Design and experiment of a human-limb driven, frequency up-converted electromagnetic energy harvester

Energy Conversion and Management 106 (2015) 393–404

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Design and experiment of a human-limb driven, frequency up-converted electromagnetic energy harvester Miah A. Halim, Hyunok Cho, Jae Y. Park ⇑ Department of Electronic Engineering, Kwangwoon University, 447-1 Wolgye-dong, Nowon-gu, Seoul 139-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 11 July 2015 Accepted 25 September 2015

Keywords: Non-resonant Frequency up-conversion Helical compression spring Human-limb motion Electromagnetic energy harvester

a b s t r a c t We present a frequency up-converted electromagnetic energy harvester that generates significant power from human-limb motion (hand-shaking). Because the power generated by a vibration energy harvester is proportional to the operating frequency, the proposed energy harvester has been designed to upconvert the applied low-frequency vibration to a high-frequency vibration by mechanical impact. Upon excitation, a freely moveable ball (non-magnetic) within a cylindrical structure periodically hits two magnets suspended on two helical compression springs located at either ends of the cylinder, allowing these to vibrate with higher frequencies. The relative motion between the magnets and coils (wrapped around the outside of the cylinder) induces e.m.f. (voltage). High-frequency oscillators have been designed through the design parameters (i.e., frequency, spring stiffness, mechanical, and electrical damping), to minimize the power loss. A prototype was fabricated and tested both using a vibration exciter and by manual hand-shaking. The fabricated device showed non-resonant behavior during the vibration exciter test. At optimum load condition, the frequency up-converted generators (FUGs) delivered 0.84 mW and 0.96 mW of average power. A maximum 2.15 mW of average power was obtained from the device with series connected FUGs while it was mounted on a smart phone and was hand-shaken. The fabricated device exhibited 0.33 mW cm
3 of average power density, which is very high compared to the current state-of-the-art devices, indicating its ability in powering portable and wearable smart devices from extremely low frequency (5 Hz) vibration. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Over past few decades, clean and regenerative energy sources have become more important due to the increasing global warnings on environmental issues. Meanwhile, advances in technologies of micro-electromechanical system technology and electronics industries mean that wireless sensors and portable and implantable smart devices are being manufactured with miniaturized, low-power consumption, and low-cost features that allow their wide potential applications and accessibility in hostile environments [1]. However, one of the major challenges is the energy source. The power required for these devices to be operated is mainly generated by conventional electrochemical batteries and micro fuel cells. Although these power sources can provide more power, they require periodic charging; they also need to be replaced due to their limited lifetime. Sometimes these power sources are inconvenient, expensive, tedious, and even impossible to recharge or replace. Moreover, disposal of the expired batteries ⇑ Corresponding author. Tel.: +82 2 940 5113; fax: +82 2 942 1502. E-mail address: [email protected] (J.Y. Park). http://dx.doi.org/10.1016/j.enconman.2015.09.065 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

and cells exacerbates environmental pollutions. Therefore, energy harvesting from environmental sources such as light, wind, ambient heat, acoustic noise, radio waves, and vibrations has attracted considerable research interest as an alternative power source for these devices [2–4]. Among these energy sources, kinetic energy in the form of vibration or motion is more attractive due to its versatility, ubiquity, inexhaustibility, and abundance in nature [5,6]. The well known techniques for harvesting energy from vibration or motion are piezoelectric, electromagnetic, electrostatic, and magnetoelectric transduction mechanisms, which have already been published extensively in the literature [7–14]. The electromagnetic mechanism has been chosen in this study. Generally, vibration harvesters are utilized by resonant systems that prolong the vibration displacement and preserve kinetic energy which is then transferred to electrical energy by any of the transduction mechanisms mentioned above. A vibration energy harvester must be customized for a specific operating condition and application because the operating conditions (e.g., the available vibration characteristics, required output voltage and power, overall harvester dimension, mass, and volume) will differ for different applications. In the work presented here, we developed

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a frequency-up converted electromagnetic energy harvester based on an application-oriented operation through the optimization of design parameters. Following the introduction, Section 2 will discuss the design choices regarding the vibration environment and intended applications, from which system architecture will be developed. The theory behind the working of the proposed system along with the parameters design will be analyzed in Section 3. The fabrication of a prototype will be discussed in Section 4. Subsequently, the realistic performance of the fabricated prototype based on the proposed system architecture and the parameter design process will be verified by carrying out a series of experiments in Section 5. Finally, a conclusion will be drawn in Section 6.

Z0 ¼

QApeak

x2r

ð2Þ

Eq. (2) shows that at the resonance, for a given acceleration amplitude, the magnitude of proof mass deflection depends on the quality factor of the resonant generator. All the resonant generators reported to date are under damped ðf < 1Þ; which have higher quality factor ðQ > 1Þ. Therefore, a relatively large proof mass deflection is obtained from a resonant generator with a high quality factor than that with a low quality factor, at a certain Apeak. In addition, the amplitude of proof mass deflection increases with the decrease in resonant frequency. For this reason, designing a small scale low frequency resonant generator with high quality factor is a challenge.

