• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 8
  • 3
  • 1
  • Tagged with
  • 17
  • 7
  • 6
  • 6
  • 5
  • 4
  • 4
  • 4
  • 4
  • 4
  • 4
  • 3
  • 3
  • 3
  • 3
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Control algorithms and flight software framework for a spacecraft guidance navigation and control system

Zhang, Jing 10 February 2012 (has links)
This thesis presents a comparison of controller designs and a system software design for a general Guidance, Navigation and Control (GNC) system. The first part of the thesis investigates four control algorithms based on Lyapunov Direct Method in conjunction with sliding mode and adaptive control. These algorithms address three practical issues in controller design: maximum actuation limitation, external disturbances, and imperfect dynamic models. Each of the algorithms is proven to be globally asymptotically stable within its constraints. A simulation is then used to model a cube-satellite attitude maneuver using each of the controllers to evaluate its performance. The second part of this thesis discusses the development of a high-level flight software architecture capable of handling common tasks, including ground station communication and attitude maneuvers, as well as power or device failures. / text
2

Analysis and order reduction of an autonomous lunar lander navigation system

Newman, Clark Patrick 18 July 2012 (has links)
A navigation system for precision lunar descent and landing is presented and analyzed. The navigation algorithm is based upon the extended Kalman Filter and employs measurements from an inertial measurement unit to propagate the vehicle position, velocity, and attitude forward in time. External measurements from an altimeter, star camera, terrain camera, and velocimeter are utilized in state estimate updates. The navigation algorithm also attempts to estimate the values of uncertain parameters associated with the sensors. The navigation algorithm also estimates the map-tie angle of the landing site which is a measure of the misalignment of the actual landing site location on the surface of the Moon versus the estimated position of the landing site. The navigation algorithm is subject to a sensitivity analysis which investigates the contribution of each error source to the total estimation performance of the navigation system. Per the results of the sensitivity analysis, it is found that certain error sources need not be actively estimated to achieve similar estimation performance at a reduced computational burden. A new, reduced-order system is presented and tested through covariance analysis and a monte carlo analysis. The new system is shown to have comparable estimation performance at a fraction of the computer run-time, making it more suitable for a real-time implementation. / text
3

Modeling and Simulation of Vehicle Performance in a UAV Swarm Using Horizon Simulation Framework

Frye, Adam J. 01 October 2018 (has links)
A UAV swarm is simulated using Horizon Simulation Framework. The asset utilized for the swarm agent is a simplified model of the MQ-1 Predator, a large fixed-wing aircraft. The simulated swarm utilizes a decentralized cooperative control approach to command the assets through the use of digital pheromones and a pheromone map. Each vehicle operates at steady-state flight conditions of 36 m/s with an altitude of 1,800 m, and utilize an LQR set-point controller to maneuver through the pheromone map. All pheromone and aircraft related models are written in Python to expand the HSF scripting capability and include airborne scenarios. The simulation study focuses in the variation of three parameters in the repelling pheromone model. The first two are the update and deposit parameters with values of 2, 10, and 18. The third is the threshold parameter with values of 1e-02, 1e-10, and 1e-18. The lower parameter values provide more time-on-target while the higher parameters allow the swarm to search the surrounding area by only visiting the grid-space once.
4

Gender Nonconforming, Transgender, and Transsexual Patient Navigation of the American Health Care System: Locating a Primary Care Provider

Vinneau, Justin M 01 January 2016 (has links)
This study explores the experiences of gender nonconforming (GNC), transgender, and transsexual (trans) people in the search for a primary care provider in the United States. The current body of literature on transgender health often discusses HIV rates, substance use, mental health/suicide, and few studies have studies primary care seeking behaviors; this study seeks to provide new insight into the primary care (pcp) seeking behaviors of GNC and trans people. The primary theoretical perspectives utilized in this study were West and Zimmerman’s (1987) “Doing Gender” and the Health Beliefs Mode (Ayers et al., 2007; Connor and Norman, 2005; Green and Murphy, 2014). In order to explore the pcp seeking behaviors among GNC and trans individuals, I designed a 45 item survey. The survey was posted on-line on three separate “sub-reddits” between March and April of 2016 and was open to all individuals who self-identified as gender nonconforming, transgender, and/or transsexual. Of 96 responses, 68 were included. Although the sample is small, the results showed that structural barriers were significantly associated with having a PCP. Those with health insurance (p=.031) and those with at least one chronic illness (p=.037) were more likely to have a regular primary care provider. Descriptive findings support the role of socio-economic factors, geographic location, and past experiences of discrimination as predictors of primary care status.
5

