석사 학위논문

Master's Thesis

대형 구조물의 건전성 모니터링을 위한 동심원형

이중 압전센서 기반의 기전 임피던스 계측

Dual Piezoelectric Transducer Based Electromechanical

Impedance Measurement for Health Monitoring of

Large Structures

송 호 민 (宋 鎬 珉 Song, Homin)

건설 및 환경공학과

Department of Civil and Environmental Engineering

KAIST

2013

대형 구조물의 건전성 모니터링을 위한 동심원형

이중 압전센서 기반의 기전 임피던스 계측

Dual Piezoelectric Transducer Based Electromechanical

Impedance Measurement for Health Monitoring of

Large Structures

Dual Piezoelectric Transducer Based Electromechanical

Impedance Measurement for Health Monitoring of

Large Structures

Advisor : Professor Hoon Sohn by

Homin Song

Department of Civil and Environmental Engineering

Korea Advanced Institute of Science and Technology

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

quirements for the degree of Master of Engineering in the Department of Civil

and Environmental Engineering. The study was conducted in accordance with

Code of Research Ethics1.

2012. 11. 29

Approved by

Professor Hoon Sohn

[Major Advisor]

1Declaration 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.

i

대형 구조물의 건전성 모니터링을 위한 동심원형

이중 압전센서 기반의 기전 임피던스 계측

송 호 민

위 논문은 한국과학기술원 석사학위논문으로

학위논문심사위원회에서 심사 통과하였음.

2012 년 11 월 29 일

심사위원장

심사위원

심사위원

심사위원

손 훈 (인)

정 형 조 (인)

홍 정 욱 (인)

노 용 래 (인)

- ii -

MCE

20113313

송 호 민. Song, Homin. Dual Piezoelectric Transducer Based Electromechanical Imped-ance Measurement for Health Monitoring of Large Structures. 대형 구조물의 건전성

모니터링을 위한 동심원형 이중 압전센서 기반의 기전 임피던스 계측. Depart-ment of Civil and Environmental Engineering. 2012. 50 p. Advisor Prof. Sohn, Hoon. Text in English

ABSTRACT

This study presents an electromechanical (EM) impedance measurement technique

using a dual piezoelectric transducer for continuous monitoring of large structures. The dual

piezoelectric transducer, composed of two separate but concentric lead zirconate titanate

(PZT) segments, can effectively measure the small response of the host structure, since it is

not influenced by the excitation-induced PZT strain. The proposed EM impedance measure-

ment technique shows high signal-to-noise ratio (SNR) and good repeatability from the

measured impedance signals in massive structures. At the same time, the proposed technique

allows low-cost and fast measurement of the impedance signatures. In this study, the dual

PZT impedance measurement is formulated theoretically and verified numerically. Then, a

series of experimental validation were carried out on a laboratory-size specimen and full-

scale bridge and building structures. The experimental results show that the proposed tech-

nique successfully measures the EM impedance signals with high SNR and good repeatabil-

ity from massive structures even in which the conventional techniques fail to do so.

Keywords: Dual piezoelectric transducer, Electromechanical impedance, Large structures,

Separate excitation and sensing, High signal-to-noise ratio, Good repeatability

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TABLE OF CONTENTS

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

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

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

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

CHAPTER 1. INTRODUCTION ·············································· 1

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

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

1.3 Contribution and uniqueness ······················································ 3

CHAPTER 2. FORMULATION OF DUAL PZT IMPEDANCE

MEASUREMENT ································································· 4

2.1 Conventional impedance measurement technique ···························· 4

2.2 Proposed dual PZT admittance measurement technique ····················· 8

2.2.1 Dual PZT admittance measurement ······································ 8

2.2.2 Formulation of the dual PZT admittance ······························· 10

2.3 Discussions on amplitude difference of active admittance between

conventional and dual PZT techniques ············································ 14

2.3.1 Sensing area difference between actuator and sensor PZTs ·········· 14

2.3.2 Structural impedance difference between conventional and dual PZT

admittances ··············································································· 15

CHAPTER 3. NUMERICAL SIMULATION ···························· 18

3.1 Simulation setup ································································· 18

3.2 Simulation results ································································ 20

CHAPTER 4. LAB-SCALE EXPERIMENTAL VALIDATION ···· 22

4.1 Experimental setup ······························································ 22

4.2 Admittance measurement results ·············································· 24

4.3 Effects of separate excitation and sensing for the dual PZT ················ 25

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CHAPTER 5. FULL-SCALE EXPERIMENTAL VALIDATION ·· 28

5.1 Admittance measurement in a cable anchor of an extra-dosed bridge ···· 28

5.1.1 Experimental setup ························································· 29

5.1.2 Admittance measurement results ········································· 30

5.2 Admittance measurement in an inclined steel column of a building

structure ············································································· 34

5.1.1 Experimental setup ························································· 34

5.1.2 Admittance measurement results ········································· 35

CHAPTER 6. APPLICATION OF DUAL PZT IMPEDANCE

TECHNIQUE TO STRUCTURAL DAMAGE DETECTION ········· 38

6.1 Experimental setup ··························································· 38

6.2 Experimental results ·························································· 39

CHAPTER 7. CONCLUSION ··············································· 43

REFERENCES ·································································· 44

SUMMARY (IN KOREAN) ·················································· 47

ACKNOWLEDMENTS (IN KOREAN) ··································· 48

CURRICULUM VITAE ······················································· 49

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LIST OF TABLES

1. Material properties of the steel beam and piezoelectric properties of the PZT

························································································ 18

2. Effective resolutions of A/D conversion ··································· 27

3. Bolt-loosening scenario for the damage detection test ··················· 39

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LIST OF FIGURES

1. A schematic representation of the conventional impedance measurement

technique. ············································································ 4

2. Analytical models of EM impedance technique. ··························· 7

3. The dual PZT impedance measurement. ····································· 8

4. A beam structure under axial and bending vibration. A dual PZT is located

on the top surface of the beam structure. ····································· 15

5. A finite element model used to validate the proposed dual PZT based imped-

ance technique. ··································································· 19

6. Admittance signals obtained from the conventional and dual PZT tech-

niques. ·············································································· 20

7. Experimental setup. ··························································· 23

8. Admittance signals obtained from the conventional and dual PZT tech-

niques. ·············································································· 24

9. Time-domain input and output signals for 1 V excitation applied. ······ 25

10. Overview of Haknarae bridge in Sejong, South Korea. ················ 28

11. Target bridge structure and the installed dual PZT. ···················· 29

12. Admittance measurement results from the field bridge test. ·········· 30

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13. Quantitative indicators of measurement performance. ················· 31

14. A roof structure at Korea Advanced Institute of Science and Technology in

South Korea. ······································································ 34

15. Admittance measurement results from the field building structure. ·· 35

16. Quantitative indicators of measurement performance. ················· 37

17. A steel H-beam where a steel gusset plate tightened with twelve bolts is ar-

ranged. ············································································· 39

18. Normalized conductance signals ·········································· 40

19. Damage index values calculated for all measurement techniques ···· 42

- viii -

CHAPTER 1 INTRODUCTION

1.1 Motivation

Structural health monitoring (SHM) has become a significant issue in civil, mechani-

cal, and aeronautical engineering, and a number of SHM techniques using vibration, acoustic

emission and ultrasonic have been investigated to assess the safety and integrity of in-situ

structures [1-4]. Among a diversity of SHM techniques, the electromechanical (EM) imped-

ance based damage detection technique is recognized as a promising approach due to its high

sensitivity to local incipient damage [5-7]. The EM impedance techniques measure the elec-

trical impedance of a surface-mounted lead zirconate titanate (PZT) transducer, which is di-

rectly coupled with the mechanical impedance of the host structure so that structural damage

can be detected by monitoring the change of the measured EM impedance signature [6].

