This article presents a fundamental investigation in which velocity amplification is employed in non-resonant structures to enhance the power harvested from ambient vibrations. Velocity amplification is achieved utilising sequential collisions between free-moving masses, and the final velocity is proportional to the number of masses and the mass ratios selected. The governing theory is discussed, particularly how the final velocity scales with the number of masses. This article examines n-mass velocity-amplified vibration energy harvesters and examines their performance relative to single-mass harvesters. Electromagnetic energy conversion is chosen as it is fundamental in allowing the free movement of the masses. Experimental results from two- and three-mass prototypes are presented that demonstrate a wider frequency response and a gain in power of 33 times compared to single-mass configurations under wideband random excitation. The volume of the devices was constrained, which resulted in the two-mass system outperforming the triple-mass system counter to expectations. This was caused by the triple-mass device experiencing an increased number of impact due to the volume constraint, leading to high losses in the system. It is recommended that in order to realise the full benefits of the triple-mass system, additional volume for mass actuation is required.

The major focus of this work is to examine the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop useful machines. For instance low-cost machines based on this principle could be used for detailed, very-high acceleration shock-testing of MEMS devices. A theoretical basis for determining the number and mass of intermediate stages in such a velocity amplifier, based on simple rigid body mechanics, is proposed. The influence of mass ratios and the coefficient of restitution on the optimisation of the system is identified and investigated. In particular, two cases are examined: in the first, the velocity of the final mass in the chain (that would have the object under test mounted on it) is maximised by defining the ratio of adjacent masses according to a power law relationship; in the second, the energy transfer efficiency of the system is maximised by choosing the mass ratios such that all masses except the final mass come to rest following impact. Comparisons are drawn between both cases and the results are used in proposing design guidelines for optimal shock amplifiers. It is shown that for most practical systems, a shock amplifier with mass ratios based on a power law relationship is optimal and can easily yield velocity amplifications of a factor 5–8 times. A prototype shock testing machine that was made using above principles is briefly introduced.

No abstract

Recent advances in micro-electro-mechanical system (MEMS) fabrication technology have resulted in proliferation of micro-scale mechanical devices, some of which are applied in environments with severe levels of shock. The objective of this paper was to investigate the use of experimental and numerical methods in quantifying the behaviour of representative MEMS devices subject to high-g impact stimuli. Micro-cantilevers were analysed under vibration and shock on a modified Hopkinson pressure bar and vibration table to determine the mechanical properties of single crystal silicon (SCS). The characteristic dimensions of the beams were of 100 lm in height/ width with beam lengths ranging from 5 to 7 mm. The experimental approach allowed non-invasive in situ monitoring of the micro-cantilevers upon impact through accelerometry and high-speed imaging. An investigation of the shock response of micro-cantilever beams indicates that orientation plays a significant role in their sensitivity to shock because of the planarity of the fabrication technique. Scanning electron microscopy identified octahedral cleavage of SCS as the dominant failure mechanism of the micro-cantilevers. Finite element analysis in conjunction with in situ high-speed imaging proved to be a viable non-invasive inverse technique to determine the loci and amplitude of tensile stress within generic micro-scale devices.

Vibrational energy harvesters (VEHs) are devices which convert ambient vibrational energy into electrical power, offering an alternative to batteries for powering wireless sensors. Detailed experimental characterisation of a 2-degree-of-freedom (2-Dof) VEH is presented in Part A of this paper, while a theoretical analysis is completed in Part B. This design employs velocity amplification to enhance the power harvested from ambient vibrations, while also seeking to increase the bandwidth over which power can be harvested. Velocity amplification is achieved through sequential collisions between free-moving masses. Electromagnetic induction was chosen as the transduction mechanism as it can be readily implemented in a velocity amplified system, although other transduction mechanisms can also be used. The VEH prototype was tested experimentally under both sinusoidal excitation and exponentially correlated Gaussian noise, with different VEH geometries. The maximum power generated under a sinusoidal excitation of arms = 0.6 g was 12.95 mW for a resistive load of 13.5 Ω at 12 Hz, while the maximum power under exponentially correlated Gaussian noise with σ = 0.8 grms, autocorrelation time τ = 0.01s and resistive load 13.5 Ω was 5.3 mW. Maximum bandwidths of 54% and 66%, relative to the central frequency, were achieved under sinusoidal and noise excitation, respectively. The device shows resonant peaks at approximately 15 and 30 Hz, while significant power is also generated in the 17–20 Hz range due to non-linear effects. The VEH component dynamics were analysed using a laser Doppler vibrometer (LDV), while Lab VIEW was used to control the electromagnetic shaker, read the LDV signal and record the VEH output voltage. The aim of this investigation is to achieve a more complete understanding of the dynamics of velocity-amplified systems to aid the optimization of velocity amplified VEH designs.