Harvesting Mechanical Vibrations using a Frequency Up-converter

Abstract

With the rise of wireless sensor networks and the internet of things, many sensors are being developed to help us monitor our environment. Sensor applications from marine animal tracking to implantable healthcare monitoring require small and non-invasive methods of powering, for which purpose traditional batteries are considered too bulky and unreasonable. If appropriately designed, energy harvesting devices can be a viable solution. Solar and wind energy are good candidates of power but require constant exposure to their sources, which may not be feasible for in-vivo and underwater applications. Mechanical energy, however, is available underwater (the motion of the waves) and inside our bodies (the beating of the heart). These vibrations are normally low in frequency and amplitude, thus resulting in a low voltage once converted into electrical signals using conventional mechanical harvesters. These mechanical harvesters also suffer from narrow bandwidth, which limits their efficient operation to a small range of frequencies. Thus, there is a need for a mechanical energy harvester to convert mechanical energy into electrical energy with enhanced output voltage and for a wide range of frequencies. In this thesis, a new mechanical harvester is introduced, and two different methods of rectifying it are investigated. The designed harvester enhances the output voltage and extends the bandwidth of operation using a mechanical frequency up-convertor. This is implemented using magnetic forces to convert low-frequency vibrations to high-frequency pulses with the help of a piezoelectric material to generate high output voltage. The results show a 48.9% increase in the output voltage at 12.2Hz at an acceleration of 1.0g, and a bandwidth increase from 0.23Hz to 11.4Hz. For the rectification, mechanical rectifiers are discussed, which would obviate the need for electrical rectification, thus preventing the losses normally caused by the threshold voltage of electronics. Two designs of mechanical rectifiers are investigated and implemented on the frequency up-converter: a static rectifier and a rotating rectifier. The results show a voltage rectification, which required a sacrifice in the bandwidth and boosted voltage.