A non-resonant, frequency up-converted electromagnetic energyharvester from human-body-induced vibration for hand-held smartsystem applications

Miah A. Halim and Jae Y. Parka)

Micro/Nano Devices and Packaging Lab, Department of Electronic Engineering, Kwangwoon University,Seoul 139-701, South Korea

(Received 5 December 2013; accepted 18 February 2014; published online 3 March 2014)

We present a non-resonant, frequency up-converted electromagnetic energy harvester that

generates significant power from human-body-induced vibration, e.g., hand-shaking. Upon

excitation, a freely movable non-magnetic ball within a cylinder periodically hits two magnets

suspended on two helical compression springs located at either ends of the cylinder, allowing those

to vibrate with higher frequencies. The device parameters have been designed based on the

characteristics of human hand-shaking vibration. A prototype has been developed and tested both

by vibration exciter (for non-resonance test) and by manual hand-shaking. The fabricated device

generated 110 lW average power with 15.4 lW cm�3 average power density, while the energy

harvester was mounted on a smart phone and was hand-shaken, indicating its ability in powering

portable hand-held smart devices from low frequency (<5 Hz) vibrations. VC 2014 Author(s). Allarticle content, except where otherwise noted, is licensed under a Creative Commons Attribution3.0 Unported License. [http://dx.doi.org/10.1063/1.4867216]

I. INTRODUCTION

Advances in technologies make human lifestyle easy

and comfortable. With the advances in the fields of micro-

electronics, it has become possible to develop miniaturized

and low power consuming portable smart devices which are

becoming very popular. However, these devices require

some source of electrical energy to operate. Although con-

ventional electrochemical batteries and micro-fuel cells can

satisfy the need for power, they have limited lifespan and

require periodic charging and/or replacement. Alternatively,

energy extraction from the ambient environment, known as

energy harvesting, is another attractive solution because of a

number of environmental sources (e.g., ambient heat, light,

radio waves, vibrations) and their availability. Among vari-

ous ambient energy sources, kinetic energy in the form of

mechanical vibration is one of the most common energy

sources available.1 Over the last few decades, there has been

growing interest in vibration energy harvesting. Recently,

energy harvesting from human motions has become a thrust

for exploration.2–5 Basic human activities (e.g., walking, run-

ning, shaking limbs, jumping, etc.) produce mechanical

vibrations that can be converted to useful electrical energy

by various transaction mechanisms such as electromagnetic,5

piezoelectric,6 and electrostatic7 mechanisms. A suitable

human-body-induced vibration energy harvester can be used

to power various consumer electronics, e.g., mobile phones,

wrist watches, audio devices, hearing aids, implanted bio-

medical devices, etc.

Vibration energy harvesters are linearly associated with

mass-spring-damper systems that show resonant behavior.

Maximum voltage and generated electrical power of a reso-

nant harvester are strongly dependent on the externally

applied vibration frequency and drops significantly at low

frequencies.8 Human-body-induced vibrations are of low fre-

quency with large amplitude, which do not allow the conven-

tional resonant harvesting devices to employ conveniently.9

In order to address the challenge of generating significant

power from low frequency ambient vibration, a number of

research groups have been working on mechanical frequency

up-conversion technique in which low frequency vibration is

converted to high frequency vibration either by mechanical

impact or by magnetic attraction/repulsion.8,10–13 In most of

the reported works, both low frequency and high frequency

oscillators used spring-mass structures. As human motion

generates extremely low-frequency and high-amplitude

vibrations of irregular nature, low frequency oscillator

desires a spring less (freely movable) structure in order to

eliminate the inconvenience in energy harvesting from

human motion.

