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Institute
- Graduate School of Science and Technology (36) (remove)
Sonstige beteiligte Institutionen
EU-Project number / Contract (GA) number
- 320377 (1)
Metallic nanostructures possess the ability to support resonances in the visible wavelength regime which are related to localized surface plasmons. These create highly enhanced electric fields in the immediate vicinity of metal surfaces. Nanoparticles with dipolar resonance also radiate efficiently into the far-field and hence serve as antennas for light. Such optical antennas have been explored during the last two decades, however, mainly as standalone units illuminated by external laser beams and more recently as electrically driven point sources, yet merely with basic antenna properties. This work advances the state of the art of locally driven optical antenna systems. As a first instance, the electric driving scheme including inelastic electron tunneling over a nanometer gap is merged with Yagi-Uda theory. The resulting antenna system consists of a suitably wired feed antenna, incorporating a tunnel junction, as well as several nearby parasitic elements whose geometry is optimized using analytical and numerical methods. Experimental evidence of unprecedented directionality of light emission from a nanoantenna is provided. Parallels in the performance between radiofrequency and optical Yagi-Uda arrays are drawn. Secondly, a pair of electrically connected antennas with dissimilar resonances is harnessed as electrodes in an organic light emitting nanodiode prototype. The organic material zinc phthalocyanine, exhibiting asymmetric injection barriers for electrons and holes, in conjunction with the electrode resonances, allows switching and controlling the emitted peak wavelength and directionality as the polarity of the applied voltage is inverted. In a final study, the near-field based transmission-line driving of rod antenna systems is thoroughly explored. Perfect impedance matching, corresponding to zero back-reflection, is achieved when the antenna acts as a generalized coherent perfect absorber at a specific frequency. It thus collects all guided, surface-plasmon mediated input power and transduces it to other nonradiative and radiative dissipation channels. The coherent interplay of losses and interference effects turns out to be of paramount importance for this delicate scenario, which is systematically obtained for various antenna resonances. By means of the here developed semi-analytical toolbox, even more complex nanorod chains, supporting topologically nontrivial localized edge states, are studied. The results presented in this work facilitate the design of complex locally driven antenna systems for optical wireless on-chip communication, subwavelength pixels, and loss-compensated integrated plasmonic nanocircuitry which extends to the realm of topological plasmonics.
Development, Simulation and Evaluation of Mobile Wireless Networks in Industrial Applications
(2023)
Manyindustrialautomationsolutionsusewirelesscommunicationandrelyontheavail-
ability and quality of the wireless channel. At the same time the wireless medium is
highly congested and guaranteeing the availability of wireless channels is becoming
increasingly difficult. In this work we show, that ad-hoc networking solutions can be
used to provide new communication channels and improve the performance of mobile
automation systems. These ad-hoc networking solutions describe different communi-
cation strategies, but avoid relying on network infrastructure by utilizing the Peer-to-
Peer (P2P) channel between communicating entities.
This work is a step towards the effective implementation of low-range communication
technologies(e.g. VisibleLightCommunication(VLC), radarcommunication, mmWave
communication) to the industrial application. Implementing infrastructure networks
with these technologies is unrealistic, since the low communication range would neces-
sitate a high number of Access Points (APs) to yield full coverage. However, ad-hoc
networks do not require any network infrastructure. In this work different ad-hoc net-
working solutions for the industrial use case are presented and tools and models for
their examination are proposed.
The main use case investigated in this work are Automated Guided Vehicles (AGVs)
for industrial applications. These mobile devices drive throughout the factory trans-
porting crates, goods or tools or assisting workers. In most implementations they must
exchange data with a Central Control Unit (CCU) and between one another. Predicting
if a certain communication technology is suitable for an application is very challenging
since the applications and the resulting requirements are very heterogeneous.
The proposed models and simulation tools enable the simulation of the complex inter-
action of mobile robotic clients and a wireless communication network. The goal is to
predict the characteristics of a networked AGV fleet.
Theproposedtoolswereusedtoimplement, testandexaminedifferentad-hocnetwork-
ing solutions for industrial applications using AGVs. These communication solutions
handle time-critical and delay-tolerant communication. Additionally a control method
for the AGVs is proposed, which optimizes the communication and in turn increases the
transport performance of the AGV fleet. Therefore, this work provides not only tools
for the further research of industrial ad-hoc system, but also first implementations of
ad-hoc systems which address many of the most pressing issues in industrial applica-
tions.
Motivated by the perceived great potential of chiral polymers, the presented work aimed at the investigation of synthesis, solubility and optical activity of chiral poly(2,4-disubstituted-2-oxazoline)s. A novel polymeric carrier based on ABA-type triblock copolymers poly(2-oxazoline)s with chiral and racemic hydrophobic blocks was developed for the formulation of chiral and achiral drugs (Fig. 5.1). Poly(2-methyl-2-oxazoline) (pMeOx) was used as hydrophilic A block, and poly(2-ethyl-4-ethyl-2-oxazoline) (pEtEtOx) and poly(2-propyl-4-methyl-2-oxazoline) (pPrMeOx) were used as hydrophobic B blocks. Curcumin (CUR), paclitaxel (PTX) and chiral/racemic ibuprofen (R/S/RS-IBU) were applied as model drugs. Nanoformulations were prepared consisting of these triblock copolymers and model drugs. ...
There is great interest in affordable, precise and reliable metrology underwater:
Archaeologists want to document artifacts in situ with high detail.
In marine research, biologists require the tools to monitor coral growth and geologists need recordings to model sediment transport.
Furthermore, for offshore construction projects, maintenance and inspection millimeter-accurate measurements of defects and offshore structures are essential.
While the process of digitizing individual objects and complete sites on land is well understood and standard methods, such as Structure from Motion or terrestrial laser scanning, are regularly applied, precise underwater surveying with high resolution is still a complex and difficult task.
Applying optical scanning techniques in water is challenging due to reduced visibility caused by turbidity and light absorption.
However, optical underwater scanners provide significant advantages in terms of achievable resolution and accuracy compared to acoustic systems.
This thesis proposes an underwater laser scanning system and the algorithms for creating dense and accurate 3D scans in water.
It is based on laser triangulation and the main optical components are an underwater camera and a cross-line laser projector.
The prototype is configured with a motorized yaw axis for capturing scans from a tripod.
Alternatively, it is mounted to a moving platform for mobile mapping.
The main focus lies on the refractive calibration of the underwater camera and laser projector, the image processing and 3D reconstruction.
For highest accuracy, the refraction at the individual media interfaces must be taken into account.
