Constrained optimal control theory for differential linear repetitive processes

Differential repetitive processes are a distinct class of continuous-discrete two-dimensional linear systems of both systems theoretic and applications interest. These processes complete a series of sweeps termed passes through a set of dynamics defined over a finite duration known as the pass length, and once the end is reached the process is reset to its starting position before the next pass begins. Moreover the output or pass profile produced on each pass explicitly contributes to the dynamics of the next one. Applications areas include iterative learning control and iterative solution algorithms, for classes of dynamic nonlinear optimal control problems based on the maximum principle, and the modeling of numerous industrial processes such as metal rolling, long-wall cutting, etc. In this paper we develop substantial new results on optimal control of these processes in the presence of constraints where the cost function and constraints are motivated by practical application of iterative learning control to robotic manipulators and other electromechanical systems. The analysis is based on generalizing the well-known maximum and $\epsilon$-maximum principles to the

Experimental and numerical study of pinching phenomena in sheet metal rolling processes

Steel sheets are an essential raw material for a wide range of applications, such as household appliances, packaging, construction, shipbuilding, industrial machinery, automotive, and energy industries. The worldwide market for steel sheets faces intense competition and increasing demand for light-weight metal components to reduce CO2 emissions. As a result, newly developed grades of advanced high-strength steel (AHSS) have gained attention, especially from the automotive industry. AHSS allows for down-gauging due to its higher strength compared to other conventional steel grades. However, due to the low thickness of the sheets, rolling of AHSS is a critical process that may suffer from instabilities, such as pinching. Pinching represents a complex type of phenomena related to inhomogeneous stress distributions in the strip, which may arise from disruptions during the rolling process. Similarly to other shape defects, pinches can be related to uneven strip deformations in the roll bite, which result in inhomogeneous stress distributions across the strip’s width. Pinching defects in steel sheets appear as surface marks, wrinkling, repetitive rippled areas, and local ruptures. In the most severe cases, the strip breaks completely, causing damage to the rolls and considerable manufacturing downtime.Controlling the stability and enhancing the performance of the rolling process are top priorities for steel manufacturers. These tasks aim to minimize the occurrence of defects, ensure consistent product quality, and enhance the efficiency of the manufacturing process. Therefore, better understanding of instability phenomena like pinching is required for determining suitable solutions to prevent them and to obtain a stable rolling process. However, despite being a commonly reported issue among steel manufacturers, pinching has been poorly understood in terms of its underlying mechanism. Currently, there is a lack of research examining the mechanisms behind pinches, both in terms of experimental and numerical investigations. Without a comprehensive understanding of these phenomena, it is unfeasible to develop effective measures to prevent pinches and ensure stable operations of rolling mills. Therefore, the aims of this study are: firstly, to identify the mechanism and possible causes of pinching, and secondly, to develop a simulation tool that can be used to analyze pinching phenomena and design guidelines for the selection of robust production settings in cold rolling mills. To this end, both experimental study and numerical modelling are performed, as presented in this work.The experimental investigation of pinching phenomena presented in this work provide an in-depth understanding of the circumstances that lead to pinching through a series of cold rolling tests and the analysis and characterization of pinching defects.To study pinching phenomena, an appropriate tool is needed to replicate and investigate actual pinching events. Simulation models are essential for predicting the occurrence of pinching during the rolling process. However, existing numerical models of rolling do not succeed to reproduce the occurrence of pinching. This is because pinching is a complex phenomenon that depends on the strong interplay between local deformations within the roll bite and the stress state outside the roll bite. To capture this complexity, a numerical tool must be capable of modeling the process both at a millimeter (or sub-millimeter) scale within the roll bite and at a meter scale outside the roll bite. Moreover, to effectively study pinching events, a three-dimensional rolling model is necessary, as the distribution of stresses and strains across the strip's width is a crucial factor. The finite element method (FEM) is a well-established numerical tool for simulating metal forming processes, and is therefore a suitable technique for analyzing and predicting defects during rolling. However, accounting for all the relevant physics of the rolling process in a conventional 3D FEM model would result in an unfeasible computational time. This work proposes a numerical strategy to decrease the computational expense of 3D sheet rolling FEM simulations. The method involves coupling a global model, which represents the behavior and stress state of the strip outside the roll bite, with a local model that reproduces the deformation mechanics inside the roll bite. The global model is a shell finite element model of the sheet, while the local model is a high resolution 2D plane strain model of the roll bite. The developed approach has been validated by comparing its results to those of a conventional full 3D rolling model under stable rolling conditions. Additionally, this model has been employed to carry out a qualitative analysis of instability phenomena that arise during thin strip rolling. Such phenomena include flatness defects that result from disruptions in the frictional conditions. The simulation results demonstrate that locally varying friction induce local variations in the thickness strain, which cause stress re-distributions in the rolled sheet, resulting in flatness defects. Therefore, the proposed model offers a cost-effective alternative to more expensive 3D FEM models in the analysis of complex instability phenomena that can lead to defects during sheet metal rolling processes

