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High-performance cycloid inspired wearable electromagnetic energy harvester for scavenging human motion energy
Applied Energy 256 (2019) 113987
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High-performance cycloid inspired wearable electromagnetic energy harvester for scavenging human motion energy
T
Pukar Maharjan, Trilochan Bhatta, M. Salauddin Rasel, Md. Salauddin, M. Toyabur Rahman, ⁎ Jae Yeong Park Micro/Nano Devices and Packaging Laboratory, Department of Electronic Engineering, Kwangwoon University, Seoul, South Korea
H I GH L IG H T S
energy harvester based on cycloid structure is proposed. • AThewearable high output performance of cycloid based energy harvester is demonstrated. • A cycloid is the path with quickest descent for an object to travel. • The cycloidcurve curve the speed of magnetic ball and increase induced emf. • 5 s of wrist motionincreases successfully powered a sports stopwatch for more than 16 min. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Electromagnetic Vibration Energy harvester Cycloid Human motion Wearable
Eco-friendly and wearable power sources are in high demand because of the hasty growth in smart wearable electronic devices including health care monitoring sensors. Here, we successfully designed and fabricated a high-performance cycloid-inspired wearable electromagnetic energy harvester (CEEH) for scavenging low frequency (≤5 Hz) human motion energy. The proposed CEEH introduces a cycloid curved structure as an energy harvester for the first time which provides the fastest descent for the freely rolling spherical magnet in the curve path, resulting an increment in the rate of cutting magnetic flux. In order to demonstrate the capability of the proposed device for harvesting electrical energy from the natural human motions such as arm swinging and vibration, hand-shaking vibration motion tests and custom-made swinging arm tests were performed. The asfabricated harvester can deliver an average power of 8.8 mW under the excitation vibration of 5 Hz at an optimum load resistance of 104.7 Ω. Moreover, it can continuously power a commercial sporty stopwatch for more than 16 min and a wristwatch for more than 34 min from just 5 s of hand motion vibration. The proposed device exhibits much enhanced performance which is more than 1.45 times higher in comparison with other different geometric structures such as straight and circular design. The outstanding energy harvesting capability of the proposed energy harvester shows huge potential as a wearable energy harvester for wrist and foot worn applications and as a sustainable power source for powering smart wearable or portable electronic devices and systems.
1. Introduction Smart wearable electronic devices and health care monitoring sensors are flourishing because of their stylish nature, compatibility, multifunctionality, robustness to hostile environment, and light weight. However, most of these devices are powered by either replaceable or rechargeable batteries, which would cause environmental issues are complicated to replace and expensive. To address these issues, kinetic energy [1] harvesting from human motion [2–5] has been studied
⁎
widely. Converting low frequency [6–8] human biomechanical energy [7–9] expensed during walking, running and other daily activities into a useful electrical energy can be a sustainable source of energy for wearable sensors and smart electronic devices [9–11]. Electromagnetic energy harvester based on Halbach array magnet [8], rolling magnet [12], magnetic spring [13], frequency upconverted [14], monostable design [15], curved structure [16,17], rotary motion [18], and other different designs are previously reported to harvest human motion energy [19–21]. Different piezoelectric energy harvesters based on
Corresponding author. E-mail address: [email protected] (J.Y. Park).
https://doi.org/10.1016/j.apenergy.2019.113987 Received 1 July 2019; Received in revised form 20 September 2019; Accepted 9 October 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
Applied Energy 256 (2019) 113987
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impact driven [22], force amplification mechanism [23], finger tapping [24], frequency upconverted [24,25], u-shaped [26], and other designs are reported for powering cardiac pace maker and other various human body applications [27,28]. Recently, triboelectric nanogenerators are also trending for self-powered sensor [29,30] and harvesting human biomechanical energy [31–34]. For increasing the capability of energy harvester power density, hybridizing different energy harvesting mechanism are also reported for wearable devices [35,36] and other portable devices [37,38]. Among them, electromagnetic mechanism generates comparatively high power because of its high output current. Most of the electromagnetic energy harvesters are based on the translational motion of freely moving magnet [16,17,35]. However, most of these energy harvesters have resonant behavior as well as have limited degree of freedom, which limits the harvester’s output performance. In order to improve the output performance at low frequencies, different designs such as straight tube [13,14,21], curve [16,34,39], and circular tube [17,37] along with various magnet-coil configurations have been exercised. Also, upconverting such low frequencies enhances the output power generation and different approaches have been already experienced regarding this [14,18,19,23,24]. Increasing the harvester’s working frequency and applied force increases rms as well as peak output voltage and current. Since, the natural motion of human body is random in behavior, the wearable energy harvester should be capable of scavenging energy in that arbitrary environment. Considering the natural characteristics of human wrist motion during walking, running, and shaking which consists of low frequency (≤5 Hz), random and irregular vibrations, the energy harvesting efficiency can be improved by optimizing the coil-magnet configuration and increasing the rate of cutting magnetic flux. In this paper, we proposed a high-performance electromagnetic energy harvester inspired from cycloid curve. The energy harvester introduces a cycloid curve for the first time in which a freely rolling spherical magnet travels along a curved path with least time resulting the highest rate of cutting magnetic flux. The proposed energy harvester can harvest energy from diverse vibration excitations during linear motion and swinging motion. The proposed energy harvester in compared with the conventional state-of-the-art energy harvesters, generates higher output power and normalized power density under the low applied frequency.
on a straight line, as shown in Fig. 1c. By following the cycloid curve, a hollow cycloid tube is designed in CAD software and fabricated using 3D printing technology, which makes the production much faster, easy and cost-effective. The proposed CEEH has broad application scenarios such as wearable energy harvester for wrist and foot, and vibrationbased energy harvester, as shown in Fig. 1d. For wearable application, the harvester device can be attached on the wrist and ankle so that the freely moving spherical magnet makes translational-rotary motion inside the tube during the motion of arm and foot and harvest the electrical energy from the human bio-mechanical energy. 2.2. Theoretical analysis By using the Faraday’s law of induction, the amount of e.m.f. induced in the coils because of rolling spherical magnet can be determined.
Emf = −N
ΔΦ Δt
(1)
where N is the total number of turns in the coil, Φ = BA is the magnetic flux, B is the magnetic field, A is the area of coil and t is the time. From Eq. (1), we can see the induced emf is directly proportional to the rate of change of magnetic flux.
Emf ∝
ΔΦ Δt
(2)
From Eq. (2), the rate of change of magnetic flux can be increased by reducing the time. The rate of change of magnetic flux depends on the velocity of magnet entering the coil, which also increases on reducing the time. Thus, the magnetic flux of rolling magnet can cut the coil at higher rate only along the path that follows the fastest descent. A brachistochrone curve is a path with quickest descent which allows an object to travel from one point to another without any friction under uniform gravitational field at least time. This issue was posed by Johann Bernoulli’s in 1696. An object travels on its own weight (force) in the curve from one point to another under the least time. However, the brachistochrone curve has two varies on vertical such that the initial level is higher than the final level. A cycloid is another curve which follows the principle of brachistochrone and has same level at two points. Therefore, a cycloid curve is implemented in this work to design the curve shaped electromagnetic energy harvester in which the spherical magnet can move across the two coils at least time. This achievement of least travel time of magnet increases the rate of cutting magnetic flux corresponding to the two coils which ultimately increases the induced emf in them. The time t required to travel from point I to point II in the cycloid path is given by,
2. Results and discussion 2.1. Harvester design and configuration Fig. 1a shows the schematic design and broken-out sectional view of the proposed cycloid inspired electromagnetic energy harvester which consists of a 3D printed hollow cycloid tube (Ø11 mm inner diameter, 0.8 mm thickness and 96 mm curve length) made up of polylactic acid (PLA), a spherical magnet (NdFeB, N35 grade, Ø10 mm diameter), and two copper coils (each 600 turns, Ø0.1 mm wire diameter) as the generators G1 and G2, winded around the tube. The two coils as two generators G1 and G2 are connected in series. The spherical magnet is placed inside the hollow cycloid tube and the tube is closed with stoppers at both ends. The photograph of as-fabricated prototype of the proposed energy harvesting device is shown in Fig. 1b. When the harvester is excited via the external vibration, the spherical magnet follows translational-rotary motion inside the hollow cycloid tube and induce electromotive force (e.m.f.) across the coils. For designing the cycloid tube, different cycloid curves were selected by varying the radius of circle which is given by equation (length = 8a, where a is the radius of circle). The average size of human wrist is around 180–200 mm (length) and most of the commercial smart wristband is based on this size. Since our proposed harvester is designed for half of the wrist size, a cycloid curve length of 96 mm is used in this work with the circle of radius 12 mm, which is almost equivalent to the half of the average human wrist size. An arc of length 96 mm is drawn by following a trajectory of a point fixed on the circumference of a circle with radius 12 mm rolling
t=
∫I
II
1 + y '2 dx 2gy
(3)
where g is the acceleration due to gravity, y' = dy/dx. In order to verify this phenomenon experimentally, three different structural energy harvesters were prepared under the same volumetric dimensions. The schematic representation of an electromagnetic energy harvester with straight structure, circular structure and cycloid structure are shown in Fig. 2a–c. Fig. 2d shows the numerical result of time required to travel from point I to III for cycloid and circular curve. The time required to travel is calculated using Eq. (3) for cycloid curve and using the integral of equation of circle for circular curve. A cycloid curve is the path through which an object will travel from rest and is accelerated by gravity without friction from one point to another point in the least time. Also, the spherical magnet travels in the cycloid tube via rolling without slipping, so that we can neglect the friction in the motion of spherical magnet. In case of the external excitation, except for the straight structure, the force on spherical magnet will be external force along with acceleration due to gravity. Therefore, we believe that for simulation of travel time, without considering friction and external 2
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Fig. 1. (a) Schematic structure of the cycloid-shaped electromagnetic energy harvester, (b) photograph of as-fabricated device, (c) illustrative representation of cycloid curve for designing the harvester, and (d) application scenario of proposed device as a wearable energy harvester.
curve length performs better with two number of coils.
force, can also verify our claim as shown in Fig. 2d. The numerical results verify that the object can travel faster in cycloid curve than in circular path. In this numerical analysis, since the object is supposed to move with its own weight force under gravity, the object cannot move on straight path. Furthermore, in support of this numerical analysis, an open-circuit voltage test was performed experimentally for different structural energy harvester device designs by using a custom swinging arm under the swing frequency of 5 Hz and 2.5 g acceleration, and the corresponding results are shown in Fig. 2e. The peak to peak opencircuit voltage of 7.8 V was generated by straight device, 8.7 V by circular device, and 11.36 V by cycloid device. The results depict that the cycloid device can generate 1.45 times more voltage than straight curve and 1.3 times more voltage than the circular device. In order to find the optimized cycloid model as human wrist wearable device, we fabricated three different model based on the radius of circle (R1 = 8 mm, R2 = 12 mm and R3 = 16 mm) for cycloid arc. These three different radius R1, R2, and R3 gives different length of cycloid arc 64 mm, 96 mm and 128 mm, respectively. The experimental result of opencircuit voltage and rms voltage for these three types of cycloid tube are shown in Fig. 2f, where the peak-peak open circuit voltage increases with the increase in arc length, however, the rms voltage seems decreasing. This might be the effect of increase in input acceleration required to move the spherical magnet inside the tube for longer length which reduces the device working frequency. Also, the perfect number of coils is also important parameter during the optimization of the cycloid model. Fig. 2f shows the variation in output voltage for different number of coils which depicts that the proposed harvester with 96 mm
2.3. Output performance of the CEEH To analyze the electrical output performance of the CEEH, a custom swinging arm controlled by servo motor was made, mimicking the human arm. The as-fabricated device is attached on the custom swinging arm whose swinging frequency can be controlled by controlling the servo motor. Fig. 3a shows the open-circuit voltage waveform of the proposed harvester under the swing frequency of 5 Hz and acceleration of 2.5 g. When the harvester device starts swinging along with the motion of swinging arm, the spherical magnet tends to roll across the coils G1 and G2 inducing an emf on both generators. These two generators G1 and G2 are connected in series in order to increase the voltage. For a single period of swing, the spherical magnet starts rolling from the generators G1 to G2 and returns through G2 to G1. The corresponding waveform for this single period of swing is shown in Fig. 3b. In order to generate voltage from both generators, the swing should be enough so that the spherical magnet can reach from one end of device to another end. If the spherical ball is allowed to fall freely inside the cycloid tube from one end, the rolling of the magnet across the tube produces a voltage waveform with a tendency of damping signal, as shown in Fig. 3c. The magnet freely rolls mimicking the pendulum motion with only force due to the gravity and stops after the total swing for 0.44 s and can generates an open circuit voltage of 5.3 V. For further analysis of the magnetic effect on the coil, we performed the finite element method simulation of magnetic field intensity of 3
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Fig. 2. Schematic structures of an electromagnetic energy harvester with (a) straight structure, (b) circular structure, and (c) cycloid structure. (d) Numerical analysis result of time taken to travel from one end to another end for an object in cycloid and circular path. (e) Experimental open-circuit voltage generated by straight, circular, and cycloid device under the same swing frequency of 5 Hz frequency and 2.5 g acceleration. (f) Experimental open-circuit voltage and rms voltage of different cycloid curve length under swing frequency of 5 Hz frequency and 2.5 g acceleration. (g) Experimental open-circuit voltage against different number of coils under swing frequency of 5 Hz frequency and 2.5 g acceleration.
commercial 6-axis gyroscope-accelerometer (K6DS3TR, STMicroelectronics) was used to measure the excitation frequency of hand-shaking vibration motion and perform the test. Fig. 4a shows the directional setup position of the harvester device and accelerometer sensor during the hand-shaking vibration and swinging test experiment. The open-circuit voltage of the harvester under the hand shaking vibration motion with the input acceleration of range 0.1–3.5 g and the frequency range of 0.5–6 Hz are shown in Fig. 4b. With the increment in the external vibration frequency and acceleration, the amount of voltage induced in the coil also increased. For the very low frequency of 0.5–2 Hz, the open-circuit voltage is less than 4 V while that for more than 3 Hz reached up to 11 V. The proposed CEEH can deliver a peak power of around 74 mW and an average power of 8.8 mW for an external hand-shaking vibration frequency of 5 Hz and peak acceleration of 2.5 g. The instantaneous peak power waveform and the average power level is shown in Fig. 4c. The relation between the RMS voltage and average power of the harvester under the varying load resistance of 1–600 Ω is shown in Fig. 4d. The figure shows that the average power of the proposed energy harvester reaches maximum value of 8.8 mW at the optimum load resistance of 104.7 Ω. Similarly, the rms voltage increases with the increase of load resistance under the constant external
spherical magnet inside the tube, as shown in Fig. 3d. Although, the spherical magnet has polarity in hemispherical volume, the flux direction during the rolling of magnet is hard to predict [34]. The simulation result shows the maximum flux density of 0.577 T for N35 grade neodymium magnet and based on the coil position, a maximum of 0.455 T flux density is available in the middle of coil’s cross-sectional area. Fig. 3e shows the distribution of the magnetic flux density under the different inclination of the spherical magnet such as 0°, 30°, 60°, and 90° inclination. The variation of the magnetic flux density inside the coil can be seen in Fig. 3f for the complete 360° inclination. The magnetic flux density was measured at the middle of the coil cross section area for the simulation, as shown in Fig. 2d. The high and low values of the magnetic flux density help to calculate the average flux density of the spherical magnet during rolling in the tube. The simulation results show that during the inclination of 30° and 210° of the spherical magnet, the magnetic flux density is found higher (0.4555 T) and lower (0.4507 T) at the inclination of 120° and 300°. Therefore, the calculated average value of the moving spherical magnet is around 0.4528 T. The electrical performance of the CEEH is also characterized through controlled excitation of hand-shaking motion. For this, a 4
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Fig. 3. (a) Open-circuit voltage waveform of the CEEH using customized swinging arm under the swing frequency of 5 Hz and acceleration of 2.5 g. (b) Focused voltage signal for a single period of external vibration. (c) Voltage signal under a freefall of the spherical magnet inside the cycloid tube showing a damping effect. (d) Finite element method simulation results of magnetic field intensity of a spherical magnet of N35 grade inside a hollow cycloid curve. (e and f) FEM simulation result for magnetic flux density under different inclination of spherical magnet.
For all the experiment, we have adjusted the accelerometer and the sensor at the same position for better analysis of the performance. Fig. 5a and b shows the input acceleration to the harvester due to wrist swinging motion during slow walking and corresponding output opencircuit voltage waveforms. For slow walking, the acceleration is higher in x-axis the moving magnet direction and the vibration direction is in same direction. During the slow walking test, the harvester can generate an open-circuit voltage of 1 V. Similarly, the proposed harvester generates an open-circuit voltage of 2.35 V for fast walking and the corresponding input acceleration and voltage waveform are shown in Fig. 5c and d. Comparing to the slow and fast walking, running activity shows better performance as the applied acceleration is higher than of those two modes. The running activity can generate an open-circuit voltage of 7.8 V and the corresponding 3-axis acceleration and voltage waveform are shown in Fig. 5e and f. Here, along with the x-axis, the harvester device also gets accelerated in the z-direction compared to slow walking test. Moreover, from the hand shaking vibration motion, the harvester can generate the highest output open-circuit voltage of 11 V. The corresponding 3-axis acceleration data also shows the high acceleration input to the harvester during the hand shaking motion, as
vibration frequency. In overall, the average output power of the CEEH under the different vibration frequency range of 0.5–5 Hz is shown in Fig. 4e. For the low frequency of 1 Hz, the harvester can generate an average power of around 2 mW and can harvest maximum average power of 8.8 mW at 5 Hz external vibration frequency. These experimental results exhibit the higher performance comparing to the previously reported state-of-the-arts electromagnetic energy harvesters. Table 1 shows the comparative analysis of the previously reported electromagnetic energy harvesters having different geometries. From the table, it clearly shows that, introducing the cycloid geometry in this kind of vibration-based energy harvester provides higher output performance and output power density. The proposed cycloid shaped energy harvester is designed for wearable energy harvester as a real-time application. For this, the harvester was tested under different types of human motions such as walking, running and vibration. The accelerometer sensor was used to analyze the applied acceleration in three axis x, y and z, during the human motion vibration. The harvester device along with accelerometer senor was worn on wrist in the position mentioned in Fig. 4a, for different arm swinging test including walking, running and shaking. 5
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Fig. 4. (a) Directional setup for harvester device and accelerometer sensor position for experiment. (b) Open-circuit peak-to-peak voltage of the CEEH under the different external vibration frequencies. (c) Instantaneous power waveform and average power generated under the vibration of 5 Hz and 2.5 g input acceleration. (d) Harvested average power and rms voltage under the various load resistance under the vibration of 5 Hz frequency and 2.5 g acceleration. (e) Average output power against various load resistances under the different external vibration frequencies. Table 1 Performance comparison among similar hand motion driven electromagnetic energy harvesters. References
Structure
Device Volume (cm3)
Operating Frequency (Hz)
Average Power Density (mW/cm3)
13 14 16 17 21 34 37
Straight block Hollow cylinder Curve tube Circular tube Hollow cylinder Curve tube Circular tube
32.76 6.47 42.2 29.19 12.7 15.93 24.87
11 5.17 2–5 5 8 5 5
0.033 0.33 0.123 0.107 0.193 0.157 0.118
This work
Cycloid tube
11.97
5
0.73
usability of wearable energy harvester, it requires a storage unit such as capacitor, to store the harvested biomechanical energy during the daily natural activities. We used a bridge rectifier circuit to convert the harvested alternating voltage to rectified dc voltage. A storage capacitor was connected at the end terminal of the rectifier and harvester at another terminal. Several capacitors (47 µF, 100 µF, 220 µF, 470 µF, and 1000 µF) were used as the storage capacitor to characterize the charging performance of the harvester, as shown in Fig. 6b. The charging characteristics of the storage capacitors were observed by digital
shown in Fig. 5g. The open-circuit voltage waveform generated during the hand shaking is shown in Fig. 5h. These experimental results show the potential application of the proposed energy harvester as an effective wearable energy harvester for harvesting human motion energy from wrist and foot induced motions. In order to explore the potentiality of the CEEH as a sustainable biomechanical energy harvesting device, variety of practical investigations were conducted. Fig. 6a shows the photograph of the fabricated harvester device. Since the proposed device shows better
6
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Fig. 5. Applied input 3-axis acceleration and open-circuit output voltage of the CEEH during (a, b) slow walking, (c, d) fast walking, (e, f) running and (g, h) hand shaking activity.
it can easily turn on those LEDs during the swinging motion of the arm. Fig. 6d shows the photograph of bright turned red LEDs powered by the energy harvester during the swinging motion. A movie of turning those LEDs by using swinging arm is included in the supplementary information M1. The purpose of developing wearable energy harvester is to supply power for wearable electronic devices and will be meaningful only when the harvester can drive these devices. A commercial sports stopwatch which is widely used by athletes to check their performance time, was successfully powered by the proposed harvester with gentle hand shaking motion, as shown in Fig. 7a. The energy harvesting device with bridge rectifier circuit and 470 µF capacitor as a storage unit was
oscilloscope (Tektronix TDS5052B). The charging results show that the low value capacitor of 47 µF takes around 0.8 s to charge up to 5 V and it takes around 3.3 s to charge 1000 µF capacitor for charging up to 5 V. The capacitor with higher capacitance takes longer time to full charge in comparison to capacitor with lower capacitance, however, there is low ripple in the charging curve of higher capacitor. For a charging period of 5 s, the maximum dc voltage across the capacitor (100 µF) is 7.8 V. To demonstrate the proposed energy harvester as an efficient power source for driving electronic load, an array of 240 LEDs connected in parallel and assembled as “APPLIED ENERGY 2019” word combination, were successfully powered by the harvester. The fabricated device was attached on the swinging arm as shown in Fig. 6c, and 7
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Fig. 6. (a) Photograph of fabricated prototype CEEH device. (b) Capacitor charging performance for different range of capacitors. (c) Photograph of an experimental setup for swinging arm with harvester device mimicking the human arm motion. (d) Photograph of 240 LEDs assembled as “APPLIED ENERGY 2019” powered by swinging motion of CEEH.
used for driving the stopwatch. The capacitor was rapidly charged up to 4 V with load (stopwatch) connected, from 5 s of hand shaking of the harvester, as illustrated in Fig. 7b. During this period, the stopwatch was easily turn on when the capacitor voltage reaches 1.8 V. After 5 s of charging then the vibration was stopped, and the storage capacitor begins discharging. For 90 s of period, it shows a quick discharge of the capacitor and for next 870 s of period with slow discharging nature, the stopwatch was successfully driven continuously. In overall, for the 5 s of handshaking, the proposed energy harvesting device can successfully drive a commercial electronic sports stopwatch continuously for 16 min. The nature of storage capacitor charging and discharging, is clearly shown in Fig. 7b. A movie demonstrating successfully driving the sports watch with CEEH is included in supplementary information M2. Moreover, a commercial wristwatch shown in Fig. 7c was also successfully driven by proposed wearable energy harvester for more than 34 min continuously. The harvester was vibrated for just 5 s with wristwatch connected across the capacitor, it began to run when the capacitor voltage reaches more than 1.2 V, as shown in Fig. 7d. The total discharge time was around 2076 s which is more than 34 min of running wristwatch. Fig. 7e shows the charging discharging capacitor voltage curve during driving a commercial digital humidity temperature (DHT) meter. The proposed CEEH can drive the DHT meter for 193 s continuously with 5 s of hand motion vibration. Finally, in order to analyze the capability of proposed energy harvester as wrist wearable energy harvester, we drive a commercial wristwatch during different human activity such as walking, running, and hand shaking vibration motion. Fig. 7f shows the performance comparison for different activity, where the activity duration is just 5 s time which can power the wristwatch for more than 5 min during slow walking, more than minutes during fast walking, more than 17 min during running and impressively more than 34 min for hand shaking vibration activity. The energy conversion efficiency of an energy harvester is very important and has significant value in the development of high-performance energy harvester. The efficiency depends on the input mechanical energy and the harvester’s output electrical energy.