This is addressed by an optimization-based calibration framework using a physical-geometric camera model derived from an analytical formulation of a ray-tracing projection model.
In addition to scanning underwater structures, this work presents the 3D acquisition of semi-submerged structures and the correction of refraction effects.
As in-situ calibration in water is complex and time-consuming, the challenge of transferring an in-air scanner calibration to water without re-calibration is investigated, as well as self-calibration techniques for structured light.
The system was successfully deployed in various configurations for both static scanning and mobile mapping.
An evaluation of the calibration and 3D reconstruction using reference objects and a comparison of free-form surfaces in clear water demonstrate the high accuracy potential in the range of one millimeter to less than one centimeter, depending on the measurement distance.
Mobile underwater mapping and motion compensation based on visual-inertial odometry is demonstrated using a new optical underwater scanner based on fringe projection.
Continuous registration of individual scans allows the acquisition of 3D models from an underwater vehicle.
RGB images captured in parallel are used to create 3D point clouds of underwater scenes in full color.
3D maps are useful to the operator during the remote control of underwater vehicles and provide the building blocks to enable offshore inspection and surveying tasks.
The advancing automation of the measurement technology will allow non-experts to use it, significantly reduce acquisition time and increase accuracy, making underwater metrology more cost-effective.
Accurate crop monitoring in response to climate change at a regional or field scale
plays a significant role in developing agricultural policies, improving food security,
forecasting, and analysing global trade trends. Climate change is expected to
significantly impact agriculture, with shifts in temperature, precipitation patterns, and
extreme weather events negatively affecting crop yields, soil fertility, water availability,
biodiversity, and crop growing conditions. Remote sensing (RS) can provide valuable
information combined with crop growth models (CGMs) for yield assessment by
monitoring crop development, detecting crop changes, and assessing the impact of
climate change on crop yields. This dissertation aims to investigate the potential of RS
data on modelling long-term crop yields of winter wheat (WW) and oil seed rape (OSR)
for the Free State of Bavaria (70,550 km2
), Germany. The first chapter of the dissertation
describes the reasons favouring the importance of accurate crop yield predictions for
achieving sustainability in agriculture. Chapter second explores the accuracy
assessment of the synthetic RS data by fusing NDVIs of two high spatial resolution data
(high pair) (Landsat (30 m, 16-days; L) and Sentinel-2 (10 m, 5–6 days; S), with four low
spatial resolution data (low pair) (MOD13Q1 (250 m, 16-days), MCD43A4 (500 m, one
day), MOD09GQ (250 m, one-day), and MOD09Q1 (250 m, 8-days)) using the spatial
and temporal adaptive reflectance fusion model (STARFM), which fills regions' cloud
or shadow gaps without losing spatial information. The chapter finds that both L-MOD13Q1 (R2 = 0.62, RMSE = 0.11) and S-MOD13Q1 (R2 = 0.68, RMSE = 0.13) are more
suitable for agricultural monitoring than the other synthetic products fused. Chapter
third explores the ability of the synthetic spatiotemporal datasets (obtained in chapter
2) to accurately map and monitor crop yields of WW and OSR at a regional scale. The
chapter investigates and discusses the optimal spatial (10 m, 30 m, or 250 m), temporal
(8 or 16-day) and CGMs (World Food Studies (WOFOST), and the semi-empiric light
use efficiency approach (LUE)) for accurate crop yield estimations of both crop types.
Chapter third observes that the observations of high temporal resolution (8-day)
products of both S-MOD13Q1 and L-MOD13Q1 play a significant role in accurately
measuring the yield of WW and OSR. The chapter investigates that the simple light use
efficiency (LUE) model (R2 = 0.77 and relative RMSE (RRMSE) = 8.17%) that required fewer input parameters to simulate crop yield is highly accurate, reliable, and more
precise than the complex WOFOST model (R2 = 0.66 and RRMSE = 11.35%) with higher
input parameters. Chapter four researches the relationship of spatiotemporal fusion
modelling using STRAFM on crop yield prediction for WW and OSR using the LUE
model for Bavaria from 2001 to 2019. The chapter states the high positive correlation
coefficient (R) = 0.81 and R = 0.77 between the yearly R2 of synthetic accuracy and
modelled yield accuracy for WW and OSR from 2001 to 2019, respectively. The chapter
analyses the impact of climate variables on crop yield predictions by observing an
increase in R2
(0.79 (WW)/0.86 (OSR)) and a decrease in RMSE (4.51/2.57 dt/ha) when
the climate effect is included in the model. The fifth chapter suggests that the coupling
of the LUE model to the random forest (RF) model can further reduce the relative root
mean square error (RRMSE) from -8% (WW) and -1.6% (OSR) and increase the R2 by
14.3% (for both WW and OSR), compared to results just relying on LUE. The same
chapter concludes that satellite-based crop biomass, solar radiation, and temperature
are the most influential variables in the yield prediction of both crop types. Chapter six
attempts to discuss both pros and cons of RS technology while analysing the impact of
land use diversity on crop-modelled biomass of WW and OSR. The chapter finds that
the modelled biomass of both crops is positively impacted by land use diversity to the
radius of 450 (Shannon Diversity Index ~0.75) and 1050 m (~0.75), respectively. The
chapter also discusses the future implications by stating that including some dependent
factors (such as the management practices used, soil health, pest management, and
pollinators) could improve the relationship of RS-modelled crop yields with
biodiversity. Lastly, chapter seven discusses testing the scope of new sensors such as
unmanned aerial vehicles, hyperspectral sensors, or Sentinel-1 SAR in RS for achieving
accurate crop yield predictions for precision farming. In addition, the chapter highlights
the significance of artificial intelligence (AI) or deep learning (DL) in obtaining higher
crop yield accuracies.