In the fundamental stage of the EM impedance techniques, the EM impedance meas-

urement has typically been performed by commercial impedance analyzers [6-7]. Commer-

cial impedance analyzers are able to measure the impedance signals with high signal-to-noise

ratio (SNR) and good repeatability, since an auto-balancing bridge technique measures the

exact electric current flowing through the device under test (DUT), and the built-in high-gain

preamplifiers enhance the amplitude of the output response [15]. However, their high price,

bulky hardware, and slow measurement speed make it less attractive to use the technique in

real-field SHM applications [16]. To overcome these limitations, the self-sensing circuit

based low-cost impedance measurement has recently been developed [9, 13-14]. The self-

- 1 -

sensing circuits using a reference capacitor and/or resistors enable light weight and low-cost

measurement, but they are hard to expect the high SNR and good repeatability in stiff struc-

tures due to lack of any noise reduction technique [9]. Here, both techniques share difficulties

in measuring impedance signals when the host structure becomes excessively large. Because

only one PZT segment is utilized for both excitation and sensing simultaneously, it measures

the excitation-induced strain as well as the structural response-induced strain. Here, the exci-

tation-induced PZT strain which is not related to the structural dynamic properties is domi-

nantly large compared to the structural response-induced strain, hence it degrades the meas-

urement sensitivity of the PZT to a small response to a small response from the massive host

structure.

1.2 Literature review

A variety of EM impedance based techniques have been investigated for SHM appli-

cations. Park et al (2005) investigated an outlier analysis framework combined with the EM

impedance methods for structural health monitoring [8]. Min et al (2010) examined the im-

pedance-based health monitoring of building and bridge structures using artificial neural

networks (ANNs) for autonomous frequency selection [9]. Lim et al (2011) developed an

impedance-based bolt-loosening detection method compensating external environmental

conditions using Kernel principal component analysis [10]. Kim et al (2011) developed a

baseline-free impedance-based crack detection technique using two collocated PZT transduc-

ers [11]. Mascarenas et al (2010) developed the impedance-based wireless sensors and sensor

nodes, and verified through laboratory and field tests [12]. However, a great part of the stud-

ies have validated the feasibility of impedance techniques in ideal laboratory-size specimens.

- 2 -

Moreover, some rare studies which applied impedance-based SHM techniques to in-service

massive structures showed a difficulty in measurement of clear EM impedance signature.

1.3 Contribution and uniqueness

To overcome the inferior applicability of the conventional impedance measurement

techniques to massive structures, this study develops a low-cost and continuous impedance

measurement technique using a dual PZT, showing high SNR and good repeatability. A dual

PZT, composed of two separate but concentric PZT segments, can effectively measure the

small response of the host structure, because it is not influenced by the excitation-induced

PZT strain, but is possible to catch the small structural response. The proposed EM imped-

ance measurement technique has following uniqueness and advantages: (1) EM impedance

measurement using a dual PZT is first developed, ensuring separate excitation and sensing;

(2) It has compatibility with light weight and low-cost measurement devices, which is similar

to the self-sensing circuit measurement; (3) Higher SNR and better repeatability than both

commercial impedance analyzers and self-sensing circuits are obtained; (4) The measurement

speed is much faster than the one of the commercial impedance analyzers, which is adequate

for the real- field applications [16].

- 3 -

CHAPTER 2

FORMULATION OF DUAL PZT IMPEDANCE MEASUREMENT

2.1 Conventional impedance measurement technique

Sensing (Iout)Excitation (Vin)

Host Structure

PZT

Fig. 1. A schematic representation of the conventional impedance measurement technique.

Piezoelectric materials create an electric response when a mechanical stress is sub-

jected to the materials (direct piezoelectric effect), and a mechanical response is conversely

produced when an electric voltage is applied to them (converse piezoelectric effect). Owing

to the unique piezoelectric properties, the piezoelectric materials have been used for various

sensor and actuator applications [17].

The EM impedance-based SHM techniques utilize a surface-mounted PZT for both

actuator and sensor as shown in Fig. 1. A sinusoidal input voltage (Vin) is applied to the PZT

and the output current (Iout) is measured. The electrical admittance (inverse of the electrical

impedance) is defined by the ratio of the output current (Iout) to the excitation voltage (Vin),

which is an electrical frequency response function. In this paper, the electrical admittance of

PZT is considered for the numerical simulation and test results on behalf of the electrical im-

pedance of PZT, since the admittance signature is shown to be more sensitive to incipient

damage than the impedance signature [7]. Note that the admittance and impedance signatures

show identical resonance frequencies of the host structure. Liang et al (1996) proposed a

- 4 -

one-dimensional (1D) analytical model of the EM impedance as shown in Fig. 2(a), and

showed that the mechanical impedance of the host structure (𝑍𝑍𝑆𝑆) is directly coupled with the

electrical impedance of the PZT (𝑍𝑍) as follows [6]:

Y(ω) = 𝑍𝑍(𝜔𝜔)−1 =𝐼𝐼𝑜𝑜𝑜𝑜𝑜𝑜(𝜔𝜔)𝑉𝑉𝑖𝑖𝑖𝑖(𝜔𝜔)

= 𝑖𝑖𝜔𝜔𝐶𝐶𝑎𝑎 �1 − 𝜅𝜅312 �1 −𝑍𝑍𝑎𝑎

𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑎𝑎�� (1)

where 𝐶𝐶𝑎𝑎 is the zero-load capacitance of the PZT, 𝜅𝜅312 is the electromechanical coupling

coefficient of the PZT and 𝑍𝑍𝑎𝑎 is the mechanical impedance of the PZT. Given that the me-

chanical impedance and material properties of the PZT are invariant, any changes of the me-

chanical impedance of the host structure (𝑍𝑍𝑆𝑆) are reflected into the change of the measured

electrical admittance of the PZT (Y), which allows monitoring of the health state of the host

structure. Here, the admittance can be further divided by a passive and an active admittances

(Y𝑃𝑃(ω) and Y𝐴𝐴(ω)) as follows [21]:

Y(ω) = Y𝑃𝑃(ω) + Y𝐴𝐴(ω) = [𝑖𝑖𝜔𝜔𝐶𝐶𝑎𝑎(1 − 𝜅𝜅312 )] + �𝑖𝑖𝜔𝜔𝐶𝐶𝑎𝑎 �𝜅𝜅312𝑍𝑍𝑎𝑎

𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑎𝑎�� (2)

The mechanical impedance of the host structure is not coupled with the passive admittance,

but with the active admittance. Thus, the change of the mechanical impedance of the host

structure is reflected into the change of the active admittance, which makes the EM imped-

ance technique sensitive to local incipient damage. Note that the passive admittance does not

change by the change of the structural impedance, but by the change of the PZT properties.

