The development of a control-oriented dynamic relative humidity model for a polymer electrolyte membrane (PEM) fuel cell stack is presented. This model is integrated with a first law based thermal model, which tracks energy flow within four defined control volumes in the fuel cell; the cathode channel, anode channel, coolant channel, and fuel cell stack body. Energy and mass conservation equations are developed for each control volume. On top of mass conservation, electro-drag and osmosis models were also implemented within the model to account for the major modes of vapor transfer through the membrane between the anode and cathode. Requisite alterations to the thermal model as well as mass flow rate calculations are also discussed. Initially, the model utilized a single lumped control volume for the calculation of all values each channel (anode and cathode). This lumped value method is computationally inexpensive, and makes the model optimal for control design. However, investigation of the mass-based Biot number showed the need for greater granularity along the length of the channels to properly capture the relative humidity dynamics. In order to improve the resolution of the model, while still minimizing the computation expense, the model was subdivided into a series of lumped value models. The cathode channel was the point of focus as it is the major concern from a controls perspective. This method captures the proper trends found in far more complex CFD models, while still maintaining a quick calculation time. Different levels are subdivision (3 and 6 submodels) are investigated, and the differences discussed. Particularly, temperature range, relative humidity range, the effect on the modeled voltage, and calculation time are compared. This control-oriented model is low order and based on lumped parameters, which makes the computational expense low. Formulation of this model enables the development of control algorithms to achieve optimal thermal and water management. / text

Design and development of future high speed aircraft require the use of advanced modeling tools early on in the design phase to study and analyze complex aeroelastic, thermoelastic, and aerothermal interactions. This phase, commonly referred to as the conceptual design phase, involves using first principle based analytical models to obtain a practical starting point for the preliminary and detailed design phases. These analytical models are expected to, firstly, capture the effect of complex interactions between various subsystems using basic physics, and secondly, minimize computational costs. The size of a typical air-breathing hypersonic vehicle can vary anywhere between 12 ft, like the NASA X-43A, to 100 ft, like the NASP demonstrator vehicle. On the other hand, the performance expectations can vary anywhere between cruising at Mach 5 @ 85; 000 ft to Mach 10 @ 110; 000 ft. Reduction of computational costs is essential to efficiently sort through such a vast design space, while capturing the various complex interactions between subsystems has shown to improve accuracy of the design estimates. This motivates the need to develop modelling tools using first principle based analytical models with "needed" fidelity, where fidelity refers to the extent of interactions captured. With the advent of multidisciplinary design optimization tools, the need for an integrated modelling and analysis environment for high speed aircraft has increased substantially over the past two decades. The ever growing increase in performance expectations has made the traditional design approach of optimize first, integrate later obsolete. Designing a closed-loop control system for an aircraft might prove to be a difficult task with a geometry that yields an optimal (L/D) ratio, a structure with optimal material properties, and a propulsion system with maximum thrust-weight ratio. With all the subsystems already optimized, there is very little freedom for control designers to achieve their high performance goals. Integrated design methodologies focus on optimizing the overall design, as opposed to individual subsystems. Control-oriented modelling is an approach that involves making appropriate assumptions while modelling various subsystems in order to facilitate the inclusion of control design during the conceptual design phase. Due to their high lift-to-drag ratio and low operational costs, air-breathing hypersonic vehicles have spurred some interest in the field of high speed aircraft design over the last few decades. Modeling aeroelastic effects for such an aircraft is challenging due to its tightly integrated airframe and propulsion system that leads to significant deflections in the thrust vector caused by flexing of the airframe under extreme aerodynamic and thermal loads. These changes in the orientation of the thrust vector in turn introduce low frequency oscillations in the flight path angle, which make control system design a challenging task. Inclusion of such effects in the vehicle dynamics model to develop accurate control laws is an important part of control-oriented modeling. The air-breathing hypersonic vehicle considered here is assumed to be a thin-walled structure, where deformations due to axial, bending, shear, and torsion are modeled using the six independent displacements of a rigid cross section. Free vibration mode shapes are computed accurately using a novel scheme that uses estimates of natural frequency from the Ritz method as initial guesses to solve the governing equations using SUPORE, a two-point boundary value problem solver. A variational approach involving Hamilton's principle of least action is employed to derive the second order nonlinear equations of motion for the flexible aircraft. These nonlinear equations of motion are then linearized about a given cruise condition, modal analysis carried out on the linearized system, and the coupling between various significant modes studied. Further, open-loop stability analysis in time domain is conducted. / Ph. D.