Accurate crop monitoring in response to climate change at a regional or field scale plays a significant role in developing agricultural policies, improving food security, forecasting, and analysing global trade trends. Climate change is expected to significantly impact agriculture, with shifts in temperature, precipitation patterns, and extreme weather events negatively affecting crop yields, soil fertility, water availability, biodiversity, and crop growing conditions. Remote sensing (RS) can provide valuable information combined with crop growth models (CGMs) for yield assessment by monitoring crop development, detecting crop changes, and assessing the impact of climate change on crop yields. This dissertation aims to investigate the potential of RS data on modelling long-term crop yields of winter wheat (WW) and oil seed rape (OSR) for the Free State of Bavaria (70,550 km2), Germany. The first chapter of the dissertation describes the reasons favouring the importance of accurate crop yield predictions for achieving sustainability in agriculture. Chapter second explores the accuracy assessment of the synthetic RS data by fusing NDVIs of two high spatial resolution data (high pair) (Landsat (30 m, 16-days; L) and Sentinel-2 (10 m, 5–6 days; S), with four low spatial resolution data (low pair) (MOD13Q1 (250 m, 16-days), MCD43A4 (500 m, one day), MOD09GQ (250 m, one-day), and MOD09Q1 (250 m, 8-days)) using the spatial and temporal adaptive reflectance fusion model (STARFM), which fills regions' cloud or shadow gaps without losing spatial information. The chapter finds that both L-MOD13Q1 (R2 = 0.62, RMSE = 0.11) and S-MOD13Q1 (R2 = 0.68, RMSE = 0.13) are more suitable for agricultural monitoring than the other synthetic products fused. Chapter third explores the ability of the synthetic spatiotemporal datasets (obtained in chapter 2) to accurately map and monitor crop yields of WW and OSR at a regional scale. The chapter investigates and discusses the optimal spatial (10 m, 30 m, or 250 m), temporal (8 or 16-day) and CGMs (World Food Studies (WOFOST), and the semi-empiric light use efficiency approach (LUE)) for accurate crop yield estimations of both crop types. Chapter third observes that the observations of high temporal resolution (8-day) products of both S-MOD13Q1 and L-MOD13Q1 play a significant role in accurately measuring the yield of WW and OSR. The chapter investigates that the simple light use efficiency (LUE) model (R2 = 0.77 and relative RMSE (RRMSE) = 8.17%) that required fewer input parameters to simulate crop yield is highly accurate, reliable, and more precise than the complex WOFOST model (R2 = 0.66 and RRMSE = 11.35%) with higher input parameters. Chapter four researches the relationship of spatiotemporal fusion modelling using STRAFM on crop yield prediction for WW and OSR using the LUE model for Bavaria from 2001 to 2019. The chapter states the high positive correlation coefficient (R) = 0.81 and R = 0.77 between the yearly R2 of synthetic accuracy and modelled yield accuracy for WW and OSR from 2001 to 2019, respectively. The chapter analyses the impact of climate variables on crop yield predictions by observing an increase in R2 (0.79 (WW)/0.86 (OSR)) and a decrease in RMSE (4.51/2.57 dt/ha) when the climate effect is included in the model. The fifth chapter suggests that the coupling of the LUE model to the random forest (RF) model can further reduce the relative root mean square error (RRMSE) from -8% (WW) and -1.6% (OSR) and increase the R2 by 14.3% (for both WW and OSR), compared to results just relying on LUE. The same chapter concludes that satellite-based crop biomass, solar radiation, and temperature are the most influential variables in the yield prediction of both crop types. Chapter six attempts to discuss both pros and cons of RS technology while analysing the impact of land use diversity on crop-modelled biomass of WW and OSR. The chapter finds that the modelled biomass of both crops is positively impacted by land use diversity to the radius of 450 (Shannon Diversity Index ~0.75) and 1050 m (~0.75), respectively. The chapter also discusses the future implications by stating that including some dependent factors (such as the management practices used, soil health, pest management, and pollinators) could improve the relationship of RS-modelled crop yields with biodiversity. Lastly, chapter seven discusses testing the scope of new sensors such as unmanned aerial vehicles, hyperspectral sensors, or Sentinel-1 SAR in RS for achieving accurate crop yield predictions for precision farming. In addition, the chapter highlights the significance of artificial intelligence (AI) or deep learning (DL) in obtaining higher crop yield accuracies.
Since the first CubeSat launch in 2003, the hardware and software complexity of the nanosatellites was continuosly increasing.
To keep up with the continuously increasing mission complexity and to retain the primary advantages of a CubeSat mission, a new approach for the overall space and ground software architecture and protocol configuration is elaborated in this work.
The aim of this thesis is to propose a uniform software and protocol architecture as a basis for software development, test, simulation and operation of multiple pico-/nanosatellites based on ultra-low power components.
In contrast to single-CubeSat missions, current and upcoming nanosatellite formation missions require faster and more straightforward development, pre-flight testing and calibration procedures as well as simultaneous operation of multiple satellites.
A dynamic and decentral Compass mission network was established in multiple active CubeSat missions, consisting of uniformly accessible nodes.
Compass middleware was elaborated to unify the communication and functional interfaces between all involved mission-related software and hardware components.
All systems can access each other via dynamic routes to perform service-based M2M communication.
With the proposed model-based communication approach, all states, abilities and functionalities of a system are accessed in a uniform way.
The Tiny scripting language was designed to allow dynamic code execution on ultra-low power components as a basis for constraint-based in-orbit scheduler and experiment execution.
The implemented Compass Operations front-end enables far-reaching monitoring and control capabilities of all ground and space systems.
Its integrated constraint-based operations task scheduler allows the recording of complex satellite operations, which are conducted automatically during the overpasses.
The outcome of this thesis became an enabling technology for UWE-3, UWE-4 and NetSat CubeSat missions.
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.
Motivated by the great potential offered by the combination of additive manufacturing technology and hydrogels, especially in the field of tissue engineering and regenerative medicine, a series of novel hybrid hydrogel inks were developed based on the recently described thermogelling poly(2-oxazoline)s-block-poly(2-oxazine)s diblock copolymers, which may help to expand the platform of available hydrogel inks for this transformative 3D printing technology (Fig. 5.1).
In the present thesis, the first reported thermogelling polymer solely consisting of POx and POzi, i.e., the diblock copolymer PMeOx-b-PnPrOzi comprising a hydrophilic block (PMeOx) and a thermoresponsive block (PnPrOzi), was selected and used as a proof-of-concept for the preparation of three novel hybrid hydrogels. Therefore, three batches of the diblock copolymers with a DP of 100 were synthesized for the study of three different hybrid hydrogels with a special focus on their suitability as (bio)inks for extrusion-based 3D printing. The PMeOx-b-PnPrOzi diblock copolymer solution shows a temperature induced reversible gelation behavior above a critical polymer concentration of 20 wt%, as described for the Pluronic F127 solution but with a unique gelation mechanism, working through the formation of a bicontinuous sponge-like structure from the physically crosslinked vesicles. Specially, its intrinsic shear thinning behavior and excellent recovery property with a certain yield point make it a promising ink candidate for extrusion-based printing technology.
