Vibrational energy harvesting for sensors in vehicles

Doctoral thesis, 2024

The miniaturization of semiconductor technology and reduction in power requirements have enabled wireless self-sufficient devices, powered by ambient energy. To date the primary application lies in generating and transmitting sensory data. The number of sensors and their applications in automotive vehicles have grown drastically in the last decade. Wireless self-powered sensors can facilitate current sensor systems by removing the need for cabling and may enable additional applications. These systems have the potential to provide new avenues of optimization in safety and performance.

This thesis delves into the topic of vibrations as ambient energy source, primarily for sensors in automotive vehicles. The transduction of small amounts of vibrational, i.e. kinetic, energy to electrical power, also known as vibrational energy harvesting, is an extensive field of research with a plethora of inventions. A short review is given for energy harvesters, in an automotive context, utilizing transduction through either the piezoelectric effect or magnetic induction. Three practical examples of kinetic energy harvesting in vehicles are described in more detail. The first is a piezoelectric beam for powering a strain sensor on the engines rotating flexplate. It makes combined use of centrifugal force, gravitational pull and random vibrations to enhance performance and reduce required system size. The simulated power output is 370 mW at a rotation frequency of 10.5 Hz, with a bandwidth of 2.44 Hz. The second example is an energy harvesting unit placed on a belt buckle to power a hall sensor measuring if the belt buckle is securely buckled in. It implements magnetic induction by the novel concept of a spring balance air gap of a magnetic circuit, to efficiently harvest minute vibrations and allow for tuning of the resonance frequency through adjusting the equilibrium air gap distance. Simulations show a resonance frequency tuning of 420 Hz/mm (on average) and the potential to achieve 52 µW under normal road conditions driving at 70 km/h. A potential improvement of this concept, by implementing a magnetostrictive component, is evaluated in a separate work. The same concept, of a spring balanced air gap in a magnetic circuit, is here implemented in a cantilever design. In this way the magnetostrictive component can easily me incorporated into the spring component, i.e. the cantilever, and can thus affect both proof mass displacement and magnetic flux. The system using magnetostriction shows a factor of 2 larger output power, compared to an equivalent system without magnetostriction, while maintaining a resonance frequency tuning of 140 Hz /mm. The third practical example is of a device for harvesting the mechanical energy from the occupant’s act of buckling and unbuckling the seat belt. The linear motion of the belt buckle insertion and extraction is in this case used to drive movement of magnets and induce voltage in a coil. The linear motion of the buckle is converted to rotation of a magnet array to increase power generation. Simulations show the device can potentially generate 4 mJ for a single buckle insertion.

Theoretical modeling of vibrational energy harvesting systems is also addressed. Fundamental descriptions of the lumped and distributed models are given. Based on the lumped models of a piezoelectric energy harvester (PEH) and an electromagnetic energy harvester (EMEH), a unified model is described and analyzed. New insights are gained regarding the pros and cons of the two types of energy harvester run at either resonance or anti-resonance. A numerical solution is given for the exact boundary of a dimensionless quality factor and a dimensionless intrinsic resistance, at which the system begins to exhibit anti-resonance. Regarding the maximum achievable power, the typical PEH is favored when running the system in anti-resonance and the typical EMEH is favored at resonance. The described modeling considers all parameters of the lumped model and thus provides a useful tool for developing vibrational energy harvester prototypes.