석사 학위논문

Master's Thesis

교량의 교통 유발 진동 에너지용

전자기 기반 2 자유도 에너지 하베스터 개발

Development of an electromagnetic based

two degree-of-freedom energy harvester

for traffic-induced vibration energy in bridges

양 수 영 (梁 秀 榮 Yang, Suyoung)

건설 및 환경공학과

Department of Civil and Environmental Engineering

KAIST

2015

교량의 교통 유발 진동 에너지용

전자기 기반 2 자유도 에너지 하베스터 개발

Development of an electromagnetic based

two degree-of-freedom energy harvester

for traffic-induced bridge vibration energy in bridges

Development of an electromagnetic based

two degree-of-freedom energy harvester

for traffic-induced vibration energy in bridges

Advisor : Professor Hoon Sohn

by

Suyoung Yang

Department of Civil and Environmental Engineering

KAIST

A thesis submitted to the faculty of KAIST in partial fulfillment of the re-

quirements for the degree of Master of Science and Engineering in the Depart-

ment of Civil and Environmental Engineering. The study was conducted in ac-

cordance with Code of Research Ethics1

2014. 12. 16

Approved by

Professor Hoon Sohn

1 Declaration of Ethical Conduct in Research: I, as a graduate student of KAIST, hereby declare that I have not

committed any acts that may damage the credibility of my research. These include, but are not limited to: falsi-fication, thesis written by someone else, distortion of research findings or plagiarism. I affirm that my thesis contains honest conclusions based on my own careful research under the guidance of my thesis advisor.

교량의 교통 유발 진동 에너지용

전자기 기반 2 자유도 에너지 하베스터 개발

양 수 영

위 논문은 한국과학기술원 석사학위논문으로

학위논문심사위원회에서 심사 통과하였음.

2014 년 12 월 16 일

심사위원장

심사위원

심사위원

손 훈 (인)

정 형 조 (인)

장 선 준 (인)

i

MCE

20134378

양 수 영. Yang, Suyoung. Development of an electromagnetic based two degree-of-freedom energy harvester for traffic-induced vibration energy in bridges.

교량의 교통 유발 진동 에너지용 전자기 기반 2 자유도 에너지 하베스터 개발.

Department of Civil and Environmental Engineering. 2015. 41 p. Advisor Prof.

Sohn, Hoon.

ABSTRACT

This paper presents an electromagnetic based two degree-of-freedom (DOF) energy

harvester for traffic-induced vibration energy in bridges. Since the developed harvester,

composed of a mechanical and magnetic spring, has two similar resonance frequencies be-

low 5 Hz, it can effectively harvest the low-frequency and –acceleration vibration energy

induced by traffic. For the optimal design of the proposed 2DOF energy harvester, nonline-

ar numerical simulation algorithm is developed and the harvester hardware is fabricated

based on the simulation. The performance of the developed harvester is validated by lab-

scale experiment using the emulated vibration data of real bridges. The simulated and

measured results show quite high similarity and averaged 0.31 mW and 0.015 mW of

power are generated respectively in Yeondae Bridge and Geumdang Bridge. Also, it is

proved that the developed harvester performs well in low-frequency and –acceleration vi-

bration by the comparison with the 1DOF harvester in the Fourier spectrum.

Keywords: Electromagnetic based 2DOF energy harvester, Traffic-induced vibration,

Magnetic spring, Nonlinear numerical simulation, Structural health monitoring, Wireless

sensor

ii

TABLE OF CONTENTS

ABSTRACT ···························································································· i

TABLE OF CONTENTS ··········································································· ii

LIST OF TABLES ·················································································· iv

LIST OF FIGURES ·················································································· v

CHAPTER 1. INTRODUCTION

1.1 Motivation ······································································· 1

1.2 Literature review ······························································· 2

1.3 Objective and uniqueness ····················································· 4

CHAPTER 2. THEORETICAL BACKGROUND

2.1 Governing equation of motion ··············································· 5

2.1.1 Nonlinear magnetic spring stiffness ···································· 5

2.1.2 Derivation of an equation of motion ··································· 7

2.2 State-space solutions ·························································· 8

2.2.1 Continuous state-space solution ········································· 8

2.2.2 Discrete state-space solution ············································ 9

2.3 Electromagnetic transduction ·············································· 12

CHAPTER 3. NUMERICAL SIMULATION

3.1 Simulation algorithm ························································ 13

3.2 Simulation results ···························································· 15

CHAPTER 4. HARVESTER HARDWARE DESCRIPTION

4.1 Magnet and spring design ··················································· 18

4.2 Casing and assembly ························································· 19

4.3 Coil manufacturing ··························································· 20

CHAPTER 5. EXPERIMENTAL VERIFICATION

5.1 Experimental setup ··························································· 21

iii

5.2 Experimental results ························································· 24

5.2.1 Traffic-induced vibration input of Yeondae Bridge ················ 24

5.2.2 Traffic-induced vibration input of Geumdang Bridge ············· 28

5.3 Comparison with the 1DOF harvester in Fourier spectrum ············ 30

CHAPTER 6. CONCLUSIONS

6.1 Executive summary ····························································· 31

6.2 Future works ····································································· 32

REFERENCES ······································································· 33

SUMMARY (IN KOREAN) ······················································· 36

ACKNOWLEDGEMENTS (IN KOREAN) ·································· 37

CURRICULUM VITAE ··························································· 38

iv

LIST OF TABLES

Table 1. Detailed parameters of magnet and spring ······························ 18

Table 2. Detailed parameters of coil ················································ 20

Table 3. Measured and simulated results of Yeondae Bridge ··················· 26

Table 4. Measured and simulated results of Geumdang Bridge ················ 29

v

LIST OF FIGURES

Figure 1. Force-displacement graph of nonlinear and linear spring ············· 6

Figure 2. Schematic diagram and free body diagram of the 2DOF energy harvester

································································································ 7

Figure 3. Flow diagram of numerical simulation algorithm ····················· 13