Increasing the polymer concentration is the most traditional approach to improve the printability of an ink material, and serve as the major strategy available to improve the printability of PMeOx-b-PnPrOzi systems prior to this work. From the analysis of rheological properties related to printability, it came a conclusion that increasing the copolymer concentration does improve the hydrogel strength and thus the printability. However, such improvement is very limited and usually leads to other problems such as more viscous systems and stringent requirements on the printers, which are not ideal for the printing process and applications especially in the cell-embedded biofabrication field.
POx-b-POzi/clay Hybrid Hydrogel
An alternative method proposed to improve the printability of this thermoresponsive hydrogel ink is through nanoclay (Laponite XLG) addition, i.e., the first hybrid hydrogel system of PMeOx-b-PnPrOzi/clay (also named shortly as POx-b-POzi/clay) in this thesis. To optimize the viscoelastic properties of the ink material, Laponite XLG acted as a reinforcement additive and a physically crosslinker was blended with the copolymers. Compared with the pristine copolymer solution of PMeOx-b-PnPrOzi, the hybrid PMeOx-b-PnPrOzi/clay solution well retained the temperature induced gelation performance of the copolymers.
The obtained hybrid hydrogels exhibited a rapid in situ reversible thermogelation at a physiological relevant Tgel of around 15 ℃ and a rapid recovery of viscoelastic properties within a few seconds. More importantly, with the addition of only a small amount of 1.2 wt% clay, it exhibited obviously enhanced shear thinning character (n = 0.02), yield stress (240 Pa) and mechanical strength (storage modulus over 5 kPa). With this novel hybrid hydrogel, real three-dimensional constructs with multiple layers and various geometries are generation with greatly enhanced shape fidelity and resolution. In this context, the thermogelling properties of the hybrid hydrogels over a copolymer concentration range of 10-20 wt% and a clay concentration of 0-4 wt% were systematically investigated, and from which a printable window was obtained from the laboratory as a reference.
In fact, the printing performance of an ink is not only determined by the intrinsic physicochemical properties of the material, but is also influenced by the external printing environments as well as the printer parameter settings. All the printing experiments in this study were conducted under a relatively optimized conditions obtained from preliminary experiments. In future work, the relationship between material rheology properties, printer parameters and printing performance could be systematically explored. Such a fundamental study will help to develop models that allows the prediction and comparison of printing results from different researches based on the parameters available through rheology, which is very beneficial for further development of more advanced ink systems.
Although the printability has been significantly improved by the addition of nanoclay Laponite XLG, the hybrid hydrogels and their printed constructs still suffer from some major limitations. For example, these materials are still thermoresponsive, which will cause the printed constructs to collapse when the environment temperature changes below their Tgel. In addition, the formed hydrogel constructs are mechanical too weak for load-bearing applications, and the allowed incubation time is very limited during media exchange/addition as it will lead to dissolution of the hydrogels due to dilution effects. Therefore, it is essential to establish a second (chemical or physical) crosslinking mechanism that allows further solidification of the gels after printing. It should be kept in mind that the second crosslinking step will eliminate the thermoresponsive behavior of the gels and thus the possibility of cell recovery. In this case, besides through the traditional approach of copolymer modification to realize further crosslinking, like one of the well-known post-polymerization modification approach Diels-Alder reaction,[430] designing of interpenetrating networks (IPN) hydrogels serves as one of the major strategy for advanced (bio)ink preparation.[311] Therefore, the second hybrid hydrogel system of PMeOx-b-PnPrOzi/PDMAA/clay (also named shortly as POx-b-POzi/PDMAA/clay) was developed in this thesis, which is a 3D printable and highly stretchable ternary organic-inorganic IPN hydrogel.
POx-b-POzi/PDMAA/clay Hybrid Hydrogel
The nanocomposite IPN hydrogel combines a thermoresponsive hydrogel with clay described above and in situ polymerized poly(N, N-dimethylacrylamide). Before in situ polymerization, the thermoresponsive hydrogel precursors exhibited thermogelling behavior (Tgel ~ 25 ℃, G' ~ 6 kPa) and shear thinning properties, making the system well-suited for extrusion-based 3D printing. After chemical curing of the 3D-printed constructs by free radical polymerization, the resulting IPN hydrogels show excellent mechanical strength with a high stretchability to a tensile strain at break exceeding 550%. The hybrid hydrogel can sustain a high stretching deformation and recover quickly due to the energy dissipation from the non-covalent interactions. With this hybrid hydrogel, integrating with the advanced 3D-printing technique, various 3D constructs can be printed and cured successfully with high shape fidelity and geometric accuracy.
In this context, we also investigated the possibility of acrylic acid (AA) and 2-hydroxyethylmethacrylate (HEMA) as alternative hydrogel precursors. However, the addition of these two monomers affected the thermogelation of POx-b-POzi in an unfavorable manner, as these monomers competed more effectively with water molecules, preventing the hydration of nPrOzi block at lower temperatures and therefore, the liquefaction of the gels. Furthermore, the influence of the printing process and direction on the mechanical properties of the hydrogel was investigated and compared with the corresponding bulk materials obtained from a mold. No significant effects from the additive manufacturing process were observed due to a homogeneously adhesion and merging between sequentially deposited layers. In the future, further studies on the specific performance differences among hydrogels fabricated at different printing directions/speeds would be of great interest to the community, as this allows for a more accurately control and better predict of the printed structures.
This newly developed hybrid IPN hydrogel is expected to expand the material toolbox available for hydrogel-based 3D printing, and may be interesting for a wide range of applications including tissue engineering, drug delivery, soft robotics, and additive manufacturing in general. However, in this case, the low toxicity from the monomer DMAA and other small molecules residuals in the polymerized hydrogels made this hybrid hydrogel not ideal for bioprinting in the field of biofabrication. For this problem, cyto-/biocompatible monomers such as polyethylene glycol diacrylate (PEGDA) can be used as an alternative, while the overall properties of the hydrogels including mechanical properties should be re-evaluated accordingly. Moreover, the swelling behavior of the hydrogels should also be taken into account, as it may most likely affect the mechanical strength and geometry size of the printed scaffold, but is often be overlooked after printing. For example, regarding the specific hybrid hydrogel POx-b-POzi/PDMAA/clay in this work, an equilibrium swelling ratio of 1100% was determined. The printed hydrogel cuboid experienced a volume increasing over 6-fold after equilibrium swelling in water, and became mechanical fragile due to the formation of a swollen hydrogel network absorbing large amount of water.