In this paper, a frequency up-converted electromagnetic

energy harvester (EMEH) is newly proposed, designed, and

demonstrated that is capable of harvesting significant power

from human-body-induced vibration such as hand-shaking. It

utilizes a freely moveable non-magnetic ball to impact peri-

odically (at low frequency) on two magnets suspended on

two separate helical compression springs, allowing them to

vibrate with higher frequency. Relative motion between

the magnet and coil (wounded around the magnets over the

structure) induces voltage across the coil terminals. As the

ball moves freely inside the harvester’s cylindrical structure,

our proposed energy harvesting approach offers non-

resonant operation. The use of two magnet-coil structures

increases the overall output power density and energy trans-

fer efficiency of the harvester. Moreover, the non-resonant

a)Author to whom correspondence should be addressed. Electronic mail:

jaepark@kw.ac.kr

0021-8979/2014/115(9)/094901/6 VC Author(s) 2014115, 094901-1

JOURNAL OF APPLIED PHYSICS 115, 094901 (2014)

operation of the device makes itself to work effectively in

powering portable hand-held smart devices from human-

body-induced vibration.

II. CHARACTERISTICS OF HAND-SHAKINGVIBRATION

As the proposed EMEH has been intended to operate

under human hand-shaking, it is obvious to characterize the

vibration behavior generated by hand-shaking. For better

understanding, we have measured and analyzed the vibration

characteristics of hand-shaking using an accelerometer

(LSM330DLC 3-axis accelerometer; ST Microelectronics)

embedded in a smart phone (Galaxy SIII; Samsung

Electronics). The accelerometer senses the acceleration from

all three directions (x, y, and z-axes) but we have taken the

data from y-axis along which the vibration was applied.

Because of almost zero acceleration in x-axis and z-axis,

data from those axes have been ignored. Gravity bias has

also been eliminated. Data have been collected for 1 min at

50 Hz sampling rate from 5 male subjects of different ages

(from 25 to 35 yr). Fig. 1(a) shows the measured acceleration

values when the phone was hand-shaken in one direction

(along y-axis of the accelerometer). The peak acceleration

values were found to be ranged from 15 ms�2 (�1.5 g) to

20 ms�2 (�2 g). Frequency components of the measured

accelerations have been analyzed by FFT (Fast Fourier

Transform) of the measured data. Fig. 1(b) shows that the

frequency of hand-shaking falls within 2.5 Hz to 6 Hz range

for different subjects. Based on the experimentally obtained

hand-shaking vibration characteristics, we have designed the

EMEH as a non-resonant device to be operated as desired.

III. SYSTEM DESIGN AND PROTOTYPE FABRICATION

The proposed frequency up-converted EMEH consists

of two helical compression springs, two NdFeB magnets,

two coils (connected in series), and a non-magnetic ball.

Fig. 2(a) shows the schematic structure of the proposed

EMEH. Each cylinder shape magnet is suspended on one

side of one spring, while the other end of the spring is

attached to the end-cover (top and/or bottom) of the cylindri-

cal device structure. A freely moveable non-magnetic ball is

placed in between the magnets. Each coil is wrapped around

outside of the cylindrical tube in a position where magnetic

flux linkage with the coil turns during relative motion

between magnet and coil is maximum. The magnet-spring

structure works as the spring-mass-damper system. When

the EMEH is shaken at low frequency with sufficient large

acceleration, the free-moving ball hits the magnet on the

spring allowing it to vibrate with higher frequency (resonant

frequency of the spring-mass system) and a voltage is

induced in the coil. In order to illustrate the proof of concept,

a macro-scale prototype of the frequency up-converted

EMEH has been fabricated and tested. Fig. 2(b) shows the

photograph of the prototype, pictured beside a standard AA

size battery for size comparison. Each NdFeB (1.18 T) mag-

net was glued to a helical compression spring made of

0.5 mm diameter steel wire with 6 turns. The coils were

FIG. 1. Human-body-induced vibra-

tion characteristics: (a) measured

acceleration of vibration generated by

hand-shaking a smart phone in one

direction and (b) frequency compo-

nents obtained by FFT.

FIG. 2. (a) Schematic structure, (b) photograph of the fabricated prototype

(size comparison with a AA size battery), (c) SDOF spring-mass-damper

model, and (d) the equivalent circuit of electromotive force with coil and

load resistance of the proposed frequency up-converted electromagnetic

energy harvester.