2. Motivation and system architecture 2.2. Frequency up-conversion 2.1. Challenges in low frequency energy harvesting Typically, most vibration energy harvesters are resonant devices employing one or more spring-mass-damper systems. These devices must be operated at their resonant frequencies in order to harvest maximum energy because they generate maximum voltages and power at their resonances. It has been observed that the average power of a vibration energy harvester decreases dramatically as the resonant frequency decreases [15]. Unfortunately, most ambient vibration sources have peak vibration amplitude at low frequencies (<100 Hz) [16]. The ambient vibration frequencies for human-body-induced motion and machine-induced motion range from 1 to 10 Hz and from 10 to 100 Hz, respectively [17]. The kinetic energy within vibration or motion varies unpredictably with cyclic movements in different directions from time to time within the environment [18]. A number of studies on the characteristics of ambient environmental vibrations generated by various sources in various locations and conditions have been published in the literature [19–23]. Generally, a mass loaded cantilever beam is used as the spring element where the loaded-mass works as the proof-mass. The undamped angular resonant frequency of a cantilever in transverse motion is

sffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 keq Ewh =4L3 xr ¼ ¼ meq ð33=140Þmb þ mt

ð1Þ

keq is the equivalent spring stiffness of the beam expressed with the length L, width w, thickness of the beam h, and Young’s modulus E of beam material; meq is the equivalent mass composed of the beam mass mb and the tip mass mt [24]. According to Eq. (1), lowering the resonant frequency increases the dimensions of the resonating element either by reducing the stiffness of the compliant spring or by introducing a heavy mass on the spring, or both. Explicitly, either decreasing keq by any of (i) increasing L, (ii) decreasing w, (iii) decreasing h or increasing mt makes the system impractical (either large or weak, or both) for any desired application. Moreover, reliability of the cantilever beam must be taken into consideration while decreasing its keq or increasing mt. Therefore, it is not easy to ensure that a typical straight cantilever beam can meet the low-frequency energy harvesting requirement. While the resonant system is subjected to a periodic base vibration of amplitude Y0, the magnitude of proof mass deflection Z0 can be expressed as Z 0 ¼ QY 0 ; where Q is the quality factor related to the dimensionless damping ratio f as Q ¼ 1=ð2fÞ [25]. Vibrations are commonly referred to in terms of acceleration amplitude Apeak rather than displacement amplitude Y0 related as Apeak ¼ x2r Y 0 . Therefore, the magnitude of proof mass deflection can be re-written as

To address the low frequency environmental energy harvesting challenges, mechanical frequency up-conversion techniques have been suggested and demonstrated by many researchers over the past few years [26–35]. A mechanical frequency up-converted energy harvester uses at least two oscillating structures, one of which (low frequency oscillator) absorbs kinetic energy from low frequency environmental vibration and transfers it to the second one (high frequency oscillator) either by mechanical impact or by non-mechanical interaction such as magnetic attraction/repulsion. The kinetic energy transferred to the high frequency oscillator is then converted into electrical energy by any of the transduction mechanisms mentioned earlier. Being excited by the first oscillator, the second oscillator oscillates freely with its damped resonant frequency, resulting in an exponentially decaying motion with time. The periodic motion of the low frequency oscillator re-excites the high frequency oscillator in each of its cycles. As the kinetic energy of the low frequency oscillator is periodically removed by the high frequency oscillator, the larger amplitude of the proof mass motion in the low frequency oscillator is restricted within an allowed geometry. A number of prototypes of mechanical frequency up-converted energy harvesters have been designed and presented in the literature. In their design, Rastegar et al. [27] presented a two stage system in which a proof mass absorbs very low frequency vibration energy and triggers the high frequency vibration of two piezoelectric cantilever beams to convert the vibration energy to electrical energy. Lee et al. [28] suggested a comb slider with low resonant frequency that excites a high resonant frequency piezoelectric cantilever beam by means of a sharp probe touching the ridges of the comb slider. Kulah and Najafi [17] and Galchev et al. [30] proposed magnetic attraction force in order to achieve frequency up-conversion. Unfortunately, each of the proposed structures mentioned above has been designed to operate at a specific resonant frequency of the low frequency oscillator. The unpredictably varying characteristics (frequency and amplitude) of the ambient vibration limit their efficient energy harvesting in low frequency vibration environments. In order to overcome this limitation, a number of research groups have proposed frequency up-converted wideband energy harvesters. Jung and Yun [31] demonstrated snap-through bulking for mechanical frequency up-conversion and the bi-stable behavior of buckled bridges in the proposed device offers wideband operation at an ambient vibration frequency. Tang et al. [32] demonstrated a broadband, bi-stable frequency up-converted energy harvester driven by non-contact magnetic repulsion. Liu et al. [33] and Halim and Park [35] investigated wideband frequency response of an impact driven piecewise linear energy harvester with mechanical stoppers. Table 1 summarizes the frequency up-conversion methods used in some of the aforementioned references and their operating

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M.A. Halim et al. / Energy Conversion and Management 106 (2015) 393–404 Table 1 Summary of frequency up-conversion methods used in the recently demonstrated low frequency energy harvesters. Reference

Transduction mechanism

Frequency up-conversion method

Harvester type

Frequency/bandwidth

Power density

Kulah and Najafi [17] Lee et al. [28] Zorlu et al. [29] Galchev et al. [30] Jung and Yun [31] Tang et al. [32] Liu et al. [33] Halim and Park [35]

Electromagnetic Piezoelectric Electromagnetic Piezoelectric Piezoelectric Piezoelectric Piezoelectric Piezoelectric

Magnetic attraction Comb slider Mechanical impact Magnetic attraction Snap through bulking Bi-stable Piecewise linear impact Piecewise linear impact

