Feedback control of two physical processes: Design and experiments

In this thesis the concepts of feedback control are applied to two important technologies. Magnetic resonance imaging is a new technology so far untouched by feedback control. This thesis reports on the first investigations in this direction and shows that feedback control is of great promise and is likely to change the design and operation of future nuclear magnetic resonance (NMR) systems. The second part of the thesis deals with the gas metal-arc welding (GMAW) process. Although much older than NMR, this process is poorly understood and presents a major challenge to control engineers.; Magnetic resonance images are obtained by observing the fluctuations in nuclear magnetization produced by sequences of RF pulses and applied magnetic field gradients. Conventional imaging is an open-loop process in the sense that measurements of the magnetization state are not utilized until after the completion of a pulse sequence. A fundamentally different approach is considered in this thesis, whereby the nuclear magnetization is controlled by using measurements of the magnetization to adjust sequence parameters between successive RF pulses. This strategy facilitates the optimization of the pulse parameters during the imaging sequence.; A systematic method for modeling and controlling the nuclear magnetization is presented in this thesis. The method is demonstrated by implementing a simple scheme for controlling the angle between the bulk magnetization and the axis of an applied static magnetic field. The theoretical and experimental results are in excellent agreement. One set of experiments is used to illustrate the difference between open-loop and closed-loop control. Studies using single and composite pulses have also been performed to show the effects of pulse imperfections in both open-loop and closed-loop experiments. The application of feedback control to magnetic resonance imaging and spectroscopy is discussed.; The second part of this thesis advances the role of control engineering in arc welding by demonstrating the applicability and benefits of multivariable and adaptive feedback control. Gas metal-arc welding is a widely used manufacturing process that joins materials through the coalescence of a consumable electrode and the materials to be joined. Welding is a process that depends on a host of parameters such as the thermal diffusivity of the material to be joined, the initial temperature of the material, the location of heat sinks along the weld path and the geometry of the joint. Subtle variations in these parameters are common and can lead to unacceptable weld joints.; Due to the lack of accurate models, the feasibility of control concepts could not be convincingly proven by simulations. Therefore, the major effort (and the most time consuming part) of this research was the design and implementation of an experimental welding facility at the United States Army Construction Engineering Research Laboratory at Champaign, Illinois.