A Deep Learning Approach to Autonomous Relative Terrain Navigation

Campbell, Tanner, Campbell, Tanner January 2017 (has links)
Autonomous relative terrain navigation is a problem at the forefront of many space missions involving close proximity operations to any target body. With no definitive answer, there are many techniques to help cope with this issue using both passive and active sensors, but almost all require high fidelity models of the associated dynamics in the environment. Convolutional Neural Networks (CNNs) trained with images rendered from a digital terrain map (DTM) of the body’s surface can provide a way to side-step the issue of unknown or complex dynamics while still providing reliable autonomous navigation. This is achieved by directly mapping an image to a relative position to the target body. The portability of trained CNNs allows “offline” training that can yield a matured network capable of being loaded onto a spacecraft for real-time position acquisition. In this thesis the lunar surface is used as the proving ground for this optical navigation technique, but the methods used are not unique to the Moon, and are applicable in general.
6

Design and Evaluation of a Photovoltaic Inverter with Grid-Tracking and Grid-Forming Controls

Rye, Rebecca Pilar 20 March 2020 (has links)
This thesis applies the concept of a virtual-synchronous-machine- (VSM-) based control to a conventional 250-kW utility-scale photovoltaic (PV) inverter. VSM is a recently-developed control scheme which offers an alternative grid-synchronization method to the conventional grid-tracking control scheme, which is based on the dq phase-locked-loop- (PLL-) oriented vector control. Synchronous machines inherently synchronize to the grid and largely partake in the stabilization of the grid frequency during power system dynamics. The purpose of this thesis is primarily to present the design of a grid-forming control scheme based on the VSM and the derivation of the terminal dq-frame ac impedance of the small-signal model of the inverter and control scheme. This design is also compared to the design of the conventional grid-tracking control structure, both from a loop design and terminal dq-frame ac impedance standpoint. Due to the inherent lax power-balance synchronization, the grid-forming control scheme results in 1 to 2 decades' lower frequency range of negative incremental input impedance in the diagonal elements, which is a favorable condition for stability. Additionally, the stability of the grid-forming control scheme is compared to the conventional grid-tracking control using the generalized Nyquist criterion (GNC) for stability under three modes of operation of active and reactive power injection. It is found that the connection is stable for both control schemes under unity power factor and fixed reactive power modes; however, the grid-forming control is able to inject twice the amount of active power under the voltage regulation mode when compared to the grid-tracking control. / Master of Science / Concerns about the current and future state of the environment has prompted government and non-profit agencies to enact regulatory legislation on fossil fuel emissions. In 2017, electricity generation comprised 28% of total U.S. greenhouse gas emissions with 68% of this generation being due to coal combustion sources. As a result, utilities have retired a number of coal power plants and have employed alternative means of power generation, specifically renewable energy sources (RES). Most RES operate as variable-frequency ac sources (wind) or dc sources (solar) and are interfaced with the power grid through ac-dc-ac or dc-ac converters, respectively, which are power-electronic devices used to control the injection of power to the grid. Conventional converters synchronize with the grid by tracking the phase of the voltage at the point of common coupling (PCC) through a phase-locked loop (PLL). While power system dynamics significantly affect the performance of a PLL, and, subsequently, inverters' operation, the initial frequency regulation during grid events is attributed to the system's inherent inertia due to the multitude of synchronous machines (SM). However, with the steady increase of RES penetration, even while retaining the number of SM units, the net inertia in the system will decrease, thus resulting in prolonged responses in frequency regulation to the aforementioned dynamics. This thesis investigates the control of variable-frequency sources as conventional synchronous machines and provides a detailed design procedure of this control structure for photovoltaic (PV) inverter applications. Additionally, the stability of the connection of the inverter to the grid is analyzed using innovative stability analysis techniques which treat the inverter and control as a black box. In this manner, the inner-workings of the inverter need not be known, especially since it is proprietary information of the manufacturer, and the operator can measure the output response of the device to some input signal. In this work, it is found that the connection between the inverter and grid is stable with this new control scheme and comparable to conventional control structures. Additionally, the control based on synchronous machine characteristics shows improved stability for voltage and frequency regulation, which is key to maintaining a stable grid.
7

