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UBC Theses and Dissertations

On the optimisation of force and position self sensing of piezoelectric stack actuators using capacitance Ahmadi, Saeed


Due to their characteristics including high resolution, high stiffness, and short response time, piezoelectric actuators facilitate applications requiring high-precision actuation. In such applications, the actuator turns an applied voltage into displacement or force. This conversion suffers by nonlinearities such as hysteresis and creep. Usually, users employ feedback sensors to compensate for these nonlinearities. Yet, these sensors can be unreliable, costly, and space-consuming. Users, instead, can apply position or force self-sensing techniques to measure displacement or force, respectively. In this research, we study the apparent capacitance of a piezoelectric stack actuator as an auxiliary parameter for self-sensing. To study the relationship between displacement and capacitance, and between force and capacitance, we developed analytical lumped parameter models to analyze the interaction between electrical and mechanical material properties, we built FEM models of the mechanical structure to provide insights into the resonance behaviour, and we performed experiments aimed at finding the individual impacts of voltage, displacement, and force on the capacitance of the actuator. The results of these investigations indicate that the voltage is the main factor affecting the capacitance. Nevertheless, the capacitance has indirect relationships to both displacement and force, making position and force self-sensing possible. We set two main criteria to optimize the sensing performance: low hysteresis, and high sensitivity. These criteria vary with the frequency of the capacitance measurement. In general, best results are obtained for both force and displacement sensing, when the capacitance measurement is sufficiently far removed from resonances.

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