Efficiency (η) =
Output Electrical Power Input Mechanical Power
The mechanical input power during the vibration of harvester device can be calculated by the given equation,
Input Power (IP ) =
Kinetic Energy = time
1 mv 2 2
t
=
ma2t 2
where m = 16.6 g (mass of device), a = 2.5 g is input acceleration, and t is time. Similarly, the harvested output electrical power (OP) of the proposed energy harvester is 74 mW. Therefore, the energy conversion efficiency (ɳ) is given by
η=
OP = 7.7% IP
The implementation of the cycloid physics for designing the efficient biomechanical energy harvester shows the high-performance electromagnetic energy harvester from the hand-shaking vibration motion. Hence, this research paves the way of implementing physics for the structural improvement of the electromagnetic energy harvester with high performance for harvesting human biomechanical energy. 3. Conclusion In summary, a cycloid curve-shaped electromagnetic energy harvester was designed, developed and demonstrated successfully as a sustainable wearable energy harvester for scavenging human body motion energy. The cycloid physics inspired hollow circular tube was fabricated using 3D printing technology which provides a fastest rolling of spherical magnet inside the tube which ultimately increases the emf induced in the coils. Compared to the conventional rectangular/circular straight and circular tube, the proposed hollow cycloid tube enhanced the energy harvesting performance by 1.45 times and 1.3 times, respectively. By effectively harvesting the human biomechanical energy based on the human hand-shaking vibration motion of 5 Hz, the fabricated energy harvesting device could deliver an average power of 8.8 mW under an optimum load resistance of 104.7 Ω with the energy 8
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Fig. 7. (a) Photograph of commercial sports stopwatch successfully powered by the hand-shaking vibration motion of the energy harvester. (b) Capacitor charging and discharging nature during powering sports stopwatch by proposed energy harvester. (c) Photograph of commercial wristwatch successfully driven by CEEH under hand shaking vibration motion. Fabricated prototype CEEH device. (d) Capacitor charging and discharging nature during powering wristwatch. (e) Capacitor charging and discharging nature during powering commercial digital humidity temperature meter. (Inset image shows photograph of digital humidity temperature meter powered by CEEH). (f) Graph showing total driving time for a commercial wrist watch powered by CEEH under different activity such as walking, running and handshaking.
Appendix A. Supplementary material
conversion efficiency of 7.7%. With just 5 s of handshaking the harvester, it could magnificently drive a commercial sports stopwatch for 16 min. This marvelous achievement proves the possibility of the proposed device for developing self-powered smart wearable electronic devices and sensors for sports, fitness, and health-care monitoring system via scavenging the wasted human biomechanical energy during daily activities.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apenergy.2019.113987. References [1] Berdy DF, Valentino DJ, Peroulis D. Kinetic energy harvesting from human walking and running using a magnetic levitation energy harvester. Sens Actuator A-Phys 2015;222:262–71. [2] Donelan JM, Li Q, Naing V, Hoffer JA, Weber DJ, Kuo AD. Biomechanical energy harvesting: generating electricity during walking with minimal user effort. Science 2018;319:807–10. [3] Yang R, Qin Y, Li C, Zhu G, Wang ZL. Converting biomechanical energy into electricity by a muscle-movement-driven nanogenerator. Nano Lett 2009;9(3):1201–5. [4] Hansen BJ, Liu Y, Yang R, Wang ZL. Hybrid nanogenerator for concurrently harvesting biomechanical and biochemical energy. ACS Nano 2010;4:3647–52. [5] Yang W, Chen J, Zhu G, Yang J, Bai P, Su Y, et al. Harvesting energy from the natural vibration of human walking. ACS Nano 2013;7:11317–24. [6] Park JY, Salauddin M, Rasel MS. Nanogenerator for scavenging low frequency vibrations. J Micromech Microeng 2019;29:053001. [7] Liu H, Hou C, Lin J, Li Y, Shi Q, Chen T, et al. A non-resonant rotational electromagnetic energy harvester for low-frequency and irregular human motion. Appl Phys Lett 2018;113:203901. [8] Salauddin M, Halim MA, Park JY. A magnetic-spring-based, low-frequency-vibration energy harvester comprising a dual halbach array. Smart Mater Struct 2016;25:095017. [9] Xie L, Li X, Cai S, Huang G, Huang L. Knee-braced energy harvester: reclaim energy and assist walking. Mech Syst Signal Proc 2019;127:172–89.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This research was supported by the Bio & Medical Technology Development Program of the NRF grant funded by the Korean government (MSIT) (NRF-2017M3A9F1031270). The authors are also grateful for the technical support and discussions with the Micro/Nano Devices and Packaging Laboratory (MiNDaP) group members of Kwangwoon University. 9
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[25] Pillatsch P, Yeatman EM, Holmes AS. A piezoelectric frequency up-converting energy harvester with rotating proof mass for human body applications. Sens Actuator A-Phys 2014;206:178–85. [26] Sun S, Tae PW. Modeling of a horizontal asymmetric U-shaped vibration-based piezoelectric energy harvester (U-VPEH). Mech Syst Signal Proc 2019;114:467–85. [27] Hwang GT, Park H, Lee JH, Oh S, Park K, Byun M, et al. Self‐powered cardiac pacemaker enabled by flexible single crystalline PMN‐PT piezoelectric energy harvester. Adv Mater 2014;26(28):4880–7. [28] Yang Z, Zhou S, Zu J, Inman D. High-performance piezoelectric energy harvester and their applications. Joule 2018;2:642–97. [29] Chen J, Zhu G, Yang W, Jing Q, Bai P, Yang Y, et al. Harmonic-resonator-based triboelectric nanogenerator as a sustainable power source and a self-powered active vibration sensor. Adv Mater 2013;25:6094–9. [30] Yi F, Wang X, Niu S, Li S, Yin Y, Dai K, et al. A highly shape-adaptive, stretchable design based on conductive liquid for energy harvesting and self-powered biomechanical monitoring. Sci Adv 2016;2:e1501624. [31] Shen J, Li Z, Yu J, Ding B. Humidity-resisting triboelectric nanogenerator for high performance biomechanical energy harvesting. Nano Energy 2017;40:282–8. [32] Wu C, Liu R, Wang J, Zi Y, Lin L, Wang ZL. A spring-based resonance coupling for hugely enhancing the performance of triboelectric nanogenerators for harvesting low-frequency vibration energy. Nano Energy 2017;32:287–93. [33] Bai P, Zhu G, Lin ZH, Jing Q, Chen J, Zhang G, et al. Integrated multilayered triboelectric nanogenerator for harvesting biomechanical energy from human motions. ACS Nano 2013;7:3713–9. [34] Maharjan P, Toyabur RM, Park JY. A human locomotion inspired hybrid nanogenerator for wrist-wearable electronic device and sensor applications. Nano Energy 2018;46:383–95. [35] Quan T, Wang X, Wang ZL, Yang Y. Hybridized electromagnetic-triboelectric nanogenerator for a self-powered electronic watch. ACS Nano 2015;9:12301–10. [36] Wang X, Wang S, Yang Y, Wang ZL. Chemical modification of polymer surfaces for advanced triboelectric nanogenerator development. ACS Nano 2015;9:4553–62. [37] Maharjan P, Cho H, Rasel MS, Salauddin M, Park JY. A fully enclosed, 3D printed, hybridized nanogenerator with flexible flux concentrator for harvesting diverse human biomechanical energy. Nano Energy 2018;53:213–24. [38] Salauddin M, Toyabur RM, Maharjan P, Rasel MS, Park JY. Miniaturized springless hybrid nanogenerator for powering portable and wearable electronic devices from human-body-induced vibration. Nano Energy 2018;51:61–72. [39] Fan K, Tan Q, Tang L. An arc-shaped electromagnetic energy harvester for ultra-low frequency vibrations and swing motions. In: Proc. SPIE 10967, active and passive smart structures and integrated systems XIII, 1096725; 2019.
[10] Proto A, Penhaker M, Conforto S, Schmid M. Nanogenerators for human body energy harvesting. Trends Biotechnol 2017;35:610–24. [11] Kuang Yang, Ruan Tingwen, Chew Zheng Jun, Zhu Meiling. Energy harvesting during human walking to power a wireless sensor node. Sens Actuators, A 2017;254:69–77. https://doi.org/10.1016/j.sna.2016.11.035. [12] Zhang LB, Dai HL, Yang YW, Wang L. Design of high-efficiency electromagnetic energy harvester based on a rolling magnet. Energy Convers Manage 2019;185:202–10. https://doi.org/10.1016/j.enconman.2019.01.089. [13] Wang W, Cao J, Zhang N, Lin J, Liao WH. Magnetic-spring based energy harvesting from human motions: design, modeling and experiments. Energy Conv. Manag. 2017;132:189–97. [14] Halim MA, Cho H, Park JY. Design and experiment of a human-limb driven, frequency up-converted electromagnetic energy harvester. Energy Convers Manage 2015;106:393–404. [15] Fan K, Cai M, Liu H, Zhang Y. Capturing energy from ultra-low frequency vibrations and human motion through a monostable electromagnetic energy harvester. Energy 2019;169:356–68. [16] Samad FA, Karim MF, Paulose V, Ong LC. A curved electromagnetic energy harvesting system for wearable electronics. IEEE Sens J 2016;16:1969–74. [17] Wu Z, Tang J, Zhang X, Yu Z. An energy harvesting bracelet. Appl Phys Lett 2016;111:013903. [18] Moss SD, Hart GA, Burke SK, Carman GP. Hybrid rotary-translational vibration energy harvester using cycloidal motion as a mechanical amplifier. Appl Phys Lett 2014;104:033506. [19] Chen J, Wang Y. A dual electromagnetic array with intrinsic frequency up-conversion for broadband vibrational energy harvesting. Appl Phys Lett 2019;114:053902. [20] Abed I, Kacem N, Bouhaddi N, Bouazizi ML. Multi-modal vibration energy harvesting approach based on nonlinear oscillator arrays under magnetic levitation. Smart Mater Struct 2016;25:025018. [21] Saha CR, O’Donnel T, Wang N, McCloskey P. Electromagnetic generator for harvesting energy from human motion. Sens Actuator A-Phys 2008;147:248–53. [22] Wei S, Hu H, He S. Modeling and experimental investigation of an impact-driven piezoelectric energy harvester from human motion. Smart Mater Struct 2013;22:105020. [23] Yang Z, Zu J. High-efficiency compressive-mode energy harvester enhanced by a multi-stage force amplification mechanism. Energy Convers Manage 2014;88:829–33. [24] Yang Z, Tan Y, Zu J. A multi-impact frequency up-converted magnetostrictive transducer for harvesting energy from finger tapping. Int J Mech Sci 2017;126:235–41.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
High-performance cycloid inspired wearable electromagnetic energy harvester for scavenging human motion energy
T
Pukar Maharjan, Trilochan Bhatta, M. Salauddin Rasel, Md. Salauddin, M. Toyabur Rahman, ⁎ Jae Yeong Park Micro/Nano Devices and Packaging Laboratory, Department of Electronic Engineering, Kwangwoon University, Seoul, South Korea
H I GH L IG H T S
energy harvester based on cycloid structure is proposed. • AThewearable high output performance of cycloid based energy harvester is demonstrated. • A cycloid is the path with quickest descent for an object to travel. • The cycloidcurve curve the speed of magnetic ball and increase induced emf. • 5 s of wrist motionincreases successfully powered a sports stopwatch for more than 16 min. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Electromagnetic Vibration Energy harvester Cycloid Human motion Wearable
Eco-friendly and wearable power sources are in high demand because of the hasty growth in smart wearable electronic devices including health care monitoring sensors. Here, we successfully designed and fabricated a high-performance cycloid-inspired wearable electromagnetic energy harvester (CEEH) for scavenging low frequency (≤5 Hz) human motion energy. The proposed CEEH introduces a cycloid curved structure as an energy harvester for the first time which provides the fastest descent for the freely rolling spherical magnet in the curve path, resulting an increment in the rate of cutting magnetic flux. In order to demonstrate the capability of the proposed device for harvesting electrical energy from the natural human motions such as arm swinging and vibration, hand-shaking vibration motion tests and custom-made swinging arm tests were performed. The asfabricated harvester can deliver an average power of 8.8 mW under the excitation vibration of 5 Hz at an optimum load resistance of 104.7 Ω. Moreover, it can continuously power a commercial sporty stopwatch for more than 16 min and a wristwatch for more than 34 min from just 5 s of hand motion vibration. The proposed device exhibits much enhanced performance which is more than 1.45 times higher in comparison with other different geometric structures such as straight and circular design. The outstanding energy harvesting capability of the proposed energy harvester shows huge potential as a wearable energy harvester for wrist and foot worn applications and as a sustainable power source for powering smart wearable or portable electronic devices and systems.
1. Introduction Smart wearable electronic devices and health care monitoring sensors are flourishing because of their stylish nature, compatibility, multifunctionality, robustness to hostile environment, and light weight. However, most of these devices are powered by either replaceable or rechargeable batteries, which would cause environmental issues are complicated to replace and expensive. To address these issues, kinetic energy [1] harvesting from human motion [2–5] has been studied
⁎
widely. Converting low frequency [6–8] human biomechanical energy [7–9] expensed during walking, running and other daily activities into a useful electrical energy can be a sustainable source of energy for wearable sensors and smart electronic devices [9–11]. Electromagnetic energy harvester based on Halbach array magnet [8], rolling magnet [12], magnetic spring [13], frequency upconverted [14], monostable design [15], curved structure [16,17], rotary motion [18], and other different designs are previously reported to harvest human motion energy [19–21]. Different piezoelectric energy harvesters based on
Corresponding author. E-mail address: [email protected] (J.Y. Park).
https://doi.org/10.1016/j.apenergy.2019.113987 Received 1 July 2019; Received in revised form 20 September 2019; Accepted 9 October 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
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impact driven [22], force amplification mechanism [23], finger tapping [24], frequency upconverted [24,25], u-shaped [26], and other designs are reported for powering cardiac pace maker and other various human body applications [27,28]. Recently, triboelectric nanogenerators are also trending for self-powered sensor [29,30] and harvesting human biomechanical energy [31–34]. For increasing the capability of energy harvester power density, hybridizing different energy harvesting mechanism are also reported for wearable devices [35,36] and other portable devices [37,38]. Among them, electromagnetic mechanism generates comparatively high power because of its high output current. Most of the electromagnetic energy harvesters are based on the translational motion of freely moving magnet [16,17,35]. However, most of these energy harvesters have resonant behavior as well as have limited degree of freedom, which limits the harvester’s output performance. In order to improve the output performance at low frequencies, different designs such as straight tube [13,14,21], curve [16,34,39], and circular tube [17,37] along with various magnet-coil configurations have been exercised. Also, upconverting such low frequencies enhances the output power generation and different approaches have been already experienced regarding this [14,18,19,23,24]. Increasing the harvester’s working frequency and applied force increases rms as well as peak output voltage and current. Since, the natural motion of human body is random in behavior, the wearable energy harvester should be capable of scavenging energy in that arbitrary environment. Considering the natural characteristics of human wrist motion during walking, running, and shaking which consists of low frequency (≤5 Hz), random and irregular vibrations, the energy harvesting efficiency can be improved by optimizing the coil-magnet configuration and increasing the rate of cutting magnetic flux. In this paper, we proposed a high-performance electromagnetic energy harvester inspired from cycloid curve. The energy harvester introduces a cycloid curve for the first time in which a freely rolling spherical magnet travels along a curved path with least time resulting the highest rate of cutting magnetic flux. The proposed energy harvester can harvest energy from diverse vibration excitations during linear motion and swinging motion. The proposed energy harvester in compared with the conventional state-of-the-art energy harvesters, generates higher output power and normalized power density under the low applied frequency.
on a straight line, as shown in Fig. 1c. By following the cycloid curve, a hollow cycloid tube is designed in CAD software and fabricated using 3D printing technology, which makes the production much faster, easy and cost-effective. The proposed CEEH has broad application scenarios such as wearable energy harvester for wrist and foot, and vibrationbased energy harvester, as shown in Fig. 1d. For wearable application, the harvester device can be attached on the wrist and ankle so that the freely moving spherical magnet makes translational-rotary motion inside the tube during the motion of arm and foot and harvest the electrical energy from the human bio-mechanical energy. 2.2. Theoretical analysis By using the Faraday’s law of induction, the amount of e.m.f. induced in the coils because of rolling spherical magnet can be determined.
Emf = −N
ΔΦ Δt
(1)
where N is the total number of turns in the coil, Φ = BA is the magnetic flux, B is the magnetic field, A is the area of coil and t is the time. From Eq. (1), we can see the induced emf is directly proportional to the rate of change of magnetic flux.
Emf ∝
ΔΦ Δt
(2)
From Eq. (2), the rate of change of magnetic flux can be increased by reducing the time. The rate of change of magnetic flux depends on the velocity of magnet entering the coil, which also increases on reducing the time. Thus, the magnetic flux of rolling magnet can cut the coil at higher rate only along the path that follows the fastest descent. A brachistochrone curve is a path with quickest descent which allows an object to travel from one point to another without any friction under uniform gravitational field at least time. This issue was posed by Johann Bernoulli’s in 1696. An object travels on its own weight (force) in the curve from one point to another under the least time. However, the brachistochrone curve has two varies on vertical such that the initial level is higher than the final level. A cycloid is another curve which follows the principle of brachistochrone and has same level at two points. Therefore, a cycloid curve is implemented in this work to design the curve shaped electromagnetic energy harvester in which the spherical magnet can move across the two coils at least time. This achievement of least travel time of magnet increases the rate of cutting magnetic flux corresponding to the two coils which ultimately increases the induced emf in them. The time t required to travel from point I to point II in the cycloid path is given by,
2. Results and discussion 2.1. Harvester design and configuration Fig. 1a shows the schematic design and broken-out sectional view of the proposed cycloid inspired electromagnetic energy harvester which consists of a 3D printed hollow cycloid tube (Ø11 mm inner diameter, 0.8 mm thickness and 96 mm curve length) made up of polylactic acid (PLA), a spherical magnet (NdFeB, N35 grade, Ø10 mm diameter), and two copper coils (each 600 turns, Ø0.1 mm wire diameter) as the generators G1 and G2, winded around the tube. The two coils as two generators G1 and G2 are connected in series. The spherical magnet is placed inside the hollow cycloid tube and the tube is closed with stoppers at both ends. The photograph of as-fabricated prototype of the proposed energy harvesting device is shown in Fig. 1b. When the harvester is excited via the external vibration, the spherical magnet follows translational-rotary motion inside the hollow cycloid tube and induce electromotive force (e.m.f.) across the coils. For designing the cycloid tube, different cycloid curves were selected by varying the radius of circle which is given by equation (length = 8a, where a is the radius of circle). The average size of human wrist is around 180–200 mm (length) and most of the commercial smart wristband is based on this size. Since our proposed harvester is designed for half of the wrist size, a cycloid curve length of 96 mm is used in this work with the circle of radius 12 mm, which is almost equivalent to the half of the average human wrist size. An arc of length 96 mm is drawn by following a trajectory of a point fixed on the circumference of a circle with radius 12 mm rolling
t=
∫I
II
1 + y '2 dx 2gy
(3)
where g is the acceleration due to gravity, y' = dy/dx. In order to verify this phenomenon experimentally, three different structural energy harvesters were prepared under the same volumetric dimensions. The schematic representation of an electromagnetic energy harvester with straight structure, circular structure and cycloid structure are shown in Fig. 2a–c. Fig. 2d shows the numerical result of time required to travel from point I to III for cycloid and circular curve. The time required to travel is calculated using Eq. (3) for cycloid curve and using the integral of equation of circle for circular curve. A cycloid curve is the path through which an object will travel from rest and is accelerated by gravity without friction from one point to another point in the least time. Also, the spherical magnet travels in the cycloid tube via rolling without slipping, so that we can neglect the friction in the motion of spherical magnet. In case of the external excitation, except for the straight structure, the force on spherical magnet will be external force along with acceleration due to gravity. Therefore, we believe that for simulation of travel time, without considering friction and external 2
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Fig. 1. (a) Schematic structure of the cycloid-shaped electromagnetic energy harvester, (b) photograph of as-fabricated device, (c) illustrative representation of cycloid curve for designing the harvester, and (d) application scenario of proposed device as a wearable energy harvester.
curve length performs better with two number of coils.
force, can also verify our claim as shown in Fig. 2d. The numerical results verify that the object can travel faster in cycloid curve than in circular path. In this numerical analysis, since the object is supposed to move with its own weight force under gravity, the object cannot move on straight path. Furthermore, in support of this numerical analysis, an open-circuit voltage test was performed experimentally for different structural energy harvester device designs by using a custom swinging arm under the swing frequency of 5 Hz and 2.5 g acceleration, and the corresponding results are shown in Fig. 2e. The peak to peak opencircuit voltage of 7.8 V was generated by straight device, 8.7 V by circular device, and 11.36 V by cycloid device. The results depict that the cycloid device can generate 1.45 times more voltage than straight curve and 1.3 times more voltage than the circular device. In order to find the optimized cycloid model as human wrist wearable device, we fabricated three different model based on the radius of circle (R1 = 8 mm, R2 = 12 mm and R3 = 16 mm) for cycloid arc. These three different radius R1, R2, and R3 gives different length of cycloid arc 64 mm, 96 mm and 128 mm, respectively. The experimental result of opencircuit voltage and rms voltage for these three types of cycloid tube are shown in Fig. 2f, where the peak-peak open circuit voltage increases with the increase in arc length, however, the rms voltage seems decreasing. This might be the effect of increase in input acceleration required to move the spherical magnet inside the tube for longer length which reduces the device working frequency. Also, the perfect number of coils is also important parameter during the optimization of the cycloid model. Fig. 2f shows the variation in output voltage for different number of coils which depicts that the proposed harvester with 96 mm
2.3. Output performance of the CEEH To analyze the electrical output performance of the CEEH, a custom swinging arm controlled by servo motor was made, mimicking the human arm. The as-fabricated device is attached on the custom swinging arm whose swinging frequency can be controlled by controlling the servo motor. Fig. 3a shows the open-circuit voltage waveform of the proposed harvester under the swing frequency of 5 Hz and acceleration of 2.5 g. When the harvester device starts swinging along with the motion of swinging arm, the spherical magnet tends to roll across the coils G1 and G2 inducing an emf on both generators. These two generators G1 and G2 are connected in series in order to increase the voltage. For a single period of swing, the spherical magnet starts rolling from the generators G1 to G2 and returns through G2 to G1. The corresponding waveform for this single period of swing is shown in Fig. 3b. In order to generate voltage from both generators, the swing should be enough so that the spherical magnet can reach from one end of device to another end. If the spherical ball is allowed to fall freely inside the cycloid tube from one end, the rolling of the magnet across the tube produces a voltage waveform with a tendency of damping signal, as shown in Fig. 3c. The magnet freely rolls mimicking the pendulum motion with only force due to the gravity and stops after the total swing for 0.44 s and can generates an open circuit voltage of 5.3 V. For further analysis of the magnetic effect on the coil, we performed the finite element method simulation of magnetic field intensity of 3
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Fig. 2. Schematic structures of an electromagnetic energy harvester with (a) straight structure, (b) circular structure, and (c) cycloid structure. (d) Numerical analysis result of time taken to travel from one end to another end for an object in cycloid and circular path. (e) Experimental open-circuit voltage generated by straight, circular, and cycloid device under the same swing frequency of 5 Hz frequency and 2.5 g acceleration. (f) Experimental open-circuit voltage and rms voltage of different cycloid curve length under swing frequency of 5 Hz frequency and 2.5 g acceleration. (g) Experimental open-circuit voltage against different number of coils under swing frequency of 5 Hz frequency and 2.5 g acceleration.
commercial 6-axis gyroscope-accelerometer (K6DS3TR, STMicroelectronics) was used to measure the excitation frequency of hand-shaking vibration motion and perform the test. Fig. 4a shows the directional setup position of the harvester device and accelerometer sensor during the hand-shaking vibration and swinging test experiment. The open-circuit voltage of the harvester under the hand shaking vibration motion with the input acceleration of range 0.1–3.5 g and the frequency range of 0.5–6 Hz are shown in Fig. 4b. With the increment in the external vibration frequency and acceleration, the amount of voltage induced in the coil also increased. For the very low frequency of 0.5–2 Hz, the open-circuit voltage is less than 4 V while that for more than 3 Hz reached up to 11 V. The proposed CEEH can deliver a peak power of around 74 mW and an average power of 8.8 mW for an external hand-shaking vibration frequency of 5 Hz and peak acceleration of 2.5 g. The instantaneous peak power waveform and the average power level is shown in Fig. 4c. The relation between the RMS voltage and average power of the harvester under the varying load resistance of 1–600 Ω is shown in Fig. 4d. The figure shows that the average power of the proposed energy harvester reaches maximum value of 8.8 mW at the optimum load resistance of 104.7 Ω. Similarly, the rms voltage increases with the increase of load resistance under the constant external
spherical magnet inside the tube, as shown in Fig. 3d. Although, the spherical magnet has polarity in hemispherical volume, the flux direction during the rolling of magnet is hard to predict [34]. The simulation result shows the maximum flux density of 0.577 T for N35 grade neodymium magnet and based on the coil position, a maximum of 0.455 T flux density is available in the middle of coil’s cross-sectional area. Fig. 3e shows the distribution of the magnetic flux density under the different inclination of the spherical magnet such as 0°, 30°, 60°, and 90° inclination. The variation of the magnetic flux density inside the coil can be seen in Fig. 3f for the complete 360° inclination. The magnetic flux density was measured at the middle of the coil cross section area for the simulation, as shown in Fig. 2d. The high and low values of the magnetic flux density help to calculate the average flux density of the spherical magnet during rolling in the tube. The simulation results show that during the inclination of 30° and 210° of the spherical magnet, the magnetic flux density is found higher (0.4555 T) and lower (0.4507 T) at the inclination of 120° and 300°. Therefore, the calculated average value of the moving spherical magnet is around 0.4528 T. The electrical performance of the CEEH is also characterized through controlled excitation of hand-shaking motion. For this, a 4
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Fig. 3. (a) Open-circuit voltage waveform of the CEEH using customized swinging arm under the swing frequency of 5 Hz and acceleration of 2.5 g. (b) Focused voltage signal for a single period of external vibration. (c) Voltage signal under a freefall of the spherical magnet inside the cycloid tube showing a damping effect. (d) Finite element method simulation results of magnetic field intensity of a spherical magnet of N35 grade inside a hollow cycloid curve. (e and f) FEM simulation result for magnetic flux density under different inclination of spherical magnet.
For all the experiment, we have adjusted the accelerometer and the sensor at the same position for better analysis of the performance. Fig. 5a and b shows the input acceleration to the harvester due to wrist swinging motion during slow walking and corresponding output opencircuit voltage waveforms. For slow walking, the acceleration is higher in x-axis the moving magnet direction and the vibration direction is in same direction. During the slow walking test, the harvester can generate an open-circuit voltage of 1 V. Similarly, the proposed harvester generates an open-circuit voltage of 2.35 V for fast walking and the corresponding input acceleration and voltage waveform are shown in Fig. 5c and d. Comparing to the slow and fast walking, running activity shows better performance as the applied acceleration is higher than of those two modes. The running activity can generate an open-circuit voltage of 7.8 V and the corresponding 3-axis acceleration and voltage waveform are shown in Fig. 5e and f. Here, along with the x-axis, the harvester device also gets accelerated in the z-direction compared to slow walking test. Moreover, from the hand shaking vibration motion, the harvester can generate the highest output open-circuit voltage of 11 V. The corresponding 3-axis acceleration data also shows the high acceleration input to the harvester during the hand shaking motion, as
vibration frequency. In overall, the average output power of the CEEH under the different vibration frequency range of 0.5–5 Hz is shown in Fig. 4e. For the low frequency of 1 Hz, the harvester can generate an average power of around 2 mW and can harvest maximum average power of 8.8 mW at 5 Hz external vibration frequency. These experimental results exhibit the higher performance comparing to the previously reported state-of-the-arts electromagnetic energy harvesters. Table 1 shows the comparative analysis of the previously reported electromagnetic energy harvesters having different geometries. From the table, it clearly shows that, introducing the cycloid geometry in this kind of vibration-based energy harvester provides higher output performance and output power density. The proposed cycloid shaped energy harvester is designed for wearable energy harvester as a real-time application. For this, the harvester was tested under different types of human motions such as walking, running and vibration. The accelerometer sensor was used to analyze the applied acceleration in three axis x, y and z, during the human motion vibration. The harvester device along with accelerometer senor was worn on wrist in the position mentioned in Fig. 4a, for different arm swinging test including walking, running and shaking. 5
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Fig. 4. (a) Directional setup for harvester device and accelerometer sensor position for experiment. (b) Open-circuit peak-to-peak voltage of the CEEH under the different external vibration frequencies. (c) Instantaneous power waveform and average power generated under the vibration of 5 Hz and 2.5 g input acceleration. (d) Harvested average power and rms voltage under the various load resistance under the vibration of 5 Hz frequency and 2.5 g acceleration. (e) Average output power against various load resistances under the different external vibration frequencies. Table 1 Performance comparison among similar hand motion driven electromagnetic energy harvesters. References
Structure
Device Volume (cm3)
Operating Frequency (Hz)
Average Power Density (mW/cm3)
13 14 16 17 21 34 37
Straight block Hollow cylinder Curve tube Circular tube Hollow cylinder Curve tube Circular tube
32.76 6.47 42.2 29.19 12.7 15.93 24.87
11 5.17 2–5 5 8 5 5
0.033 0.33 0.123 0.107 0.193 0.157 0.118
This work
Cycloid tube
11.97
5
0.73
usability of wearable energy harvester, it requires a storage unit such as capacitor, to store the harvested biomechanical energy during the daily natural activities. We used a bridge rectifier circuit to convert the harvested alternating voltage to rectified dc voltage. A storage capacitor was connected at the end terminal of the rectifier and harvester at another terminal. Several capacitors (47 µF, 100 µF, 220 µF, 470 µF, and 1000 µF) were used as the storage capacitor to characterize the charging performance of the harvester, as shown in Fig. 6b. The charging characteristics of the storage capacitors were observed by digital
shown in Fig. 5g. The open-circuit voltage waveform generated during the hand shaking is shown in Fig. 5h. These experimental results show the potential application of the proposed energy harvester as an effective wearable energy harvester for harvesting human motion energy from wrist and foot induced motions. In order to explore the potentiality of the CEEH as a sustainable biomechanical energy harvesting device, variety of practical investigations were conducted. Fig. 6a shows the photograph of the fabricated harvester device. Since the proposed device shows better
6
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Fig. 5. Applied input 3-axis acceleration and open-circuit output voltage of the CEEH during (a, b) slow walking, (c, d) fast walking, (e, f) running and (g, h) hand shaking activity.
it can easily turn on those LEDs during the swinging motion of the arm. Fig. 6d shows the photograph of bright turned red LEDs powered by the energy harvester during the swinging motion. A movie of turning those LEDs by using swinging arm is included in the supplementary information M1. The purpose of developing wearable energy harvester is to supply power for wearable electronic devices and will be meaningful only when the harvester can drive these devices. A commercial sports stopwatch which is widely used by athletes to check their performance time, was successfully powered by the proposed harvester with gentle hand shaking motion, as shown in Fig. 7a. The energy harvesting device with bridge rectifier circuit and 470 µF capacitor as a storage unit was
oscilloscope (Tektronix TDS5052B). The charging results show that the low value capacitor of 47 µF takes around 0.8 s to charge up to 5 V and it takes around 3.3 s to charge 1000 µF capacitor for charging up to 5 V. The capacitor with higher capacitance takes longer time to full charge in comparison to capacitor with lower capacitance, however, there is low ripple in the charging curve of higher capacitor. For a charging period of 5 s, the maximum dc voltage across the capacitor (100 µF) is 7.8 V. To demonstrate the proposed energy harvester as an efficient power source for driving electronic load, an array of 240 LEDs connected in parallel and assembled as “APPLIED ENERGY 2019” word combination, were successfully powered by the harvester. The fabricated device was attached on the swinging arm as shown in Fig. 6c, and 7
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Fig. 6. (a) Photograph of fabricated prototype CEEH device. (b) Capacitor charging performance for different range of capacitors. (c) Photograph of an experimental setup for swinging arm with harvester device mimicking the human arm motion. (d) Photograph of 240 LEDs assembled as “APPLIED ENERGY 2019” powered by swinging motion of CEEH.
used for driving the stopwatch. The capacitor was rapidly charged up to 4 V with load (stopwatch) connected, from 5 s of hand shaking of the harvester, as illustrated in Fig. 7b. During this period, the stopwatch was easily turn on when the capacitor voltage reaches 1.8 V. After 5 s of charging then the vibration was stopped, and the storage capacitor begins discharging. For 90 s of period, it shows a quick discharge of the capacitor and for next 870 s of period with slow discharging nature, the stopwatch was successfully driven continuously. In overall, for the 5 s of handshaking, the proposed energy harvesting device can successfully drive a commercial electronic sports stopwatch continuously for 16 min. The nature of storage capacitor charging and discharging, is clearly shown in Fig. 7b. A movie demonstrating successfully driving the sports watch with CEEH is included in supplementary information M2. Moreover, a commercial wristwatch shown in Fig. 7c was also successfully driven by proposed wearable energy harvester for more than 34 min continuously. The harvester was vibrated for just 5 s with wristwatch connected across the capacitor, it began to run when the capacitor voltage reaches more than 1.2 V, as shown in Fig. 7d. The total discharge time was around 2076 s which is more than 34 min of running wristwatch. Fig. 7e shows the charging discharging capacitor voltage curve during driving a commercial digital humidity temperature (DHT) meter. The proposed CEEH can drive the DHT meter for 193 s continuously with 5 s of hand motion vibration. Finally, in order to analyze the capability of proposed energy harvester as wrist wearable energy harvester, we drive a commercial wristwatch during different human activity such as walking, running, and hand shaking vibration motion. Fig. 7f shows the performance comparison for different activity, where the activity duration is just 5 s time which can power the wristwatch for more than 5 min during slow walking, more than minutes during fast walking, more than 17 min during running and impressively more than 34 min for hand shaking vibration activity. The energy conversion efficiency of an energy harvester is very important and has significant value in the development of high-performance energy harvester. The efficiency depends on the input mechanical energy and the harvester’s output electrical energy.
Efficiency (η) =
Output Electrical Power Input Mechanical Power
The mechanical input power during the vibration of harvester device can be calculated by the given equation,
Input Power (IP ) =
Kinetic Energy = time
1 mv 2 2
t
=
ma2t 2
where m = 16.6 g (mass of device), a = 2.5 g is input acceleration, and t is time. Similarly, the harvested output electrical power (OP) of the proposed energy harvester is 74 mW. Therefore, the energy conversion efficiency (ɳ) is given by
η=
OP = 7.7% IP
The implementation of the cycloid physics for designing the efficient biomechanical energy harvester shows the high-performance electromagnetic energy harvester from the hand-shaking vibration motion. Hence, this research paves the way of implementing physics for the structural improvement of the electromagnetic energy harvester with high performance for harvesting human biomechanical energy. 3. Conclusion In summary, a cycloid curve-shaped electromagnetic energy harvester was designed, developed and demonstrated successfully as a sustainable wearable energy harvester for scavenging human body motion energy. The cycloid physics inspired hollow circular tube was fabricated using 3D printing technology which provides a fastest rolling of spherical magnet inside the tube which ultimately increases the emf induced in the coils. Compared to the conventional rectangular/circular straight and circular tube, the proposed hollow cycloid tube enhanced the energy harvesting performance by 1.45 times and 1.3 times, respectively. By effectively harvesting the human biomechanical energy based on the human hand-shaking vibration motion of 5 Hz, the fabricated energy harvesting device could deliver an average power of 8.8 mW under an optimum load resistance of 104.7 Ω with the energy 8
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Fig. 7. (a) Photograph of commercial sports stopwatch successfully powered by the hand-shaking vibration motion of the energy harvester. (b) Capacitor charging and discharging nature during powering sports stopwatch by proposed energy harvester. (c) Photograph of commercial wristwatch successfully driven by CEEH under hand shaking vibration motion. Fabricated prototype CEEH device. (d) Capacitor charging and discharging nature during powering wristwatch. (e) Capacitor charging and discharging nature during powering commercial digital humidity temperature meter. (Inset image shows photograph of digital humidity temperature meter powered by CEEH). (f) Graph showing total driving time for a commercial wrist watch powered by CEEH under different activity such as walking, running and handshaking.
Appendix A. Supplementary material
conversion efficiency of 7.7%. With just 5 s of handshaking the harvester, it could magnificently drive a commercial sports stopwatch for 16 min. This marvelous achievement proves the possibility of the proposed device for developing self-powered smart wearable electronic devices and sensors for sports, fitness, and health-care monitoring system via scavenging the wasted human biomechanical energy during daily activities.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements This research was supported by the Bio & Medical Technology Development Program of the NRF grant funded by the Korean government (MSIT) (NRF-2017M3A9F1031270). The authors are also grateful for the technical support and discussions with the Micro/Nano Devices and Packaging Laboratory (MiNDaP) group members of Kwangwoon University. 9
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