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When an oscillating mechanical structure comes into contact with a fluid, frequency and damping of the vibration change depending on the physical properties of the surrounding medium. This principle is applied in dynamic fluid sensing, where feedback control is used to hold the oscillation at resonance, thus providing continuous measurement output of the fluid parameters. Currently numerous applications exist in research and industry, such as process devices, portable hand-held systems or lab-on-a-chip applications, where usually one single frequency is controlled, mainly to determine the viscosity and density of simple Newtonian fluids. However, often non-Newtonian fluids, especially viscoelastic fluids, are the subject of interest. In this case, the dependence of the fluid parameters on the frequency is of particular importance hence the fluid characterization requires the use and interpretation of various frequencies. The present thesis is concerned with the development of a measurement system that uses various resonance frequencies of an oscillator. In order to obtain continuous measurement output, the corresponding natural vibrations are to be excited and controlled simultaneously. Alongside the improvement of common resonance-based viscometers in terms of robustness and accuracy, the development of a process rheometer is considered as the main goal of this thesis. Furthermore, the analysis and design of appropriate control concepts shall be emphasized. The thesis is structured in three parts: design and model of the oscillator, analysis of the vibration control and applications in fluid sensing. The design of the oscillator is based on a commercially available process viscometer which uses torsional oscillations of a rod-like structure. In order to obtain as many torsional resonance frequencies as possible in an accessible frequency range, the design is adapted accordingly using a mechanical model of the structure. An analytical description of the relationship between measurement parameters and physical fluid parameters is presented that is valid both for Newtonian and linear viscoelastic fluids. Simple, explicit equations can be derived for weakly elastic fluids, while implicit equations represent the interaction between structure and strongly elastic fluids. Further, the analytical model is used to investigate the influence of the immersion level of the sensor as well as temperature effects. The second part of the thesis presents the control concept that is used to evaluate multiple resonance frequencies and modal damping values of an oscillator. The central part of the control is a phase-locked loop (PLL), which stabilizes the phase shift between excitation and response of the oscillator by adapting the excitation frequency. First, phase controlled systems with one degree of freedom are investigated. Since the stability and dynamics of the control loop strongly influences the measurement quality, an analytical

Multi-Frequency Phase Control of a Torsional Oscillator for Applications in Dynamic Fluid Sensing