094901-2 M. A. Halim and J. Y. Park J. Appl. Phys. 115, 094901 (2014)

formed of 0.14 mm copper wire with 200 turns of each. The

combination was assembled within a 2 mm thick hollow

acrylic tube of 11 mm inner diameter. The parameters of the

macro-scale device are given in Table I.

As the free moving ball impacts on the magnets (upon

excitation), each high-frequency resonator of the proposed

system can be modeled as a single degree of freedom

(SDOF) forced spring-mass-damper system as shown in

Fig. 2(c) in order to derive the generated voltage and power,

and the dynamic equation of motion where m, k, and c are

the equivalent mass, spring stiffness, and damping constant,

respectively. f(t) is the applied harmonic force of amplitude

f0 and angular frequency x. Fig. 2(d) shows the equivalent

electrical circuit of induced electromotive force with coil

and load resistance. The induced open circuit emf voltage,

Vem is then obtained by14

Vem ¼ �NBld

dtxðtÞ ¼ �NBl _xðtÞ; (1)

where N is the number of coil turns, B is the magnetic field

strength, l is the coil length, and x(t) is the relative displace-

ment of the magnet. The derivative of x(t) gives the relative

velocity term that can be determined from the dynamic

motion equation of the SDOF forced spring-mass-damper

system as15

_xðtÞ ¼ � f0xn

kffiffiffiffiffiffiffiffiffiffiffiffiffi1� f2

p e�fxntsinðxdtÞ; (2)

where xn is the natural frequency of the spring, f is the

damping ratio, and xd is the damped natural frequency

defined as xd ¼ xn

ffiffiffiffiffiffiffiffiffiffiffiffiffi1� f2

p. The stiffness of the helical

compression spring is determined from its material parame-

ters and geometry as k ¼ Gd4=8nD3, where G is the shear

modulus of spring material, d is the spring wire diameter, nis the number of active spring coil, and D is the spring diam-

eter. Now, the instantaneous power generated by frequency

up-converted EMEH can be expressed as

P ¼ NBlð Þ2Rload

2 Rcoil þ Rloadð Þ2f0xn

kffiffiffiffiffiffiffiffiffiffiffiffiffi1� f2

p e�fxntsinðxdtÞ" #2

: (3)

According to maximum power transfer theorem, the gener-

ated power is maximum when load resistance matches the

coil resistance.

Equation (2) reveals that the amplitude of magnet/mass

vibration decays exponentially due to damping. As a result,

the output voltage and generated power will also be expo-

nentially decayed signals. Generated output from one elec-

tromagnetic generator diminishes before the next impact

occurs on it, which, in turn, reduces the overall voltage and

power. Use of two series connected generators at two ends of

the system prevents it, as both generators are actuated in one

cycle of the ball vibration. Damping (mechanical and electri-

cal) is the main parameter responsible for the amplitude

decay, which needs to be controlled. It is linearly related to

the velocity of the oscillator, which increases with the

increase in frequency. In a frequency up-converting system,

the resonant frequency of the high-frequency oscillator must

be optimized to compensate the mechanical damping, which

can be achieved by optimal spring constant (depends on

spring material and geometry) and the amount of mass

attached to the spring. In order to obtain maximum output

power, mechanical damping should be minimized and equal

to electrical damping. The device has been designed taking

into account of those design parameters, as well as the

behavior of the operating environment, feasibility in desired

application and reliability. MATLAB simulation has been

carried out to predict the output voltage and power of the

proposed frequency up-converted EMEH. Fig. 3 shows the

simulation results of the open circuit voltage and generated

power of the proposed frequency up-converted EMEH. Two

consecutive maximum peaks are generated in one cycle of

the ball movement because it impacts on the magnets

TABLE I. Design parameters of the frequency up-converted EMEH.

Parameter Value

Magnet dimension Ø10� 3 mm

Spring dimension Ø8� 10 mm

Ball (non-magnetic) material SUS-316

Ball diameter 10.3 mm

Coil inner diameter 13 mm

Coil outer diameter 13.5 mm

Coil length 5 mm

Overall device dimension Ø13.5� 50 mm

FIG. 3. Simulated (a) open circuit volt-

age and (b) instantaneous power deliv-

ered to 17 X optimum load resistance

at 5 Hz excitation frequency and

20 ms�2 accelerations.

094901-3 M. A. Halim and J. Y. Park J. Appl. Phys. 115, 094901 (2014)

consecutively. Moreover, the peak amplitudes decay with

time due to damping of the spring vibration. The mechanical

and electrical damping ratios were found to be 0.024 and

0.013, respectively, which were measured by flick test (flick-

ing the spring-mass system and examining the logarithm of

the ratio of succeeding amplitudes of the decaying signal).16

IV. TEST RESULTS AND DISCUSSION

The fabricated prototype has been tested in the labora-

tory based on the analysis of vibration characteristics gener-

ated by hand-shaking. But, vibrations of low frequency

(<10 Hz) and higher acceleration (>10 ms�2) could not be

applied to the prototype due to the limitation of the test

equipment (vibration exciter). This is why vibration exciter

test was started from 12 Hz frequency. Moreover, in order to

meet practical applications, output from the frequency

up-converted EMEH has been measured by hand-shaking the

fabricated device mounted on a smart phone. Fig. 4 shows

the experimental setup for both the vibration exciter test and

hand-shaking test. In order to minimize the gravity effect on

the inertia of the freely moveable ball, vibration was applied

horizontally in both cases.

Fig. 5(a) illustrates the frequency response behavior of

the fabricated device. Vibrations of three different accelera-

tions (based on the accelerations generated by hand-shaking)

were applied. The highest acceleration (in this case,

20 ms�2) could not be generated below 14 Hz frequency due

to the limitation of the vibration exciter. Results show that

the device generates almost constant open circuit rms

voltages (33.71, 40.11, and 45.29 mV average Vrms at 15, 18,

and 20 ms�2, respectively) up to 22 Hz frequency. It gener-

ates random voltages with a decreasing trend within 23 Hz to

39 Hz frequency range due to the random movement of the

freely movable ball within the cylinder, impacting on the

magnets randomly. At the higher end of the applied fre-

quency range, i.e., within 40 Hz to 60 Hz, the ball does not

move significantly (due to its inertia) to hit the magnets. As a

result, the generated voltages are very low. These voltages

are generated from the self-response of the high-frequency

oscillators (linear resonant behavior) to the applied vibration.

Within this frequency range, a maximum voltage has been

found at 51 Hz frequency, which is the resonant frequency of

each high-frequency oscillator. As the frequency compo-

nents of human-body-induced vibration exist within 25 Hz

frequency,4,5 the frequency response result shows the

non-resonant behavior of the device for energy harvesting

from human-body-induced vibration. The maximum

peak-peak open circuit voltage obtained was 194 mV at

20 ms�2 acceleration, which is almost equal to the predicted

value (201 mV). Fig. 5(b) shows the rms load voltages and

average powers delivered to the load resistances at 15 Hz fre-

quency under 20 ms�2 acceleration. The output of the

EMEH was connected with a continually adjustable load re-

sistor and the resistance values were swept from 8 X to 40 Xrange. The voltage across the load increases as the value of

load resistance increases. However, the average powerFIG. 4. Experimental setup of the (a) vibration exciter test and (b) hand-

shaking test for the frequency up-converted EMEH.

FIG. 5. Vibration exciter test results: (a) open circuit rms voltage, Voc vs.

frequency at different input accelerations, and (b) output rms voltage and av-

erage power vs. load resistances at 15 Hz excitation frequency and 20 ms�2

accelerations.

094901-4 M. A. Halim and J. Y. Park J. Appl. Phys. 115, 094901 (2014)

delivered to the load has a maximum value 104 lW at 17 Xmatched load resistance. The generated power is experimen-

tally equal to V2load=Rload, where Vload is the rms voltage

across the load resistance Rload.

The fabricated frequency up-converted EMEH was

mounted on the backside of a smart phone and the output

voltage across the optimum load resistance was measured by

hand-shaking the smart phone (along with the EMEH). The

applied acceleration (peak value, 20 ms�2) was also meas-

ured at the same time by the accelerometer embedded within

the smart phone. Fig. 6 shows the instantaneously generated

voltage and power waveforms that the maximum peak-peak

voltage is 180 mV and the corresponding peak power is

475 lW. Both the voltage and power are attenuated exponen-

tially with time, as predicted by the simulation result. In

practice, the attenuation is not perfectly exponential because

of the process variation in mounting the magnet on the spring

and assembling, which, in turn, reduces the value of average

power (110 lW) than the expected value (271 lW), as indi-

cated in Fig. 6(b). As the frequency up-converted EMEH

produces decaying waveform rather than producing symmet-

ric periodic waveform produced by common vibration

energy harvesters, the generated average power drops very

much as compared to the peak power. Therefore, more atten-

tion needs to be paid to reduce the damping during designing

the high frequency resonator in order to prevent fast decay of

the generated waveform. Analyzing (by FFT) both the wave-

forms (voltage and acceleration) as shown in Fig. 6(a), it has

been found that the frequency of the generated output volt-

age is 51 Hz (calculated frequency, 50.13 Hz), whereas the

frequency of the applied (hand-shaking) vibration is 4.6 Hz.

It clearly indicates the frequency up-conversion behavior of

the proposed EMEH which can be used to generate signifi-

cant amount of power from human-body-induced vibration.

The impact of movable mass can induce higher frequency at

the high frequency resonator, but it introduces more damping

which reduces the average power. This is why we have

designed the high-frequency resonator to vibrate around

50 Hz. The corresponding average power density of the de-

vice is 15.4 lW cm�3 and the energy transfer efficiency,

defined as17 the ratio of electrical damping to the total (both

mechanical and electrical) damping, is 35%. The perform-

ance of the frequency up-converted EMEH is a significant

advancement of the current state-of-the-art in energy har-

vesting from human-body-induced vibration, especially from

handshaking. The values of up-converted frequency, gener-

ated voltage, and power differ from the values predicted by

the simulation because of the deviation from the calcula-

tions; such that the damping ratio, magnetic field values,

magnetic flux densities, coil position, effect of coil induct-

ance, etc. Significant improvement in the damping (both me-

chanical and electrical) is still required to improve the

performance of the proposed device. More sophisticated

spring design can reduce the mechanical damping. Careful

design of the magnetic circuit can guide the route of flux

lines, reduce the eddy currents and hysteresis losses, which

reduce the electrical damping.18 A device with further opti-

mization would be able to generate much higher voltage,

power, and efficiency.

V. CONCLUSION

In summary, a non-resonant and frequency up-

converted electromagnetic energy harvester to harvest

energy from human-body-induced vibration has been pre-

sented. Proposed system has been designed and verified

with both vibration exciter test and manual vibration (hand-

shaking) test in macro-scale (volume, 7.16 cm3). The vibra-

tion exciter test results prove its non-resonant operation and

feasibility of the frequency up-conversion technique.

Although the generated voltage (and power) level was not

sufficient to power up an electronic circuit, manual vibra-

tion test results showed its ability in powering portable

hand-held smart devices from hand-shaking. The output

voltage and power can be increased by increasing the num-

ber of coil turns and using thicker magnet, without increas-

ing the volume of the device, which, in turn, will increase

the power density. With further improvements in design pa-

rameters (e.g., spring stiffness, damping), it is possible to

improve the performance of the proposed electromagnetic

energy harvesting device.

ACKNOWLEDGMENTS

The authors are grateful to acknowledge the support

from the research grant of Kwangwoon University in 2013,

Basic Science Research Program (2010-0024618), and the

Pioneer Research Center Program (2010-0019313) through

the National Research Foundation of Korea (NRF) funded by

the Ministry of Education, Science and Technology, Korea.

FIG. 6. (a) Output voltage waveform and (b) instantaneous power waveform

across 17 X optimum load resistance when the frequency up-converted

EMEH prototype was mounted on a smartphone and was hand-shaken.

094901-5 M. A. Halim and J. Y. Park J. Appl. Phys. 115, 094901 (2014)

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