NASA patent abstracts bibliography: A continuing bibliography. Section 1: Abstracts (supplement 38)

Abstracts are provided for 132 patents and patent applications entered into the NASA scientific and technical information system during the period July 1990 through December 1990. Each entry consists of a citation, an abstract, and in most cases, a key illustration selected from the patent or patent application

A study of the applicability/compatibility of inertial energy storage systems to future space missions

The applicability/compatibility of inertial energy storage systems like the homopolar generator (HPG) and the compensated pulsed alternator (CPA) to future space missions is explored. Areas of CPA and HPG design requiring development for space applications are identified. The manner in which acceptance parameters of the CPA and HPG scale with operating parameters of the machines are explored and the types of electrical loads which are compatible with the CPA and HPG are examined. Potential applications including the magnetoplasmadynamic (MPD) thruster, pulsed data transmission, laser ranging, welding and electromagnetic space launch are discussed

Active Stimuli-Responsive Polymer Surfaces and Thin Films: Design, Properties and Applications: Active Stimuli-Responsive Polymer Surfaces and Thin Films: Design, Properties and Applications

Design of 2D and 3D micropatterned materials is highly important for printing technology, microfluidics, microanalytics, information storage, microelectronics and biotechnology. Biotechnology deserves particular interest among the diversity of possible applications because its opens perspectives for regeneration of tissues and organs that can considerably improve our life. In fact, biotechnology is in constant need for development of microstructured materials with controlled architecture. Such materials can serve either as scaffolds or as microanalytical platforms, where cells are able to self-organize in a programmed manner. Microstructured materials, for example, allow in vitro investigation of complex cell-cell interactions, interactions between cells and engineered materials. With the help of patterned surfaces it was demonstrated that cell adhesion and viability as well as differentiation of stem cells1 depend of on the character of nano- and micro- structures 2 as well as their size. There are number of methods based on optical lithography, atomic force microscopy, printing techniques, chemical vapor deposition, which have been developed and successfully applied for 2D patterning. While each of these methods provides particular advantages, a general trade-off between spatial resolution, throughput, “biocompatibility of method” and usability of fabricated patterned surfaces exists. For example, AFM-based techniques allow very high nanometer resolution and can be used to place small numbers of functional proteins with nanometer lateral resolution, but are limited to low writing speeds and small pattern sizes. Albeit, the resolution of photolithography is lower, while it is much faster and cheaper. Therefore, it is highly desirable to develop methods for high-resolution patterning at reasonably low cost and high throughput. Although many approaches to fabricate sophisticated surface patterns exist, they are almost entirely limited to producing fixed patterns that cannot be intentionally modified or switched on the fly in physiologic environment. This limits the usability of a patterned surface to a single specific application and new microstructures have to be fabricated for new applications. Therefore, it is desirable to develop methods for design of switchable and rewritable patterns. Next, the high-energy of the ultraviolet radiation, which is typically used for photolithography, can be harmful for biological species. It is also highly important to develop an approach for photopatterning where visible light is used instead of UV light. Therefore, it is very important for biotechnological applications to achieve good resolution at low costs, create surface with switchable and reconfigurable patterns, perform patterning in mild physiologic conditions and avoid use of harmful UV light. 3D patterning is experimentally more complicated than 2D one and the applicability of available techniques is substantially limited. For example, interference photolithography allows fabrication of 3D structures with limited thickness. Two-photon photolithography, which allows nanoscale resolution, is very slow and highly expensive. Assembling of 3D structures by stacking of 2D ones is time consuming and does not allow fabrication of fine hollow structures. At the same time, nature offers an enormous arsenal of ideas for the design of novel materials with superior properties. In particular, self-assembly and self-organization being the driving principles of structure formation in nature attract significant interest as promising concepts for the design of intelligent materials 3. Self-folding films are the examples of biomimetic materials4. Such films mimic movement mechanisms of plants 5-7 and are able to self-organize and form complex 3D structures. The self-folding films consist of two materials with different properties. At least one of these materials, active one, can change its volume. Because of non-equal expansion of the materials, the self-folding films are able to form a tubes, capsules or more complex structure. Similar to origami, the self-folding films provide unique possibilities for the straightforward fabrication of highly complex 3D micro-structures with patterned inner and outer walls that cannot be achieved using other currently available technologies. The self-folded micro-objects can be assembled into sophisticated, hierarchically-organized 3D super-constructs with structural anisotropy and highly complex surface patterns. Till now most of the research in the field of self-folding films was focused on inorganic materials. Due to their rigidity, limited biocompatibility and non-biodegradability, application of inorganic self-folding materials for biomedical purposes is limited. Polymers are more suitable for these purposes. There are many factors, which make polymer-based self-folding films particularly attractive. There is a variety of polymers sensitive to different stimuli that allows design of self-folding films, which are able to fold in response to various external signals. There are many polymers changing their properties in physiological ranges of pH and temperature as well as polymers sensitive to biochemical processes. There is a variety of biocompatible and biodegradable polymers. These properties make self-folding polymer highly attractive for biological applications. Polymers undergo considerable and reversible changes of volume that allows design of systems with reversible folding. Fabrication of 3D structures with the size ranging from hundreds of nanometers to centimeters is possible. In spite of their attractive properties, the polymer-based systems remained almost out of focus – ca 15 papers including own ones were published on this topic (see own review 8, state October 2011). Thereby the development of biomimetic materials based on self-folding polymer films is highly desired and can open new horizons for the design of unique 3D materials with advanced properties for lab-on-chip applications, smart materials for everyday life and regenerative medicine

