Vibration-based electromagnetic energy harvester : energy performance, vibration control, and frequency tuning

Abstract

Vibration-based energy harvesting is an emerging technique that can convert vibration energy into electrical energy. Energy harvesters adopting various types of energy transducers have been developed to extract energy from different vibration sources, wherein the extracted energy can be applied to different objectives, such as structural health monitoring and semi-active/active control. However, the typical power levels in most micro-scale vibration-based harvesters are too low to meet the power demands of the aforementioned applications. This thesis investigates two important topics on vibration-based energy harvesters: the leveraging of the dynamic coupling effect between energy harvesters and vibration sources to realize both vibration control and energy harvesting functions, and the introduction of the frequency tuning functions into an energy harvester in the electrical and mechanical domains to realize broader energy harvesting bandwidth, whereby frequency tuning can enhance the energy harvesting performance of typical vibration-based energy harvesters. Developed vibration-based harvesters fall into two categories and comprise two key components: an electromagnetic (EM) transducer and an energy harvesting circuit (EHC). An ad hoc resistance-emulation EHC was designed. Its equivalent resistance characteristic and the vibration damping performance when it was connected to an electromagnetic damper (EMD) were verified through experiments. An energy-harvesting EMD (EHEMD), a simple type of dual-function damper, was fabricated and employed in an experiment on a full-scale bridge stay cable as an energy-harvesting passive vibration control device. Subsequently, similar EHEMDs were numerically applied to the secondary suspension of a high-speed train to realize an energy-harvesting adaptive control strategy. These numerical and experimental cases illustrate the effectiveness of the proposed EHEMD device for providing the optimal vibration damping and energy harvesting function in the meantime. Furthermore, a coupled analysis of structural vibrations and EM energy harvesters was performed, wherein broadband random excitations were applied, and the structures were assumed to respond within the elastic range. The optimization objectives were set as the minimization of the structural kinetic energy and the maximization of the power transferred into a dual-function device. Different systems with a single-degree-of-freedom structure coupled with an EHEMD, an energy-harvesting tuned mass damper (EHTMD), and an energy-harvesting tuned inerter damper (EHTID) were investigated analytically and numerically. The general consistency between vibration control and energy harvesting was demonstrated. The second category of EM energy harvester typically contains an oscillating structure without a significant coupling effect on the vibration source; that is, the device operates as a pure energy harvester. An equivalent circuit model for such a case was developed based on the dynamic electromechanical analogy. Moreover, an overall impedance optimization theory is proposed for the first time by considering different excitation types, coupling effect strengths, and oscillator complexities. A potential electrical frequency tuning method was validated through numerical case studies. Subsequently, a novel design of an energy harvester with tunable low frequency, termed double-mass-pendulum (DMP) oscillator, is proposed. The mathematical model of the DMP oscillator was established in terms of different base motions. The nonlinear characteristic, frequency tuning function, and energy harvesting performance of the DMP oscillator were evaluated through free vibration tests and shake table tests. In addition, the oscillator was enclosed in a floating-point absorber and then tested in a wave flume to evaluate its potential applications in wave energy converters. Through the combination of theoretical, numerical, and experimental studies, this work demonstrates the promising prospects of the developed two categories of vibration-based energy harvesters for functionality and performance enhancement. Some challenges are also discussed based on the outcome of the work.