Despite the high sensitivity of the EM impedance signature to local incipient damage,

simultaneous excitation and sensing of the PZT makes it difficult to catch a small response

from massive and/or stiff host structures. Since the PZT measures the vibration response

- 5 -

while exciting the host structure, the excitation-induced PZT strain is included in the meas-

ured output signal and it worsens the measurement sensitivity of the PZT. Moreover, the

dominantly large amplitude of the excitation-induced PZT strain compared to the structural

response-induced PZT strain is unfavorable to measure the small response, unless the data

acquisition system is equipped with a high-bit analog-to-digital (AD) converter. That is be-

cause the larger the output voltage range is, the poorer effective resolution of the AD conver-

sion can be obtained under fixed bits of the AD converter. That follows [29]:

∆𝑉𝑉𝑒 =𝐹𝑉𝑉𝑆𝑆2𝑖𝑖

=𝑉𝑉𝑠𝑠𝑎𝑎𝑥𝑥 − 𝑉𝑉𝑠𝑠𝑖𝑖𝑖𝑖

2𝑖𝑖 (3)

where, ∆𝑉𝑉𝑒, 𝐹𝑉𝑉𝑆𝑆 and 𝑛 represent the effective resolution of the AD conversion, the full

scale voltage range and the number of bits for the AD converter, respectively. Also, 𝑉𝑉𝑠𝑠𝑎𝑎𝑥𝑥

and 𝑉𝑉𝑠𝑠𝑖𝑖𝑖𝑖 are maximum and minimum output voltages, respectively. This handicap of the

conventional impedance measurement techniques have been restricting the application of the

impedance-based SHM technique to large scale structures, such as bridge components, pipe-

line structures, and wind turbine blades.

- 6 -

Actuator(a)

z

x

hala

Structure

KM

C

(a)

Actuator(a)

z

xhb

ha

lb

la

Structure

K

Sensor(b)

M

C

(b)

Fig. 2. Analytical models of EM impedance technique: (a) Conventional 1-D model [6], and (b) the proposed

dual PZT impedance model.

- 7 -

2.2 Proposed dual PZT admittance measurement technique

2.2.1 Dual PZT admittance measurement

(a)

(b)

Fig. 3. The dual PZT impedance measurement: (a) the configuration of the dual PZT composed of concentric

ring and circular PZTs, and (b) schematic diagram of the dual PZT impedance measurement.

The dual PZT for the proposed EM impedance measurement technique consists of

concentric ring and circular PZT segments as shown in Fig. 3(a) [18]. These two concentric

but separate PZT components can be activated independently or simultaneously for ultrasonic

wave excitation and sensing. In previous studies, dual PZTs were in mainly use for the Lamb

wave mode decomposition to identify individual Lamb wave modes [19], and baseline-free

damage detection to discriminate mode conversions of the Lamb waves propagating within

thin metallic structures [20].

- 8 -

Fig. 3(b) illustrates the schematic diagram of the dual PZT impedance measurement

technique. The EM impedance is measured in a pulse-echo mode by exciting the ring seg-

ment and measuring the response using the inner circular segment of the dual PZT. The dual

PZT impedance measurement technique ensures enhanced measurement efficiency, by sepa-

rating the excitation and sensing parts of the PZT transducer. The separate excitation and

sensing is favorable to obtain a small response from massive structures [28], because it is

possible to catch the small response in the absence of the excitation-induced PZT strain. It is

noted that the full scale voltage of the output response measured by the dual PZT (𝐹𝑉𝑉𝑆𝑆𝑑𝑑𝑜𝑜𝑎𝑎𝑙𝑙)

is much smaller than the one by the conventional single PZT (𝐹𝑉𝑉𝑆𝑆𝑐𝑐𝑜𝑜𝑖𝑖𝑐𝑐), resulting in that the

much higher resolution of the AD conversion can be obtained.

The electrical admittance Y(ω) obtained using the dual PZT is calculated by dividing

the excitation voltage applied to the outer ring PZT (Vin) by the output current measured

from the inner circular PZT (Iout) as follows:

Y(ω) = 𝑍𝑍(𝜔𝜔)−1 =𝐼𝐼𝑜𝑜𝑜𝑜𝑜𝑜(𝜔𝜔)𝑉𝑉𝑖𝑖𝑖𝑖(𝜔𝜔)

= 𝑖𝑖𝜔𝜔𝐶𝐶𝑏𝑏𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜(𝜔𝜔)𝑉𝑉𝑖𝑖𝑖𝑖(𝜔𝜔)

(4)

where, Vout and Cb are the measured output voltage and zero-load capacitance of the circular

sensing PZT, respectively. Note that similar electrical admittance can also be obtained by ex-

citing the inner circular PZT and measuring the output current from the outer ring PZT, as

well. In this study, however, the outer ring PZT-excitation and inner circular PZT-sensing are

used since a larger PZT area attached on the host structure is preferred for excitation and a

smaller area for sensing [19].

There has been little attention to a transfer impedance measurement which use sepa-

rate actuator and sensor PZTs so far, and its physical characteristics has not been described in

- 9 -

depth [13-14]. Moreover, the measurement sensitivity of the transfer impedance is decreased

when the distance between the actuator and sensor PZTs gets long apart due to the attenua-

tion of the excitation waves [5, 29]. In the proposed dual PZT technique, the separate sensor

PZT is located inside the ring-shape actuator PZT so that the distance between the actuator

and sensor PZTs cannot be an issue. Also, the physical characteristics of the dual PZT tech-

nique are thoroughly described by the theoretical formulation, numerical simulation and a

series of experiments in following chapters.

2.2.2 Formulation of the dual PZT admittance

A 1D analytical model of the proposed dual PZT admittance measurement is derived,

based on the 1D model of Liang et al [6]. Fig. 2(b) illustrates the analytical model of the pro-

posed dual PZT impedance measurement technique. The main difference of the dual PZT

admittance model from the Liang’s model is that two PZT segments interact with the struc-

ture whereas only one PZT actuator interacts with the host structure in the Liang’s model.

The impedance measurement model using separate actuator and sensor PZTs is theoretically

derived in this study at first.

Considering the actuator and sensor PZTs in Fig. 2(b), the electric field is applied to

the actuator PZT in the z-direction, and it is assumed that the actuator and sensor PZTs ex-

pand and contract only in x-direction. The constitutive equation of the PZTs can be expressed

as follows:

𝑆𝑆1 = �̅�𝑠11𝐸𝐸 𝑇𝑇1 + 𝑑𝑑31𝐸𝐸3 (5)

𝐷𝐷3 = 𝜀𝜀3̅3𝑇𝑇 𝐸𝐸3 + 𝑑𝑑31𝑇𝑇1 (6)

- 10 -

where 𝑆𝑆1 is strain and 𝐷𝐷3 is the electric displacement. 𝑇𝑇1, 𝐸𝐸3, �̅�𝑠11𝐸𝐸 , 𝜀𝜀3̅3𝑇𝑇 and 𝑑𝑑31are the

stress, the applied electric field, the complex compliance, the complex dielectric constant,

and the piezoelectric constant, respectively. Note that an overhead bar indicates its spatial

component, other than the case representing the complex material properties, such as �̅�𝑠11𝐸𝐸

and 𝜀𝜀3̅3𝑇𝑇 . Here, the subscripts 1 and 3 represent the x- and z- direction.

