Scavenging power from ultra-low frequency and large amplitude vibration source through a new non-resonant electromagnetic energy harvester

Highlights

• A new non-resonant and free/impact motion based EMEH with steel frame for ultra-low frequency and large amplitude vibration application is proposed.

• Mathematical model and simulation procedure are established and well validated.

• Experimental results are analyzed and show the valuable features of the proposed EMEH.

Abstract

In this paper, a new non-resonant, free motion based electromagnetic energy harvester is proposed for harvesting energy from low-frequency vibration sources. In this design, steel frame is beneficially utilized to enhance the flux and consequently the power density of the harvester. The steel frame can also function as a magnetic shield to protect the harvester from external magnetic field interferences. Prototype of this harvester is constructed and characterized experimentally. A dedicated model and simulation method including non-linear contact characteristics is presented in this paper for predicting the dynamic response of this harvester at different vibration frequencies and amplitudes, and different operation modes are characterized. The measured results have well validated the behaviors predicted by the proposed model. In particular, the developed prototype of this harvester can generate a power density 748 μW/c${\mathrm{m}}^3$ at a low-frequency of 4.5 Hz and 40 mm vibration amplitude. Compared to recently reported low-frequency energy harvesters summarized in Table 3, this new harvester has the superiority of high-power density. It is a non-resonant harvester so it can harvest energy for a wide frequency band.

Introduction

Using surrounding energy sources to generate electricity that can be applied to power miniature electronic systems including wearable devices [1], [2], medical implants [3], [4], and wireless sensors [5], [6], has attracted increasing popularity over the last several decades. The use of batteries for supplying power to these electronic systems has brought environmental issues and not been eco-friendly [7]. Especially, for implantable devices and hazardous industrial applications, it is risky and inconvenient for periodic battery replacement and frequent maintenance. Fortunately, benefited from the progress of power electronics technology, more miniature electronic systems can now work in milli-watt (mW), micro-watt (μW) and even lower power levels, and this has motivated an increasing focus on converting the environmental energy into electricity as a maintenance-free and permanent power supply [8]. In our surrounding environment, there are many energy sources (e.g., solar, thermal, chemical, kinetic) that can be utilized to generate electricity. Among them, the kinetic energy, particularly the vibrated energy, has appeared to be an attractive alternative power source in small-scale systems according to recent studies [9], [10].

The common energy-conversion mechanisms including piezoelectric [11], [12], electrostatic [13], triboelectric [14] and electromagnetic [15], [16], [17], could convert kinetic energy into electricity through energy harvesting systems. In terms of the application circumstances, the majority of miniature electronic systems (wearable devices, medical implants, et.) could easily access the low-frequency vibration sources (e.g., the human body induced motion). In the recent few decades, plenty of work on low-frequency vibration energy harvesting has been carried out. Based on multi-directional vibration excitation from human motion, Wu et al. [18] had proposed a piezoelectric energy harvester with spring pendulum oscillator, which could successfully generate 13.29 mW at 2.03 Hz ultra-low frequency excitation. In another recent research, a biomechanical energy harvester consisting of cable-pulley mechanism and traditional rotated generator had been investigated to generate electricity from human motion energy [4]. The harvester had been capable of producing 4.1 W exhibiting a high-power density. Moreover, a hybrid electromagnetic triboelectric energy harvester using Halbach magnet array had been fabricated to generate power from human motions for potential low-frequency applications in portable electronics [19]. This harvester could produce 11.75 mW output power at 5 Hz resonant frequency and 0.5 g acceleration.

Existing studies have shown clearly that low-frequency vibration energy harvesting systems can serve as promising power supplies for the electronic systems around us. Most of the mechanical vibration sources in the surrounding environment have multiple degrees of freedom (DOF) motions, which could be converted to several single DOF problems to study. One of the issues in vibration-based energy harvesters is that it is difficult to efficiently scavenge energy from ambient vibration sources at a broadband low frequency spectrum. Many recent researchers have tried to extend the operating bandwidth of vibration-based energy harvesters. Stephen [20] had investigated a common magnetic levitation based electromagnetic energy harvester (EMEH) and verified its nonlinear stiffness behavior benefiting for broad frequency operation. Additionally, Chiacchiari et al. [21] had proposed a bi-stable nonlinear EMEH with a weakly damped linear oscillator, which could harvest energy from the broadband vibrations. This harvester utilized the activation of large-orbit inter-well oscillations to attain up to 40 mW mean power at the range of (0.05–0.45) m/s excitation. Nevertheless, multi-stable energy harvesters have also shown that the oscillations could not be uniformly activated over a broad bandwidth and generally include chaotic behaviors. Although the solution that active tuning of stiffness or damping coefficients through external equipment had been investigated [22], the additional volume and power dissipation of subsidiary devices was unsuitable for miniature electronic systems. Further, these resonant energy harvesters lost the advantage of resonant amplification in limited volume of miniature electronic systems, especially, when excited by high amplitude vibration such as human motion.

Non-resonant energy harvesters are proposed to expand the operating bandwidth and incorporate properly with human powered devices. Haroun et al. [23] designed a non-resonant energy harvester based on free/impact motion, and it could efficiently generate up to 113.3 μW power under 5 Hz via the simple frame combined with a permanent magnet and a tube-carrying coil. For increasing the magnetic flux rate in the above mentioned non-resonant energy harvesters, Zhang et al. [24] introduced the rolling magnet to replace the commonly used sliding magnet. Its experiment result had shown that an average power of 1.02 mW could be captured from 3.1 Hz handshaking. Another non-resonant energy harvester with impact-based piezoelectric mechanism had been analyzed in [25], and it generated 511 μW in the principle that a rolling steel ball moved between two piezoelectric cantilever ends. However, the piezoelectric energy harvester has an innate disadvantage of large inner impedance, which could result in that the current was too low to drive the electronic system. On the other hand, from the discussion in the previous literatures, most EMEH operated in low frequency excitation had the drawback of low voltage that is not sufficient to power electronic devices. For improving the output voltage of EMEH in low-frequency vibration conditions, the concept of frequency-up conversion had been proposed by [26], [16], [2]. These two prototypes used cantilever beams to up-convert large-amplitude and low-frequency vibration to small-amplitude and high-frequency vibration through magnetic/mechanical coupling. However, the extra cantilever beam had remarkably increased the volume and been unsuitable for miniature applications. Appliance of the ferromagnetic materials to enhance the magnetic flux was another considerable voltage-up solution, which was generally adopt in traditional generators. Askari et al. [27] had imposed this methodology to their hybridized electromagnetic-triboelectric self-powered sensor, which could generate 50.81 W/${\mathrm{m}}^3$ for electromagnetic component at 1 Hz excitation.

A non-resonant and free/impact motion based EMEH with steel frame for low-frequency and large-amplitude application is presented in this work. Its new design, dedicated operation mode characterization and proposed mathematical model and simulation methods with solid validation that can well benefit the study of other energy harvesters, are the new contributions of this paper.

In this paper, a prototype design of the proposed EMEH is introduced in Section 2. The mathematical modelling covering both mechanical and electromagnetic behaviors of the proposed device is given in Section 3, together with the predicted performance and motion profiles. Experimental setup and detailed tests are described in Section 4. Comparison between simulation and experimental results, performance analysis under variable load resistances and comparison to other low-frequency energy harvesters reported in recent literatures are discussed in Section 5.