Figure 4. Comparison of simulated results between 1DOF and 2DOF energy har-

vester: (a) open circuit voltage, (b) generated power on load ( 296.92 ) ·· 15

Figure 5. Schematic of an electromagnetic based 2DOF energy harvester ·· 17

Figure 6. Actual images of (a) magnet (two magnets, which have 15 mm height,

stick to each other) and, (b) spring ···················································· 18

Figure 7. Actual images of (a) plastic pipe, (b) plastic stopper, and (c) assembled

casing ······················································································· 19

Figure 8. Actual images of coil ······················································ 20

Figure 9. Overall experimental setup ··············································· 21

Figure 10. Vibration data of Yeondae Bridge: (a) traffic-induced displacement da-

ta, (b) acceleration data in time domain, and (c) in frequency domain ··········· 23

Figure 11. Comparison with measured and simulated results of 2DOF energy har-

vester in Yeondae Bridge: (a) open circuit voltage, and (b) generated power on load

······························································································· 25

vi

Figure 12. Comparison with measured and simulated results of 1DOF magnetic

spring energy harvester in Yeondae Bridge ··········································· 26

Figure 13. Vibration data of Geumdang Bridge: (a) traffic-induced displacement

data, (b) acceleration data in time domain, and (c) in frequency domain ········· 27

Figure 14. Comparison with measured and simulated results of 2DOF energy har-

vester in Geumdang Bridge: (a) open circuit voltage, and (b) generated power on

load ·························································································· 28

Figure 15. Comparison with the 1DOF harvester in Fourier spectrum of open cir-

cuit voltage ················································································· 30

- 1 -

CHAPTER 1

INTRODUCTION

1.1 Motivation

During the past decades, wireless sensors have been widely used in a variety of

monitoring applications. Especially in structural health monitoring (SHM) applications,

they are receiving a growing interest since they provide advantages as cabling convenience,

continuous and remote monitoring, low price, and etc [1]. They require no wires but batter-

ies for power, which need periodic replacement. This is a serious problem because the

maintenance cost for battery replacement can be over the cost of sensor itself due to labor

expenses [2]. Therefore, developing energy harvesting devices are attracting considerable

attention for supplying power to wireless sensor systems in civil applications.

Bridges are one of the best options for the wireless sensor based SHM system [3].

There have been a tremendous number of researches that harvest ambient energy from

bridge environment such as sunlight, wind, temperature gradient, and vibration. Among

these, vibration based energy harvesting system is regarded as the most promising due to

the fact that vibration source like traffic almost constantly exist around a bridge site [4, 5].

Generally, the characteristics of traffic-induced vibration of a bridge are that its frequency

and acceleration are below 8 Hz and 1 m/s2 [6, 7], and scavenging this kind of low-

frequency and –acceleration vibration energy is quite difficult compared with other vibra-

tions such as factory machines, vehicles and human motions. Therefore, the energy harvest-

ing device capturing traffic-induced vibration energy is in need for wireless sensor based

bridge SHM systems.

- 2 -

1.2 Literature review

Few researches have studied on harvesting bridge vibration energy induced by traf-

fic or wind. Elvin et al (2006) developed piezoelectric (PZT) transducer based vibration

energy harvesting system and its average power output was 0.25 μW at 8.7 Hz and 0.61

m/s2 [8]. However, PZT badly performs below 1000 Hz frequency vibration so that without

any special technique like frequency up-conversion [9], this is unsuitable for the bridge ap-

plication.

Instead of the PZT approach, electromagnetic method, which follows Faraday’s law

with permanent magnets and coils, is more prospective. Many researchers developed one

degree-of-freedom (DOF) magnetic spring devices for harvesting low-frequency vibration

energy [10-13]. Due to the low spring constant of the magnetic springs, they are proper for

low-frequency application, but they still require high level of displacement. Their target ap-

plication is actually not on a bridge but human motion. These kinds of resonant spring-mass

approach are utilized to vibrate a moving body with magnets of the generator. The designed

natural frequency of the moving part has to be similar to the main resonant frequency of the

target structure to magnify the movement of a moving part. For the bridge application,

however, the movement of a moving part cannot reach available level even in resonant con-

dition due to its inherent characteristic of traffic-induced vibration.

Besides, the dissertation of Galchev (2010) and Galchev et al. (2011) proposed an

electromagnetic based parametric frequency-increased generator (PFIG) and it provides av-

erage power of 2.3 μW at 2.0 Hz and 0.54 m/s2 [14, 15], but the output power is too low to

operate general wireless sensors. Rotational energy harvester utilizing wind-induced vibra-

tion had been fabricated by Jung et al. (2011) and Kim et al. (2013) improved it [4, 16]. It

- 3 -

generates 2.6 mW average power at 3.4 Hz and 1.13 m/s2, but this high acceleration level

hardly occurs in traffic-induced vibration. Kwon et al. (2014) proposed electromagnetic an

energy harvester with repulsively stacked multilayer magnets and 118 μW of average power

is scavenged at 3.7 Hz and 1 m/s2, which is in still high acceleration level compared with

real bridge vibration [17].

- 4 -

1.3 Objective and uniqueness

To overcome disadvantages of the conventional energy harvesting devices men-

tioned in previous subchapter, this study develops an electromagnetic based 2DOF energy

harvester, which contains two resonance frequencies from the linear spring and magnetic

spring. This will magnify the movement of a moving part twice by matching its resonance

frequencies to resonance frequency of target structure, so that it will show greater output

power than conventional harvesters. The proposed harvester will be designed by means of a

nonlinear numerical simulation based on discrete state-space solution, and then be verified

in lab-scale experiment with emulated real bridge vibration.

The uniqueness of this study lies in: (1) development of an energy harvester for traf-

fic-induced vibration, which is low-frequency and –acceleration, in bridges, (2) remarkable

increase of output power compared with 1DOF magnetic spring energy harvester, and (3)

performance of a nonlinear numerical simulation.