The equation of motion for the actuator PZT vibrating in x-direction can be expressed

as follows:

𝑌𝑌�11𝐸𝐸𝜕𝜕2𝑢𝑢(𝑥𝑥, 𝑡𝑡)𝜕𝜕𝑥𝑥2

= 𝜌𝜌𝜕𝜕2𝑢𝑢(𝑥𝑥, 𝑡𝑡)𝜕𝜕𝑡𝑡2

(7)

where 𝑢𝑢(𝑥𝑥, 𝑡𝑡) is the displacement in the x-direction, 𝑌𝑌�11𝐸𝐸 is the complex Young’s modulus,

and ρ is the density of the PZT.

Solving Eq. (7) yields [6]:

𝑢𝑢𝑥𝑥=𝑙𝑙𝑎𝑎 = 𝑢𝑢�𝑥𝑥=𝑙𝑙𝑎𝑎𝑒𝑒𝑖𝑖𝑖𝑖𝑜𝑜 =

𝑑𝑑31𝐸𝐸�𝑘𝑘

𝑍𝑍𝑎𝑎𝑍𝑍𝑇𝑇 + 𝑍𝑍𝑎𝑎

sin𝑘𝑘𝑙𝑙𝑎𝑎cos𝑘𝑘𝑙𝑙𝑎𝑎

𝑒𝑒𝑖𝑖𝑖𝑖𝑜𝑜 (8)

where 𝑘𝑘 is the wavenumber of the wave excited by the actuator PZT and 𝑍𝑍𝑎𝑎 is the mechan-

ical impedance of the actuator PZT. Note that this solution is valid for the actuator PZT and

the host structure at the coupled point at 𝑥𝑥 = 𝑙𝑙𝑎𝑎. Here, the mechanical impedance of the

structure (𝑍𝑍𝑆𝑆) in the solution of Liang’s model is replaced by the total mechanical impedance

(𝑍𝑍𝑇𝑇). Assume that the mechanical impedance of the structure (𝑍𝑍𝑆𝑆) and sensor PZT (𝑍𝑍𝑏𝑏) are

connected in parallel for the proposed technique, 𝑍𝑍𝑇𝑇 can be expressed as the sum of 𝑍𝑍𝑆𝑆 and

𝑍𝑍𝑏𝑏 as follows:

- 11 -

𝑍𝑍𝑇𝑇 = 𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑏𝑏 = �𝑅𝑅𝑠𝑠𝜔𝜔 −𝐾𝑠𝑠(1 + 𝜂𝑠𝑠𝑖𝑖)

𝜔𝜔𝑖𝑖� + �−

𝐾𝑏𝑏(1 + 𝜂𝑏𝑏𝑖𝑖)𝜔𝜔

𝑖𝑖� (9)

where, 𝐾𝑠𝑠 and 𝜂𝑠𝑠 are the static stiffness and the mechanical loss factor of the structure and

𝐾𝑏𝑏 and 𝜂𝑏𝑏 are the static stiffness of the sensor PZT and the mechanical loss factor of the

sensor PZT. The mass of the sensor PZT can be ignored.

It is assumed that the displacement solution of Eq. (8) is valid for the sensor PZT at the

coupled point at 𝑥𝑥 = 𝑙𝑙𝑏𝑏, due to the concentric characteristic of the dual PZT [19]. Then the

output displacement and strain of the sensor PZT yield by substituting 𝑙𝑙𝑎𝑎 in the nominator

in Eq. (8) into 𝑙𝑙𝑏𝑏, as follows:

𝑢𝑢�𝑥𝑥=𝑙𝑙𝑏𝑏 =𝑑𝑑31𝐸𝐸�𝑘𝑘

𝑍𝑍𝑎𝑎𝑍𝑍𝑇𝑇 + 𝑍𝑍𝑎𝑎

sin𝑘𝑘𝑙𝑙𝑏𝑏cos 𝑘𝑘𝑙𝑙𝑎𝑎

(10)

𝑆𝑆̅ = 𝑑𝑑31𝐸𝐸�𝑍𝑍𝑎𝑎

𝑍𝑍𝑇𝑇 + 𝑍𝑍𝑎𝑎cos 𝑘𝑘𝑥𝑥cos 𝑘𝑘𝑙𝑙𝑎𝑎

(11)

The stress and electric displacement field is calculated using Eq. (5) and (6), by cooperating

with the condition that no electric field is applied to the sensor PZT (𝐸𝐸3 = 0). That follows:

𝑇𝑇�1 = 𝑌𝑌�11𝐸𝐸 𝑆𝑆1̅ = 𝑑𝑑31𝑌𝑌�11𝐸𝐸 𝐸𝐸�𝑍𝑍𝑎𝑎

𝑍𝑍𝑇𝑇 + 𝑍𝑍𝑎𝑎cos 𝑘𝑘𝑥𝑥cos 𝑘𝑘𝑙𝑙𝑎𝑎

(12)

𝐷𝐷�3 = 𝑑𝑑31𝑇𝑇�1 = 𝑑𝑑312𝑌𝑌�11𝐸𝐸 𝐸𝐸�

𝑍𝑍𝑎𝑎𝑍𝑍𝑇𝑇 + 𝑍𝑍𝑎𝑎

cos𝑘𝑘𝑥𝑥cos 𝑘𝑘𝑙𝑙𝑎𝑎

(13)

The electric current is then calculated as:

- 12 -

𝐼𝐼 = 𝐼𝐼�̅�𝑒𝑖𝑖𝑖𝑖𝑜𝑜 = 𝑖𝑖𝜔𝜔 � � 𝐷𝐷3𝑑𝑑𝑥𝑥𝑑𝑑𝑑𝑑𝑤𝑤𝑏𝑏𝑙𝑙𝑏𝑏

(14)

where

𝐼𝐼 ̅ = 𝑖𝑖𝜔𝜔𝐸𝐸�𝑤𝑤𝑏𝑏𝑙𝑙𝑏𝑏 �𝑑𝑑312𝑌𝑌�11𝐸𝐸

𝑍𝑍𝑎𝑎𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑎𝑎 + 𝑍𝑍𝑏𝑏

sin𝑘𝑘𝑙𝑙𝑏𝑏cos𝑘𝑘𝑙𝑙𝑎𝑎

1𝑘𝑘𝑙𝑙𝑏𝑏

� (15)

Considering the electric field can be expressed as 𝐸𝐸� = 𝑉𝑉�𝑖𝑖𝑖𝑖/ℎ, the electrical admittance ob-

tained from the sensor PZT, 𝑌𝑌 = 𝐼𝐼�̅�𝑜𝑜𝑜𝑜𝑜/𝑉𝑉�𝑖𝑖𝑖𝑖, is brought to a conclusion as:

Y = 𝑍𝑍−1 = 𝑖𝑖𝜔𝜔𝑤𝑤𝑏𝑏𝑙𝑙𝑏𝑏ℎ𝑎𝑎

�𝑑𝑑312𝑌𝑌�11𝐸𝐸

𝑍𝑍𝑎𝑎𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑎𝑎 + 𝑍𝑍𝑏𝑏

sin𝑘𝑘𝑙𝑙𝑏𝑏cos 𝑘𝑘𝑙𝑙𝑎𝑎

1𝑘𝑘𝑙𝑙𝑏𝑏

� (16)

Here, Eq. (16) can be further simplified by two conditions that the actuator and sensor PZTs

have identical thickness and sin𝑘𝑘𝑙𝑙𝑏𝑏cos𝑘𝑘𝑙𝑙𝑎𝑎

1𝑘𝑘𝑙𝑙𝑏𝑏

is close to unity in the frequency range of interest in

most EM impedance applications. That follows:

Y = 𝑖𝑖𝜔𝜔𝐶𝐶𝑏𝑏 �𝜅𝜅312𝑍𝑍𝑎𝑎

𝑍𝑍𝑆𝑆 + 𝑍𝑍𝑎𝑎 + 𝑍𝑍𝑏𝑏� (17)

where 𝐶𝐶𝑏𝑏 is the zero-load capacitance of the sensor PZT.

The formulated dual PZT admittance has following properties: (1) the dual PZT ad-

mittance shares identical resonance frequencies with the conventional admittance. (2) The

dual PZT admittance does not include the passive admittance, 𝑖𝑖𝜔𝜔𝐶𝐶(1 − 𝜅𝜅312), whereas ex-

- 13 -

isting in the conventional admittance shown in Eq. (1) [21]. Note that absence of the passive

admittance is due to no electric field applied to the sensor PZT as shown in Eq. (12). (3) The

dual PZT admittance shows high SNR and good repeatability due to the absence of the pas-

sive admittance which degrades the measurement sensitivity to the small structural response.

The first two properties are verified by numerical simulation and lab-scale experiment in fol-

lowing chapters, and the last property is validated by a series of full-scale experiments.

2.3 Discussions on amplitude difference of active admittance between con-

ventional and dual PZT techniques

Although the resonant frequencies of the active admittance from the conventional and

the dual PZT techniques are identical, there can be a little difference in their amplitudes. In

this subchapter, the possible reasons of the amplitude difference are discussed. Here, the con-

ventional and dual PZT techniques are assumed to use identical actuator PZT (outer ring PZT

segment).

2.3.1 Sensing area difference between actuator and sensor PZTs

Compared to the conventional admittance techniques, the dual PZT admittance tech-

nique utilizes different PZT segment for sensing the response of the host structure. The dual

PZT technique uses the separate inner circular sensor PZT for sensing the response, thus the

output current is obtained by integration of the electrical displacement over the area of sensor

PZT as shown in Eq. (14). However, the conventional technique uses the actuator PZT for

simultaneous excitation and sensing, so that the output current can be obtained by integration

- 14 -

of the electrical displacement over the area of the actuator PZT [6]. This area difference of

the actuator and sensor PZTs results in that the different electric capacitances are reflected

into each admittance equation. As shown in Eq. (1) and (17). The conventional admittance

model includes the electric capacitance of the actuator PZT (𝐶𝐶𝑎𝑎), whereas the dual PZT ad-

mittance shows the sensor PZT capacitance (𝐶𝐶𝑏𝑏). This electric capacitance difference causes

the overall amplitude difference between each active admittance signature.

2.3.2 Structural impedance difference between conventional and dual PZT

admittances

Amplitudes of the structural mechanical impedance (𝑍𝑍𝑆𝑆) coupled with the conven-

tional and dual PZT admittance are a little different due to the measured strain field differ-

ence between the actuator and sensor PZTs. This variation of the structural impedance ampli-

tude can be explained using a beam structure under axial and bending vibration.

Fig. 4. A beam structure under axial and bending vibration. A dual PZT is located on the top surface of the

beam structure.

Fig. 4 shows the beam structure, where a dual PZT is situated on the top surface.

Considering the beam structure vibrates with axial and bending modes and the outer ring

- 15 -

segment of the dual PZT is used for simultaneous excitation and sensing, the mechanical im-

pedance coupled with the conventional admittance (𝑍𝑍𝑆𝑆,𝑐𝑐𝑜𝑜𝑖𝑖𝑐𝑐) can be expressed as [22]:

𝑍𝑍𝑆𝑆,𝑐𝑐𝑜𝑜𝑖𝑖𝑐𝑐(𝜔𝜔) = −𝜌𝜌𝜌𝜌𝜔𝜔𝑖𝑖 ��

�𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑎𝑎 + 𝑙𝑙𝑎𝑎) − 𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑎𝑎)�2

𝜔𝜔𝑖𝑖𝑢𝑢2 + 2𝑖𝑖𝜁𝜁𝑖𝑖𝑢𝑢𝜔𝜔𝑖𝑖𝑢𝑢𝜔𝜔 − 𝜔𝜔2𝑖𝑖𝑢𝑢

+ �ℎ2�2

��𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑎𝑎 + 𝑙𝑙𝑎𝑎) −𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑎𝑎)�

2

𝜔𝜔𝑖𝑖𝑤𝑤2 + 2𝑖𝑖𝜁𝜁𝑖𝑖𝑤𝑤𝜔𝜔𝑖𝑖𝑤𝑤𝜔𝜔 − 𝜔𝜔2𝑖𝑖𝑤𝑤

�

−1

(18)

where, 𝜌𝜌, 𝜌𝜌, ℎ, subscripts u and w represent the density, cross-section area, height, axial

and bending modes of the host structure, respectively. Also, 𝑈𝑈𝑖𝑖, 𝑊𝑊′𝑖𝑖, 𝜔𝜔𝑖𝑖𝑢𝑢, 𝜔𝜔𝑖𝑖𝑤𝑤, 𝜁𝜁𝑖𝑖𝑢𝑢 and

𝜁𝜁𝑖𝑖𝑤𝑤are the axial mode shapes, the spatial derivative of bending mode shapes, the axial and

bending natural frequencies, and the axial and bending modal damping ratios of the host

structure, respectively. Here, 𝑥𝑥𝑎𝑎 and 𝑙𝑙𝑎𝑎 represent the left side location of the actuator PZT

and length of the actuator PZT as shown in Fig. 4. The mechanical impedance coupled with

the dual PZT admittance (𝑍𝑍𝑆𝑆,𝑑𝑑𝑜𝑜𝑎𝑎𝑙𝑙) yields as Eq. (19), considering the actuator and sensor

PZTs are separate each other and situated at the difference location on the host structure.

𝑍𝑍𝑆𝑆,𝑑𝑑𝑜𝑜𝑎𝑎𝑙𝑙(𝜔𝜔)

= −𝜌𝜌𝜌𝜌𝜔𝜔𝑖𝑖 ��

�𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑎𝑎 + 𝑙𝑙𝑎𝑎) − 𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑎𝑎)��𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑏𝑏 + 𝑙𝑙𝑏𝑏)− 𝑈𝑈𝑖𝑖𝑢𝑢(𝑥𝑥𝑏𝑏)�𝜔𝜔𝑖𝑖𝑢𝑢2 + 2𝑖𝑖𝜁𝜁𝑖𝑖𝑢𝑢𝜔𝜔𝑖𝑖𝑢𝑢𝜔𝜔 − 𝜔𝜔2

𝑖𝑖𝑢𝑢

+ �ℎ2�2

��𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑎𝑎 + 𝑙𝑙𝑎𝑎) −𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑎𝑎)��𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑏𝑏 + 𝑙𝑙𝑏𝑏)−𝑊𝑊′𝑖𝑖𝑤𝑤(𝑥𝑥𝑏𝑏)�

𝜔𝜔𝑖𝑖𝑤𝑤2 + 2𝑖𝑖𝜁𝜁𝑖𝑖𝑤𝑤𝜔𝜔𝑖𝑖𝑤𝑤𝜔𝜔 − 𝜔𝜔2𝑖𝑖𝑤𝑤

�

−1

(19)

- 16 -

where, 𝑥𝑥𝑏𝑏 and 𝑙𝑙𝑏𝑏 are the left side location of the sensor PZT and length of the sensor PZT,

respectively. As shown in the nominators in Eq. (18) and (19), the amplitudes of the mechan-

ical impedance coupled with the conventional and dual PZT admittance differs due to the

sensor PZT location which is different with the actuator PZT location. However, it is noted

that the resonant modes of the host structure identically occur at the same frequencies for

both admittances, because the structural parameters in the denominators, such as natural fre-

quencies (𝜔𝜔𝑖𝑖𝑢𝑢 and 𝜔𝜔𝑖𝑖𝑤𝑤) and modal damping ratios (𝜁𝜁𝑖𝑖𝑢𝑢 and 𝜁𝜁𝑖𝑖𝑤𝑤), do not vary at all.

