Institut für Produkt und Prozessinnovation (PPI)
The transition of our energy system towards a generation by renewables, and the corresponding developments of wind power technology enlarge the requirements that must be met by a wind turbine control scheme. Within this thesis, the role of modern, model-based control approaches in providing an answer to present and future challenges faced by wind energy conversion systems is discussed. While many different control loops shape the power system in general, and the energy conversion process from the wind to the electrical grid specifically, this work addresses the problem of power output regulation of an individual turbine. To this end, the considered control task focuses on the operation of the turbine on the nonlinear power conversion curve, which is dictated by the aerodynamic interaction of the wind turbine structure and the current inflow. To enable a power tracking functionality, and thereby account for requirements of the electrical grid instead of operating the turbine at maximum efficiency constantly, an extended operational range is explicitly considered in the implemented control scheme. This allows for an adjustment of the produced power depending on the current state of the electrical grid and is one component in constructing a reliable and stable power system based on renewable generation. To account for the nonlinear dynamics involved, a linear matrix inequalities approach to control based on Takagi-Sugeno modeling is investigated. This structure is capable of integrating several degrees of freedom into an automated control design, where, additionally to stability, performance constraints are integrated into the design to account for the sensitive dynamical behavior of turbines in operation and the loading experienced by the turbine components. For this purpose, a disturbance observer is designed that provides an estimate of the current effective wind speed from the evolution of the measurements. This information is used to adjust the control scheme to the varying operating points and dynamics. Using this controller, a detailed simulation study is performed that illustrates the experienced loading of the turbine structure due to a dynamic variation of the power output. It is found that a dedicated controller allows wind turbines to provide such functionality. Additionally to the conducted simulations, the control scheme is validated experimentally. For this purpose, a fully controllable wind turbine is operated in a wind tunnel setup that is capable of generating reproducible wind conditions, including turbulence, in a wide operational range.
This allows for an assessment of the power tracking performance enforced by the controller and analysis of the wind speed estimation error with the uncertainties present in the physical application. The controller showed to operate the turbine smoothly in all considered operating scenarios, while the implementation in the real-time environment revealed no limitations in the application of the approach within the experiments. Hence, the high flexibility in adjusting the turbine operating trajectories and structural design characteristics within the model-based design allows for efficient controller synthesis for wind turbines with increasing functionality and complexity.
This thesis aims to develop a FE-based model of a dieless wire drawing process for wires made from magnesium alloys. To this end a general material model of pure magnesium and a model of the dieless wire drawing process are developed. Based on the general pure magnesium model an alloy specific model for AZ31 wire is developed. The performance of both models is assessed using experimental data generated on a dieless wire drawing prototype.
The process model is conceptionally split into the thermal and mechanical response of the wire. The thermal model is validated by axial temperature profiles and the mechanical model is vali-dated by CSA-reduction and wire force. Both behaviours are validated separately before combin-ing the thus created models into a thermomechanical model of the dieless wire drawing process. The thermal material model is developed for pure magnesium. An initial assumption of limited correlation between content of alloying elements and thermal behaviour, was disproven. As a results in addition to alloy-specific mechanical data, thermo-electric data is recorded to achieve thermal validity of the model. This is done by identifying the experimental maximum temperature of the drawn wire for a given heating power and calculating the necessary input power of the in-duction heating device to achieve this temperature in simulation. The mechanic material model is based on experimental stress-strain curves recorded for each investigated wire materials in addi-tion to pure magnesium data, based on literature.
Results show the thermomechanical magnesium models to be mostly valid, provided process parameters stay within the range of available data on the mechanic material performance. Where the model is forced to extrapolate material behaviour, simulation quality drops. This ap-plies for wire temperature and CSA-reduction. Estimations of wire force are shown to be invalid. For AZ31 wire the thermal model generated valid temperature profiles of the wire. The thermo-mechanical model for AZ31 is shown invalid as both CSA-reduction and wire force deviate from experimental results.
The doctoral dissertation deals with the problems of the diagnosis of rolling bearings using recurrence analysis. The main topic is the influence of radial internal clearance on the change of dynamics in a self-aligning double-row ball bearing with a tapered bore, in which the axial preload can control this parameter in a wide range. The dissertation began with an analysis of the state of knowledge, where the works related to the analyzes of the impact of radial clearance on the dynamics of rolling bearings have been cited so far. In the next part of the dissertation, the thesis was formulated and activities related to its proving were defined. The theoretical part was supplemented with the basics related to vibroacoustic diagnostics of rolling bearings and presented methods that can be used for their diagnostics. The research on proving the thesis was started with the preparation of a mathematical model in which a change in the damping coefficient in the field of radial clearance was adopted, a difference in the clearance value for a given row of balls was proposed, and the influence of shape errors and radial shaft endplay on the dynamics of the tested bearing was taken into account. During the dynamics tests, the radial clearance was adopted as a bifurcation parameter, and on the basis of the bifurcation diagram, it was possible to indicate the characteristic areas of bearing operation due to the radial internal clearance. In order to verify the model, experimental tests were carried out with a series of bearings in which the radial clearance was changed in a wide range possible to be physically realized. Recurrence analysis was used for both the dynamic response obtained from model and experimental studies. Owing to the comparative analysis of the dynamic response, recurrence quantificators were selected that are most susceptible to changes in radial clearance to bearing dynamics. Moreover, as a result of the research, it was possible to select a narrow range of radial clearance, ensuring the smoothest operation of the tested bearing.