- 5 -

CHAPTER 2

THEORETICAL BACKGROUND

2.1 Governing equation of motion

To predict the output power and find the optimal design of 2DOF energy harvester,

analytical or numerical approach is necessary and, this involves the formulation, which de-

scribes the physical situation of the harvester. In this subchapter, nonlinearity of magnetic

spring stiffness and the formulation of a governing equation of motion are presented

2.1.1 Nonlinear magnetic spring stiffness

The definition of stiffness is the extent to which it resists deformation in response to

an applied force [18]. By Hook’s law, F −kx, the force F needed to extend or compress

a mechanical spring by some distance x is proportional to that distance, where k is a

spring stiffness. This spring stiffness is commonly used for constant since the force-

displacement graph is linear (Blue line in Fig. 1(a)).

- 6 -

Figure 1. Force-displacement graph of nonlinear and linear spring

In case of magnetic (nonlinear) spring, however, its force-displacement graph shows

high nonlinearity (Red line in Fig. 1(b)). This is because the interaction between magnets

has nonlinear relationship [12], inversely proportional to the square of the distance. Here,

the gradient of force-displacement graph at any position will be used for the magnetic

spring stiffness kM as shown in Eq. (1).

𝑘 𝑑𝐹

𝑑𝑥

(1)

- 7 -

2.1.2 Derivation of an equation of motion

Figure 2. Schematic diagram and free body diagram of the 2DOF energy harvester

Fig. 2 represents the schematic diagram and free body diagram of the 2DOF energy

harvester, where y0 is base displacement, y1 is the harvester casing displacement, y2 is

the moving magnet displacement, m1 is the mass of the harvester casing, m2 is the mass

of the moving magnet, c1 is damping coefficient for m1, c2 is damping coefficient for

m2, k is spring coefficient of the mechanical spring (Constant), and kM is spring coeffi-

cient of the magnetic spring as shown in Eq. (1). The equation of motion is formulated by

force equilibrium as Eq. (2).

[𝑀][�̈�] + [𝐶][�̇�] + [𝐾][𝑦] [𝑓] (2)

[𝑀] [𝑚1 00 𝑚2

] , [𝐶] *𝑐1 + 𝑐2 −𝑐2−𝑐2 𝑐2

+ , [𝐾] [𝑘 + 𝑘𝑀 −𝑘𝑀−𝑘𝑀 𝑘𝑀

]

[𝑓] *𝑐1 ∗ 𝑦0̇ + 𝑘 ∗ 𝑦0

0+ , [𝑦] *

𝑦1𝑦2+

- 8 -

2.2 State-space solutions

2.2.1 Continuous state-space solution

Eq. (2) expresses the equation of motion of the 2DOF magnetic energy harvester,

and this can be written as

[�̇��̈�] [

𝑂 𝐼−[𝑀]−1[𝐶] −[𝑀]−1[𝐾]

] *𝑦�̇�+ + [

𝑂[𝑀]−1

] [𝑓] (3)

[𝐴] [𝑂 𝐼

−[𝑀]−1[𝐶] −[𝑀]−1[𝐾]] , [𝐵] [

𝑂[𝑀]−1

]

In case of continuous problem, then, we rewrite Eq. (3) in the customary state form

[�̇�(𝑡)] [𝐴][𝑌(𝑡)] + [𝐵][𝑓(𝑡)] (4)

where [𝑌(𝑡)] [𝑦(𝑡) �̇�(𝑡)]𝑇.

- 9 -

2.2.2 Discrete state-space solution

If the set of equations is to be computed on a digital computer, it must be discretized

[19]. Because

�̇� lim𝑇 →0

𝑦(𝑡 + 𝑇) − 𝑦(𝑡)

𝑇

we can approximate Eq. (4) as

[𝑌(𝑡 + 𝑇)] [𝑌(𝑡)] + [𝐴][𝑌(𝑡)]𝑇 + [𝐵][𝑓(𝑡)]𝑇 (5)

If we compute [𝑌(𝑡)] only at t kT for k 0, 1, 2,⋯, then Eq. (5) become

[𝑌((𝑘 + 1)𝑇)] (𝐼 + 𝑇[𝐴] )[𝑌(𝑘𝑇)] + 𝑇[𝐵][𝑓(𝑘𝑇)] (6)

This is a discrete-time state-space equation and can easily be computed on a digital com-

puter. This discretization is the easiest to carry out but yields the least accurate results for

the same T.

If an input [𝑓(𝑡)] is generated by a digital computer followed by a digital-to-

analog converter, then [𝑓(𝑡)] will be piecewise constant. This situation often arises in

computer control systems. This input changes values only at discrete-time instants. For this

input, Eq. (4) still equals

- 10 -

[𝑌(𝑡)] 𝑒[𝐴]𝑡[𝑌(0)] + ∫ 𝑒[𝐴](𝑡−𝜏)[𝐵][𝑓(𝜏)]𝑑𝜏𝑡

0

(7)

Computing Eq. (7) at t kT and t (k + 1)T yields

[𝑌(𝑘𝑇)]: 𝑌[𝑘] 𝑒[𝐴]𝑘𝑇𝑌[0] + ∫ 𝑒[𝐴](𝑘𝑇−𝜏)[𝐵][𝑓(𝜏)]𝑑𝜏𝑘𝑇

0

[𝑌((𝑘 + 1)𝑇)]: 𝑌[𝑘 + 1] 𝑒[𝐴](𝑘+1)𝑇𝑌[0] + ∫ 𝑒[𝐴]((𝑘+1)𝑇−𝜏)[𝐵][𝑓(𝜏)]𝑑𝜏(𝑘+1)𝑇

0

(8)

Eq. (8) can be written as

𝑌[𝑘 + 1] 𝑒[𝐴]𝑇(𝑒[𝐴]𝑘𝑇𝑌[0] + ∫ 𝑒[𝐴](𝑘𝑇−𝜏)[𝐵][𝑓(𝜏)]𝑑𝜏𝑘𝑇

0

)