POx-b-POzi/Alg/clay Hybrid Hydrogel
In the final part of this dissertation, to enable the cell-loaded bioprinting and long-term cell culture, the third hybrid hydrogel system POx-b-POzi/Alg/clay was introduced by replacing the monomer DMAA to the natural polysaccharides alginate. Initially, detailed rheological characterization and mechanical tests were performed to evaluate their printability and mechanically properties. Subsequently, some simple patterns were printed with the optimized hydrogel precursor solutions for the preliminary filament fusion and collapse test before proceeding to more complex printings. The fibers showed a sufficient stability which allows the creation of large structures with a height of a few centimeters and a suspended filament up to centimeter. Accordingly, various 3D constructs including suspended filaments were printed successfully with high stackability and shape fidelity. The structure after extrusion was physical crosslinked easily by soaking in CaCl2 solution and, thereafter exhibited a good mechanical flexibility and long-term stability. Interestingly, the mechanical strength and geometry size of the generated scaffolds were well maintained over a culture period of weeks in water, which is of great importance for clinical applications. In addition, the post-printing ionic crosslinking of alginate could also be realized by other di/trivalent cations such as Fe3+ and Tb3+.
Subsequently, the cell-laden printing with this hybrid hydrogel and post-printing crosslinking by Ca2+ ions highlighting its feasibility for 3D bioprinting. WST-1 assay of fibroblast suggested no-dose dependent cytocompatibility of the hydrogel precursor solution. The cell distribution was uniform throughout the printed construct, and proliferated with high cell viability during the 21 days culture. The presented hybrid approach, utilizing the beneficial properties of the POx-b-POzi base material, could be interesting for a wide range of bioprinting applications and potentially enabling also other biological bioinks such as collagen, hyaluronic acid, decellularized extracellular matrix or cellulose based bioinks. Although the results look promising and the developed hydrogel is an important bioink candidate, the long-term in vitro cell studies with different cell lines and clinical model establishment are still under investigation, which remains a long road but is of great importance before realizing real clinical application.
Last but not least, the improvement to the printability of thermogelling POx/POzi-based copolymers by the clay Laponite XLG was also demonstrated in another thermogelling copolymer PEtOx-b-PnPrOzi. This suggests that the addition of clay may be a general strategy to improve the printability of such polymers. Despite these advances in this work which significantly extended the (bio)material platform of additive manufacturing technology, the competition is still fierce and more work should be done in the further to reveal the potential and limitations of this kind of new and promising candidate (bio)ink materials. It is also highly expected for further creative works based on the thermogelling POx/POzi polymers, such as crosslinking in Ca2+ solution containing monomer acrylamide to prepare printable and mechanically tough hydrogels, research on POx-based support bath material, and print of clinically more relevant sophisticated structures such as 3D microvascular networks omnidirectionally.
Overview of the Organolead Trihalide Perovskite Crystal Area
Studies of perovskite single crystals with high crystallographic quality is an important technological area of the perovskite research, which enables to estimate their full optoelectronic potential, and thus to boost their future applications [26]. It was therefore essential to grow high-quality single crystals with lowest structural as well as chemical defect densities and with a stoichiometry relevant for their thin-film counterparts [26]. Optoelectronic devices, e.g. solar cells, are highly complex systems in which the properties of the active layer (absorber) are strongly influenced by the adjacent layers, so it is not always easy to define the targeted properties and elaborate the design rules for the active layer. Currently, organolead trihalide perovskite (OLTP) single crystals with the structure ABX3 are one of the most studied crystalline systems. These hybrid crystals are solids composed of an organic cation such as methylammonium (A = MA+) or formamidinium (A = FA+) to form a three-dimensional periodic lattice together with the lead cation (B = Pb2+) and a halogen anion such as chloride, bromide or iodide (X = Cl-, Br- or I-) [23]. Among them are methylammonium lead tribromide (MAPbBr3), methylammonium lead triiodide (MAPbI3), as well as methylammonium lead trichloride (MAPbCl3) [62, 63]. Important representatives with the larger cation FA+ are formamidinium lead tribromide (FAPbBr3) and formamidinium lead triiodide (FAPbI3) [23, 64]. Besides the exchange of cations as well as anions, it was possible to grow crystals containing two halogens to obtain mixed crystals with different proportions of chlorine to bromine and bromine to iodine, as it is shown in Figure 70. By varying the mixing ratio of the halogens, it was therefore possible to vary the colour and thus the absorption properties of the crystals [85], as it can be done with thin polycrystalline perovskite films. In addition, since a few years it is also doable to grow complex crystals that contain several cations as well as anions [26, 80, 81]. These include the perovskites double cation – double halide formamidinium lead triiodide – methylammonium lead tribromide (FAPbI3)0.9(MAPbBr3)0.1 (FAMA) [26, 80] and formamidinium lead triiodide – methylammonium lead tribromide – caesium lead tribromide (FAPbI3)0.9(MAPbBr3)0.05(CsPbBr3)0.05 (CsFAMA) [81], which have made a significant contribution to increase the power conversion efficiency (PCE) in thin-film photovoltaics [47, 79, 182]. The growth of crystals to this day is performed exclusively from solution [23, 26, 56, 62]. Important preparation methods are the cooling acid-based precursor solution crystallisation [22], the inverse temperature crystallisation (ITC) [62], and the antisolvent vapour-assistant crystallisation (AVC) [137]. In the cooling crystallisation, the precursor salts AX and PbX2 are dissolved in an aqueous halogen-containing acid at high temperatures [56]. Controlled and slow cooling finally results in a supersaturated precursor solution, which leads to spontaneous nucleation of crystal nuclei, followed by subsequent crystal growth. The ITC method is based on the inverse or retrograde solubility of a dissociated perovskite in an organic solvent [23, 64]. With increasing temperature, the solubility of the perovskite decreases and mm-sized crystals can be grown within a few hours [23]. In the AVC method, the precursors are also dissolved in an organic solvent as well [137]. By slow evaporation of a so-called antisolvent [137], the solubility of the perovskite in the now present solvent mixture decreases and it finally precipitates. In addition, there are many other methods with the goal of growing high quality and large crystals in a short period of
time [60, 61, 233, 310].