Microfabrication and development of multi-scaled metallic surfaces using direct laser interference patterning

Die Kontrolle physikalischer Phänomene auf Oberflächen durch bestimmte Topographien ist eines der Ziele oberflächentechnischer Verfahren. Die Oberflächentopographie kann durch oberflächenmodifizierende Verfahren wie Direkte Laserinterferenzstrukturieren (DLIP) und das Direkte Laserschreiben (DLW) verändert werden. Dadurch können definierte und kontrollierte Mikro- und Nanostrukturen auf verschiedenen Materialien erzeugt werden. Darüber hinaus können spezifische Topographien entworfen und großflächig nachgebildet werden, welche die gleichen Oberflächeneigenschaft gewährleisten können. Diese Arbeit schlägt neue Ansätze zur Verbesserung der Mikro- und Nano-Oberflächenstrukturen vor, die durch DLIP auf Metalloberflächen erzeugt werden. DLIP Experimente werden in der Zweistrahlkonfiguration entweder mit infraroten Nano- oder Pikosekundenlasern durchgeführt. Damit werden die Möglichkeiten zur Verbesserung und Kontrolle von Oberflächeneigenschaften durch die Mikrofertigung mit Strukturperioden von 0,2 µm bis 7,2 µm erweitert. Anschließend wird die Homogenität der Oberflächentextur auf Basis der Pulsverteilung und der Laserparameter optimiert. Ein quantitatives Messschema der Homogenität, das auf etablierten Parametern wie mittlere Strukturhöhe, seiner Standardabweichung und Kurtosis basiert, wird vorgestellt. Darüber hinaus wird die Herstellung hierarchischer linien- und säulenartiger Mikrostrukturen mittels DLIP in Abhängigkeit von der Anzahl der Pulse und der Fluenz untersucht. Zusätzlich zu den Mikrostrukturen, die der Interferenzverteilung entsprechen, wurden gleichzeitig laserinduzierte periodische Oberflächenstrukturen (LIPSS) erzeugt, die zu hierarchischen Mikro- und Nanostrukturen führen. Überdies wird als weitere Technologie das DLW eingesetzt, um Mikrozellen im Bereich von 17 µm bis 50 µm zu generieren. Anschließen werden Mikro- und Nanostrukturen mittels DLIP auf den Mikrozellen hergestellt. Die finale Topographie besteht aus multiskaligen hierarchischen Mikro- und Nanostrukturen. Um den Durchsatz des DLIP-Verfahrens zu verbessern, wird ein Ablationsmodell entwickelt und mit experimentellen Daten verifiziert. Das Modell ermöglicht die Berechnung von Strukturtiefe in Abhängigkeit von optimalen Laserprozessparametern. Darüber hinaus wird die Benetzbarkeit auf den Mikrosäulen im Rahmen des Füllfaktors und der Kombination von hierarschischen und einskalen Strukturen ausgewertet. Dazu wird ein hydrophobes Lösungsmittel auf die hierarchischen Strukturen aufgetragen, um den Wasserkontaktwinkel auf bis zu 152 ° ± 2 ° und die Kontaktwinkelhysterese von 4 ° ± 2 ° zu erreichen. Mikrosäulen mit einer Periode von 5,20 µm werden auf einer Flugzeugtragfläche hergestellt. Auf diese Weise wird der mögliche Einfluss von Mikrostrukturen auf die Ermüdungseigenschaften untersucht. Schließlich werden Mikrosäulen mit ca. 40 % geringeren Reibungskoeffizienten