+ ∫ 𝑒[𝐴]((𝑘𝑇+𝑇−𝜏)[𝐵][𝑓(𝜏)]𝑑𝜏(𝑘+1)𝑇

𝑘𝑇

(9)

which becomes, after substituting Eq. (8) and introducing new variable α kT + T − τ,

𝑌[𝑘 + 1] 𝑒[𝐴]𝑇𝑌[𝑘] + (∫ 𝑒[𝐴]𝛼𝑑𝛼𝑇

0

) [𝐵]𝑓[𝑘] (10)

Thus, if an input changes values only at discrete-time instants kT and if we compute only

the response at t kT. Then Eq. (4) become

𝑌[𝑘 + 1] 𝐴𝑑𝑌[𝑘] + 𝐵𝑑𝑓[𝑘] (11)

where

- 11 -

𝐴𝑑 𝑒[𝐴]𝑇 , Bd (∫ 𝑒

[𝐴]𝜏𝑑𝜏𝑇

0) [𝐵] [𝐴]−1(𝐴𝑑 − 𝐼)[𝐵]

Using this discrete state-space approach, the displacement response (y1, 𝑦2) from Eq. (2)

can be easily obtained.

- 12 -

2.3 Electromagnetic transduction

Once mechanical motion, vibration, is coupled to the generator, a transduction

mechanism is required to convert this vibration energy into electricity. An electromagnetic

based energy harvester uses a magnetic field to convert mechanical vibration energy to

electrical energy. A permanent magnet attached to the oscillating mass moves through a sta-

tionary coil. The magnet moves through the coil causes varying magnetic flux through coil

and an electromotive voltage is induced according to Faraday’s law [20]. The relative am-

plitude of the oscillation depends on a quality of the resonance mechanism and level of vi-

bration affects the relative movement which provides harvested power [21]. The induced

voltage depends on velocity and length of coil and it is usually small and must therefore be

increased to viably source energy. The induced voltage across the coil and instantaneous

generated power are given by

𝑣 −𝐵𝐿𝑐�̇� (12)

𝑃𝑖 𝑐𝑒𝑧2̇ (13)

where B is magnetic flux density, Lc is the length of coil, �̇� is net velocity of moving

magnet, and ce is electromagnetic damping coefficient, which is given by

𝑐𝑒 (𝐿𝑐𝐵)

2

𝑅𝑙 + 𝑅𝑐 + 𝑗𝑤𝐿𝑐

(14)

where l, c, and Lc are the load resistance, coil resistance, and coil inductance, respec-

tively.

- 13 -

CHAPTER 3

NUMERICAL SIMULATION

3.1 Simulation algorithm

Eq. (11) in previous chapter implies that response at time t (k + 1)T can be ob-

tained using response and input at time t kT. Before applying Eq. (11), matrices Ad and

Bd have to be updated at every time step T because it includes nonlinear stiffness matrix

[K], and this changes with the position of moving magnet as mentioned in Chapter 2.1.1.

Then, it is obvious that in case initial condition Y[0], and input 𝑓[𝑘] are given, response

Y[𝑘] is numerically acquired. Using discrete state-space solution, simulation algorithm for

this nonlinear problem is developed, and Fig. 3 shows its flow diagram.

Figure 3. Flow diagram of numerical simulation algorithm

- 14 -

The response Y[𝑘] [𝑦1[𝑘] 𝑦2[𝑘] 𝑦1̇[𝑘] 𝑦2̇[𝑘]]𝑇 is numerically derived

through this procedure, and net displacement z y2 − 𝑦1 and velocity �̇� 𝑦2̇ − 𝑦1̇ of

moving magnet is obtained. Finally, the induced voltage on the coil and generated power

can be provided using Eq. (12) and Eq. (13).

- 15 -

3.2 Simulation results

(a)

(b)

Figure 4. Comparison of simulated results between 1DOF and 2DOF energy harvester: (a)

open circuit voltage, (b) generated power on load ( 296.92 )

Discrete state-space numerical simulation algorithm is implemented with MATLAB

coding. Using this, outputs of the 2DOF energy harvester are simulated and they are com-

pared with those of 1DOF magnetic spring energy harvester, which has same size, to show

the performance superiority of 2DOF energy harvester. Also, the hardware parameters are

decided through this simulation. For the simulation input, previously measured vibration

- 16 -

data in Yeondae Bridge is used and the details about it will be presented in chapter 5.

Blue and red line means the results of 1DOF and 2DOF energy harvester, respec-

tively. As shown in Fig. 4, 2DOF energy harvester shows higher performance than that of

1DOF magnetic spring energy harvester.

- 17 -

CHAPTER 4

HARVESTER HARDWARE DESCRIPTION

Figure 5. Schematic of an electromagnetic based 2DOF energy harvester

The hardware of 2DOF energy harvester consists of magnets, spring, casing with

additional mass, and coil. Total mass of the harvester is 1.16 kg including 1 kg of addi-

tional mass. Fig. 5 presents the schematic of the harvester. Intimate information for each

component is explained in following subchapter.

- 18 -

4.1 Magnet and spring design

To have resonance frequencies of the harvester below 5 Hz, parameters for magnet

and spring magnet are decided. Following the simulation mentioned in chapter 3, the radius

and height of magnet and spring constant are 7.5 and 30 mm and around 800 N/m, re-

spectively. Totally, three identical magnets are used in the harvester, one is for moving

mass, and the others are for magnetic spring. Detailed parameters and actual images of

magnet and spring are shown in Table 1 and Fig. 5.

Figure 6. Actual images of (a) magnet (two magnets, which have 15 mm height, stick to

each other) and, (b) spring

Table 1. Detailed parameters of magnet and spring

Magnet

Material NdFeB (N35)

Residual magnetic flux density 1.18 T

Coercive force 860 kA/m

Density 7.47 × 103𝑘𝑔/𝑚3

Dimension ϕ15 × 30 mm

Spring

Material Stainless steel (STS304)

Constant 797.7 N/m

Dimension ϕ21.6 × 15 mm

- 19 -

4.2 Casing and assembly

(a)

(c)

(b)

Figure 7. Actual images of (a) plastic pipe, (b) plastic stopper, and (c) assembled casing

A 200 mm length of plastic pipe is used for casing of the harvester. The length is

determined to achieve low resonance frequency. Top and bottom side of the pipe, magnets

with plastic stopper are located facing the surface have the same pole with the moving

mass. They push the moving mass, so this performs as a magnetic spring. In the bottom

stopper, there is a hole for screw that additional mass can be hanged, so that the resonance

frequency is easily tuned. The plastic pipe and stoppers are assembled by glue. More infor-

mation of casing is in Fig. 7.

- 20 -

4.3 Coil manufacturing

A wire-wound copper coil is wrapped horizontally around the outside of the plastic

pipe. The thickness of coil wire is 0.08 mm and the number of coil turn is 1000. Fig. 7 and

Table 2 show more about the coil.

Figure 8. Actual images of coil

Table 2. Detailed parameters of coil

Coil

Material Copper

Wire thickness 0.08 mm (AWG40)

Inner diameter 20.1 mm

Outer diameter 24.0 mm

Height 5 mm

Number of turn 1000

Resistance 271.36

Spacing between coil and moving magnet 2.1 mm

- 21 -

CHAPTER 5

EXPERIMENTAL VERIFICATION

To verify the performance of the developed 2DOF energy harvester, lab-scale test is

conducted. Using a modal shaker, previously measured traffic-induced vibration data of real

bridges are excited and generated voltage is measured by a data acquisition system with a

simple electric circuit. The performance is compared with the simulation results.

5.1 Experimental setup

Figure 9. Overall experimental setup

- 22 -

Fig. 9 shows the overall experimental setup for the lab-scale test. Previously meas-

ured traffic-induced displacement data of real bridges are stored in the arbitrary waveform

generator (AWG; Agilent 33220A) and the generator is connected with the modal shaker

controller. The modal shaker (APS 400 Electro-seis) replicates those traffic-induced vibra-

tions through the controller. By the oscillation of the developed harvester, induced voltage

is generated on coil winding. This open circuit voltage and generated power are measured

by the digitizer (NI-PXI 5122) in the data acquisition system (DAQ; NI-PXI 1031). While

coil is directly cabled to the DAQ system to measure open circuit voltage, the measurement

of generated power needs electric circuit with single resistor. By triggering, DAQ system

and AWG are synchronized.

- 23 -

(a)

(b)

(c)

Figure 10. Vibration data of Yeondae Bridge: (a) traffic-induced displacement data, (b) ac-

celeration data in time domain, and (c) in frequency domain

- 24 -

5.2 Experimental results

Two traffic-induced vibration data are measured in real bridges, one is Yeondae

Bridge (steel girder box bridge) and the other is Geumdang Bridge (prestressed concrete

plate girder bridge), both are located in Jungbunaeryuk Expressway, Yeoju, Korea. A truck

of 15 ton runs at 100 km/h on the bridges, and the displacement is obtained by dis-

placement meter (Tokyo Sokki Kenkyujo CDP-10) [7].

5.2.1 Traffic-induced vibration input of Yeondae Bridge

Fig. 10 shows the displacement and acceleration data of Yeondae Bridge in time and

frequency domain. Maximum displacement, and acceleration are less than 5 mm, and

0.6 m/s2 respectively, and the resonance frequencies are below 5 Hz. From this vibration,

the measured peak and the root-mean-square (RMS) open circuit voltage on coil are 2.10 V

and 0.74 V respectively, and measured peak and the average power on the load, which

resistor value is 296.92 , are 2.80 mW and 0.31 mW respectively. Fig. 11 describes

them with simulated results.

- 25 -

(a)

(b)

Figure 11. Comparison with measured and simulated results of 2DOF energy harvester in

Yeondae Bridge: (a) open circuit voltage, and (b) generated power on load

Same experiment is performed by getting rid of a spring, which implies a 1DOF

magnetic spring energy harvester. Its results are in Fig. 12 (Open circuit voltage is not pre-

sented because of low signal to noise ratio). Compared with this, the developed 2DOF en-

ergy harvester performs better. The reason of this huge difference from the simulation is

inferred that the simulation does not reflect the initial friction and high air damping. Table

3. summarizes measured and simulated results.

- 26 -

Figure 12. Comparison with measured and simulated results of 1DOF magnetic spring en-

ergy harvester in Yeondae Bridge

Table 3. Measured and simulated results of Yeondae Bridge

1DOF 2DOF

Power (μW) Open circuit voltage (V) Power (mW)

Peak Average Peak RMS Peak Average

Measured 0.80 0.05 2.10 0.74 2.80 0.31

Simulated 56.0 8.44 3.09 1.20 3.20 0.53

- 27 -

(a)

(b)

(c)

Figure 13. Vibration data of Geumdang Bridge: (a) traffic-induced displacement data, (b)

acceleration data in time domain, and (c) in frequency domain

- 28 -

5.2.2 Traffic-induced vibration input of Geumdang Bridge

(a)

(b)

Figure 14. Comparison with measured and simulated results of 2DOF energy harvester in

Geumdang Bridge: (a) open circuit voltage, and (b) generated power on load

Fig. 13 shows the displacement and acceleration data of Geumdang Bridge in time

and frequency domain. Maximum displacement, and acceleration are less than 2.5 mm,

and 0.4 m/s2 respectively, and the resonance frequencies are below 6 Hz. From this vi-

bration, the measured peak and the root-mean-square (RMS) open circuit voltage on coil

are 0.54 V and 0.16 V respectively, and measured peak and the average power on the

- 29 -

load, which resistor value is 296.92 , are 0.18 mW and 0.015 mW respectively. Fig.

14 describes them with simulated results.

Same experiment is performed by getting rid of a spring, which implies a 1DOF

magnetic spring energy harvester. Because of very small displacement, however, the 1DOF

harvester does not respond. Table 4. summarizes measured and simulated results.

Table 4. Measured and simulated results of Geumdang Bridge

1DOF 2DOF

Power (μW) Open circuit voltage (V) Power (mW)

Peak Average Peak RMS Peak Average

Measured - - 0.54 0.16 0.18 0.015

Simulated 2.09 0.18 0.60 0.25 0.23 0.039

- 30 -

5.3 Comparison with the 1DOF harvester in Fourier spectrum

To figure out the enhancement of the developed 2DOF harvester against 1DOF har-

vester, the open circuit voltage from the 1DOF and 2DOF harvester are analyzed in Fourier

spectrum. The open circuit voltages are plotted in frequency domain by using fast Fourier

transform. Fig. 15 shows the induced voltage from Yeondae Bridge vibration data, and this

presents that the developed 2DOF harvester performs much better than the 1DOF harvester

in traffic-induced vibration.

Figure 15. Comparison with the 1DOF harvester in Fourier spectrum of open circuit voltage

- 31 -

CHAPTER 6

CONCLUSIONS

6.1 Executive summary

In this study, an electromagnetic based 2DOF energy harvester is proposed for scav-

enging traffic-induced vibration energy in bridges. The hardware of the developed harvester

is presented, and it is designed by using a nonlinear numerical simulation based on a dis-

crete state-space solution. The performance of the developed harvester is validated by lab-

scale experiment using emulated vibration data from Yeondae Bridge and Geumdang

Bridge. The simulated and measured results show quite high similarity and averaged

0.31 mW and 0.015 mW of power are generated respectively in Yeondae Bridge and Ge-

umdang Bridge. By the construction of the Fourier spectrum of induced voltage, it is also

proved that this harvester performs well in low-frequency and –acceleration vibration.

- 32 -

6.2 Future works

For the performance improvement and real field application of the developed elec-

tromagnetic based 2DOF energy harvester, more work should be studied in the future:

1. New nonlinear numerical simulation algorithm, able to reflect the high damping

case (i.e. critically or over damped) and initial friction of the casing, should be

developed to design the harvester optimally. Also, reducing the friction between

moving magnet and casing should be achieved to increase the harvesting effi-

ciency.

2. Additional electric circuit and rechargeable battery should be integrated with the

energy harvester. The fresh energy from the energy harvester cannot operate a

wireless sensor because of low stability and dissatisfaction of the sensor re-

quirement. Therefore, electric circuit for rectification and rechargeable battery

for stable energy storage should be accomplished.

3. Neatly designed and packaged harvester including previously mentioned electric

circuit and battery should be provided. Otherwise, the applicability in field and

durability will dramatically decrease.

4. After achieving all above, field test should be conducted. To apply the harvester

for powering real wireless sensor, field test is necessary. Yeondae Bridge, in

Yeoju, Korea, and Yeongjong Grand Bridge, in Incheon, Korea, are considered

as potential test-beds.

- 33 -

REFERENCES

[1] Lynch, J. P., Wang, Y., Law, K. H., Yi, J. H., Lee, C. G., and Yun, C. B. (2005).

“Validation of a large-scale wireless structural monitoring system on the Geumdang

Bridge.” International Conference on Safety and Structural Reliability, Rome, Italy,

pp.19-23.

[2] US Department of Energy, Energy Efficiency and Renewable Energy. (2008).

“Low-Cost Vibration Power Harvesting for Wireless Sensors.” pp.1-2.

[3] Williams, C. B., Pavic, A., Crouch, R. S., and Woods, R. C. (1997). “Feasibility

study of vibration-electric generation for bridge vibration sensors.” 16th

Internation-

al Modal Analysis Conference, Orlando, US, pp.1111-1117.

[4] Jung, H. J., Kim, I. H., and Jang, S. J. (2011). “An energy harvesting system using

wind-induced vibration of a stay cable for powering a wireless sensor node.” Smart

Materials and Structures, 20(7), pp.1-9.

[5] Jung, H. J., Park, J., and Kim, I. H. (2012). “Investigation of applicability of elec-

tromagnetic energy harvesting system to inclined stay cable under wind load.”

IEEE Transactions on Magnetics, 48(11), pp.3478-3481.

[6] Bachmann, H., and Ammann, W. (1987). Vibrations in structures induced by man

and machines, Structural Engineering Document 3e of International Association for

Bridge and Structural Engineering, Zurich.

[7] Kim, K., and Sohn, H. (2014). “Dynamic displacement estimation based on two-

- 34 -

stage Kalman filter and multi-rate data fusion.” 6th

World Conference on Structural

Control and Monitoring, Barcelona, Spain.

[8] Elvin, N. G., Lajnef, N., and Elvin, A. A. (2006). “Feasibility of structural health

monitoring with vibration powered sensors.” Smart Materials and Structures, 15(4),

pp.977-986.

[9] Kulah, H., and Najafi, K. (2008). “Energy scavenging from low-frequency vibra-

tions by using frequency up-conversion for wireless sensor applications.” IEEE

Sensors Journal, 8(3), pp.261-268.

[10]Saha, C. R., O’Donnell, T., Wang, N., and McCloskey, P. (2008). “Electromagnetic

generator for harvesting energy from human motion.” Sensors and Actuators A:

Physical, 147(1), pp.248-253.

[11]Cheng, S., and Arnold, D. P. (2010). “A study of multi-pole magnetic generator for

low-frequency vibration energy harvesting.” Journal of Micromechanics and Mi-

croengineering, 20(2), 025015.

[12]Foisal, A. R. M., Hong, C., and Chung, G. S. (2012). “Multi-frequency electromag-

netic energy harvester using a magnetic spring cantilever.” Sensors and Actuators

A: Physical, 182, pp.106-113.

[13]Zhang, Q., Wang, Y., and Kim, E. S. (2014). “Power generation from human body

motion through magnet and coil arrays with magnetic spring.” Journal of Applied

Physics, 115, 064908.

[14]Galchev, T. (2010). “Energy scavenging from low frequency vibrations.” Ph.D.

Dissertation, The University of Michigan, Mcichigan, US, 228 pages.

- 35 -

[15]Galchev, T., McCullagh, J., Peterson, R. L., and Najafi, K. (2011). “Harvesting traf-

fic-induced vibrations for structural health monitoring of bridges.” Journal of Mi-

cromechanicals and Microengineering, 21(10), pp.1-13.

[16]Kim, I. H., Jang, S. J., and Jung, H. J. (2013). “Performance enhancement of a rota-

tional energy harvester utilizing wind-induced vibration of an inclined stay cable.”

Smart Materials and Structures, 22(7), pp.1-7.

[17]Kwon, S. D., Park, J., and Law, K. (2013). “Electromagnetic energy harvester with

repulsively stacked multilayer magnets for low frequency vibrations.” Smart Mate-

rials and Structures, 22(5), 055007.

[18]Baumgart, F. (2000). “Stiffness – an unknown world of mechanical science?” Inju-

ry, 31(2), pp.14-23.

[19]Chen, C. T. (1984). Linear system theory and design, Oxford University Press, Ox-

ford

[20]Hadas, Z., Zouhar, J., Singule, V., and Ondrusek, C. (2008). “Design of energy har-

vesting generator based on rapid prototyping parts.” 13th

Power Electronics and

Motion Control Conference, Poznan, Poland, pp.1665-1669.

[21]Hadas, Z., Zouhar, J., Singule, V., and Ondrusek, C. (2007). Recent Advances in

Mechatronics: Simulation of Vibration Power Generator, Springer, New York.

- 36 -

요 약 문

교량의 교통 유발 진동 에너지용

전자기 기반 2 자유도 에너지 하베스터 개발

본 연구에서는 교량의 교통 유발 진동 에너지 수확을 위한 전자기 기반

2 자유도 에너지 하베스터를 개발하였다. 개발된 2 자유도 에너지 하베스터는

선형 및 비선형 (자석) 스프링으로 이루어져 있으며, 두 공진 주파수가 5 Hz

이하가 되도록 설계되어 저주파수 및 저진폭의 특성을 가지는 교량의 교통

유발 진동에 적합하다. 2 자유도 에너지 하베스터의 최적 설계를 위하여 비선형

수치 시뮬레이션 알고리즘을 고안했으며, 이를 토대로 하베스터의 하드웨어를

제작하였다. 제작된 하베스터의 성능을 평가하기 위하여 미리 계측한 실제

교량의 데이터를 이용한 실험실 규모의 검증 실험을 수행하였다.

중부내륙고속도로에 위치하고 있는 연대교, 금당교의 진동 데이터를 실험에

사용하였으며, 각각 평균 0.31 mW 및 0.015 mW 의 출력을 나타내었다. 또한,

푸리에 스펙트럼 (Fourier spectrum) 분석을 통하여 개발된 2 자유도

하베스터가 기존의 1 자유도 하베스터보다 월등히 높은 성능을 가지는 것을 알

수 있었다.

핵심어: 전자기 기반 2 자유도 에너지 하베스터, 교통 유발 진동, 자석 스프링,

비선형 수치 시뮬레이션, 구조물 건전성 모니터링, 무선 센서

- 37 -

감 사 의 글

지난 2 년간의 석사과정 생활을 마무리하며 도움을 주신 많은 분들께

감사의 글을 전하고자 합니다. 먼저, 지도교수님이신 손훈 교수님께 깊은

감사의 뜻을 전합니다. 저의 부족한 점을 채워주시고 끊임 없는 가르침을 주신

교수님 덕분에 석사과정을 무사히 끝맺을 수 있었습니다. 더불어 바쁜 일정

중에서도 시간을 내어 학위논문 심사에 참여해주신 KAIST 정형조 교수님,

이노베이션 KR 장선준 박사님께도 감사의 말씀을 전합니다. 또한 본 연구를

수행할 수 있도록 지원해주신 다차원스마트 IT 융합 시스템 연구단 (CISS-

2011-0031870) 에도 깊은 감사의 뜻을 전합니다.

생각했던 것과는 달리 대학원 생활에서 많은 어려움과 힘든 일이

있었지만, 항상 제 편에서 힘을 북돋아주시는 가족이 있었기에 이렇게 감사의

글을 쓸 수 있었습니다. 그리고 매일 아침부터 밤 늦게까지 같이 생활하며 크고

작은 도움을 준 연구실 구성원들에게도 감사한 마음을 표현하고 싶습니다. 이

연구를 진행하는 과정에서 가장 큰 도움을 주신 기영이형, 날카로운 지적과

좋은 조언을 해주는 형진이형, 어려움이 있을 때마다 본인의 일처럼 같이

고민해준 병진이형, 처음 연구실에 들어와서 많은 것을 가르쳐 준 진열이형,

항상 열심히 하는 모습으로 자극이 되어준 지민이형, 좋은 이야기를 많이

해주는 민구형, 할 수 있다는 자신감을 북돋아 준 Peipei 형, 서로 의지하며

석사 생활을 같이 보낸 동기 준우형, 같이 탁구를 치며 많은 이야기를 나누는

정말 착한 상민이형, 귀찮았을 텐데도 내색하지 않고 그림 그리는 걸 도와준

- 38 -

준이형, 디펜스 준비 때 같이 늦게까지 연구실에 남아 의지가 되어준 재묵이형

순규형, 칭찬을 해주며 긍정적인 생각을 가질 수 있게 해준 승환이형, 이상한

장난에도 늘 박장대소 해주는 병주, 그리고 이름을 일일이 거론하기 힘들지만

대학원 생활을 즐겁게 할 수 있는 활력소가 되어준 건설 및 환경공학과

선후배님들과 직원 분들에게도 감사의 마음을 전합니다. 그리고 축구를 하며

함께 땀 흘렸던 현재형과 916 식구들, 멀리 있지만 항상 의지가 되고 언제

봐도 즐거운 찬호, 찬솔, 동언이 그리고 충북과학고 19 기 친구들에게도

감사하다는 말을 드립니다.

