Ultrasonic Generators for Energy Harvesting Applications: Self-Excitation and Mechanical Frequency Transformation

Abstract

The main objective in the field of vibration-based energy harvesting is to convert the kinetic energy from an ambient energy source into an useable electrical form in the most efficient way. The intention is to provide power for low-powered electronic devices, such as intelligent sensors for structural health monitoring, in order to make an external power source or periodic battery replacement redundant and thus lower the costs. Applications of this technology can be found in the automotive and aerospace industry as well as in civil and mechanical engineering. One of the main challenges in the area of vibration-based energy harvesting is to design an energy harvesting device generating a significant amount of electrical power across varying vibration inputs. Due to the design, most energy harvesters are subject to forced excitation and have therefore the drawback that the performance strongly depends on the uncertain excitation parameters. Furthermore, to achieve a high power density of the piezoceramics used for the energy conversion, it is required to generate a high-frequency operation of the piezoceramics from a low-frequency vibration source. Such frequency transformation is, for example, exploited in ultrasonic motors, but has never been examined in the inverse direction for ultrasonic generators. In this thesis, a new concept of piezoelectric generators is studied in detail with respect to its applicability for energy harvesting systems. To this end, electromechanical models of two different ultrasonic motors are derived in order to study their convertibility of the operating direction. Based on the analytical models, the influence of the main parameters on the dynamic behavior as well as the characteristic steady-state operation are determined. Experiments are carried out to validate this concept.