326? measured from velocity vector of the upperstage.Deploying at zero pitch has the effect that the phase between the cross-track and radial separation is half the orbital period. This has the consequence that, when either the cross-track separation is zero, the radial separation has its maximum and visa versa. This can also be identified as (anti-)parallel alignment of relative eccentricity and inclination vectors. Synchronization of the motion at times where the deployment is half or equal to the orbital period should be avoided. For these satellites the amplitude radial and cross-track separations are small. Furthermore, the relative perturbations between these satellites is large, decreasing the offset of the radial motion. This causes the radial and cross-track separation reach zero at equal time. Synchronized satellite increase the collision probability significantly. Both Patera’s method and the line-integral method are applied to a full-scale simulation for the QB50 mission. Multiple scenario’s are chosen for the full-scale simulation. The two scenario’s with the lowest probability are sequential deployment at one of the ideal angles and alternating deployment between two opposite ideal angles. These fall below the threshold value of 10?4, a value used by the German Space Operations Center (GSOC).Space Systems EngineeringSpace FlightAerospace Engineerin
Autonomous Formation Flying in Low Earth Orbit
No full textFormation flying is commonly identified as the collective usage of two or more cooperative spacecraft to exercise the function of a single monolithic virtual instrument. The distribution of tasks and payloads among fleets of coordinated smaller satellites offers the possibility to overcome the classical limitations of traditional single-satellite systems. The science return is enhanced through observations made with larger, configurable baselines and an improved degree of redundancy can be achieved in the event of failures. Different classes of formation flying missions are currently under discussion within the engineering and science community: technology demonstration missions, synthetic aperture interferometers and gravimeters for Earth observation, multi-spacecraft interferometers in the infrared and visible wavelength regions as a key to new astrophysics discoveries and to the direct search for terrestrial exoplanets. These missions are characterized by different levels of complexity, mainly dictated by the payload metrology and actuation needs, and require a high level of on-board autonomy to satisfy the continuously increasing demand of relative navigation and control accuracy.
This dissertation presents the first realistic demonstration of a complete guidance, navigation and control (GNC) system for formation flying spacecraft in low Earth orbit. Numerous technical contributions have been made during the course of this research in the areas of formation flying guidance, GPS-based relative navigation, and impulsive relative orbit control, but the primary contribution of this thesis does not lie in one or more of these disciplines. The innovation and originality of this work stems from the design and implementation of a comprehensive formation flying system through the successful integration of various techniques. This research has led to the full development, testing and validation of the GNC flight code to be embedded in the on-board computer of the active spacecraft of the PRISMA technology demonstration. Furthermore key guidance and control algorithms presented here are going to be demonstrated for the first time in the TanDEM-X formation flying mission. Overall this thesis focuses on realistic application cases closely related to upcoming missions. The intention is to realize a practical and reliable way to formation flying: a technology that is discussed and studied since decades but is still confined in research laboratories. Hardware-in-the-loop real-time simulations including a representative flight computer and the GPS hardware architecture show that simple techniques, which exploit the natural orbit motion to full extent, can meet the demanding requirements of long-term close formation-flying
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