이렇게 많은 분들과 미처 언급하지 못한 분들의 도움과 관심이

없었더라면 석사과정을 무사히 마칠 수 없었을 것입니다. 항상 감사하는 마음을

가지고 앞으로 더 정진하는 모습을 보이도록 하겠습니다. 다시 한번 더

감사하다는 말씀을 드리며 감사의 글을 마치겠습니다. 감사합니다.

2015 년 1 월 KAIST 에서 양수영 올림.

- 39 -

CURRICULUM VITAE

Suyoung Yang

Research Assistant

Department of Civil and Environmental Engineering

Korea Advanced Institute of Science and Technology (KAIST)

291 Daehak-Ro, Guseong-Dong, Yuseong-Gu, Daejeon, Republic of Korea, 305-701

Tel.: (82)+42-350-3665, Fax: (82)+42-350-3610

Email: suyoung091@kaist.ac.kr

RESEARCH INTERESTS

Electromagnetic based vibration energy harvesting for traffic-induced vibration in bridges

Fatigue crack detection using nonlinear ultrasonics and wireless sensor node

Impedance based structural health monitoring

EDUCATION

2013 – 2015

M.S., Dept. of Civil and Environmental Eng., Korea Advanced Institute of Science

and Technology (KAIST), Republic of Korea

2009 – 2013

B.S., Dept. of Civil and Environmental Eng., Korea Advanced Institute of Science

and Technology (KAIST), Republic of Korea

JOURNAL PUBLICATIONS

* The corresponding author is underlined

1. Peipei Liu, Hyung Jin Lim, Suyoung Yang, Hoon Sohn, and Yi Yung, “Development of a

stick-and-detect wireless sensor node for fatigue crack detection,” in preparation for An Inter-

national Journal of Structural Health Monitoring, 2014

2. Peipei Liu, Hoon Sohn, Suyoung Yang, and Tribikram Kundu, “Fatigue crack localization us-

ing noncontact laser ultrasonics and state space attractors,” Submitted to Journal of the Acousti-

cal Society of America, 2014

3. Jinyeol Yang, Peipei Liu, Suyoung Yang, Hyeonseok Lee, and Hoon Sohn, “Laser based im-

pedance measurement for pipe corrosion and bolt-loosening detection,” Accepted to Smart

Structures and Systems, 2014

4. Peipei Liu, Hoon Sohn, Tribikram Kundu, and Suyoung Yang, “Noncontact detection of fa-

mailto:suyoung091@kaist.ac.kr

- 40 -

tigue cracks by laser nonlinear wave modulation spectroscopy (LNWMS),” NDT&E Interna-

tional, Vol. 66, pp. 106-116, 2014.

5. Hyeonseok Lee, Suyoung Yang, Jinyeol Yang, and Hoon Sohn, “Monitoring of pipelines in

nuclear power plants by measuring laser-based mechanical impedance,” Smart Materials and

Structures, Vol. 23, No. 6, 065008, 2010.

PATENTS

* The corresponding author is underlined

1. Hoon Sohn, Peipei Liu, and Suyoung Yang, “Diagnosis method of structure and diagnosis sys-

tem,” Korea patent application (10-2014-0123524), September 17th, 2014

2. Hoon Sohn, Hyung Jin Lim, Suyoung Yang, and Peipei Liu, “Wireless inspection apparatus of

a structure using nonlinear ultrasonic wave modulation and inspecting method using the appa-

ratus,” Korea patent publication (10-1414520-0000), June 26th, 2014

3. Hoon Sohn, Hyung Jin Lim, Suyoung Yang, and Peipei Liu, “Wireless inspection apparatus of

a structure using nonlinear ultrasonic wave modulation and inspecting method using the appa-

ratus,” International PCT patent application (PCT/KR2013/012037), December 23rd

, 2013

TECHNOLOGY TRANSFER

1. Hoon Sohn, Hyung Jin Lim, Suyoung Yang, and Peipei Liu, “Wireless sensor using nonlinear

ultrasonic wave modulation technique,” transferred to TM E&C Co. Ltd, November 2014

BOOK & BOOK CHAPTERS

* The corresponding author is underlined

1. Hoon Sohn, Hyung Jin Lim, and Suyoung Yang, “Fatigue crack detection methodology,” a

book chater in CISS: Smart sensors for health and environmental monitoring, Springer, in

preparation

CONFERENCE PROCEEDINGS

* 1 PCT patent application, 1 Korea patent publication, and 1 Korea patent application

1. Peipei Liu, Hyung Jin Lim, Suyoung Yang, Hoon Sohn, Hyung Chul Park, Yung Yi, and Dong

Sam Ha, “Development of an active wireless sensor node for fatigue crack detection using non-

linear wave modulation,” the 2nd

International Conference on Structural Health Monitoring and

- 41 -

Integrity Management, September 24-26, Nanjing, China, 2014

2. Jinyeol Yang, Suyoung Yang, Hyeonseok Lee, and Hoon Sohn, “Wireless impedance-based

pipe corrosion monitoring using MFC transducers under temperature variations,” the 6th Inter-

national Conference on Structural Health Monitoring of Intelligent Infrastructure, December 9-

11, Hong Kong, 2013

PARTICIPANT PROJECTS

1. Development of as self-sufficient wireless sensor node based structural health monitoring

system for civil infrastructure : Global Frontier Project (CISS) at National Research Founda-

tion of Korea (Funded: 608,508,000 KRW (608,508 USD) for 09/29/11 to 08/31/20)

2. A smart scanning system for green energy infrastructure : Korean Federation of Science

and Technology Societies (Funded: 327,000,000 KRW (327,000 USD) for 04/17/13-08/16/14)

EDUCATIONAL EXPERIENCES

Fall

2013

Teaching Assistant, CE207, “Elementary Structural Engineering and Laboratory” ,

KAIST, 3 unit undergraduate course

HONORS & AWARDS

Scholarships for outstanding students granted KAIST (2010-2012) : The three prominent students

in the department of civil and environmental engineering of KAIST were selected as recipients. This

scholarship provided 2000$ per year for three years