Systematic investigation of the Friction Stir Welding (FSW) process shows that a fixed rotational velocity and feed rate may not yield uniform mechanical properties along the length of a weldment. Nevertheless, correlations between process parameters and post-weld material properties have successfully demonstrated that peak temperature and cooling rate drive post-weld properties. There have been many reported methodologies for controlling friction stir welding, with varying degrees of cost to implement and effectiveness. However, comparing data from uncontrolled FSW of AA 6111-T4 sheet with controlled FSW at temperatures ranging from 375 °C to 450 °C demonstrates that a simplified methodology of a single-loop PID controlling with spindle speed may be used to effectively control temperature. This methodology can be simply used with any machine that already has the ability to actively control spindle speed, and has been previously shown to be able to be auto-tuned with a single weld. Additionally, implementation of this method compared to uncontrolled FSW in AA6111 at linear weld speeds of 1-2 meters per minute showed improved mechanical properties and greater consistency in properties along the length of the weld under temperature control. Further results indicate that a minimum spindle rpm may exist above which tensile specimens did not fracture within the weld centerline, regardless of temperature. This work demonstrates that a straight-forward, PID-based implementation of temperature control at high weld rates can produce high quality welds with auto-tuned gains. This method also shows promise in application to other processes in the Friction Stir family, and preliminary results in an application to the Additive Friction Stir Deposition (AFSD) process are also presented.

Temperature control in friction stir welding (FSW) is of interest because of the potential to improve the mechanical and microstructure characteristics of a weld. Two types of active temperature control have been previously implemented for steady-state friction stir welding conditions: PID Feedback Control and Model Predictive Control. The start-up portion of a weld is an obstacle for these types of active control.To date, only minimal exploratory research has been done to develop an active temperature controller for the start-up portion of the weld. The FSW temperature controller presented in this thesis, a Position-Velocity-Acceleration (PVA) controller implemented with gain-scheduling, is capable of active control during the start-up portion of a weld. The objectives of the controller are (1) to facilitate fully-automated active temperature control during the entire welding process, (2) to minimize the rise time, the settling time, the percentage maximum post-rise error (overshoot calculated as a percentage of the settling band half-width), and the post-settled root-mean-square (RMS) of the temperature error, and (3) to maintain the steady state performance of previous control methods.For welds performed in 6.35 mm plates of 7075-T651 Aluminum with controller gains identified through a manual tuning process, the mean controller performance is a rise time of 10.82 seconds, a settling time of 11.35 seconds, a percentage maximum post-rise error of 69.86% (as a percentage of the 3◦C settling band half-width), and a post-settled RMS error of 0.92◦C.Tuning of the start-up controller for operator-specified behavior can be guided through construction of regression models of the weld settling time, rise time, percent maximum post-rise error, and post-settled RMS error. Characterization of the tuning design space is performed through regression modeling. The effects of the primary controller tuning parameters and their interactions are included. With the exception of the post-settled RMS error model, these models are inadequate to provide useful guidance of the controller tuning, as significant curvature is present in the design space. Exploration of higher-order models is performed and suggests that regression models including quadratic terms can adequately characterize the design space to guide controller tuning for operator-specified behavior.

New heating techniques are required to better control heat transfer between heating tools and a composite towpreg in the automated fiber placement process. This dissertation suggests new heating techniques with liquid and rigid contact heat sources, and compares them with a widely used gas heat source for the fiber placement process. A thin towpreg composite model needs to be developed to describe the heat transfer. Subsequently, the response of the towpreg with each heat source was compared from manufacturing speed and energy efficiency viewpoints. The most promising heat source was developed for heat transfer modeling between a moving towpreg and dynamic heat source in the automated fiber placement. Through the heat transfer model, both the temperature controllability of the towpreg and manufacturing speed could be investigated. In addition, an accurate compaction process is needed in response to the growing demand for better composite processing. Since the errors in compaction mechanisms and robotic machinery in fiber placement have not been discussed in the literature, experimental investigation to address possible reasons for the variations in the compaction force was conducted with a compaction mechanism. A clearer understanding of the physical compaction process can lead to controllable process parameters for consistent ply compaction, such that the final parts quality can be enhanced. Even though this dissertation investigates the thin thermoset fiber placement process, the proposed approach could be universally applicable to other composite-fabrication processes. / Ph. D.

