Micro electromagnetic vibration energy harvester based on free/impact motion for low frequency–large amplitude operation

https://doi.org/10.1016/j.sna.2015.01.025Get rights and content

Highlights

  • New non-resonant micro-electromagnetic energy harvester based on combined free/impact motion is presented.

  • It shows a unique dynamic behaviour.

  • The power generated increases with both input amplitude and frequency.

  • The harvester simple design promotes size minimization.

  • The harvester is well suited for human-powered micro devices.

Abstract

This paper presents design, simulation, and experimentation of a novel micro-electromagnetic vibration energy harvester based on free/impact motion. Power harvesting is simply achieved from relative oscillation between a permanent magnet allowed to move freely inside a tube-carrying electrical coil with two end stoppers and directly connected to the vibration source. The proposed harvester with free/impact motion shows a non-resonant behaviour in which the output power continuously increase with the input frequency and/or amplitude. In addition, the allowable free motion permits significant power scavenging at low frequencies. Hence, the proposed harvester is well suited for the applications involved variable large amplitude–low frequency vibrations such as human-powered devices. A nonlinear mathematical model of the proposed harvester including electromagnetic and impact characteristics is derived and used further for a case study model prediction. A unique way of oscillation is observed, in which four modes of magnet/tube relative motion appear over the range of exciting amplitudes and frequencies. Two experiments are conducted on different fabricated prototypes. The first shows the effect of different magnet shapes on the harvesting performance, and the second is carried out to investigate the performance of two different size prototypes with variable large amplitude-low frequency vibrations. A harvester with cylindrical total size of D9 × L12 mm can generate RMS power of 71.8 μW at (2.5 Hz and 5.2 ms−2) and 113.3 μW at (3.33 Hz and 12.38 ms−2). Another of D7 × L12 mm size can generate RMS power of 28.4 μW at (2.5 Hz and 5.2 ms−2) and 82.9 μW at (3.33 Hz and 12.38 ms−2). Comparison with some previously fabricated low frequency energy harvesters is made which shows the advantageous of the new harvester in size minimization as well as the significant power raise with the input amplitude.

Introduction

Recently, significant growth has been shown in the field of miniature wireless sensors such as body implantable and wearable sensors, industrial structure health monitoring and embedded sensors in aerospace, machines and automotive applications. One physical feature of those sensors is to be completely embedded in the equipment or body without any physical connection to the outside world. Consequently, those sensors have to have their own power supply unit. Chemical batteries minimization is possible by using micro battery. However, the life time still limited which needs a periodic replacement. Adding a micro generator to the system is considered one solution to this problem, which depends on generation the required electrical power through converting one of the environmental energy sources. Mechanical vibration is one of the most available energy sources in the machine medium and moving systems. However, challenge is faced when dealing with vibration energy harvesting from human-induced motion. Human vibrations are low frequency–large amplitude in natural [1], [2], and continuously varying with time. The most common energy harvesting architectures are resonant systems, which consist of a proof mass vibrates inside a frame with a spring suspension. The relative oscillation can then be converted to electrical energy through three transduction mechanisms; namely electromagnetic [3], [4], electrostatic [5], [6], and piezoelectric [7], [8]. The resonant harvester can work effectively only within narrow range of exciting frequencies. In addition it is difficult to match low frequencies while keeping size minimization [9]. Further, the amplitude of the body motion is relatively large compare to the mass limit displacement within the harvester frame, which makes the resonant amplification useless.

Non-resonant harvesters can propose a solution for such problem and become more suited for human-powered devices. One of the earlier works in this way is the electrostatic Coulomb-force parametric generator (CFPG) [9], [10]. The relative displacement in CFPG is allowed only at the maximum input acceleration. This can be achieved by adjusting a holding electrostatic force to an optimum value, which is slightly below the maximum inertia force. Exciting CFPG with a high acceleration allow a higher holding force to be adjusted and consequently higher power harvesting. In fact, holding force can be easily adjusted by setting the prime voltage to an optimal value for a given source of acceleration. However, for a variable acceleration source such as human body motion, optimization should be carried out dynamically, which required an optimization power supply module [11]. MEMS-CFPG is studied, and analyzed in [10], fabricated and tested by a low frequency shaker in the range of 10–100 Hz [11] as well as by actual human walking motion [12]. However, some problems are observed in the experiment such tilting of the proof mass, late landing of the moving plate due to air damping, etc., which need further improvements [11].

Bowers and Arnold [13], and Rao et al. [14] utilize the free oscillation to achieve low frequency energy harvesting. They construct an electromagnetic non-resonant architecture which consists of a ball magnet allowed to move freely within a spherical cavity wrapped with copper coil windings. The second tested a prototype of 100 cm3, which shows an average power of 300 μW when attached to human ankle during walking. The harvester output power is quite large, however its large size remains an obstacle for the usage with human body applications.

Lee and Chung [15] present an electromagnetic harvester of 21 cm3 and 30 g mass which consists of planner spring, NdFeB permanent magnets, and a copper coil. The proposed harvester is able to generate a maximum power of 65.33 μW at a resonance frequency of 8 Hz and 1.96 ms−2.

A principle of frequency-up conversion [16], [17] is also introduced for improving power generation at low frequencies [18]. Galchev et al. [19] uses frequency-up conversion technique in an electromagnetic generator. An average power of 2.3 μW is obtained at 0.54 m/s2 and 2 Hz. Pillatsch et al. [20] achieve the frequency-up conversion through plucking of a piezoelectric beam by a magnetic coupling with a free rotating mass. The rotation motion can be preferred to human body movement. A prototype of 30 mm diameter disc and 7 mm thickness can generate an electrical power at low range of frequencies with a peak value of 43 μW at 2 Hz and 20 ms−2.

Other ways of improving low frequency energy harvesting is presented. Naruse et al. [21] propose a micro-electrostatic generator based on new electret structure, which can deliver 40 μW at 2 Hz and 0.4 g acceleration. Jo et al. [22] utilize the magnetic spring and planner coil structure to fabricate a low frequency electromagnetic energy harvester to be used for human body vibrations. A prototype of approximately 25 mm × 25 mm × 16 mm size is able to generate a maximum power of 430 μW at 8 Hz resonant frequency. Magnetoelectric laminated structure is utilized in vibration energy harvesting [23], [24], [25]. An improvement in the energy harvesting performance is shown [26], [27], where the nonlinear motion not only leads to a broadband energy harvesting but also resulted in a double power peak, as well as the ability of bi-directional energy harvesting.

A way of introducing impact in piezoelectric energy harvesting is presented [28] for either achieving non-resonant behaviour or up converting the input frequency. For instance, Renaud et al. [29] achieve a non-resonant behaviour from repeated impact of a free ball on two piezoelectric plates. Gu and Livermore [30] present an impact-driven, resonant, frequency-up converting macro-piezoelectric harvester that comprise a low frequency driven beam and high frequency generating beam. Vibro-impact mechanism with end stops is introduced in vibration energy harvesting with either piezoelectric [31], [32] or electromagnetic [33] transductions, where a broadband energy harvesting and increase in the total output power could be obtained. However, Vanderwater and Moss [34] recently carried out a theoretical modelling with experimental validation of a vibro-impact energy harvester implemented as ball-bearing/permanent magnet arrangement. The study shows there is a trade-off between the operating bandwidth and output power.

Impact with end stops is also introduced and modelled in some beam-based resonant harvesters that work with a large amplitude source. The aim of using end stops is to limit the internal displacement of the proof mass as well as broaden the bandwidth by introducing impact nonlinearity [35], [36], [37]. They are also integrated in some electrostatic harvesters to limit the mass displacement [38] and in other cases used as slave motion harvesters as well [39].

In this work, a non-resonant micro-electromagnetic energy harvester or micro-electromagnetic generator based on free/impact motion (FIMG) is presented. A proof magnet mass is allowed to move freely inside a tube-carrying electrical coil, and collide with the tube end stoppers when it reaches the extreme positions. Power harvesting is achieved from tube/magnet relative motion by electromagnetic induction. The free motion promotes power harvesting at low frequencies while free/impact motion leads to non-resonant harvesting behaviour, in which the output power increases with both input amplitude and frequency. A mathematical model including the mechanical and electromagnetic properties is derived. The impact force is modelled according to Lankarani and Nikravesh [40], which considers the elastic and damping natural of the collided bodies. The obtained mathematical model is nonlinear which can be solved numerically. A case study of FIMG with certain parameters is studied and analyzed using the derived model. Uncommon way of oscillation is observed in which four different modes of relative motion appear over the range of input amplitudes and frequency. In addition, a continuous raise of the output power is observed with either input amplitude or frequency.