Establishing discipline in the contemporary classroom is a challenge to most educators. The real challenge lies in the implementation of discipline measures and procedures that uphold order in the classroom with understanding and compassion and more importantly, in the development of self-discipline in learners. The researcher adopted a qualitative approach to understand the phenomenon classroom discipline and to answer the research questions that sought to explore the meaning that is attached to the word “discipline” by individual educators, the challenges that educators are faced with in their classrooms, as well as the discipline strategies that they employ to establish discipline. A case study involving three high schools was conducted. Data was collected through interviews and observations. It has emerged in the findings that educators face a daily struggle in terms of establishing discipline in their classrooms; educators attach different meanings to the word “discipline” and the meaning that individual educators attach to “discipline” impacts on their choice of discipline strategies. Most of the discipline strategies employed by educators are control-oriented and thus hinge on rewards and punishment. The study also revealed that when these control-oriented strategies are employed to establish discipline, learners engage in various coping mechanisms, which ultimately render these strategies ineffective, and thus minimise any chance the child has to develop self-discipline. Essentially, learners who have been coerced usually show very little self-control when they are outside the influence of the controller. Recommendations based on findings and conclusions of this study are discussed and revolve mainly around the use of proactive discipline strategies that are geared to promote self-discipline and thus inner control. The recommendations outline proactive discipline strategies that could be employed by educators to establish discipline in their classrooms and suggest the creation of a good educator-learners relationship, the empowerment of learners to be in charge of their behaviour, responsibility training, inculcation of values, character development, modelling good behaviour, and strengthening of partnership with parents and other support structures in behaviour management. Benchmarking for best practices with other schools and conducting internal workshops for educators to share classroom discipline issues and solutions, as well as skills development programmes for training and development of educators are recommended. The study hopes to contribute to the existing body of knowledge and will be useful to educators by enabling them to find more constructive ways of building a culture of discipline among learners. It will also help educators develop personal systems of discipline tailored to their individual philosophies as well as to the needs and social realities of their schools and communities. / Dissertation (MEd (Education Management, Law and Policy Studies))--University of Pretoria, 2008. / Education Management and Policy Studies / unrestricted

No description available.

Complex helicopter models that include effects typically ignored in control models, such as an analytical formulation for fuselage aerodynamics, blade lead-lagging and flexibility, and tail rotor aerodynamics, are derived. The landing gear, horizontal tailplane, a fully articulated main rotor, main rotor downwash, and blade flapping are also modeled. The modeling process is motivated by the desire to build control oriented, physics based models that directly result in ordinary differential equations (ODE) models which are sufficiently rich in dynamics information. A physics based model simplification procedure, which is called new ordering scheme, is developed to reduce the number of terms in these large nonlinear ODE models, while retaining the same number of governing equations of motion. The resulting equations are trimmed and linearized around several flight conditions (i.e. straight level flight, level banked turn, and helical turn) using Maple and Matlab. The resulting trims and model modes are validated against available literature data. The linearized models are first used for the design of variance constrained controllers with inequality constraints on outputs or inputs, output variance constrained controllers (OVC) and input variance constrained controllers (IVC), respectively. The linearized helicopter models are also used for the design of online controllers which exploit the constrained model predictive control (MPC) theory. The ability of MPC to track highly constrained, heterogeneous discontinuous trajectories is examined. The performance and robustness of all these controllers (e.g. OVC, IVC, MPC) are thoroughly investigated with respect to several modeling uncertainties. Specifically, for robustness studies, variations in the flight conditions and helicopter inertial properties, as well as blade flexibility effects, are considered. Furthermore, the effectiveness of adaptive switching between controllers for the management of sensor failure during helicopter operations is studied using variance constrained controllers. Finally, the simultaneous design of the helicopter and control system is examined using simultaneous perturbation stochastic approximation in order to save active control energy. / Ph. D.

No description available.

No description available.