Resonant Resonant Resonant Resonant Wideband Wideband Wideband Wideband

25 Hz 60 Hz 10 Hz 10 Hz 12–38 Hz 10–22 Hz 12–26 Hz 7–14.5 Hz

– 225 lW cm
2 – 2.7 lW cm
3 – 8.4 lW cm
3 159.4 lW cm
3 38.8 lW cm
3

frequency ranges. It is observed that all the frequency up-converted energy harvesters show varied ability to be operated at low frequency ranges with a minimum frequency of 7 Hz. It is important to note that all these devices used a spring-mass system as a low frequency oscillator having specific resonant frequency. However, for a frequency lower than 7 Hz that belongs to human motion vibration frequency, employing a spring-mass system as a low frequency oscillator becomes unrealistic. Even though proper design of a frequency up-converted vibration energy harvester addresses the challenges aforementioned, a number of research issues still need to be addressed before its commercialization (e.g., efficient power management of the decayed output voltage, micro-fabrication requires special processes due to complicated design, reliability of the oscillating elements in order to withstand the force created during impact or non-impact interaction within it, etc.). 2.3. System architecture and its operation A number of attempts have been made to harvest energy from basic human activities such as walking, running, finger and elbow/shoulder movement, and shaking limbs [36–41]. While most of these devices are wearable on different places of the human body, they are quite uncomfortable. In this case, we intend to design an electromagnetic energy harvester to be applied for powering or recharging the batteries of portable smart devices from human-limb motion (hand-shaking vibration). We have

observed the basic human activities e.g., walking, running, jumping, exercising, etc. However, people only perform these activities occasionally and/or sporadically since most of their time is spent on their regular working tasks. Moreover, any vibration energy harvester performs better in harmonic excitation rather than random excitation. Most of the human activities such as running, jumping, and exercising cannot produce harmonic excitation; it produces random excitation in different directions. We recently reported characteristics of hand-shaking vibration [34], showing that the vibration is nearly harmonic with low frequency (2.5–6 Hz) and high amplitude (15–20 ms
2 peak acceleration). In order to address the challenge of harvesting energy from vibration of frequency below 6 Hz (human hand-shaking vibration), a frequency up-converted, non-resonant electromagnetic energy harvester architecture has newly been designed. As per the definition of ‘Frequency Up-conversion’, the proposed device must accept and convert low-frequency vibration to high-frequency vibration in order to act as a linear motion transformer. The design of a system showing this behavior can be achieved using the basic mechanical vibration theory
when under-damped structures are excited by an initial condition such as displacement or velocity; their response will be an oscillatory motion with exponential decay [31]. In order to realize such a design, the necessary condition is to construct a mechanism that periodically excites the generator. As shown in Fig. 1(a), the harvester is designed with a spring-less structure (a freely movable non-magnetic ball) as a low frequency oscillator in the middle of

Fig. 1. (a) Schematic structure and (b) operation principle of the proposed energy harvester.

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two high frequency oscillators. The ball acts as an inertial mass that couples the applied low frequency vibration into the high frequency spring-mass structure by direct mechanical impact and transfers the kinetic energy to the structures. The high frequency oscillator then converts the kinetic energy into electrical energy via electromagnetic induction. It should be noted that the high frequency structures placed above and below the ball act as stoppers to accommodate the large amplitude of the ball’s vibration. Each of the high-frequency generators (FUGs) consists of a helical compression spring, where one end is attached to the end-cover of the cylindrical-shaped housing and an NdFeB cylinder magnet is attached to the other end. Two pick-up coils are wound around the magnets over the cylindrical structure. The operation of the proposed energy harvester is illustrated in Fig. 1(b). For ease of explanation, we consider the ball is at the middle position. The ball starts moving in response to the applied periodic vibration. The ball impacts on the FUG-1 magnet with a certain force at its peak displacement and bounces back during the first half-cycle. As a result of impact, the magnet on the spring vibrates with a high damped resonant frequency due to the compression and rarefaction of the spring. The kinetic energy is transferred from the ball to the spring and is then converted to electrical energy by the relative motion between the magnet and coil. The process is repeated for the next half cycle of the ball displacement for FUG-2, and continues periodically.

Fig. 2. (a) Mechanical model and (b) the equivalent circuit of electromotive force with coil and load resistance for each FUG of the proposed energy harvester.

3. Theoretical modeling and parameters design 3.1. Modeling of the proposed energy harvester The proposed frequency up-converted electromagnetic energy harvester architecture is composed of two force-driven single degree of freedom (SDOF) spring-mass damper systems which are excited by a periodic force FðtÞ ¼ F 0 sin ðxtÞ; where F0 is the amplitude of force generated by the inertial mass M and x is the angular frequency of its vibration. This periodic force F(t) acts on the high-frequency oscillator with amplitude F0 when the ball impacts periodically on the mass (magnet) of the oscillator. The impact can be classified as rigid body impact since the contact area between the bodies (spherical ball and plane surface of a cylindrical magnet) is small in comparison with all section dimensions. The mechanical model of the harvester is shown in Fig. 2(a), where two FUGs are presented by masses m1 and m2, spring constants k1 and k2, damping coefficients c1 and c2 for FUG-1 and FUG-2, respectively. Upon excitation, the inertial mass M vibrates periodically with an acceleration A and impacts on m1 or m2 with force F 0 ¼ MA, allowing the masses m1 and m2 to displace x1 or x2 from their equilibrium positions, respectively. It should be noted that the impact period between M and m1 or m2 is too short to be ignored because the collision between them (M and m1, or M and m2) is considered to be completely elastic. According to Newton’s second law, the standard form of the differential equation of motion of each force-driven damped SDOF system with singlefrequency periodic excitation is [42]

mi €xi ðtÞ þ ci x_ i ðtÞ þ ki xi ðtÞ ¼ FðtÞ

ð3Þ

where i ¼ 1; 2 indicates the corresponding variables to be associated with FUG-1 and FUG-2, respectively. The relative displacement of the mass mi can be obtained with the initial conditions xi ð0Þ ¼ x0 and x_ i ð0Þ ¼ 0 is expressed as:

xi ðtÞ ¼

  qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi F 0 e
fi xri t qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fi sinðxdi tÞ þ ð1
f2i Þ cosðxdi tÞ ki ð1
f2i Þ

