Small-scale vibration energy harvesters that respond efficiently at low frequencies are challenging to realize. This paper describes the design and implementation of one such harvester, which achieves a high volumetric Figure of Merit (FoM v ¼ 2.6% at 11.50 Hz) at the scale of a C-type battery and outperforms other state-of-the-art devices in the sub 20 Hz frequency range. The device employs a 2 Degree-of-Freedom velocity-amplified approach and electromagnetic transduction. The harvester comprises two masses oscillating one inside the other, between four sets of magnetic springs. Collisions between the two masses transfer momentum from the heavier to the lighter mass, exploiting velocity amplification. The paper first presents guidelines for designing and optimizing the transduction mechanism, before a nonlinear numerical model for the system dynamics is developed. Experimental characterisation of the harvester design is then presented to validate both the transducer optimization and the dynamics model. The resulting high FoM V demonstrates the effectiveness of the device for low frequency applications, such as human motion. V C 2016 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4939545] Wireless sensor networks are currently widespread in many aspects of everyday life.1 Typically, each sensor is independently powered by batteries, which potentially leads to some major issues: batteries have a limited lifetime, and their disposal is polluting. Moreover, their replacement in a large network can be costly due to their high numbers and practically difficult, because they may be embedded in structures, and so difficult to reach.2 Battery limitations have led to the interest in converting energy which is already present in the environment into electrical energy. Among all the possible sources, kinetic energy from ambient vibrations is one of the most common forms. Conventional Vibrational Energy Harvesters (VEHs) are based on simple linear spring-mass resonator designs, for which the resonant frequency of the device has to be tuned to the dominant frequency of the ambient vibration. To overcome this problem, a 2 Degree-ofFreedom (2DoF) electromagnetic velocity amplified VEH is presented in this paper. These configurations have been shown to naturally enhance the frequency response and power generated in VEHs due to the nonlinear effects introduced by impacts within the device, which enable momentum transfer between masses.3-8 To enable effective operation at the low frequencies typical of human motion applications (typically under 5 Hz), a nonlinear contribution to the system dynamics of the device described in this paper was added through the use of magnetic springs. Such an approach results in an efficient but small-scale VEH device that couples the high power and wide frequency response of the velocity amplified VEHs but enables operation at low frequencies and leads to high volumetric Figure of This paper outlines a design methodology to electrically optimize the harvester. A numerical model for predicting the...

In recent years, the development of small and low power electronics has led to the deployment of Wireless Sensor Networks (WSNs) that are largely used in military and civil applications. Vibrational energy harvesting can be used to power these sensors in order to obviate the costs of battery replacement. Vibrational energy harvesters (VEHs) are devices that convert the kinetic energy present in the ambient into electrical energy using three principal transduction mechanisms: piezoelectric, electromagnetic or electrostatic. The investigation presented in this paper specifically aims to realize a device that converts vibrations from different ambient sources to electrical energy for powering autonomous wireless sensors. A “C-battery” scale (25.5 mm diameter by 57.45 mm long, 29.340 cm3) two Degree-of-Freedom (2-DoF) nonlinear electromagnetic energy harvester, which employs velocity amplification, is presented in this paper. Velocity amplification is achieved through sequential collisions between two free-moving masses, a primary (larger) and a secondary (smaller) mass. The nonlinearities are due to the use of multiple masses and the use of magnetic springs between the primary mass and the housing, and between the primary and secondary masses. Part A of this paper presents a detailed experimental characterization of the system dynamics, while Part B describes the design and verification of the magnet/coil interaction for optimum prototype power output. The harvester is characterized experimentally under sinusoidal excitation for different geometrical configurations and also under the excitation of an air-compressor. The maximum output power generated under sinusoidal excitation of arms = 0.4 g is 1.74 mW across a resistive load of 9975 Ω, while the output rms voltage is 4.2 V. Under the excitation of the compressor, the maximum peak power across a load resistance of 8660 Ω is 1.37 mW, while the average power is 85.5 μW.

Vibration energy harvesting extracts energy from the environment and can mitigate reliance on battery technology in wireless sensor networks. This article presents the nonlinear responses of two multi-mass vibration energy harvesters that employ a velocity amplification effect. This amplification is achieved by momentum transfer from larger to smaller masses following impact between masses. Two systems are presented that show the evolution of multi-mass vibration energy harvester designs: (1) a simplified prototype that effectively demonstrates the basic principles of the approach and (2) an enhanced design that achieves higher power densities and a wider frequency response. Various configurations are investigated to better understand the nonlinear dynamics and how best to realise future velocity-amplified vibration energy harvesters. The frequency responses of the multi-mass harvesters show that these devices have the potential to reduce risks associated with deploying vibration energy harvester devices in wireless sensor network applications; the wide frequency response reduces the need to re-tune the harvesters following frequency variations of the source vibrations.

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