- 17 -

CHAPTER 3

NUMERICAL SIMULATION

The proposed dual PZT admittance model is validated with a numerical simulation to

investigate two main characteristics of the proposed dual PZT impedance: (1) The dual PZT

admittance shares identical resonant frequencies with the conventional admittance. (2) The

dual PZT admittance does not contain passive admittance due to the separation of excitation

and sensing.

3.1 Simulation setup

Table 1. Material properties of the steel beam and piezoelectric properties of the PZT.

Parameter Value

Steel beam

Density, ρ (kg/m3) 7800

Young’s modulus, 𝑌𝑌11𝐸𝐸 (Gpa) 210

Poisson’s ratio, ν 0.3

Structural damping coefficient 0.01

PZT

Density, ρ (kg/m3) 7800

Young’s modulus, 𝑌𝑌11𝐸𝐸 (Gpa) 66

Poisson’s ratio, ν 0.35

Piezoelectric constant, d31 (pC/N) -190 × 10-12

Dielectric constant, 𝜀𝜀33𝑇𝑇 (Frad/m) 1.6 × 10-8

- 18 -

Fig. 5. A finite element model used to validate the proposed dual PZT impedance measurement technique. A

dual PZT is placed at the center of the simply supported beam.

A 2D plane strain steel beam of dimensions 300 mm × 40 mm was constructed using

eight nodes, CPE8R biquadratic plane strain elements in commercial finite element analysis

(FEA) program, ABAQUS/CAE 6.11, as shown in Fig. 5. The dual PZT, composed of 8 mm

× 0.5 mm inner circle segment and two 4mm × 0.5 mm outer ring segments, was modeled

using eight nodes, CPE8RE biquadratic plane strain piezoelectric elements. The dual PZT is

assumed to be perfectly bonded to the host structure and the mechanical and electrical damp-

ing of the dual PZT is not considered. The material properties of the steel beam and the pie-

zoelectric properties of the dual PZT are shown in Table 1. In dual PZT impedance technique,

the ring part of the dual PZT were in use for the excitation and the inner circle part were used

for the sensing, whereas only the outer ring part were used for both excitation and sensing in

conventional impedance technique. A sinusoidal voltage with 1V amplitude was applied

across the ring segments of the dual PZT along the z-direction. The corresponding complex

electrical response was then obtained from the steady-state dynamic analysis.

The frequency range was selected from 0 to 50 kHz which may be lower than the

resonance frequencies of the dual PZT attached. A simply supported (pin-pin) boundary con-

dition was applied to the steel beam. Meshes of size 1 mm × 0.5 mm, 1 mm × 1 mm and

5mm × 4 mm were adopted for the dual PZT, the structural region where PZT is attached and

the rest of the structure, respectively. Here, the relatively fine mesh sizes for the PZT and the

- 19 -

PZT-attached structural region were set to obtain accurate output results, and the relatively

coarse mesh size for the rest of the structure were selected to reduce the computational loads.

Note that 1 mm mesh size was found to be appropriate enough for extracting all structural

vibration modes by simulation of modal analysis, and the 5mm × 4 mm mesh size of the rest

structural region provided almost identical results within 0.02 percent error in the modal analysis.

3.2 Simulation results

(a)

(b)

Fig. 6. Admittance signals obtained from the conventional and dual PZT techniques: (a) Conductance signals

(the real part of admittance) and (b) susceptance signals (the imaginary part of admittance). The blue solid line

and the green dotted line in the legend represent the admittance signals obtained from the conventional and dual

PZT techniques, respectively.

0 10 20 30 40 50

0

5

10

x 10-5

Frequency (kHz)

Con

duct

ance

(S)

Conventional Dual PZT

0 10 20 30 40 50

0

10

20x 10-4

Frequency (kHz)

Susc

epta

nce

(S)

Conventional Dual PZT

- 20 -

Fig. 6(a) compares the conductance signals (real part of admittance) of each technique,

showing that the dual PZT and conventional admittances share same resonance frequencies

with some amplitude variation. The amplitude variation may be due to the reasons described

in the previous chapter: (1) sensing area difference between the actuator and sensor PZTs,

and (2) measured strain field difference between the actuator and sensor PZTs. In addition,

the susceptance (imaginary part of admittance) signal of the dual PZT does not include pas-

sive admittance (the capacitance slope) as shown in Fig. 6(b) in that the excitation and sens-

ing is performed separately using the outer ring and inner circular segments.

- 21 -

CHAPTER 4

LAB-SCALE EXPERIMENTAL VALIDATION

A lab-scale experiment is performed for the further validation of the dual PZT tech-

nique, by comparing the dual PZT admittance and the conventional admittance measured by

a commercial impedance analyzer and a self-sensing circuit. Time-domain input and output

signals measured by the dual PZT and the self-sensing circuit are also compared to investi-

gate the effects of separate excitation and sensing for the dual PZT.

4.1 Experimental setup

Fig. 7 shows the overall experimental setup for the test. An aluminum beam of dimen-

sions, 300 mm × 50 mm × 3 mm, where a packaged dual PZT (EBL#2 type, Metis design

corp.) is surface-bonded, was prepared as shown in Fig. 7(a). The outer and inner diameters

of the ring segment of the dual PZT, the diameter of the inner circular PZT segment, and the

thickness are 18mm, 10mm, 8mm and 0.3 mm, respectively. Admittance signals were ob-

tained using (1) a commercial impedance analyzer (Agilent 4294A), (2) a self-sensing circuit

and (3) a dual PZT. Fig. 7(b) illustrates the data acquisition system for the test. In the cases of

the dual PZT and the self-sensing circuit, a multi-functional measurement system composed

of a controller (CTRL, NI-PXI 8105), an arbitrary waveform generator (AWG, NI-PXI 5421)

and a digitizer (DIG, NI-PXI 5122) was used. In the case of the self-sensing circuit, a 4.7 nF

reference capacitor was used as shown in Fig. 7(c) [23]. In a similar manner with the previ-

ous chapter, the outer ring and the inner circular PZT segments were used for the excitation

- 22 -

and sensing, respectively in dual PZT admittance measurement and only outer ring segment

of the dual PZT was used in conventional admittance measurement techniques. In excitation,

1 V frequency-swept input from 10 to 100 kHz with 10 Hz increment was applied to the out-

er ring PZT segment for all measurement techniques [24]. Admittance signals were measured

in the frequency range of the excitation with 10 Hz resolution. Also, ten admittance signals

obtained from each technique were averaged to improve the SNR.

