A cantilever-plucked and vibration-driven rotational energy harvester with high electric outputs

Highlights

• A cantilever-plucked rotor is proposed to realize vibration-to-rotation conversion.

• The proposed motion-conversion mechanism features extremely simple structure.

• High-speed rotation is achieved by the rotor under low-level vibrations.

• A harvester with the rotor achieves high power output under low-level vibrations.

• The harvester generates sufficient electric energy for several electronics.

Abstract

The ubiquitous and renewable vibration energy has been recognized as a promising energy source for the increasing number of low-power electronics, but its efficient harvesting is still an open issue. Here, an innovative vibration-to-rotation conversion mechanism (which we name ‘cantilever-plucked rotor’) is reported for converting ambient vibrations to uni-directional and high-speed rotation through a new plectrum design. Featuring a simple configuration and being attached on a cantilever beam, the proposed plectrum consists of a rigid part for spinning the rotor and a flexible part for reducing the frictional resistance acting on the rotor, enabling the plectrum to switch its effective stiffness automatically according to its motion with respect to the rotation of the rotor. An electromagnetic rotational energy harvester constructed with the cantilever-plucked rotor generates high output power of 4 mW under a harmonic vibration of 0.4 g (1 g = 9.8 m/s²) at 8.6 Hz, which is several times higher than that provided by a conventional electromagnetic energy harvester without the rotor. Under low-level vibrations (≤0.8 g), the constructed energy harvester can also generate sufficient electric energy for the continuous operation of some commercial electronics. This work demonstrates the promising potential of the cantilever-plucked rotational energy harvester in the efficient scavenging of ambient vibrations.

Introduction

With the quick development of wireless sensor networks and Internet of Things, a wide range of small electronics, such as radio frequency identification devices, Global Positioning Systems, infrared sensors and other low-power electronics, have been utilized to connect all items to one network for easy identification and management [1], [2]. Featuring low power consumption, these small electronics can be run by conventional electrochemical batteries. However, the excessive use of batteries may contaminate the environment due to the high difficulty in dealing with the huge number of expired batteries. Moreover, it is difficult or even impossible to recharge/replace the batteries in the small electronics when they are distributed in hard-to-reach locations or come in tremendously large numbers. One promising way to overcome the downsides of conventional batteries is the vibration energy harvesting technology, which converts the omnipresent but mostly wasted mechanical vibrations in the ambient environment into electricity through various transduction mechanisms, such as piezoelectric effect [3], [4], electrostatic induction [5], [6], triboelectric effect [7], [8] and electromagnetic induction [9], [10].

Piezoelectric vibration energy harvesters (VEHs) can be easily achieved by bonding piezoelectric materials onto linear mechanical oscillators (e.g., cantilever beams) [11]. The vibration energy with the frequency close to the resonant frequency of the linear piezoelectric VEHs can be scavenged through the direct piezoelectric effect of the piezoelectric materials whose stresses/strains change along with the vibration of the mechanical oscillators [12], [13]. The electrostatic VEHs consist mainly of two electrode plates and an electret [14]. One of the two electrode plates is normally affixed to a mechanical oscillator to engender the relative movement (in-plane [15] or out-of-plane [5]) between the two electrodes, and the electret is employed to provide a constant electric field that drives the induced charges to flow between the two electrodes via the outer circuit. The triboelectric VEHs are based on the triboelectrification effect and electrostatic induction [16]. Compared with the electrostatic VEHs, the electret in the triboelectric VEHs is dispensable since the charges can be generated and replenished through the contact/separation or the relative sliding between two materials with dissimilar electron affinity. The electromagnetic VEHs are generally constructed by attaching magnets (or pick-up coils) to a mechanical oscillator [17]. Under ambient excitations, the vibration of the mechanical oscillator gives rise to the relative movement between the magnets and the coils, which engenders the change of the magnetic flux through the coils and then the electricity flowing through the external load circuit.

No matter which transduction mechanism is employed, the linear VEH usually works well only in the proximity of its resonant frequency, leading to poor energy harvesting performance under ambient broadband vibrations [18]. To tackle this issue, several strategies have been proposed to broaden the operation bandwidth of VEHs, including tuning mechanisms [19], [20], multi-modal approaches [21], [22], and nonlinear dynamics [23], [24]. The core of tuning mechanisms is to change the natural frequency of a VEH by adjusting the mechanical domain parameters (e.g., the effective stiffness and axial stress) and/or the electrical domain parameters (e.g., load resistance) to make the VEH resonate with the ambient vibrations. Drawbacks of tuning mechanisms are the great difficulty and extra energy dissipation for implementing the automatic tuning. The multi-modal VEHs can be realized with a multi-degree-of-freedom (M-DOF) mechanical oscillator [25], [26] or an array of mechanical oscillators with different resonant frequencies [27], [28] to cover a wide frequency region. The wide bandwidth of multi-modal VEHs is achieved at the cost of diminishing the power amplitudes or decreasing the power density. In recent years, introducing the nonlinear magnetic interaction into the VEH has been deemed as one of the most promising broadband strategies [11], [26], [29]. Depending on the number and configuration of magnets, the VEH can be made with monostability [30], [31], bistability [32], [33] and even multi-stability [34], [35], and the VEH with the introduced nonlinearity can respond to the ambient vibrations in a hardening [36] or softening way [30], enabling the VEH to function well within an extended frequency range. In addition, the conjunction of different broadband strategies (e.g., the multi-modal approach and the nonlinear dynamics) has also been proposed to improve the VEH energy harvesting performance over an extended frequency range [18], [37]. With these broadband strategies, the VEH can generate useful electricity within a wider frequency range, however the extended bandwidth is still insufficient for practical applications. Moreover, most of broadband strategies contribute little to the enhancement of the power amplitude, hindering the efficient exploitation of ambient vibration energy.

It should be noted that, as a dominant and straightforward approach for exploiting ambient vibrations, the VEH has also been utilized to harvest other mechanical motions (e.g., the rotation of rotators [38], [39], rolling of spheres or cylinders [40], [41], and swings of human limbs [33], [42]) by transforming them to vibrations of mechanical oscillators. However, limited by the dynamics of mechanical oscillators, the finite working bandwidth and low power level still impede the VEH as a practical power source. There is little doubt that the performance of the VEH will be improved gradually by exploring other dynamic characteristics (e.g., internal resonance [37] and nonlinear energy sink [43]) of mechanical oscillators, but it may be difficult to solve the disadvantages of VEHs from the root in the short term.

Recently, the vibration-driven rotational energy harvester (V-REH), a basically different vibration energy harvesting scheme, has been proposed to achieve efficient extract of ambient mechanical energy [44], [45]. Distinguishing from the VEH that vibrates in response to the ambient excitation, the V-REH converts vibrations or compressive forces to the rotation of a rotor and may provide an alternative option for overcoming the drawbacks of VEHs. The V-REH differs from the conventional REH [46], [47], [48] in that the former normally does multi-circle and high-speed rotation instead of low-speed swing motion, which enables the V-REH to generate high electric outputs as excited by vibrations. For example, the V-REH developed by Zhang et al. [44] utilizes a twist-driving structure to realize the vibration-to-rotation conversion and achieves 7.7 mW power as excited at 5 Hz. Fan et al. [49] reported a semi-flexible rotor to convert vibrations to the rotation of a rotator, and a V-REH constructed with the rotor generates 9.7 mW power at 1.5 Hz. Although the two types of V-REHs can generate high output power from ambient vibrations, they work well only under ultralow-frequency (<5 Hz) vibrations and require comparatively large driving (compressive) forces, exhibiting limited applicability.

To overcome the limitations of the aforementioned V-REHs, this paper reports a high-performance V-REH realized with a new cantilever-plucked rotor for generating high electric outputs at relatively high vibration frequencies (greater than5 Hz). At the core of the V-REH is an innovative plectrum with variable stiffness, which consists of a rigid part for plucking the rotor to rotate and a flexible part for reducing the frictional resistance acting on the rotor. Enabled by the plectrum, the vibration of a cantilever beam can drive the rotor in the V-REH to rotate uni-directionally and continuously with high speeds via the accelerative force of the proof mass attached on the beam. An electromagnetic V-REH is designed and prototyped to reveal its energy harvesting performance, which can deliver 4 mW power to an electric load of 950 Ω when excited by a low-level harmonic vibration of 0.4 g (1 g = 9.8 m/s²) at 8.6 Hz. The fabricated V-REH prototype has also been utilized to power a wireless hygrothermograph, a Timer and a calculator, which demonstrates the potential application of the V-REH in realizing self-sustained electronics with the renewable energy harvested from ambient vibrations.