Metallic nano-optical systems allow to confine and guide light at the nanoscale,
a fascinating ability which has motivated a wide range of fundamental as well
as applied research over the last two decades. While optical antennas provide
a link between visible radiation and localized energy, plasmonic waveguides
route light in predefined pathways. So far, however, most experimental demonstrations
are limited to purely optical excitations, i.e. isolated structures are
illuminated by external lasers. Driving such systems electrically and generating
light at the nanoscale, would greatly reduce the device footprint and pave the
road for integrated optical nanocircuitry. Yet, the light emission mechanism as
well as connecting delicate nanostructures to external electrodes pose key challenges
and require sophisticated fabrication techniques. This work presents various
electrically connected nano-optical systems and outlines a comprehensive
production line, thus significantly advancing the state of the art. Importantly,
the electrical connection is not just used to generate light, but also offers new
strategies for device assembly. In a first example, nanoelectrodes are selectively
functionalized with self-assembled monolayers by charging a specific electrode.
This allows to tailor the surface properties of nanoscale objects, introducing an
additional degree of freedom to the development of metal-organic nanodevices.
In addition, the electrical connection enables the bottom-up fabrication of tunnel
junctions by feedback-controlled dielectrophoresis. The resulting tunnel barriers
are then used to generate light in different nano-optical systems via inelastic
electron tunneling. Two structures are discussed in particular: optical Yagi-Uda
antennas and plasmonic waveguides. Their refined geometries, accurately fabricated
via focused ion beam milling of single-crystalline gold platelets, determine
the properties of the emitted light. It is shown experimentally, that Yagi-Uda
antennas radiate light in a specific direction with unprecedented directionality,
while plasmonic waveguides allow to switch between the excitation of two
propagating modes with orthogonal near-field symmetry. The presented devices
nicely demonstrate the potential of electrically connected nano-optical systems,
and the fabrication scheme including dielectrophoresis as well as site-selective
functionalization will inspire more research in the field of nano-optoelectronics.
In this context, different future experiments are discussed, ranging from the
control of molecular machinery to optical antenna communication.
The aim of this thesis was the preparation of a biomaterial ink for the fabrication of chemically crosslinked hydrogel scaffolds with low micron sized features using melt electrowriting (MEW). By developing a functional polymeric material based on 2-alkyl-2-oxazine (Ozi) and 2-alkyl-2-oxazoline (Ox) homo- and copolymers in combination with Diels-Alder (DA)-based dynamic covalent chemistry, it was possible to achieve this goal. This marks an important step for the additive manufacturing technique melt electrowriting (MEW), as soft and hydrophilic structures become available for the first time. The use of dynamic covalent chemistry is a very elegant and efficient method for consolidating covalent crosslinking with melt processing. It was shown that the high chemical versatility of the Ox and Ozi chemistry offers great potential to control the processing parameters. The established platform offers straight forward potential for modification with biological cues and fluorescent markers. This is essential for advanced biological applications. The physical properties of the material are readily controlled and the potential for 4D-printing was highlighted as well. The developed hydrogel architectures are excellent candidates for 3D cell culture applications. In particular, the low internal strength of some of the scaffolds in combination with the tendency of such constructs to collapse into thin strings could be interesting for the cultivation of muscle or nerve cells. In this context it was also possible to show that MEW printed hydrogel scaffolds can withstand the aspiration and ejection through a cannula. This allows the application as scaffolds for the minimally invasive delivery of implants or functional tissue equivalent structures to various locations in the human body.
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.
Remote sensing time series is the collection or acquisition of remote sensing data in a
fixed equally spaced time period over a particular area or for the whole world. Near
daily high spatial resolution data is very much needed for remote sensing applications
such as agriculture monitoring, phenology change detection, environmental
monitoring and so on. Remote sensing applications can produce better and accurate
results if they are provided with dense and accurate time series of data. The current
remote sensing satellite architecture is still not capable of providing near daily
or daily high spatial resolution images to fulfill the needs of the above mentioned
remote sensing applications. Limitations in sensors, high development, operational
costs of satellites and presence of clouds blocking the area of observation are some
of the reasons that makes near daily or daily high spatial resolution optical remote
sensing data highly challenging to achieve. With developments in the optical sensor
systems and well planned remote sensing satellite constellations, this condition
can be improved but it comes at a cost. Even then the issue will not be completely
resolved and thus the growing need for high temporal and high spatial resolution
data cannot be fulfilled entirely. Because the data collection process relies on satellites
which are physical system, these can fail unpredictably due to various reasons
and cause a complete loss of observation for a given period of time making a gap
in the time series. Moreover, to observe the long term trend in phenology change
due to rapidly changing environmental conditions, the remote sensing data from
the present is not just sufficient, the data from the past is also important. A better
alternative solution for this issue can be the generation of remote sensing time series
by fusing data from multiple remote sensing satellite which has different spatial and
temporal resolutions. This approach will be effective and efficient. In this method
a high temporal low spatial resolution image from a satellite such as Sentinel-2 can
be fused with a low temporal and high spatial resolution image from a satellite such
as the Sentinel-3 to generate a synthetic high temporal high spatial resolution data.
Remote sensing time series generation by data fusion methods can be applied to
the satellite images captured currently as well as the images captured by the satellites
in the past. This will provide the much needed high temporal and high spatial
resolution images for remote sensing applications. This approach with its simplistic
nature is cost effective and provides the researchers the means to generate the
data needed for their application on their own from the limited source of data available
to them. An efficient data fusion approach in combination with a well planned
satellite constellation can offer a solution which will ensure near daily time series of
remote sensing data with out any gap. The aim of this research work is to develop
an efficient data fusion approaches to achieve dense remote sensing time series.
After examining suitable parameters for a newly designed system, dynamic SIPGP could be developed. For the first time, SIPGP was performed while applying a constant flow of monomer solution through the reaction system. This added a new parameter: the flow rate (rfl). Accordingly, this parameter was examined, comparing dynamic to static SIPGP. It could be shown, that by applying higher rfl to the system, the contact angle increases, which indicates a slower coating. The flow patterns inside the reactor were then modelled and calculated. These calculations indicated, that, due to higher flow velocities, the contact angle on the coated samples would be lower on the sides of the sample and higher in the middle. This finding was verified by contact angle measurements. The influence of dynamic SIPGP on the temperature inside the reaction chamber during the reaction was examined by temperature sensors inside the reactor. This showed, that the constant flow of monomer solution can be utilized to decrease the warming of the reaction solution during the reaction. Finally it was shown, that dynamic SIPGP can decrease the formation of bulk polymer on the sample, which is forming during the reaction. This enables SIPGP to fabricate more homogeneous coatings by applying a constant monomer flow.