ð4Þ

where xri and xdi are the resonant and damped resonant frequencies, respectively, and fi is the damping ratio of the corresponding spring-mass-damper system which is defined as:

sffiffiffiffiffiffi ki xri ¼ ; mi

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

c

i xdi ¼ xri ð1
f2i Þ and fi ¼ pffiffiffiffiffiffiffiffiffi

2 ki mi

ð5Þ

The relative velocity of the mass mi is then obtained by the derivative of Eq. (4) resulting in

F 0 xri e
fi xri t x_ i ðtÞ ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sinðxdi tÞ ki ð1
f2i Þ

ð6Þ

The equivalent electrical circuit of each FUG includes an e.m.f. voltage source V EMi , a coil having resistance RCi , and inductance LCi , and a load resistance RLi connected across the coil terminals, as shown in Fig. 2(b). Ii is the current induced in the coil by electromagnetic induction. Considering low inductance of the electrical circuitry, the induced open circuit e.m.f. voltage V EMi generated across the coil is given by Faraday’s law as [43]

V EMi ¼
N i

R ! !  d Bi dAi dt

¼
Ni Bi li x_ i ðtÞ

ð7Þ

The term in the integral indicates the net magnetic flux ! ! ! ðUBi ¼ B i  d A i Þ through the differential element area d A i of the magnet-coil assembly. N i is the number of coil turns, Bi is the magnetic field strength, li is the coil length, and x_ i ðtÞ is the relative velocity of the magnet (mass mi ) with respect to the coil. It is clear from Eq. (7) that for a specific electromagnetic circuit assembly, relative velocity is the main variable that depends on the mechanical design of the system. Substituting x_ i ðtÞ from Eqs. (6) and (7) can be expanded as:

F 0 xri V EMi ¼ Ni Bi li qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e
fi xri t sinðxdi tÞ ki ð1
f2i Þ

ð8Þ

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The generated power delivered to the load resistance RLi is expressed by the following

Pi ¼ I2i RLi ¼

 2 1 V EMi RLi 4 RLi þ RCi

ð9Þ

which can be expanded by using Eq. (8) as



Pi ¼

1 Ni Bi li 4 RLi þ RCi

2

2

32

6 F 0 xri 7 RLi 4 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e
fi xri t sinðxdi tÞ5 2 ki ð1
fi Þ

ð10Þ

According to the maximum power transfer theorem, maximum power is obtained when the load resistance matches the coil resistance. It is to be noted that, as the amplitude of mass (magnet) vibration decays exponentially (due to damping: both mechanical and electrical), the output load voltage and generated power will also be exponentially decayed signals. 3.2. Mechanical parameters design The dynamic behavior of the proposed energy harvester depends on its mechanical parameters that mainly include mass, spring constant, and damping coefficients of the FUG’s oscillatory system. An initial design analysis must be carried out before finalizing the design parameters that must be taken into consideration: (i) intended application, (ii) desired volume, (iii) vibration characteristics, (iv) components availability, (v) fundamental limits on various parameters, and (vi) reliability. First, since the conversion mechanism of a resonant transducer depends on frequency, the promising resonant frequency of the high-frequency oscillator must be determined. Spreemann et al. [44] identified the frequency bands that hold most energy for conversion by calculating the power spectral density (PSD) of the measured acceleration profile. Additionally, by performing full transient simulation of the mechanical model, it was shown that the advantageous operating frequencies in the considered vibration profile were clearly below 100 Hz (around 70 Hz). Based on this analysis, we designed a high-frequency oscillator having a resonant frequency below 100 Hz. Second, mechanical damping must be controlled to decrease the fast rate of the amplitude decay of the high-frequency oscillator. Mechanical damping is linearly related to the velocity of the oscillator, which increases with the increase in frequency. It occurs due to airflow force, squeeze force, internal friction, and support loss, which cannot be easily controlled [45]. In a frequency upconverting system, the resonant frequency of the high-frequency oscillator must be optimized which can be achieved by optimal spring constant (depends on spring material and geometry) and the amount of mass attached to the spring. Third, the displacement of the mass (which is a magnet in the case of an electromagnetic energy harvester), must be limited within the boundaries of the electromagnetic interaction between the magnet and coil. Parameters that control the mass displacement are the spring constant and damping factor that also depends on the spring constant as discussed earlier. Analysis of optimal damping factor can also be found in [46]. The spring constant is determined from the spring material and geometry. In our design, we used a helical compression spring instead of a cantilever beam, as shown schematically in Fig. 3. The spring constant of such a spring is determined as

k¼

Gd

4

8nD3

ð11Þ

where G is the shear modulus of the spring material, d is the spring wire diameter, n is the number of active spring coils, and D is the

Fig. 3. Mass-loaded helical compression spring as high-frequency oscillator.

mean spring diameter. Mass displacement occurs due to the deflection of the spring while subjected to an external force. The force F 0 generated by a low frequency oscillator (freely movable ball) acts as the axial load on the mass of the high frequency oscillator and allows the spring to deflect. Deflection of the spring is then expressed using Hook’s law as

d¼

8F 0 D3 n Gd

4

¼

8MAD3 n 4

Gd

ð12Þ

Finally, the minimum distance between the magnets of the front facing high-frequency oscillators should be optimized so that they do not interfere (attract or repel) with each other. The interference between two magnets affects the vibration of the high-frequency oscillator which results in the performance degradation of the energy harvester. For this reason, the minimum length of the hollow cylindrical tube must be taken into consideration. Moreover, the length of the tube should not be too large. A longer tube leaves large gap between the front facing magnets. It requires a larger displacement for the ball placed in that gap to impact on the magnets which, in turn, requires larger base vibration (applied vibration) amplitude Y0 as well as higher acceleration amplitude for the overall system (device) to operate. In our design, we have chosen the minimum length (40 mm) of the tube so that sufficient gap (20 mm) is left between the magnets to avoid their interaction to each other. 3.3. Electrical parameters design Besides the mechanical design, the generation of the highest possible voltage and power from an electromagnetic energy harvester depends on its electrical parameter design, which includes magnetic field distribution, solenoid coil characteristics, electrical damping, and the optimal equilibrium position of the magnet and coil. The spatial flux density distribution of a cylindrical permanent magnet is equivalent to that of a single layered cylindrical induction coil. The field vector is assumed to be in line with the direction as the surface normal to the coil turns, and causes a magnetic flux change in the coil turns while a relative movement

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between the magnet and coil occurs. The axial component of the magnetic flux density B of a cylinder magnet is expressed as

2 B¼

3

Br 6 dþh d 7 4qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 5 2 2 2 2 2 d þ r ðd þ hÞ þ r

ð13Þ

where Br is the residual flux density, d is the distance from the magnet, and r and h are the radius and height of the magnet, respectively. The coil has important characteristic parameters to be considered during electrical design, which include the number of turns N and the coil resistance RC. These parameters also depend on the wire cross-sectional area /2 and copper filling factor Ҕ. The filling factor is defined as the ratio of the overall wire area to the cross-section winding area. In most instances, when a wire diameter is less than 0.2 mm, the filling factor is considered to be p=4 (0.79). A coil with inner radius rI, outer radius rO, and coil length L has a resistance value calculated as

RC ¼



4q r 2O
r 2I L

ð14Þ

£4

Here, q is the specific resistivity of the wire material. The coil resistance RC has an effective role in electrical damping which occurs due to power generation through load RL. It can be expressed as (at low frequencies of <1 kHz, the coil inductance is considerably low) 2

fe ¼

ce ðNBlÞ ¼ 2mxr 2mxr ðRL þ RC Þ

ð15Þ

where fe is the electrical damping ratio and ce is the electrical damping coefficient. Eq. (15) shows that ce can be adjusted by

varying the load RL. In order to obtain maximum output power, fe should be adjusted as much as possible to match fm. In the design process, the optimum equilibrium position of the coil with respect to the magnet must be investigated in order to achieve maximum possible output. At this position, the interaction between the magnetic flux lines and the coil turns is maximum, which in turn generates maximum e.m.f. within the coil. In this case, it has been experimentally found that a maximum open circuit voltage is obtained when the cylinder magnet (poled in its thickness direction) is immerged inside the hollow cylindrical coil with 40% magnet-coil overlapping distance, as shown in Fig. 4. The thickness of the magnet and length of the coil were the same. The air gap between the magnet and coil was 1 mm. 3.4. Simulation From the above discussions on modeling and parameters design, we determined the parameters of the proposed electromagnetic energy harvester and performed time domain simulations using an appropriate simulation tool (MATLAB) in order to predict the output voltage and power. Table 2 shows the parameters used for simulation. The simulation parameters have been calculated from the geometry and material parameters of the device components which will be discussed in the ‘Prototype fabrication’ section. It was considered that the ball starts moving from the middle position and the gravity effect was ignored. Fig. 5 shows the simulation results of the open circuit voltages of each FUG at 5 Hz excitation frequency and 20 ms
2 peak acceleration. We assumed that the ball starts moving from the middle of the two front facing FUGs. As shown in the figure, FUG-2 starts generating voltage at the middle of the positive half cycle of the acceleration waveform when the ball impacts on the FUG-1

Fig. 4. (a) Schematic of magnet-coil assembly and (b) measured open circuit voltage (normalized value) for a number of equilibrium positions of the coil with respect to the magnet in axial direction.

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Table 2 Parameters for mathematical simulation of the frequency up-converted EM energy harvester. Parameter

Value

Spring stiffness Spring resonant frequency Force on magnet by impact Magnetic field strength (at d =
2 mm) Coil length Coil turns Coil resistance Mechanical damping ratio Overall damping ratio

297 N/m 50.9 Hz 0.92 N 0.569 T 5 mm 400 25.2 X 0.021 0.039

Fig. 6. Simulated instantaneous power of both FUGs delivered to a matched load resistance at 5 Hz operating frequency under 20 ms
2 peak acceleration.

Fig. 5. Simulated voltage waveforms of both FUGs at 5 Hz operating frequency under 20 ms
2 peak acceleration.

magnet. FUG-2 starts generating voltage at the middle of the negative half cycle of the acceleration waveform when the ball returns and impacts on the FUG-2 magnet. This is repeated periodically. Two consecutive maximum peaks are generated in one cycle of the ball movement because it impacts the magnets consecutively while excited with low frequency (5 Hz). The time difference between two consecutive impacts on two magnets is 50% of the time period of the low frequency vibration. Fig. 6 shows the generated power waveforms delivered to a matched load resistance under the same operating condition. In both cases, the peak amplitudes of the generated waveforms decay exponentially with time due to the damping of the spring vibration, resulting in a decrease in its average value. Damping in the open circuit voltage waveforms only constitutes mechanical damping, while damping in the instantaneous power waveform includes both mechanical damping and electrical damping. The mechanical and electrical damping ratios were found to be 0.021 and 0.018, respectively, which were measured using a flick test [47]. 4. Prototype fabrication In order to illustrate the proof of a concept, a macroscale prototype of the frequency up-converted electromagnetic energy harvester (EMEH) was fabricated and tested. As shown in Fig. 7(a), the components of the EMEH include a hollow cylindrical aluminum tube of 11 mm inner diameter and 0.5 mm wall thickness, two aluminum end covers of 14 mm diameter and 1 mm thickness, two NdFeB (1.35 T) cylinder magnets, and two closed-end helical compression springs made of 0.41 mm diameter stainless steel wire having four turns. Two high-frequency oscillators (springmass systems) were made by assembling the magnets on one

end of the springs, the other end being attached to the end covers using adhesive, as shown in Fig. 7(b). Two coils were wrapped around the outside of the hollow cylindrical tube at a position of 2 mm from the magnet-coil overlap. Each coil was made of 0.12 mm diameter laminated copper wire having 400 turns. A non-magnetic spherical ball was left between the two frontfacing high-frequency oscillators which were placed at the ends of the hollow cylindrical tube. Fig. 7(c) shows the fabricated prototype after assembling the components with its size shown in comparison with a standard AA size battery. The fabricated prototype is small, compact in size and convenient as it can be held between two fingers, as shown in Fig. 7(d). The geometric and material parameters of the macroscale prototype are given in Table 3. 5. Experimental results and discussion 5.1. Experimental procedure As we intended to operate the proposed frequency upconverted EMEH within the characteristics of hand-shaking vibration for small and portable hand-held smart system applications, the fabricated prototype was tested using both a vibration exciter test and a manual vibration test. The vibration exciter test was carried out by mounting the prototype on an electrodynamic vibration exciter, while the manual vibration test was performed by mounting the prototype on the backside of a smart phone (Galaxy SIII, Samsung Electronics) and shaking manually to observe its output response on low frequency vibration of high acceleration. Fig. 8 shows a schematic diagram of the experimental setup with a photograph of the prototype energy harvester under a vibration exciter test. In order to minimize the gravity effect on the inertia of the freely moveable ball, vibration was applied in the horizontal direction, as shown in the photograph of the figure. Fig. 9 shows the photograph of the manual vibration test. The output terminals of the prototype were connected to the digital storage oscilloscope via variable load resistors to observe and record the output voltage waveforms of the prototype under a manual vibration test. 5.2. Vibration exciter test results According to the discussion in Section 2.3, we intended to test our device under a very low frequency range that is generated by a human-body-induced vibration such as a hand-shaking vibration.

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Fig. 7. Photographs of the proposed frequency up-converted electromagnetic energy harvester: (a) individual components before assembling, (b) copper coils wound over the cylindrical tube, non-magnetic ball, and high-frequency spring-mass systems, (c) assembled prototype harvester alongside a standard AA battery for size comparison, and (d) prototype held between two fingers.

Table 3 Geometric and material parameters of the proposed frequency up-converted electromagnetic energy harvester prototype. Parameter

Value

Magnet dimension Mass of the magnet Spring dimension Spring material Shear modulus of spring material Ball (non-magnetic) material Ball diameter Mass of the ball Coil inner diameter Coil outer diameter Coil length Overall prototype dimension Overall mass of the prototype

Ø10 mm  5 mm 2.9 gram Ø6.63 mm  5.72 mm Stainless steel 80 GPa SUS-316 10.3 mm 4.36 g 12 mm 14 mm 5 mm Ø14 mm  42 mm 18.6 g

During the vibration exciter test for the frequency response of the device, vibrations were applied to the prototype within a frequency range of from 14 Hz to 25 Hz. Because of its operating limitation, the vibration exciter cannot produce vibrations with low frequency (<10 Hz) and higher acceleration (20 ms
2) for application to the prototype. Moreover, the test frequency limit of up to 25 Hz was chosen because the frequency components of the measured acceleration from the human-body-induced vibration lie within 1–25 Hz [21]. Fig. 10 shows the frequency response of the device at 20 ms
2 applied acceleration, illustrating its non-resonant behavior, while the outputs were measured from FUG-1, FUG-2, and both FUGs connected in series. It was observed that the generated peak-peak open circuit voltages for different frequencies are almost constant having average values of 1.88 V, 1.92 V, and 2.41 V for FUG-1, FUG-2, and series connected FUGs, respectively. These small changes in output voltages with frequency occur due to the random variations of the gravity effect on the inertia of the ball and the small changes of force generated

by the impact of the ball caused by its random friction at the inner wall of the cylindrical tube. It is to be noted that, since both frequency up-converted generators are independent and placed at a significant distance (16 mm, axial distance), we assumed that there is no significant interaction between them. Therefore, the effect of current (other than the current generated by electromagnetic interaction of each generator) on the generated voltage waveform (for series connected FUGs) due to the mutual inductance between the coils was ignored. Fig. 11 shows the peak–peak load voltages and peak powers delivered to various load resistances by FUG-1, FUG-2, and series connected FUGs at 15 Hz frequency under 20 ms
2 acceleration. The outputs were connected to continually adjustable load resistors and the resistance values were swept in a range from 10 X to 100 X. The voltage across the load increases as the value of load resistance increases. However, the values of maximum power delivered to the optimal load resistances of 25 X, 25 X, and 50 X were obtained as 9.23 mW, 9.62 mW, and 11.58 mW for FUG-1, FUG-2, and series connected FUGs, respectively. Also, the corresponding voltages are 0.96 V, 0.98 V, and 1.52 V. The generated power is experimentally equal to V 2L =4RL , where V L is the peak–peak voltage across the load resistance RL . Values of peak–peak voltage and peak power obtained from the series connected FUGs are somewhat lower than the sum of those of the individual FUGs because the amplitude of the voltage waveform decreases exponentially and the phase difference exists between the individual outputs of FUG-1 and FUG-2. However, the value of average power generated by the series connected FUGs is impressive compared to the individual FUGs, which will be discussed in the next sub-section.

5.3. Manual vibration test results The fabricated prototype was mounted on the backside of a smart phone (as shown in Fig. 9) and the output voltages across

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Fig. 8. Schematic diagram of the vibration exciter test and a photograph of the fabricated prototype energy harvester under test condition.

