Immersion and invariance adaptive control for discrete-time systems in strict feedback form with input delay and disturbances

This work presents a new adaptive control algorithm for a class of discrete-time systems in strict-feedback form with input delay and disturbances. The immersion and invariance formulation is used to estimate the disturbances and to compensate the effect of the input delay, resulting in a recursive control law. The stability of the closed-loop system is studied using Lyapunov functions, and guidelines for tuning the controller parameters are presented. An explicit expression of the control law in the case of multiple simultaneous disturbances is provided for the tracking problem of a pneumatic drive. The effectiveness of the control algorithm is demonstrated with numerical simulations considering disturbances and input-delay representative of the application

Beam-Steerable and Reconfigurable Reflectarray Antennas for High Gain Space Applications

Reflectarray antennas uniquely combine the advantages of parabolic reflectors and phased array antennas. Comprised of planar structures similar to phased arrays and utilizing quasi-optical excitation similar to parabolic reflectors, reflectarray antennas provide beam steering without the need of complex and lossy feed networks. Chapter 1 discusses the basic theory of reflectarray and its design. A brief summary of previous work and current research status is also presented. The inherent advantages and drawbacks of the reflectarray are discussed. In chapter 2, a novel theoretical approach to extract the reflection coefficient of reflectarray unit cells is developed. The approach is applied to single-resonance unit cell elements under normal and waveguide incidences. The developed theory is also utilized to understand the difference between the TEM and TE10 mode of excitation. Using this theory, effects of different physical parameters on reflection properties of unit cells are studied without the need of full-wave simulations. Detailed analysis is performed for Ka-band reflectarray unit cells and verified by full-wave simulations. In addition, an approach to extract the Q factors using full-wave simulations is also presented. Lastly, a detailed study on the effects of inter-element spacing is discussed. Q factor theory discussed in chapter 2 is extended to account for the varying incidence angles and polarizations in chapter 3 utilizing Floquet modes. Emphasis is laid on elements located on planes where extremities in performance tend to occur. The antenna element properties are assessed in terms of maximum reflection loss and slope of the reflection phase. A thorough analysis is performed at Ka band and the results obtained are verified using full-wave simulations. Reflection coefficients over a 749-element reflectarray aperture for a broadside radiation pattern are presented for a couple of cases and the effects of coupling conditions in conjunction with incidence angles are demonstrated. The presented theory provides explicit physical intuition and guidelines for efficient and accurate reflectarray design. In chapter 4, tunable reflectarray elements capacitively loaded with Barium Strontium Titanate (BST) thin film are shown. The effects of substrate thickness, operating frequency and deposition pressure are shown utilizing coupling conditions and the performance is optimized. To ensure minimum affects from biasing, optimized biasing schemes are discussed. The proposed unit cells are fabricated and measured, demonstrating the reconfigurability by varying the applied E-field. To demonstrate the concept, a 45 element array is also designed and fabricated. Using anechoic chamber measurements, far-field patterns are obtained and a beam scan up to 25o is shown on the E-plane. Overall, novel theoretical approaches to analyze the reflection properties of the reflectarray elements using Q factors are developed. The proposed theoretical models provide valuable physical insight utilizing coupling conditions and aid in efficient reflectarray design. In addition, for the first time a continuously tunable reflectarray operating at Ka-band is presented using BST technology. Due to monolithic integration, the technique can be extended to higher frequencies such as V-band and above

Inference of time-varying networks through transfer entropy, the case of a Boolean network model

Inferring network topologies from the time series of individual units is of paramount importance in the study of biological and social networks. Despite considerable progress, our success in network inference is largely limited to static networks and autonomous node dynamics, which are often inadequate to describe complex systems. Here, we explore the possibility of reconstructing time-varying weighted topologies through the information-theoretic notion of transfer entropy. We focus on a Boolean network model in which the weight of the links and the spontaneous activity periodically vary in time. For slowly-varying dynamics, we establish closed-form expressions for the stationary periodic distribution and transfer entropy between each pair of nodes. Our results indicate that the instantaneous weight of each link is mapped into a corresponding transfer entropy value, thereby affording the possibility of pinpointing the dominant weights at each time. However, comparing transfer entropy readings at different times may provide erroneous estimates of the strength of the links in time, due to a counterintuitive modulation of the information flow by the non-autonomous dynamics. In fact, this time variation should be used to scale transfer entropy values toward the correct inference of the time evolution of the network weights. This study constitutes a necessary step toward a mathematically-principled use of transfer entropy to reconstruct time-varying networks.This work was supported by the National Science Foundation under Grant Nos. CMMI 1433670, CMMI 1561134, and CBET 1547864, the US Army Research Office under Grant No. W911NF-15-1-0267 with Dr. Samuel C. Stanton and Dr. Alfredo Garcia as the program managers, and Ministerio de Economía y Competitividad de Espana and FEDER funds under Grant No. ECO2015-65637-P. This study is part of the collaborative activities carried out under the program Groups of Excellence of the Region of Murcia, the Fundacion Seneca, Science and Technology Agency of the Region of Murcia Project No. 19884/GERM/15

Data-driven modeling of an oscillating surge wave energy converter using dynamic mode decomposition

