Design of Piezoelectric Energy Harvesters on Strain and Stress Amplification for IoT Sensor System

Abstract

The Fourth Industrial Revolution commonly refers the widespread availability of digital technologies represented by artificial intelligence (AI), Internet of Things (IoT), robotics, cloud computing, virtual reality, and self-driving cars. In previous Third Industrial Revolution, productivity was increased through development of automation systems that input information in one-way, but in the Fourth Industrial Revolution, a system that learns and communicates through mutual data exchange will be introduced. In addition, as the quality of human life improves with advances in technology, the number of applications required is also increasing, along with the need to collect various data from the surrounding environment. Internet of Things (IoT) sensor technology is the basis for building such data collecting network. In order to build IoT sensor systems, numerous sensor nodes must be installed to collect large amounts of data. However, these kinds of expansion are dependent on technology scale of existing power grids and batteries. To address this, energy harvesting research for independent power of IoT sensor systems is essential. In this thesis, a study has been conducted to apply piezoelectric energy harvesting technology converting mechanical energy that can normally be collected from surrounding environment. The input sources for utilization are mainly classified as intermittent and continuous sources, and general constraints are analyzed, and design directions have been presented that can overcome these constraints to improve electrical performance. The resulted improvement in electrical performance presented piezoelectric energy harvesting technology usable as an independent power source for IoT systems by focusing on controlling the variables of the stress and the strain applied to the piezoelectric element within certain constraint condition. First, a method to overcome constraints where vertical pressing force has intermittent inputs but are sufficient in terms of power, while also the input displacement limited to certain values, is proposed. In order to obtain a high amount of power generation through the piezoelectric device, increased stress or strain is demanded. In general, as the stress increases, the strain also increases, but when the input displacement is limited, the strain of the piezoelectric device is also limited. Therefore, to overcome this limitation, it was confirmed that the electrical performance can be improved by fixing the piezoelectric device at both ends and concentrating the stress applied to the piezoelectric device through an active bar in the center. Second, another method to overcome the constraints by increasing the strain was proposed. Although the input displacement is limited similarly, it was confirmed that both stress and strain applied to the piezoelectric device could be increased by increasing the displacement applied to the internal element through a lever and adding an auxiliary structure for amplifying the strain. It was also confirmed that the increase in stress and strain also directly improves the output electrical performance. Third, a method to overcome constraints where there is sufficient rotational force but low rotational speed among continuous inputs is proposed, by increasing the strain of a piezoelectric device through a tip magnet and tip mass based on resonance frequency matching. In the non-resonant region, it was confirmed that the output of the piezoelectric device could be improved through the tip magnet, and experimentally shown that the output of the piezoelectric device could be further amplified in the resonant region, and the factors that should be prioritized in design were suggested. Fourth, a method to overcome constraints where there is sufficient rotational speed but weak rotational force among continuous inputs is proposed, by implementing the use of a soft ferrite. By combining a soft ferrite, which has weak coercivity and low self-reversal requirement, with a tip magnet, it was confirmed that the electrical performance could be significantly improved compared to a simple tip magnet system, with the same magnetic field. Finally, a method to further expand the previous method was presented. By adding an electromagnetic coil to method, it was confirmed that the electrical performances of both piezoelectric and electromagnetic were improved, and a design that could obtain synergy effect was proposed. The improvement in the electrical performance of the piezoelectric energy harvester means an expansion of the range of applications that can be used in the near future. This thesis contributes to the Fourth Industrial Revolution and rapidly developing IoT sensor system constructions by suggesting methods which can increase the adaptation of piezoelectric energy harvesting technology in various situations with difficult constraints.