ODonoghue_2017_characterisation.pdf (17.6 MB)

# The characterisation of velocity amplified vibration energy harvesters

thesis

posted on 2022-11-08, 16:10 authored by Declan O'DonoghueVibration energy harvesters (VEHs) scavenge ambient vibrational energy, offering
an alternative to batteries for the autonomous operation of low power electronics.
VEHs are typically spring-mass-dampers that extract mechanical energy from a
vibrating source, converting it into useful electrical energy. A number of transduction
mechanisms can be utilised, with electromagnetic induction of interest herein.
Velocity amplification, a technique used to increase velocity through impacts, is
employed in this thesis to improve the power output and operational bandwidth
of multiple-degree-of-freedom (multi-DoF) piecewise linear (PWL) VEHs, compared
to linear resonators. Such a harvester is referred to as a velocity ampli ed
electromagnetic generator (VAEG), with a gain in power achieved by increasing
the relative velocity between the magnet and coil in the transducer.
In this thesis, VAEGs were investigated numerically and experimentally under
sinusoidal excitation, for a range of parameters. An analysis of the in
uence of
mass con guration on multi-DoF VAEGs was undertaken. It was determined
that under forced excitation, contrary to velocity ampli cation theory, 2-DoF
con gurations achieve higher RMS velocities and, hence, voltages than systems
with greater numbers of DoFs. With increasing mass ratio, despite the RMS
velocity increasing, the RMS voltage actually decreases, as the increase in velocity
does not compensate for the reduction in transducer size.
A 2-DoF VAEG with a mass ratio of R = 3 was selected for in-depth investigation.
The harvester was characterised with frequency sweeps for a range
of base acceleration levels and gap lengths|a key geometric parameter. A shift
in peak output power towards lower frequencies is observed with increasing gap,
due to the decreasing e ective sti ness, while RMS velocity also increases. The
acceleration level required to achieve large amplitude oscillations increases with
gap, however. An optimisation of the 2-DoF VAEG is presented, resulting in the
prediction of a relatively high volume gure of merit (FoMV = 2:83%) at high
accelerations (10 m=s2) and low frequencies (16.4 Hz).
It is demonstrated that the hysteresis behaviour and dependence of RMS response
on initial conditions associated with non-linear VEHs is not present in the
VAEGs herein. Consequently, the frequency responses presented are independent
of initial conditions, which is signi cant for the applicability of VAEGs.
To determine the in
uence of scale on the harvester response, the 2-DoF
VAEG was fabricated at three length scales (s = V olume1=3), with the electrical
and mechanical systems considered separately|a number of deviations from linear
scaling methodologies were required to achieve this. It was determined that
the gap does not scale, while the load power is predicted to scale as PL x s5:51,
suggesting that achieving high power densities in a VAEG at low device volumes
is extremely challenging.
VAEG con gurations with 2-DoFs and low mass ratios demonstrate the highest
power densities, while the optimal gap is dependent on the excitation conditions
and increases with increasing acceleration amplitude, at the optimal frequency.
VAEGs are found to be most suitable for applications with high acceleration
levels and low frequencies, where high power densities can be achieved.

## Funding

## History

## Degree

- Doctoral

## First supervisor

Punch, Jeff## Second supervisor

Frizzell, Ronan## Note

peer-reviewed## Other Funding information

IRC## Language

English## Also affiliated with

- Stokes Research Institute

## Department or School

- School of Engineering