Resonance frequency tuning in vibration-based energy harvesting systems

Energy harvesting technologies that rely on the conversion of ambient vibration into a usable form of energy have become the subject of significant research in recent years . The most common types of transduction methods are piezoelectric, electromagnetic and electrostatic. Among these three methods, piezoelectric convertors have been recognized to offer more benefits. They have presented a potential solution to the problem of power systems which have a short battery life and high maintenance costs. Battery replacement is more of a problem for Micro Electro Mechanical Systems (MEMS). For some applications, often it is not practical to replace the dead battery because they are not easily accessible. Therefore, the concept of low-power MEMS devices that are able to scavenge, or harvest energy from their operating environment have gained growing attention over recent years. In this thesis, an overview of energy harvesting technology based on different transduction methods is presented and discussed in detail. Most energy harvesters are designed to work at resonance frequency in order to obtain maximum output power, and they are usually manufactured to have resonance frequencies that match the frequencies of excitation. However, in some cases, there is a mismatch between the resonance and excitation frequencies due to manufacturing errors or changes in the working environment. Particularly, in MEMS devices due to the fabrication process such as mask alignment, deposition, photolithography, etching and drying, manufacturing tolerances are generally high and, in some cases, can be higher than ±10% of nominal values. Therefore, parameter uncertainty can significantly affect the performance of MEMS devices. To overcome this problem, a MEMS piezoelectric harvester with electrostatically adjustable resonance frequency is proposed. The main aim is to control the resonance frequency of the piezoelectric harvester with the application of a DC voltage to the electrostatic system in order to maximize the harvested power. Based on the voltage applied to the electrostatic system, the resonance frequency of the harvester can be adjusted through hardening and softening mechanisms. The problem addressed in this thesis is non-linear due to electrostatic forces. Moreover, by considering uncertainty in the model parameters; we are dealing with a dynamic problem with the effects of both nonlinearities and uncertainties which has not received significant attention in the literature. In this study, for the first time to our knowledge, the shooting method in conjunction with Monte Carlo Simulation has been used to solve a nonlinear uncertain problem. In addition, due to the similarity between electrostatic and electromagnetic forces, an experimental set-up based on the nonlinear electromagnetic forces has been designed to show the concept of the proposed model in macro scales. The experimental results have been verified numerically and it has been shown that the proposed model has great potential in practical applications