Fig. 10. Peak–peak open circuit voltage as a function of frequency under 20 ms
2 input acceleration. Fig. 9. Photograph of the manual vibration test while the frequency up-converted EMEH prototype was mounted on the backside of a smartphone.

the corresponding optimum load resistances of FUG-1, FUG-2, and series connected FUGs were measured by hand-shaking the smart phone (along with the prototype). The acceleration generated by manual shaking (peak value, 20.2 ms
2) was also measured at the same time by the accelerometer (LSM330DLC 3-axis accelerometer; ST Microelectronics) embedded within the smart phone. Fig. 12 shows the waveforms of the instantaneously generated voltage (along with the applied acceleration waveform) across the corresponding optimum load resistances connected to individual and series connected FUGs. The maximum peak–peak load voltages are 0.96 V, 0.98 V, and 1.52 V, for the FUG-1, FUG-2, and series connected FUGs, respectively. However, the respective rms voltages are reduced to 0.16 V, 0.15 V, and 0.33 V because of the exponentially decaying behavior of the waveforms, as predicted by simulation. We determined the frequencies of the generated output voltages and the applied vibration by Fast Fourier Transform (FFT) analysis of the waveforms, which are shown in Fig. 13. It was found from the FFT analysis that the frequencies of the generated voltage waveforms are 53.8 Hz (FUG-1), 52.8 Hz (FUG-2), and 50.7 Hz (series connected FUGs), whereas that of the applied vibration is 5.2 Hz. This clearly indicates the frequency up-conversion behavior of the proposed energy harvester. The slight changes in up-converted frequencies are due to the process variations in mounting and assembling the device components.

Fig. 14 shows the instantaneous power waveforms which have the maximum peak values of 9.78 mW, 9.84 mW, and 11.89 mW for FUG-1, FUG-2, and series connected FUGs, respectively. These waveforms are also attenuated exponentially with time, though not perfectly, as predicted by simulation. This occurs due to the imperfect assembling of the magnet on the spring, which does not allow the spring to vibrate at large, resulting in imperfect damping behavior. It is observed from the figure that the peak values of the power waveforms are nearly zero for almost half of the impact cycles for both FUG-1 and FUG-2, which in turn reduce the values of average power dramatically. The average powers for FUG1 and FUG-2 have been calculated as 0.96 mW and 0.86 mW, respectively. Even though both FUGs have the same characteristics, these values differ due to variations in their damping behaviors. In the case of individual FUGs, the ball impacts on the corresponding magnet once during its one half-cycle. The output amplitude decays significantly during the next half-cycle and reduces to zero (almost) while the next impact occurs in the next cycle. The amplitude decays are recovered because two impacts occur in a full cycle when the outputs of both FUGs are connected in series, resulting in a significant improvement in the average power. In such a case, the maximum average power generated by the device is 2.15 mW and the average power density is 0.33 mW cm
3. It is observed that the experimentally obtained results are slightly different than the

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Fig. 11. Peak–peak output voltage and peak power versus load resistance at 15 Hz excitation frequency and 20 ms
2 input acceleration.

Fig. 13. Frequency components (by FFT) of the output voltage waveforms generated by (a) FUG-1, FUG-2 and series connected FUGs of the prototype and (b) the applied excitation obtained from manual vibration test.

Fig. 14. Instantaneous power waveforms across the corresponding optimum load resistances generated by (a) FUG-1, (b) FUG-2, and (c) series connected FUGs of the prototype obtained from manual vibration test.

Fig. 12. Output voltage waveforms across the corresponding load resistances of (a) FUG-1, (b) FUG-2, and (c) series connected FUGs of the prototype resulting from the application of manual hand-shaking.

results predicted by simulation. It occurs due to imperfect assembling of the device components. Since most electronic devices run on dc voltage/current, power generated from the energy harvester cannot be used directly; a self-driven power management circuit (PCM) is necessary as the

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Fig. 15. (a) Schematic diagram of a complete energy harvesting system, (b) photograph of manual vibration test with PMC, and (c) LEDs as electronic load.

Table 4 Comparison of this work with previously reported works (on macroscale) based on frequency up-conversion mechanism. References

Transduction mechanism

Operating frequency (Hz)

Applied Acceleration (g)

Max. peak Output power (mW)

Prototype volume (cm3)

Normalized avg. power density (lW cm
3 g
2)

Kulah and Najafi [17] Ashraf et al. [18] Zorlu et al. [29] Galchev et al. [30] Tang et al. [32] Gu et al. [48] This work

Electromagnetic Electromagnetic Electromagnetic Piezoelectric Piezoelectric Piezoelectric Electromagneticb

1 10 10 10 10 8.2 5.17

– 1 – 1 1 0.6 2.06

1.2  10
4 1.8 0.0687(rms) 0.1 0.101 0.429 11.89

– 27.38 – 1.2 1.67 a – 6.47

– 65.74 – 2.7 8.4 70.83 78.47

g = 9.8 ms
2. a Functional volume. b Non-resonant operation.

interface between the energy harvesting device and the electronic load. The PCM consists of an AC–DC converter (to convert instantaneous AC voltage in either pulse, periodic or irregular form), a booster (DC–DC converter to step up the converted DC voltage), a power controller (switching the circuit between charging and discharging modes), and an energy storage capacitor as shown in Fig. 15(a). Fig. 15(b) shows the photograph of manual vibration test with the PMC. Maximum 2.25 V DC voltage was obtained across the storage capacitor (100 lF) that was able to turn on a number of LEDs to observe the working state of the electronic load as shown in Fig. 15(c). Further details of the PMC and its operation will be discussed in the next stage of the work soon.

human-body-induced vibration. Under such a vibration condition, a non-resonant device is desirable. Moreover, the device offers a simple structure, smaller size, low fabrication cost, and is easy to excite by hand-shaking. The device also offers reliable operation since metallic helical compression springs (instead of cantilever beam as high-frequency oscillator) have been used. Mechanical impact with high force (due to high acceleration) on the high frequency oscillating element of a cantilevered energy harvester causes quick damage to the transducer element (especially, in a piezoelectric device). Besides, the NdFeB magnet is able to withstand against the forces caused by the repetitive mechanical impacts of the ball. We kept our prototype harvester under experiment for long, no significant damage of the magnet was observed.