Two different experiments are conducted. The first is carried out on four fabricated prototypes with identical sizes, however with different magnet shapes. This experiment shows the effect of different magnet shapes on the harvesting performance. The second is performed on two different size prototypes with the same magnets. Finally, comparison is made between some FIMG prototypes presented in this work and some low frequency energy harvesters stated in literature. The comparison shows the advantageous of FIMG for size minimization as well as its improved performance with large input amplitudes

Section snippets

System configuration and fabrication

The schematic of the micro-electromagnetic generator based on free/impact motion (FIMG) is shown in Fig. 1. It is simply consists of a thin walled tube with a cylindrical permanent magnet inside and small gap in between (the magnet can also takes the shape of ball or double ball as shown in Section 5). The tube is closed at both ends by two thin washers. Enamelled copper wire is wounded over the tube to form an electrical coil and secured in position by the tube flanges. The coil consists of

Mechanical system

By giving an input vibration yi(t) to the tube, the equation of motion of the permanent magnet can be expressed as:My+Cz˙+z|z|FN+z˙|z˙|μMg=0where M is the magnet mass, y is the absolute magnet displacement, z is the magnet relative displacement (z = y  yi), FN is the impact force with stoppers, C is the total viscous damping coefficient which includes parasitic damping coefficient (Ca), and electrical damping coefficient (Ce) [43], μ is the coulomb's friction coefficient.

The contact between the

Model predictions

The mathematical model of FIMG is described by nonlinear equations, which can be solved numerically. The aim of model predictions in this work is to study the behaviour of the micro-electromagnetic energy harvester with the free/impact vibration motion associated with the proposed design, and investigate the energy harvesting performance with variable frequency and amplitude vibrations. Model predictions of a case study with certain parameters listed in Table 1, Table 2 are obtained, followed

Experimentation

Two experiments are conducted in this work. The first is carried out on four fabricated prototypes having identical sizes and coils, but different magnet shapes. The second is performed on two different size prototypes with the same magnets and different number of coil turns. The aim of the first experiment is to examine the effect of magnet shape on the harvesting performance, which is difficult and less effective to be analyzed by modelling, as well as verifying the presented theoretical

Comparison and evaluation

The criterion of evaluating vibration energy harvester for human-power applications is the ability to match variable low frequency–large amplitude vibrations with large power output and a small size system. The best evaluation of FIMG can be done by comparing the power generated by it with the power generated by other low frequency energy harvesters with the same size and input vibration. However, the date published in previous works diverse extensively (different harvester sizes and input

Conclusion and future work

In this work, a new micro-electromagnetic energy harvester based on free/impact motion is presented. Free relative motion is allowed between tube-carrying an electrical coil directly connected to the vibration source and a permanent magnet inside it. Possible impacts appear between the magnet and the tube end stoppers during oscillation. The resulted tube/magnet relative motion has a unique style, in which four different modes of motion appears over the range of exciting frequencies and

Ahmed Haroun was born in Cairo, Egypt in December 1986. He received his B.S and M.S degrees in Mechanical design and production engineering from Cairo University in 2008 and 2011 respectively. He is currently pursuing his Ph.D. degree in mechanical engineering at University of Tokyo. From 2008 to 2011 he was a teaching and research assistant with department of mechanical design and production engineering, Cairo University. From 2012 till now he is a lecturer assistant at the same department.

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      To address the aforementioned issue, a few authors introduced piecewise linear hardening behavior to broaden the operating bandwidth of mechanical frequency increased generators. Haroun et al. [12] proposed a micro-electromagnetic harvester based on free/impact motion. The free motion increases the harvester's efficiency at low frequencies, while the free/impact combination makes the output power increase with both input amplitude and frequency.

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    Ahmed Haroun was born in Cairo, Egypt in December 1986. He received his B.S and M.S degrees in Mechanical design and production engineering from Cairo University in 2008 and 2011 respectively. He is currently pursuing his Ph.D. degree in mechanical engineering at University of Tokyo. From 2008 to 2011 he was a teaching and research assistant with department of mechanical design and production engineering, Cairo University. From 2012 till now he is a lecturer assistant at the same department. His research interests include MEMS sensors, powering of wireless sensor nodes, vibration energy harvesting, and dynamics of multibody systems.

    Ichiro Yamada received his Ph.D. in mechanical engineering from The University of Tokyo in 1985. He was a director of NTT (Nippon Telegraph and Telephone Corp.) Lifestyle and Environmental Technology Laboratories, and is presently a professor of Graduate School of Frontier Sciences, The University of Tokyo. He has worked on research and development of optical MSS (mass storage system), fuel-cell energy system, wearable sensors and their applications, etc. He is promoting the research of Human and Environmental Informatics, and is presently interested in wearable sensing systems for preventive healthcare monitoring.

    Shin’ichi Warisawa received his B.Eng., M.Eng., and D.Eng. in Mechanical Engineering from the University of Tokyo in 1989, 1991, 1994, respectively. He is presently an associate professor of Graduate School of Frontier Science, the University of Tokyo. He is currently focusing on Nanomechanics includes design, fabrication, measurement and application of Nano Electromechanical Systems (NEMS), and also promoting its application to wearable sensors utilized in a preventive healthcare service. He is a member of Japan Society of Mechanical Engineers, Japan Society of Precision Engineering, Japan Society of Applied Physics, Robotics Society of Japan and more.

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