D-Q Frame Impedance Based Small-signal Stability Analysis of PV Inverters in Distribution Grids

Tang, Ye 18 January 2021 (has links)
With development of renewable energies worldwide, power system is seeing higher penetration of Utility-scale photovoltaic (PV) farms at distributed level as well as transmission level. Power electronics converters present negative incremental impedance characteristics at their input while under regulated output control, which brings in the possibility of system instability. Recent evidence suggests that large-scale penetration of PV inverters increases the probability of instability. While IEEE standard 1547 newest version requires PV inverters to have reactive power control, there have been few investigations into the small-signal stability impact of PV inverters on distribution systems especially with reactive power control. In addition, the existing studies either use the conventional way of state space equations and eigenvalues or use time-domain simulation methodology, which are based on the assumptions that detailed models of the grid and the PV inverters are accessible. Different from the previous literatures, this research employs Generalized Nyquist Criterion (GNC) method based on measured impedances in d-q frames at connection interfaces. GNC method has the advantage that interconnection stability can be judged without knowing the grid and PV generator model details. This work first demonstrates the advantage of volt-var droop mode control among all different local reactive power control modes for PV inverters in the aspects of static impact on grid voltage profiles and power loss in a 12kV test-bed distribution system. Then it is discovered that d-q frame impedance of PV inverter under volt-var droop mode control shows a significant difference from other reactive power control modes. The d-q frame impedance derived from the small-signal model of PV generator is validated by both MATLAB simulation results and hardware experiments. Based on the d-q frame impedances, GNC is utilized to analyze the stability connection of a single PV farm and multiple PVs into the grid. GNC stability assessment results match with time-domain simulations and reveal the stability problem related to volt-var droop mode control. Furthermore, considering the unbalance of the distribution system, a new impedance model in d-q frame is proposed to capture both the dynamics of PV inverter operating in unbalanced points and the dynamics of three-phase unbalanced grid. The new impedance model is a combination of positive-negative sequence impedance and conventional d-q frame impedance. A procedure is designed for the measurement of the extended d-q frame impedance and the GNC application to predict small-signal stability of the unbalanced grid, which are justified by both time domain simulation and hardware experiments. / Doctor of Philosophy / To overcome the limited fossil fuel reserve on the earth and global warming, renewable energies become more and more popular worldwide. Centralized thermal power generators in the transmission system are gradually being replaced by distributed energy resources (DER) which are connection to the distribution system, bringing more challenges to the safe and stable operation of the power system. This work focuses on the small-signal stability impact of photovoltaic (PV) generators in the distribution system, which basically analyzes into whether the connection of PV generator to the distribution system will end up in an expected steady operation state with high resistance to any relatively small disturbances. The stability assessment tool is based on impedance measurement which treats both sides as black boxes and bridges the information gap between Utility operators and PV generator vendors. A major finding of this work is that while PV generators in the distribution system help to provide grid-support functions of voltage regulation, they may cause voltage small-signal stability problems due to the high grid impedance, which is worse if more PV inverters are put in parallel. Even PV farms connected to different branches of the complicated radial distribution system may have interactions with each other. So the design of control strategy and parameters of PV generator should consider the impact of other PV generators. GNC method based on impedances measurement is feasible and accurate for stability assessment of a distribution system with multiple PV farms. The impedance based method is upgraded and extended to be applied for the connection of power electronics devices to the three-phase unbalanced distribution systems.
8

Guidance strategies for the boosted landing of reusable launch vehicles / Strategier för motor-reglerad landning av återanvändbara bärraketer

Carpentier, Agathe January 2019 (has links)
This document presents the results of the master thesis conducted from April 2019 to October 2019 under the direction of CNES engineer Eric Bourgeois, as part of the KTH Master of Science in Aerospace Engineering curriculum. Within the framework of development studies for the Callisto demonstrator, this master thesis aims at studying and developing possible guidance strategies for the boosted landing. Two main approaches are described in this document : • Adaptive pseudo-spectral interpolation • Convex optimization The satisfying results yielded give strong arguments for choosing the latter as part of the Callisto GNC systems and describe possible implementation strategies as well as complementary analyses that could be conducted. / Denna rapport presenterar resultaten av ett examensarbete som genomfördes från april till oktober 2019 under ledning av CNES-ingenjören Eric Bourgeois, som en del av en masterexamen i flyg- och rymdteknik från KTH, Kungliga tekniska högskolan. Inom ramen för utvecklingsstudier för bärraketen Callisto syftar detta arbete att studera och utveckla möjliga reglerstrategier för Callistos landing som kontrolleras med raketer. Två huvudsakliga metoder beskrivs: • Adaptiv pseudospektral interpolering • Konvex optimering Resultaten ger starka argument för att välja den senare av dessa två metoder för Callistos reglersystem och beskriver möjliga implementeringsstrategier samt vilka kompletterande analyser som bör genomföras
9

Avaliação global dos modos energéticos de transporte do gás natural inclusive como energia secundária / Global Evaluation of Energy Ways of Natural Gás Transport, as Well as Secondary Energy

Rigolin, Pascoal Henrique da Costa 15 June 2007 (has links)
O objetivo do trabalho é fazer a avaliação global de alguns dos modos energéticos de transporte do gás natural (gasodutos, GNL e GNC), inclusive como energia secundária (eletricidade e GTL). Para a escolha do melhor modo de transporte, além dos custos de cada projeto, serão considerados outros três fatores, que são: impactos ambientais, impactos sociais e riscos políticos. Para isto foi escolhida uma ferramenta de análise conhecida como Avaliação de Custos Completos (ACC), sendo que esta considera os quatro fatores citados anteriormente como de mesma importância na avaliação para um planejamento que vise a sustentabilidade. Para melhor visualização e entendimento do trabalho, foi feito um estudo de caso para o transporte de energia (gás natural e/ou eletricidade) para atender mercados consumidores na região de fronteira entre o Brasil e a Bolívia. Fundamentalmente o estudo do transporte foi feito em solo boliviano, correspondente entre os campos produtores de gás, localizados em Margarita, até a região de fronteira próxima a Corumbá no Mato Grosso do Sul (MS), e também próxima a uma região com grandes jazidas de minério de ferro conhecida como El Mutún. Metodologicamente dentro das quatro dimensões consideradas, foram usados alguns subcritérios mais relevantes para esta análise. Para todos os sub-critérios foram atribuídas notas específicas, de acordo com a região e com o tipo de transporte de energia, e após isto os dados foram passados para um software de tomada de decisão chamado Decision Lens, obtendo assim um rank das melhores opções transporte para a região. Como resultado, o mais coerente para atender a região, são gasodutos de transporte para as demandas elétricas e de gás natural (consumo do MS mais Mutún) e trens carregados com derivados líquidos do gás natural (GTL) para atender a demanda de diesel do MS. / The objective of this work is to make a global evaluation of some energy ways of transport of natural gas (gas-pipelines, LNG and CNG), as well as secondary energy (electricity and GTL). For the selection of a better way of transport, beyond the costs of each project, others three factors were considered: environmental impacts, social impacts and political risks. To perform this, a tool of analysis known as Evaluation of Complete Costs (ACC in portuguese) was chosen. This tool considers the four factors above mentioned as having the same importance in the evaluation process for planning that aims towards sustainability. For better visualization and understanding of the work, a case study for energy transport was made (natural gas and/or electricity) to supply the consumer markets in the region bordering Brazil and Bolivia. Basically the study of the transport was made in Bolivian territory; at the producing gas fields located in Margarita as far as the region bordering Corumbá in the Mato Grosso do Sul area, and also next to a region with great known iron ore deposits known as El Mutún. Methodologically, within the four dimensions above, some more relevant sub-criteria for the analysis were used. For all the sub-criteria considered specific ranks were taken, in accordance with the region and type of transported energy, and after this the data was fed to a software called Decision Lens, thus getting a rank of the best options to carry gas to the region. The results, most coherent way to supply gas to the region, happens to be gas-pipelines of transport for the electric and natural gas demands (consumption in MS plus Mutún) and trains with liquid derivatives of natural gas (GTL) to supply care of the demand of diesel of MS.
10

Guidance and Navigation Linear Covariance Analysis for Lunar Powered Descent

Moesser, Travis J. 01 May 2010 (has links)
A linear covariance analysis is conducted to assess closed-loop guidance, navigation, and control system (GN&C) performance of the Altair vehicle during lunar powered descent. Guidance algorithms designed for lunar landing are presented and incorporated into the closed-loop covariance equations. Navigation-based event triggering is also included in the covariance formulation to trigger maneuvers and control dispersions. Several navigation and guidance trade studies are presented demonstrating the influence of triggering and guidance and study parameters on the vehicle GN&C performance.

Page generated in 0.1166 seconds