Tensegrity-membrane systems are a class of new bar-tendon-membrane systems. Such novel systems can be treated as extensions of tensegrity structures and are generally lightweight and deployable. These two major advantages enable tensegrity-membrane systems to become one of the most promising candidates for lightweight space structures and gossamer spacecraft. In this dissertation, modeling and control of tensegrity-membrane systems is studied. A systematic method is developed to determine the equilibrium conditions of general tensegrity-membrane systems. Equilibrium conditions can be simplified when the systems are in symmetric configurations. For one-stage symmetric systems, analytical equilibrium conditions can be determined. Three mathematical models are developed to study the dynamics of tensegrity-membrane systems. Two mathematical models are developed based on the nonlinear finite element method. The other model is a control-oriented model, which is suitable for control design. Numerical analysis is conducted using these three models to study the mechanical properties of tensegrity-membrane systems. Two control strategies are developed to regulate the deployment process of tensegrity-membrane systems. The first control strategy is to deploy the system by a nonlinear adaptive controller and use a linear H∞ controller for rapid system stabilization. The second control strategy is to regulate the dynamics of tensegrity-membrane systems using a linear parameter-varying (LPV) controller during system deployment. A gridding method is employed to discretize the system operational region in order to carry out the LPV control synthesis. / Ph. D.

A recent development in the design of control system for a jet engine is to use a suitable, fast and accurate model running on board. Development of linear models is particularly important as most engine control designs are based on linear control theory. Engine control performance can be significantly improved by increasing the accuracy of the developed model. Current state-of-the-art is to use piecewise linear models at selected equilibrium conditions for the development of set point controllers, followed by scheduling of resulting controller gains as a function of one or more of the system states. However, arriving at an effective gain scheduler that can accommodate fast transients covering a wide range of operating points can become quite complex and involved, thus resulting in a sacrifice on controller performance for its simplicity. This thesis presents a methodology for developing a control oriented analytical linear model of a jet engine at both equilibrium and off-equilibrium conditions. This scheme requires a nonlinear engine model to run onboard in real time. The off-equilibrium analytical linear model provides improved accuracy and flexibility over the commonly used piecewise linear models developed using numerical perturbations. Linear coefficients are obtained by evaluating, at current conditions, analytical expressions which result from differentiation of simplified nonlinear expressions. Residualization of the fast dynamics states are utilized since the fast dynamics are typically outside of the primary control bandwidth. Analytical expressions based on the physics of the aerothermodynamic processes of a gas turbine engine facilitate a systematic approach to the analysis and synthesis of model based controllers. In addition, the use of analytical expressions reduces the computational effort, enabling linearization in real time at both equilibrium and off-equilibrium conditions for a more accurate capture of system dynamics during aggressive transient maneuvers. The methodology is formulated and applied to a separate flow twin-spool turbofan engine model in the Numerical Propulsion System Simulation (NPSS) platform. The fidelity of linear model is examined by validating against a detailed nonlinear engine model using time domain response, the normalized additive uncertainty and the nu-gap metric. The effects of each simplifying assumptions, which are crucial to the linear model development, on the fidelity of the linear model are analyzed in detail. A case study is performed to investigate the case when the current state (including both slow and fast states) of the system is not readily available from the nonlinear simulation model. Also, a simple model based control is used to illustrate benefits of using the proposed modeling approach.

Orientador: Prof. Dr. Alfeu Joãozinho Sguarezi Filho / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2015. / Inicialmente, este trabalho apresenta aspectos de viabilidade e técnicos para a construção de sistemas de geração eólicos. Nele é disposto uma breve literatura de alguns dos conversores elétricos utilizados para diferentes tipos de geradores. Por fim, este trabalho propõe um método de ajuste de ganhos dos controladores proporcionais-integrais através do emprego do método por compensação de polos da função de transferência do sistema em malha fechada, para o conversor do lado do rotor, com objetivo de garantir controle estável do fluxo das potências ativa e reativa de estator do GIRB. Também, serão apresentadas as malhas de controle do conversor conectado à rede elétrica. Resultados obtidos através de simulação computacional deste sistema de controle são apresentados para validar a proposta. / This work includes feasibility and technical aspects for the construction of wind generation systems. In it, there is disposed a brief literature some electrical converters used for different types of generators. It also presents a design method for the gains of the proportional-integral controllers used in power vector control of DFIG connected to the grid in wind power generation systems where GIRB is connected directly to the power grid and its rotor through a converter called "back to back". The rotor controllers will be adjusted with the use of the method for compensating poles of the system's transfer function in closed loop. Also, the converter control loops connected to the grid will be displayed. Results from computer simulation of this control system are presented to validate the proposal.