A new laser based flow interrogation system, capable of simultaneous measurement of planar three-component velocity data, was constructed and tested. The Virginia Tech Doppler Global Velocimeter (DGV) system was designed for use in the Virginia Tech Stability Wind Tunnel as a tool for investigating complex three-dimensional separated flow regions. The systems was designed for robustness, ease of use, and for acquisition of low uncertainty velocity data. A series of tests in the Stability Tunnel were conducted to determine the how well the new DGV system met these goals. Extensive calibration tests proved the system is capable of measuring the frequency shifts of scattered laser light, and therefore velocity. However, equipment failures and inadequate flow seed density prevented accurate velocity measurements in the separated wake region behind a 6:1 prolate spheroid. Detailed uncertainty analysis techniques demonstrated that, under the proper conditions, the system is capable of making velocity measurements with approximately +/- 2m/s uncertainty. / Master of Science

<p> The work in this thesis focuses on the temperature control of a semi-batch polymerization reactor. The system is published by Chylla and Haase (1993) as an Industrial Challenge and is typical of reactors at S.C. Johnson Wax. The challenge is to find a single controller that can adequately regulate reactor temperature despite changing process conditions. The multi-product nature of the system makes it a particularly interesting problem. Several different controllers are implemented and evaluated in this thesis. The controllers are in part chosen to quantify the amount process information (large or small) required in a controller structure in order to achieve satisfactory control. Two of the most promising controllers are a PID with feedforward compensation and a Nonlinear Adaptive algorithm. It is found that in many cases, there may be little incentive to go to a complex model based controller as the simpler feedback algorithm provides adequate control. However, the nonlinear adaptive controller is more easily extended to multi-batch or multi-product situations because of its more general nature. The PID with feedforward compensation requires retuning for each new situation in order to maintain satisfactory control. </p> / Thesis / Master of Engineering (MEngr)

Due to the rapid and prosperous development of information technology, data centers are widely used in every aspect of social life, such as industry, economy or even our daily life. This work considers the idea of developing a data-driven model based model predictive control (MPC) to regulate temperature for a class of single-rack data centers (DCs). An auto-regressive exogenous (ARX) model is identified for our DC system using partial least square (PLS) to predict the behavior of multi-inputs-single-output (MISO) thermal system. Then an MPC controller is designed to control the temperature inside IT rack based on the identified ARX model. Moreover, fuzzy c-means (FCM) is employed to cluster the measured data set. Based on the clustered data sets, PLS is adopted to identify multiple locally linear ARX models which will be combined by appropriate weights in order to capture the nonlinear behavior of the highly-nonlinear thermal system inside the IT rack. The effectiveness of the proposed method is illustrated through experiments on our single-rack DC and it is also compared with proportional-integral (PI) control. / Thesis / Master of Applied Science (MASc)

Transient modelling of heat fluxes and temperatures in structures was conducted to examine the effect of various characteristics on the temperature response during unusual operating and extreme weather conditions. The analytical model was validated using published experimental data and numerical results from well-known computer codes. The effect of including radiation heat transfer between interior surfaces, using the Mean Radiative Temperature method, on the temperature response was investigated and found to be negligible for a typical commercial building and a house during winter and summer power outages. The effect of thermal mass in the interior and exterior walls on the inside temperature drift after an HVAC system cutoff or a power outage was presented. The inside air temperature response curve is presented for different wall (exterior or interior) constructions of buildings. The effect of insulation position in exterior walls was also shown for several R values. The effect of exterior wall emissivity, sky temperature, outside vertical convective coefficient, furnishings, and ground temperature on the interior temperature response during winter and summer power outages were examined for buildings. The effect of infiltration on the temperature drift in buildings was investigated during winter and summer power outages. Restarting the HVAC after the power outage was examined during both seasons for typical buildings. Outside temperature profiles exceeding the 97.5 design temperature criterion were used to study the effect of extreme weather on the interior temperature of buildings with the HVAC system operating. / Ph. D.