5.4. Performance comparison 6. Conclusions The performances of the vibration based energy harvesters are most commonly compared using the normalized power density. Table 4 presents a performance comparison of this work with those previously reported. As we have implemented our proposed frequency up-converted electromagnetic energy harvester in macroscale, the comparison was carried out with similar works (frequency up-conversion mechanism in macroscale). The comparison shows that for low frequency vibration energy harvesting, our proposed device exhibits outstanding performance compared to the other reported works in terms of both operating frequency and power density. Most of the works reported to-date are based on resonant devices. However, our proposed device offers a non-resonant operation which is intended to operate under

A frequency up-converted and non-resonant electromagnetic energy harvester was presented. The aim of the study was to develop, analyze, and implement a suitable and reliable electromagnetic energy harvester to scavenge significant power from human-limb motion such as hand-shaking which would be able to supply power to portable and wearable smart devices. The proposed system was designed and verified using both a vibration exciter test and a manual vibration (hand-shaking) test at macroscale (volume, 6.47 cm3). The vibration exciter test results proved the non-resonant behavior and feasibility of the frequency upconversion technique, whereas manual vibration test results showed its ability in generating somewhat higher power from

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extremely low frequency. These experimental results were in good agreement with the theoretical calculations. The average power generated from a hand-shaking vibration of frequency 5.17 Hz and acceleration 20.2 ms
2 was 2.15 mW. The device offered 0.33 mW cm
3 power density, which is considerably higher than the current state-of-the-art devices in the field of frequency upconverted energy harvesting at low frequency (<20 Hz) vibrations. Acknowledgments This research was partially supported by a Research Grant of the Kwangwoon University in 2014, the Basic Science Research Program (2013R1A1A2A10064810), and the Pioneer Research Center Program (20100019313) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning, Korea. The authors are grateful to the Micro/Nano Devices and Packaging Lab (MiNDaP) group members of Kwangwoon University. References [1] Sodano HA, Inman DJ, Park G. A review of power harvesting from vibration using piezoelectric materials. Shock Vib Dig 2004;36(3):197–205. [2] Roundy S, Leland ES, Baker J, Carleton E, Reilly E, Lai E, et al. Improving power output for vibration-based energy scavengers. Pervasive Comput 2005;4 (1):28–36. [3] Anton S, Sodano H. A review of power harvesting using piezoelectric materials (2003–2006). Smart Mater Struct 2007;16(3):R1–R21. [4] Joseph AD. Energy harvesting projects. Pervasive Comput 2005;4(1):69–71. [5] Cook-Chennault KA, Thambi N, Sastry A. Powering MEMS portable devices-a review of non-regenerative and regenerative power supply system with special emphasis on piezoelectric energy harvesting systems. Smart Mater Struct 2008;17(4):043001. [6] Mitcheson PD, Yeatman EM, Rao GK, Holmes AS, Green TC. Energy harvesting from human and machine motion for wireless electronic devices. Proc IEEE 2008;96(9):1457–86. [7] Liu J-Q, Fang H-B, Xu Z-Y, Mao X-H, Shen X-C, Chen D, et al. A MEMS-based piezoelectric power generator array for vibration energy harvesting. Microelectron J 2008;39(5):802–6. [8] Saadon S, Sidek O. A review of vibration based MEMS piezoelectric energy harvesters. Energy Convers Manage 2011;52(1):500–4. [9] Sari I, Balkan T, Kullah H. An electromagnetic micro-power generator for wideband environmental vibrations. Sens Actuators A 2008;145–146:405–13. [10] Yang B, Lee C, Xiang W, Xie J, He JH, Kotlanka RK, et al. Electromagnetic energy harvesting from vibrations of multiple frequencies. J Micromech Microeng 2009;19(3):035001. [11] Naruse Y, Matsubara N, Mabuchi K, Izumi M, Suzuki S. Electrostatic micro power generation from low frequency vibration such as human motion. J Micromech Microeng 2009;19(9):094002. [12] Sakane Y, Suzuki Y, Kasagi N. The development of a high performance per fluorinated polymer electrets and its application to micro power generation. J Micromech Microeng 2008;18(10):104011. [13] Dai X, Wen Y, Li P, Yang J, Zhang G. Modeling, characterization and fabrication of vibration energy harvester using Terfenol-D/PZT/Terfenol-D composite transducer. Sens Actuators A 2009;156(2):350–8. [14] Byrashev A, Robbins WP, Ziaie B. Low frequency wireless powering of microsystems using piezoelectric-magnetostrictive laminate composites. Sens Actuators A 2004;114(2-3):244–9. [15] Williams CB, Yates RB. Analysis of a micro-electric generator for microsystems. Sens Actuators A 1996;52(1–3):8–11. [16] Reilly EK, Miller LM, Fain R, Wright PK. A study of ambient vibrations for piezoelectric energy conversion. In: Proc 9th int workshop on micro and nanotechnology for power generation and energy conversion applications (PowerMEMS); 2009. p. 312–5. [17] Kulah H, Najafi K. Energy scavenging from low frequency vibrations by using frequency up-conversion for wireless sensor applications. IEEE Sens J 2008;8 (3):261–8. [18] Ashraf K, Khir MHM, Dennis JO, Baharuddin Z. A wideband, frequency upconverting bounded vibration energy harvester for a low-frequency environment. Smart Mater Struct 2013;22(2):025018. [19] Paci D, Schipani M, Bottarel V, Miatton D. Optimization of a piezoelectric energy harvester for environmental broadband vibrations. In: Proc 15th IEEE int conf on electronics, circuits and systems (IEEE-ICECS); 2008. p. 177–81.

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