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...

Electromagnetic Vibration Energy Harvesting (EM-VEH) is an attractive alternative to batteries as a power source for wireless sensor nodes that enable intelligence at the edge of the Internet of Things (IoT). Industrial environments in particular offer an abundance of available kinetic energy, in the form of machinery vibrations that can be converted into electrical power through energy harvesting techniques. These ambient vibrations are generally broadband, and multi-modal harvesting configurations can be exploited to improve the mechanical-to-electrical energy conversion. However, the additional challenge of energy conditioning (AC-to-DC conversion) to make the harvested energy useful brings into question what specific type of performance is to be expected in a real industrial application. This paper reports the operation of two practical IoT sensor nodes, continuously powered by the vibrations of a standard industrial compressor, using a multi-modal EM-VEH device, integrated with customised power management. The results show that the device and the power management circuit provide sufficient energy to receive and transmit data at intervals of less than one minute with an overall efficiency of about 30%. Descriptions of the system, test-bench, and the measured outcomes are presented.

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.

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.

Vibrational energy harvesting has become relevant as a power source for the reduced power requirement of electronics used in wireless sensor networks (WSNs). Vibrational energy harvesters (VEHs) are devices that can convert ambient kinetic energy into electrical energy using three principal transduction mechanisms: piezoelectric, electromagnetic and electrostatic. In this paper, a macroscopic two degree-of-freedom (2Dof) nonlinear energy harvester, which employs velocity amplification to enhance the power scavenged from ambient vibrations, is presented. Velocity amplification is achieved through sequential collisions between free-moving masses, and the final velocity is proportional to the mass ratio and the number of masses. Electromagnetic induction is chosen as the transduction mechanism because it can be readily implemented in a device which uses velocity amplification. The experimental results are presented in Part A of this paper, while in Part B three theoretical models are presented: (1) a coupled model where the two masses of the non-linear oscillator are considered as a coupled harmonic oscillators system; (2) an uncoupled model where the two masses are not linked and collisions between masses can occur; (3) a model that considers both the previous cases. The first two models act as necessary building blocks for the accurate development of the third model. This final model is essential for a better understanding of the dynamics of the 2-Dof device because it can represent the real behaviour of the system and captures the velocity amplification effect which is a key requirement of modelling device of interest in this work. Moreover, this model is essential for a future optimization of geometric and magnetic parameters in order to develop a MEMS scale multi-degree-of-freedom device.