50

t = 3

Dual PZT

Unit: mm

95 205

NI-PXI

Impedance Analyzer

CTRL AWG DIG

(a) (b)

Connected to PZT

(+): Vin

(-): Iout

Reference capacitor (4.7 nF)

Vout

(+)(-)-separated wire

(c) (d)

Fig. 7. Experimental setup: (a) An aluminum beam, (b) DAQ configurations, (c) a self-sensing circuit using a

reference capacitor and (d) a temperature chamber.

- 23 -

4.2 Admittance measurement results

A validation of the dual PZT admittance was investigated using an aluminum beam

specimen, which is extension of the numerical validation. Admittance signals were measured

in room temperature (20 ºC) first, and then three measurement tests were also performed in

different temperature conditions (0 ºC, 50 ºC and 70 ºC), using a temperature chamber (Fig.

7(d)).

(a)

32 34 36 38 400

5

10

15

20x 10-4

Frequency (kHz)

Susc

epta

nce

(S)

0ºC 20ºC 50ºC 70ºC

Analyzer

Circuit

Dual

(b)

Fig. 8. Admittance signals obtained from the conventional and dual PZT techniques: (a) Active conductance

signals for three measurement cases at 20 ºC and (b) Susceptance signals at different temperature conditions.

Fig. 8(a) indicates the active conductance signals obtained by the three measurement

techniques in the frequency range of 32-40 kHz. It is clearly shown that the dual PZT admit-

32 33 34 35 36 37 38 39 40

0

10

20x 10-5

Frequency (kHz)

Act

ive

cond

ucta

nce

(S)

Analyzer Circuit Dual

- 24 -

tance signal shares identical resonant frequencies with the conventional admittance signals.

Moreover, the dual PZT admittance does not include the electric capacitance-related passive

slope (the passive admittance) as shown in Fig. 8(b), which is the same observation with the

numerical simulation results. Note that the dual PZT admittance also shows some amplitude

variation under varying temperature conditions mainly due to the sensor PZT capacitance

change by the temperature variation, but the capacitive slope does not occur whereas the

conventional admittance shows both amplitude variation and capacitive slope change. Here,

the resonance peak shift under varying temperature conditions is due to the material soften-

ing effect of the host structure [25].

4.3 Effects of separate excitation and sensing for the dual PZT

0 0.02 0.04 0.06 0.08 0.1

-2

-1

0

1

Time (sec)

Volta

ge (V

)

Input Output

0 0.02 0.04 0.06 0.08 0.1

-2

-1

0

1

Time (sec)

Volta

ge (V

)

Input Output

(a) (b)

Fig. 9. Time-domain input and output signals with 1 V excitation: Signals using (a) the self-sensing circuit

and (b) the dual PZT.

Fig. 9 compares the time-domain input and output signals using the self-sensing cir-

cuit and the dual PZT when 1 V frequency-swept input of its range from 10 to 100 kHz is

applied. Here, the time-domain signals for the impedance analyzer are not considered, since

commercial impedance analyzers do not provide any time-domain signal. However, the fun-

damental working principles of the impedance analyzer and the self-sensing circuit are iden-

- 25 -

tical, which is the use of one PZT segment and a shunted electric circuit element. This simi-

larity between two techniques allows the time-domain signals of the self-sensing circuit to

fill in for the ones of the impedance analyzer. Fig. 9(a) shows the input and output signals of

the self-sensing circuit, indicating that the output signal includes the down-scaled input sig-

nal (excitation-induced PZT strain). The structural response is relatively very small compared

to the excitation-induced PZT strain, which cannot be observed until zooming in the output

signal. Fig. 9(b) shows the input and output signals of the dual PZT when the identical input

signal with the self-sensing circuit is applied. They clearly indicate that there is no excitation-

induced PZT strain in the measured output signal.

The excitation-induced PZT strain is unfavorable to measure the small response from

large structures, since it degrades the effective resolution of A/D conversion. Table 2 com-

pares the effective resolutions between the self-sensing circuit and the dual PZT, calculated

using Eq. (3). In the lab-scale admittance measurement, the effective resolution of A/D con-

version for the dual PZT is much finer compared to the one for the self-sensing circuit. It is

noted that the structural response becomes much smaller when the host structure comes to be

large. Then, the conventional single PZT scheme cannot effectively measure the small re-

sponse, since the PZT undergoes the excitation-induced strain degrading the A/D conversion

resolution. However, the dual PZT can measure the small structural response with the fine

resolution of A/D conversion, because it is not influenced by the excitation-induced PZT

strain. The superior sensing ability of the dual PZT in the admittance measurement is proved

by a series of full-scale experiments in the following chapter.

- 26 -

Table 2. Effective resolutions of A/D conversion

Experiments

∆𝑽𝑽𝒆𝒆 (mV) Ratio

�∆𝑽𝑽𝒆𝒆𝒄𝒊𝒓𝒄𝒖𝒊𝒕

∆𝑽𝑽𝒆𝒆𝒅𝒖𝒂𝒍� Self-sensing circuit Dual PZT

Lab-scale

(Al. beam) 89.53 14.43 6.20

Full-scale

(Bridge) 114.28 0.87 131.40

Full-scale

(Building) 103.69 1.64 62.97

- 27 -

CHAPTER 5

FULL-SCALE EXPERIMENTAL VALIDATION

For further investigation into the measurement performance of the proposed technique

in massive structures, such as SNR and repeatability, a series of admittance measurement

tests in full-scale bridge and building structures are performed.

5.1 Admittance measurement in a cable anchor of an extra-dosed bridge

The feasibility of the proposed technique was tested at Haknarae bridge in Sejong,

South Korea. This 740 m-long extra-dosed bridge is composed of four V-shape concrete py-

lons and extension cables supporting the pre-stressed concrete (PSC) box girders as shown in

Fig. 10. This bridge was recently opened (June, 2012), and underwent noticeable vibrations

due to the heavy traffic.

Inspected spot

Dual PZT

Target structure

(a) (b)

Fig. 10. Overview of Haknarae bridge in Sejong, South Korea: (a) An inspected spot and (b) inside box girder.

- 28 -

5.1.1 Experimental setup

250

500

290

Unit: mm

250

(a) (b)

Fig. 11. Target bridge structure and the installed dual PZT: (a) A cable anchor head and a steel distribution

plate and (b) a dual PZT installed at the edge of the distribution plate.

Fig. 11(a) shows the target structure, which is a steel distribution plate (500 mm × 500

mm × 80 mm) buried in a PSC box girder. A packaged dual PZT, same with the one used in

the previous chapter (Fig. 7(a)), was installed at the edge of the distribution plate as shown in

Fig. 11(b). The data acquisition system (Fig. 7(b)) and the self-sensing circuit using a 4.7 nF

reference capacitor (Fig. 7(c)) are identical to the ones used in the previous lab-scale test. In

the similar fashion with the lab-scale test, the outer ring and the inner circular PZT segments

were used for the excitation and sensing, respectively in dual PZT admittance measurement

and outer ring PZT segment was used for conventional admittance measurement. To investi-

gate the effect of the increasing excitation voltage on the SNR, an enhanced excitation volt-

age of 6 V was also applied for the dual PZT and self-sensing circuit techniques, as well as

the 1 V-excitation. Note that the excitation voltage of the impedance analyzer is limited to 1

V due to the own characteristics of the device itself. Therefore, 6 V-excitation of the imped-

ance analyzer is not considered in the admittance measurement results. The EM admittance

signals were measured over a frequency range of 10-100 kHz with 10 Hz increment.

