The characterisation of velocity amplified vibration energy harvesters

Abstract

Vibration 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.