als die Referenz in einem grenzflächengeschmierten Bereich getestet. Zusammenfassend kann ausgesagt werden, dass die durch DLIP erzeugten Mikrosäulen eine vielversprechende und gut realisierbare Struktur für die Oberflächenfunktionalisierung von Metallen darstellen.:Selbstständigkeitserklärung Abstract Kurzfassung Acknowledgments Symbols and abbreviations 1 Motivation 2 Theoretical background 2.1 Laser-matter interactions 2.2 Principle of interference 2.3 Wetting on solid surfaces 2.4 Introduction to friction 2.5 Introduction to fatigue 3 State of the art 3.1 Properties of natural surfaces 3.2 Texturing techniques for creating micro/nanoroughness 3.3 Surface microstructuring of metals using pulsed laser sources 3.3.1 Direct Laser Writing 3.3.2 Direct Laser Interference Patterning 3.3.3 Laser-Induce Periodic Surface Structures 3.3.4 Challenges for laser surface texturing methods 3.4 Surface properties affected by laser micro/nano texturing on metals 3.4.1 Impact of laser surface textures and chemistry on wettability 3.4.2 Control of the friction coefficient 3.4.3 Impact on fatigue performance 4 Materials and methods 4.1 Materials 4.2 Direct Laser Writing 4.3 Direct Laser Interference Patterning 4.4 Surface chemical treatment 4.5 Characterisation methods 4.5.1 Water Contact Angle 4.5.2 White Light Interferometry and Confocal Microscopy 4.5.3 Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy 4.5.4 Raman Spectroscopy 4.5.5 X-ray Photoelectron Spectroscopy 4.5.6 Tribological test 4.5.7 Fatigue test 5 Results and discussion 5.1 Interference structuring of Ti6Al4V using nanosecond laser pulses 5.1.1 Strategy to fabricate homogeneous DLIP line-like structures 5.1.2 Development of topographical parameters for homogeneity quantification 5.1.3 Impact of process parameters on surface structure homogeneity 5.2 Interference structuring of stainless steel using picosecond laser pulses 5.2.1 Fabrication of hierarchical periodic micro/nanostructures 5.2.2 Control of nanostructure orientation 5.2.3 Fabrication of hierarchical pillar-like microstructures 5.2.4 Control of nanostructures on hierarchical periodic microstructures 5.3 Fabrication of multi-scale periodic structures by DLW and DLIP 5.3.1 Laser surface texturing of Ti6Al4V 5.3.2 Laser surface texturing of Al2024 5.4 Structuring of a large aircraft surface for a flight test 6. Development of an analytical ablation model for ps-DLIP 7. Surface properties of textured materials 7.1 Determination of wetting behaviour 7.1.1 Wetting transition on single and hierarchical microstructures 7.1.2 Surface chemistry influence on wetting 7.1.3 Wetting response after the chemical surface modification 7.2 Wetting on multi-scale periodic structures fabricated by DLW and DLIP 7.3 Tribological properties of laser treated surfaces 7.4 Influence of laser treated surfaces on fatigue 8. Conclusions and outlook References Curriculum Vitae List of publication

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact