Browsing by Subject "Closed-loop"
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Item Closed-loop control of shock location to prevent hypersonic inlet unstart(2014-08) Ashley, Jonathan Michael; Akella, Maruthi Ram, 1972-Hypersonic inlet unstart remains a major technical obstacle in the successful implementation of hypersonic air-breathing propulsion systems such as ramjets and scramjets. Unstart occurs when combustor-induced pressure fluctuations lead to rapid expulsion of the shock system from the isolator, and is associated with loss of thrust. The research presented here attempts to mitigate this behavior through the design and implementation of a closed-loop control scheme that regulates shock location within a Mach 1.8 wind tunnel isolator test section. To localize the position of the shock within the isolator, a set of high frequency Kulite pressure transducers are used to measure the static pressure at various points along the wind tunnel test section. A novel Kalman filter based approach is utilized, which fuses the estimates from two distinct shock localization algorithms running at 250 Hz to determine the location of the shock in real time. The primary shock localization algorithm is a geometrical shock detection scheme that can estimate the position of the shock system even when it is located between pressure transducers. The second algorithm utilizes a sum-of-pressures technique that can be calibrated by the geometrical algorithm in real time. The closed-loop controller generates commands every 100 ms to actuate a motorized flap downstream of the test section in an effort to regulate the shock to the desired location. The closed-loop control implementation utilized a simple logic-based controller as well as a Proportional-Integral (PI) and a Proportional-Derivative (PD) Controller. In addition to the implementation of control algorithms, the importance of various design criteria necessary to achieve satisfactory control performance is explored including parameters such as pressure transducer spacing, shock localization speed, flap-motor actuation speed and actuator resolution. Experimental results are presented for various test scenarios such as regulation of the shock location in the presence of stagnation pressure disturbances as well as tracking of time-varying step inputs. Performance and robustness properties of the tested control implementations are discussed. Further areas of improvement for the closed-loop control system in both hardware and software are discussed, and the need for reduced-order dynamics-based controllers is presented.Item A critical evaluation of modern low-thrust, feedback-driven spacecraft control laws(2012-12) Hatten, Noble Ariel; Ocampo, Cesar; Akella, MaruthiLow-thrust spacecraft trajectory optimization is often a difficult and time-consuming process. One alternative is to instead use a closed-loop, feedback-driven control law, which calculates the control using knowledge of only the current state and target state, and does not require the solution of a nonlinear optimization problem or system of nonlinear equations. Though generally suboptimal, such control laws are attractive because of the ease and speed with which they may be implemented and used to calculate feasible low-thrust maneuvers. This thesis presents the theoretical foundations for seven modern low-thrust control laws based on control law "blending" and Lyapunov control theory for a particle spacecraft operating in an inverse-square gravitational field. The control laws are evaluated critically to determine those that present the best combinations of thoroughness of method and minimization of user input required. The three control laws judged to exhibit the most favorable characteristics are then compared quantitatively through three numerical simulations. The simulations demonstrate the effectiveness of feedback-driven control laws, but also reveal several situations in which the control laws may perform poorly or break down altogether due to either theoretical shortcomings or numerical difficulties. The causes and effects of these issues are explained, and methods of handling them are proposed, implemented, and evaluated. Various opportunities for further work in the area are also described.Item Stimulus-responsive delivery systems for enabling the oral delivery of protein therapeutics exhibiting high isoelectric point(2015-05) Koetting, Michael Clinton; Peppas, Nicholas A., 1948-; Contreras, Lydia M; Ellison, Christopher J; Stachowiak, Jeanne C; Truskett, Thomas MProtein therapeutics offer numerous advantages over small molecule drugs and are rapidly becoming one of the most prominent classes of therapeutics. Unfortunately, they are delivered almost exclusively by injection due to biological obstacles preventing high bioavailability via the oral route. In this work, numerous approaches to overcoming these barriers are explored. PH-Responsive poly(itaconic acid-co-N-vinylpyrrolidone) (P(IA-co-NVP)) hydrogels were synthesized, and the effects of monomer ratios, crosslinking density, microparticle size, protein size, and loading conditions were systematically evaluated using in vitro tests. P(IA-co-NVP) hydrogels demonstrated up to 69% greater equilibrium swelling at neutral conditions than previously-studied poly(methacrylic acid-co-N-vinylpyrrolidone) hydrogels and a 10-fold improvement in time-sensitive swelling experiments. Furthermore, P(IA-co-NVP) hydrogel microparticles demonstrated up to a 2.7-fold improvement in delivery of salmon calcitonin (sCT) compared to methacrylic acid-based systems, with a formulation comprised of a 1:2 ratio of itaconic acid to N-vinylpyrrolidone demonstrating the greatest delivery capability. Vast improvement in delivery capability was achieved using reduced ionic strength conditions during drug loading. Use of a 1.50 mM PBS buffer during loading yielded an 83-fold improvement in delivery of sCT compared to a standard 150 mM buffer. With this improvement, a daily dose of sCT could be provided using P(IA-co-NVP) microparticles in one standard-sized gel capsule. P(IA-co-NVP) was also tested with larger proteins urokinase and Rituxan. Crosslinking density provided a facile method for tuning hydrogels to accommodate a wide range of protein sizes. The effects of protein PEGylation were also explored. PEGylated sCT displayed lower release from P(IA-co-NVP) microparticles, but displayed increased apparent permeability across a Caco-2 monolayer by two orders of magnitude. Therefore, PEG-containing systems could yield high bioavailability of orally delivered proteins. Finally, a modified SELEX protocol for cellular selection of transcellular transport-initiating aptamers was developed and used to identify aptamer sequences showing enhanced intestinal perfusion. Over three selection cycles, the selected aptamer library showed significant increases in absorption, and from an initial library of 1.1 trillion sequences, 5-10 sequences were selected that demonstrated up to 10-fold amplification compared to the naïve library. These sequences could provide a means of overcoming the significant final barrier of intestinal absorption.