A linear model of an annealing furnace is developed using a black-box system identification approach, and used when testing three different control strategies to improve temperature control. The purpose of the investigation was to see if it was possible to improve the temperature control while at the same time  decrease the switching frequency of the  burners. This will lead to a more efficient process as well as less maintenance, which has both economic and environmental benefits. The estimated model has been used to simulate the furnace with both the existing controller and possible new controllers such as a split range controller and a model predictive controller (MPC). A split range controller is a control strategy which can be used when more than one control signal affect the output signal, and the control signals have different range. The main advantage with MPC is that it can take limitations and constraints into account for the controlled process, and with the use of integer programming, explicitly account for the discrete switching behavior of the burners. In simulation both new controllers succeed in decreasing the switching and the MPC also improved the temperature control. This suggest that the control of the furnace can be improved by implementing one of the evaluated controllers.

The main objectives of this dissertation is to investigate the prospects of embedded thermoelectric devices integrated in a chip package and to develop a design methodology aimed at taking advantage of the on-chip on-demand cooling capabilities of the thermoelectric devices. First a simulation framework is established and validated against experimental results, which helps to study the cooling capabilities of embedded thermoelectric coolers (TEC) in both a transient and steady state. The potential for up to 15°C of total cooling has been shown. The thermal simulation framework allows for rapid assessment of TEC and system level thermal performance. Next, the thesis develops a co-simulation environment that is capable of simulating the thermal and electrical domain and couples them to design intelligent TEC controllers. These controllers are implemented on chip and can leverage the transient cooling capability of the device. The controllers are simulated within the co-simulation environment and their potential to control high power chip events are thoroughly investigated. The system level overheads are considered and discussions on implementation techniques are presented. The co-simulation framework is also extended to allow for simulation of real predictive technology microprocessor cores and their workloads. Finally the thesis implements a fully on-chip autonomous energy system that takes advantage of the TEC in its reverse energy harvesting mode and uses the same device to harvest energy and use the energy to power the on-chip cooling circuit. This increases the overall energy efficiency of the cooler and verifies the TEC control methods.

Les ultrasons focalisés de haute intensité (HIFU) guidés par IRM et combinés à la thermométrie basée sur la fréquence de résonance du proton (PRF) sont une technique prometteuse pour l’ablation non invasive de tumeurs, le dépôt local de médicaments et l’activation des transgènes. Ce travail présente de nouveaux développements dans le domaine de la thermométrie PRF en présence de mouvement physiologique périodique associé aux variations du champ magnétique. De nouvelles stratégies de correction sont proposées et exploitent la méthode multi-baseline établie en incluant un modèle de variation de phase. Elles sont illustrées avec des exemples de thermométrie dans le sein et dans le cœur humain. De plus, d’autres facteurs influençant la thermométrie PRF, notamment la présence de graisse dans le sein et le flux sanguin dans le cœur, sont étudiés. Dans la seconde partie de ce travail a été abordée la problématique du contrôle précis de la température. Une première approche propose un algorithme de contrôle proportionel, intégral et dérivatif (PID) amélioré utilisant des paramétres de contrôle adaptatifs. En étendant ce concept à un contrôle 3D de la température, une implémentation de chauffage volumétrique est proposée. Par ailleurs, une nouvelle méthode de repositionnement dynamique de la coupe d’imagerie permet de fournir des informations volumétriques sur l’anatomie et la température en temps réel. La combinaison avec la compensation 2D de mouvement et l’adaptation du faisceau ultrasonore permet la réalisation d’un chauffage volumétrique suivant une courbe de température ou de dose thermique prédéfinie qui fonctionne même en présence de mouvements. / MR-guided high-intensity focused ultrasound (HIFU) using proton resonance frequency (PRF) based thermometry is a promising technique for non-invasive ablations in tumor therapy as well as for targeted drug delivery and the activation of transgenes. This work presents further developments in the field of PRF thermometry in the presence of periodical physiological motion and the associated magnetic field variations. Using the examples of thermometry in the human breast and the human heart, new correction strategies are presented which extend the established multi-baseline phase correction to include a model of the phase variation and external sensor readings from a pencil-beam navigator. In addition further factors, namely the presence of fat in the breast and blood flow in the heart influencing the performance of MR thermometry in these organs are examined.In the second part of this work, the issue of precise temperature control has been approached in two ways. First, an improved proportional, integral and derivative (PID) controller using adaptive control parameters is developed. By expanding the concept of temperature control to 3D, an implementation of volumetric heating is presented. A novel slice sweep technique provides volumetric anatomic and temperature information in near-real time. The combination with 2D motion compensation and adaptation of the ultrasound beam position allows to achieve volumetric heating according to a pre-defined target temperature or thermal dose value even in the presence of motion.