This thesis encompasses the development of the additive manufacturing technology melt electrowriting, in order to achieve the improved applicability in biomedical applications and design of scaffolds. Melt electrowriting is a process capable of producing highly resolved structures from microscale fibres. Nevertheless, there are parameters influencing the process and it has not been clear how they affect the printing result. In this thesis the influence of the processing and environmental parameters is investigated with the impact on their effect on the jet speed, fibre diameter and scaffold morphology, which has not been reported in the literature to date and significantly influences the printing quality. It was demonstrated that at higher ambient printing temperatures the fibres can be hampered to the extent that the individual fibres are completely molten together and increased air humidity intensifies this effect. It was also shown how such parameters as applied voltage, collector distance, feed pressure and polymer temperature influence the fibre diameter and critical translation speed. Based on these results, a detailed investigation of the fibre diameter control and printing of scaffolds with novel architectures was made. As an example, a 20-fold diameter ratio is obtained within one scaffold by changing the collector speed and the feed pressure during the printing process. Although the pressure change caused fibre diameter oscillations, different diameter fibres were successfully integrated into two scaffold designs, which were tested for mesenchymal stromal cell suspension and adipose tissue spheroid seeding. Further design and manufacturing aspects are discussed while jet attraction to the printed structures is illuminated in connection with the fibre positioning control of the multilayer scaffolds. The artefacts that appear with the increasing scaffold height of sinusoidal laydown patterns are counteracted by layer-by-layer path adjustment. For the prediction of a printing error of the first deposited layer, an algorithm is developed, that utilizes an empirical jet lag equation and the speed of fibre deposition. This model was able to predict the position of the printing fibre with up to ten times smaller error than the of the programmed path. The same model allows to qualitatively assess the fibre diameter change along the nonlinear pattern as well as to indicate the areas of the greatest pattern deformation with the growing scaffold height. Those results will be used in the later chapters for printing of the novel MEW structures for biomedical applications. In the final chapter the concept of multimodal scaffold was combined with the suspended fibre printing, for the manufacturing of the MEW scaffolds with controlled pore interconnectivity in three dimensions. Those scaffolds were proven to be a promising substate for the control of the neurite spreading of the chick DRG neurons.
Nanoelectronics is an essential technology for down-scaling beyond the limit of silicon-based electronics. Single-Wall Carbon Nanotubes (SWNT) are semiconducting components that exhibit a large variety of properties that make them usable for sensing, telecommunication, or computational tasks. Due to their high surface to volume ratio, carbon nanotubes are strongly affected by molecular adsorptions, and almost all properties depend on surface adsorption. SWNT with smaller diameters (0.7-0.9nm) show a stronger sensitivity to surface effects. An optimized synthesis route was developed to produce these nanotubes directly. They were produced with a clean surface, high quality, and large lengths of 2 μ m. The results complement previous studies on larger diameters (0.9-1.4nm). They allow performing statistically significant assumptions for a perfect nanotube, which is selected from a subset of nanotubes with good emission intensity, and high mechanical durability. The adsorption of molecules on the surface of carbon nanotubes influences the motion and binding strength of chargeseparated states in this system. To gain insight into the adsorption processes on the surface with a minimum of concurrent overlapping effects, a microscopic setup, and a measurement technique were developed. The system was estimated to exhibit excellent properties like long exciton diffusion lengths (>350nm), and big exciton sizes (8.5(5)nm), which was substantiated by a simulation. We studied the adsorption processes at the surface of Single-Wall Carbon Nanotubes for molecules in the gas phase, solvent molecules, and surfactant molecules. The experiments were all carried out on suspended individualized carbon nanotubes on a silicon wafer substrate. The experiments in the gas-phase showed that the excitonic emission energy and intensity experiences a rapid blue shift during observation. This shift was associated with the spontaneous desorption of large clusters of gaseous molecules caused by laser heat up. The measurement of this desorption was essential for creating a reference to an initially clean surface and allows us to perform a comparison with previous measurements on this topic. Furthermore, the adsorption of hydrogen on the nanotube surface at high temperatures was investigated. It was found that a new emission mode arises slightly red-shifted to the excitonic emission in these systems. The new signal is almost equally strong as the main excitonic peak and was associated with the brightening of dark excitons at sp3-defects through a K-phonon assisted pathway. The finding is useful for the direct synthesis of spintronic devices as these systems are known to act as single-photon emitters. The suspended nanotubes were further studied to estimate the effect of solvent adsorption on the excitonic states during nanotube dispersion for each nanotube individually. A significant quantum yield loss is observable for hexane and acetonitrile, while the emission intensity was found to be the strongest in toluene. The reference to a clean surface allowed us to estimate the exact influence of the dielectric environment of adsorbing solvents on the excitonic emission energy. Solvent adsorption was found to lead to an energy shift that is almost twice as high as suggested in previous studies. The amount of this energy shift, however, was comparably similar for all solvents, which suggests that the influence of the distinct dielectric constant in the outer environment less significantly influences the energy shift than previously thought. An interesting phenomenon was found when using acetonitrile as a solvent, which leads to greatly enhanced emission properties. The emission is more than twice as high as in the same air-suspended nanotubes, which suggests a process that depends on the laser intensity. In this study, it was reasonably explained how an energy down-conversion is possible through the coupling of the excitonic states with solvent vibrations. The strength of this coupling, however, also suggests adsorptions to the inside of the tubular nanotube structure leading to a coupled vibration of linear acetonitrile molecules that are adsorbed to the inner surface. The findings are important for the field of nanofluidics and provide an excellent system for efficient energy down-conversion in the transmission window of biological tissue. Having separated the pure effect of solvent adsorption allowed us to study the undisturbed molecular adsorption of polymers in these systems. The addition of polyfluorene polymer leads to a slow but stepwise intensity increase. The intensity increase is overlapping with a concurrent process that leads to an intensity decrease. Unfortunately, observing the stepwise process has a low spacial resolution of only 100-250nm, which is in the range of the exciton diffusion length in these systems and hinders detailed analysis. The two competing and overlapping processes processes are considered to originate from slow π-stacking and fast side-chain binding. Insights into this process are essential for selecting suitably formed polymers. However, the findings also emphasize the importance of solvent selection during nanotube dispersion since solvent effects were proven to be far more critical on the quantum yield in these systems. These measurements can shed light on the ongoing debate on polymers adsorption during nanotube individualization and allow us to direct the discussion more towards the selection of suitable solvents. This work provides fundamental insights into the adsorption of various molecules on the surface of individually observed suspended Single-Wall Carbon Nanotubes. It allows observing the adsorption of individual molecules below the optical limit in the solid, liquid, and gas phases. Nanotubes are able to act as sensing material for detecting changes in their direct surrounding. These fundamental findings are also crucial for increasing the quantum yield of solvent-dispersed nanotubes. They can provide better light-harvesting systems for microscopy in biological tissue and set the base for a more efficient telecommunication infrastructure with nano-scale spintronics devices and lasing components. The newly discovered solvent alignment in the nanotube surrounding can potentially also be used for supercapacitors that are needed for caching the calculation results in computational devices that use polymer wrapped nanotubes as transistors. Although fundamental, these studies develop a strategy to enlighten this room that is barely only visible at the bottom of the nano-scale.