Modeling wave energy converters (WECs) to accurately predict their hydrodynamic behavior has been a challenge for the wave energy field. Often, this results in either low-fidelity, linear models that break down in energetic seas, or high-fidelity numerical models that are too computationally expensive for operational use. To bridge this gap, we propose the use of dynamic mode decomposition (DMD) as a purely data-driven technique that generates an accurate and computationally efficient model of an oscillating surge WEC (OSWEC). Our goal is to model and predict the behavior of the OSWEC in monochromatic and polychromatic seas without knowledge of the governing equations or incident wave field. We generate the data for the algorithm using a semi-analytical model and the open-source code WEC-Sim, then evaluate how well DMD can describe past dynamics and predict future state behavior. We consider realistic challenges including noisy sensor measurements, nonlinear WEC dynamics, and irregular wave forcing. In each of these cases, we generate accurate models for past and future OSWEC behavior using DMD, even with limited sensor measurements. These findings provide insight into the use of DMD on systems with limited time-resolved data and present a framework for applying similar analysis to lab- or field-scale experiments

Survey of Army/NASA rotorcraft aeroelastic stability research

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability are considered. Results of parametric investigations of system behavior are presented, and correlations between theoretical results and experimental data from small- and large-scale wind tunnel and flight testing are discussed

Rotorcraft aeroelastic stability

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low-frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability in hover and forward flight, and analysis of tilt-rotor dynamic stability are considered. Results of parametric investigations of system behavior are presented, and correlation between theoretical results and experimental data from small and large scale wind tunnel and flight testing are discussed

Characterization And Application Of Isolated Attosecond Pulses

Tracking and controlling the dynamic evolution of matter under the influence of external fields is among the most fundamental goals of physics. In the microcosm, the motion of electrons follows the laws of quantum mechanics and evolves on the timescale set by the atomic unit of time, 24 attoseconds. While only a few time-dependent quantum mechanical systems can be solved theoretically, recent advances in the generation, characterization, and application of isolated attosecond pulses and few-cycle femtosecond lasers have given experimentalists the necessary tools for dynamic measurements on these systems. However, pioneering studies in attosecond science have so far been limited to the measurement of free electron dynamics, which can in most cases be described approximately using classical mechanics. Novel tools and techniques for studying bound states of matter are therefore desired to test the available theoretical models and to enrich our understanding of the quantum world on as-yet unprecedented timescales. In this work, attosecond transient absorption spectroscopy with ultrabroadband attosecond pulses is presented as a technique for direct measurement of electron dynamics in quantum systems, demonstrating for the first time that the attosecond transient absorption technique allows for state-resolved and simultaneous measurement of bound and continuum state dynamics. The helium atom is the primary target of the presented studies, owing to its accessibility to theoretical modeling with both ab initio simulations and to model systems with reduced dimensionality. In these studies, ultrafast dynamics – on timescales shorter than the laser cycle – are observed in prototypical quantum mechanical processes such as the AC Stark and ponderomotive energy level shifts, Rabi oscillations and electromagnetically-induced absorption iv and transparency, and two-color multi-photon absorption to “dark” states of the atom. These features are observed in both bound states and quasi-bound autoionizing states of the atom. Furthermore, dynamic interference oscillations, corresponding to quantum path interferences involving bound and free electronic states of the atom, are observed for the first time in an optical measurement. These first experiments demonstrate the applicability of attosecond transient absorption spectroscopy with ultrabroadband attosecond pulses to the study and control of electron dynamics in quantum mechanical systems with high fidelity and state selectivity. The technique is therefore ideally suited for the study of charge transfer and collective electron motion in more complex systems. The transient absorption studies on atomic bound states require ultrabroadband attosecond pulses − attosecond pulses with large spectral bandwidth compared to their central frequency. This is due to the fact that the bound states in which we are interested lie only 15-25 eV above the ground state, so the central frequency of the pulse should lie in this range. On the other hand, the bandwidth needed to generate an isolated 100 as pulse exceeds 18 eV – comparable to or even larger than the central frequency. However, current methods for characterizing attosecond pulses require that the attosecond pulse spectrum bandwidth is small compared to its central frequency, known as the central momentum approximation. We therefore explore the limits of attosecond pulse characterization using the current technology and propose a novel method for characterizing ultrabroadband attosecond pules, which we term PROOF (phase retrieval by omega oscillation filtering). We demonstrate the PROOF technique with both simulated and experimental data, culminating in the characterization of a world-record-breaking 67 as pulse

Process stability of a novel roughing-finishing end mill

In this paper, stability investigations of a novel roughing-finishing end mill are carried out. This tool possesses two sharp finishing teeth and two radially recessed, chamfered roughing teeth. By applying the same tool for roughing and finishing operations, tool changes and process time can be reduced. For the stability investigations, the semi-discretization method for calculating stability charts was extended and made applicable for the novel tool concept by taking into account the radial recession of the chamfered cutting teeth. This is necessary because the radial recession leads to varying time-delays during the tooth engagement. Stability charts were then calculated for roughing-finishing tools with different radial recession as well as for conventional finishing and roughing tools. Furthermore, experimental stability charts were created. The results show a good agreement between calculated and experimental stability charts for the finishing tool. However, the calculated stability limits of the roughing-finishing tool and the roughing tool do not met with the experimental stability limits, which is attributed to inaccuracies in the modelling of process damping. Nevertheless, calculated as well as experimental stability charts indicate a significant increase of the stability limit of the roughing-finishing tool compared to the finishing tool