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.
Motivated by the great potential which is offered by the combination of additive manufacturing and tissue engineering, a novel polymeric bioink platform based on poly(2 oxazoline)s was developed which might help to further advance the young and upcoming field of biofabrication. In the present thesis, the synthesis as well as the characteristics of several diblock copolymers consisting of POx and POzi have been investigated with a special focus on their suitability as bioinks.
In general, the copolymerization of 2-oxazolines and 2-oxazines bearing different alkyl side chains was demonstrated to yield polymers in good agreement with the degree of polymerization aimed for and moderate to low dispersities.
For every diblock copolymer synthesized during the present study, a more or less pronounced dependency of the dynamic viscosity on temperature could be demonstrated. Diblock copolymers comprising a hydrophilic PMeOx block and a thermoresponsive PnPrOzi block showed temperature induced gelation above a degree of polymerization of 50 and a polymer concentration of 20 wt%. Such a behavior has never been described before for copolymers solely consisting of poly(cyclic imino ether)s.
Physically cross linked hydrogels based on POx b POzi copolymers exhibit reverse thermal gelation properties like described for solutions of PNiPAAm and Pluronic F127. However, by applying SANS, DLS, and SLS it could be demonstrated that the underlying gel formation mechanism is different for POx b POzi based hydrogels. It appears that polymersomes with low polydispersity are formed already at very low polymer concentrations of 6 mg/L. Increasing the polymer concentration resulted in the formation of a bicontinuous sponge like structure which might be formed due to the merger of several vesicles. For longer polymer chains a phase transition into a gyroid structure was postulated and corresponds well with the observed rheological data.
Stable hydrogels with an unusually high mechanical strength (G’ ~ 4 kPa) have been formed above TGel which could be adjusted over a range of 20 °C by changing the degree of polymerization if maintaining the symmetric polymer architecture. Variations of the chain ends revealed only a minor influence on TGel whereas the influence of the solvent should not be neglected as shown by a comparison of cell culture medium and MilliQ water.
Rotationally as well as oscillatory rheological measurements revealed a high suitability for printing as POx b POzi based hydrogels exhibit strong shear thinning behavior in combination with outstanding recovery properties after high shear stress.
Cell viability assays (WST-1) of PMeOx b PnPrOzi copolymers against NIH 3T3 fibroblasts and HaCat cells indicated that the polymers were well tolerated by the cells as no dose-dependent cytotoxicity could be observed after 24 h at non-gelling concentrations up to 100 g/L.
In summary, copolymers consisting of POx and POzi significantly increased the accessible range of properties of POx based materials. In particular thermogelation of aqueous solutions of diblock copolymers comprising PMeOx and PnPrOzi was never described before for any copolymer consisting solely of POx or POzi. In combination with other characteristics, e.g. very good cytocompatibility at high polymer concentrations and comparably high mechanical strength, the formed hydrogels could be successfully used for 3D bioprinting. Although the results appear promising and the developed hydrogel is a serious bioink candidate, competition is tough and it remains an open question which system or systems will be used in the future.
It is the aim of this thesis to present a visual body weight estimation, which is suitable for medical applications. A typical scenario where the estimation of the body weight is essential, is the emergency treatment of stroke patients: In case of an ischemic stroke, the patient has to receive a body weight adapted drug, to solve a blood clot in a vessel. The accuracy of the estimated weight influences the outcome of the therapy directly. However, the treatment has to start as early as possible after the arrival at a trauma room, to provide sufficient treatment. Weighing a patient takes time, and the patient has to be moved. Furthermore, patients are often not able to communicate a value for their body weight due to their stroke symptoms. Therefore, it is state of the art that physicians guess the body weight. A patient receiving a too low dose has an increased risk that the blood clot does not dissolve and brain tissue is permanently damaged. Today, about one-third gets an insufficient dosage. In contrast to that, an overdose can cause bleedings and further complications. Physicians are aware of this issue, but a reliable alternative is missing.
The thesis presents state-of-the-art principles and devices for the measurement and estimation of body weight in the context of medical applications. While scales are common and available at a hospital, the process of weighing takes too long and can hardly be integrated into the process of stroke treatment. Sensor systems and algorithms are presented in the section for related work and provide an overview of different approaches.
The here presented system -- called Libra3D -- consists of a computer installed in a real trauma room, as well as visual sensors integrated into the ceiling. For the estimation of the body weight, the patient is on a stretcher which is placed in the field of view of the sensors. The three sensors -- two RGB-D and a thermal camera -- are calibrated intrinsically and extrinsically. Also, algorithms for sensor fusion are presented to align the data from all sensors which is the base for a reliable segmentation of the patient.
A combination of state-of-the-art image and point cloud algorithms is used to localize the patient on the stretcher. The challenges in the scenario with the patient on the bed is the dynamic environment, including other people or medical devices in the field of view.
After the successful segmentation, a set of hand-crafted features is extracted from the patient's point cloud. These features rely on geometric and statistical values and provide a robust input to a subsequent machine learning approach. The final estimation is done with a previously trained artificial neural network.
The experiment section offers different configurations of the previously extracted feature vector. Additionally, the here presented approach is compared to state-of-the-art methods; the patient's own assessment, the physician's guess, and an anthropometric estimation. Besides the patient's own estimation, Libra3D outperforms all state-of-the-art estimation methods: 95 percent of all patients are estimated with a relative error of less than 10 percent to ground truth body weight. It takes only a minimal amount of time for the measurement, and the approach can easily be integrated into the treatment of stroke patients, while physicians are not hindered.
Furthermore, the section for experiments demonstrates two additional applications: The extracted features can also be used to estimate the body weight of people standing, or even walking in front of a 3D camera. Also, it is possible to determine or classify the BMI of a subject on a stretcher. A potential application for this approach is the reduction of the radiation dose of patients being exposed to X-rays during a CT examination.
During the time of this thesis, several data sets were recorded. These data sets contain the ground truth body weight, as well as the data from the sensors. They are available for the collaboration in the field of body weight estimation for medical applications.