- 29 -

5.1.2 Admittance measurement results

(a)

(b)

Fig. 12. Admittance measurement results from the field bridge test: (a) Normalized conductance signals at 1

V-excitation, and (b) normalized conductance signals at 6 V-excitation.

Fig. 12(a) represents the normalized conductance signals obtained from the distribu-

tion plate in the frequency range of 70 to 100 kHz, when 1 V-excitation was applied. The ac-

tive conductance signals for all measurement techniques are divided by their maximum val-

ues to qualitatively compare the signal-to-noise level. The dual PZT and impedance analyzer

succeeded to obtain resonance peaks from the massive host structure, whereas the self-

sensing circuit failed to obtain meaningful admittance signal. When the excitation voltage

was increased by 6 V, the dual PZT technique shows more distinct conductance signal,

70 75 80 85 90 95 100

-1

0

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Circuit Analyzer Dual

70 75 80 85 90 95 100

-1

0

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Circuit Dual

- 30 -

whereas the self-sensing circuit technique failed to obtain the conductance signal as shown in

Fig. 12(b). The normalized conductance signal obtained from the self-sensing circuit is

shown to be very noisy, which is difficult to distinguish the resonant modes of the host struc-

ture and the background noise. The measurement failure of the self-sensing circuit can be an-

swered by the poor effective resolution of A/D conversion as shown in Table 2. Compared to

the dual PZT, the effective resolution of the self-sensing circuit is 131.40 times poorer, which

may be not favorable to measure the small structural response.

Here, the amplitude difference between the normalized conductance obtained from

the impedance analyzer and the dual PZT may result from the measured strain field differ-

ence between the actuator and sensor PZTs as described in the previous chapter. The host

structure of this test is relatively very stiff compare to the one of typical laboratory-size spec-

imens, and only small portion of the host structure can converge to the steady-state regime so

that the measured strain field difference between the actuator and sensor PZTs can play a sig-

nificant role of the difference of the active conductance amplitudes.

Analyzer Circuit Dual Circuit Dual-10

0

10

20

30

SNR

[dB

]

1 V Excitation 6 V Excitation

(a)

- 31 -

(b)

(c)

Fig. 13. Quantitative indicators of measurement performance: (a) Signal-to-noise ratios (SNRs), (b) cross-

correlation coefficients (CCs) and (c) root mean square deviations (RMSDs).

To quantitatively compare the impedance measurement performance, signal-to-noise

ratios (SNRs) and the cross-correlation coefficients (CCs) of the active conductance signals

are computed and compared as shown in Fig. 13(a) and (b). Here, the SNR, a signal-to-noise

ratio in frequency domain, is calculated as:

𝑆𝑆𝑆𝑆𝑆𝑆 = 20 log𝑆𝑆𝑅𝑅𝑆𝑆(𝑌𝑌𝑠𝑠𝑠𝑠𝑜𝑜𝑅𝑅𝐸𝐸 )

𝑆𝑆𝑅𝑅𝑆𝑆(𝑌𝑌𝑅𝑅𝐸𝐸 − 𝑌𝑌𝑠𝑠𝑠𝑠𝑜𝑜𝑅𝑅𝐸𝐸 ) (19)

where, 𝑆𝑆𝑅𝑅𝑆𝑆(𝑌𝑌𝑅𝑅𝐸𝐸) is the root mean square of the active conductance signal, 𝑌𝑌𝑅𝑅𝐸𝐸, and 𝑌𝑌𝑠𝑠𝑠𝑠𝑜𝑜𝑅𝑅𝐸𝐸

is the smoothed active conductance signal by Savitzky-Golay digital smoothing filter [26]. In

2 4 6 8 100.6

0.8

1

Case No.

CC

Circuit Analyzer Dual

2 4 6 8 100

0.5

1

Case No.

RM

SD

Circuit Analyzer Dual

- 32 -

addition, the CC and RMSD as indicators of the repeatability of obtained active conductance

signals are computed as:

𝐶𝐶𝐶𝐶 =1𝑆𝑆∑ �𝑌𝑌1𝑅𝑅𝐸𝐸(𝜔𝜔𝑖𝑖) − 𝑌𝑌1𝑅𝑅𝐸𝐸�������𝑌𝑌2𝑅𝑅𝐸𝐸(𝜔𝜔𝑖𝑖) − 𝑌𝑌2𝑅𝑅𝐸𝐸������𝑁𝑁𝑖𝑖=1

𝜎𝜎𝑌𝑌1𝑅𝑅𝑅𝑅𝜎𝜎𝑌𝑌2𝑅𝑅𝑅𝑅 (20)

𝑆𝑆𝑅𝑅𝑆𝑆𝐷𝐷 = �∑ �𝑌𝑌1𝑅𝑅𝐸𝐸(𝜔𝜔𝑖𝑖) − 𝑌𝑌2𝑅𝑅𝐸𝐸(𝜔𝜔𝑖𝑖)�

2𝑁𝑁𝑖𝑖=1

∑ �𝑌𝑌1𝑅𝑅𝐸𝐸(𝜔𝜔𝑖𝑖)�2𝑁𝑁

𝑖𝑖=1

(21)

where, 𝑌𝑌1𝑅𝑅𝐸𝐸����� and 𝑌𝑌2𝑅𝑅𝐸𝐸����� are the mean values of two active conductance signals of 𝑌𝑌1𝑅𝑅𝐸𝐸(𝜔𝜔)

and 𝑌𝑌2𝑅𝑅𝐸𝐸(𝜔𝜔); and 𝜎𝜎𝑌𝑌1𝑅𝑅𝑅𝑅 and 𝜎𝜎𝑌𝑌2𝑅𝑅𝑅𝑅 are the standard deviations. Ten cases of CCs and

RMSDs are computed using five conductance signals at each technique in 1 V-excitation. As

shown in Fig. 13(a), the SNR values for the dual PZT technique at 1 V-excitation are larger

than those of the impedance analyzer and the self-sensing circuit. Also, it should be noted

that the SNR value for the dual PZT technique jumps over when the excitation voltage is in-

creased by 6 V. Similarly with this observation, the CC values for the dual PZT technique

also show much larger values than those of the impedance analyzer and the self-sensing cir-

cuit. Also, the RMSD values for the dual PZT technique show much smaller values than the

two conventional techniques, meaning that higher repeatability of the admittance measure-

ment can be assured.

Despite the use of only one PZT segment for both actuation and sensing, the imped-

ance analyzer shows a relatively large SNR and CC values and smaller RMSD values com-

pared to the ones of the self-sensing circuit as shown in Fig. 13, which is mainly due to the

auto-balancing bridge technique and the built-in high-gain preamplifiers. However, bulky

device, high-cost and slow measurement speed of the impedance analyzer makes it less at-

tractive in the real-time filed applications. In this test results, it is found that the dual PZT

- 33 -

technique shows possibility of light and cost-effective measurement and the enhanced admit-

tance measurement performance without using any external noise reduction devices.

5.2 Admittance measurement in an inclined column of a building structure

Next, an additional admittance measurement in an inclined steel column of a building

structure is conducted for the validation of further applicability of the proposed technique to

full-scale massive structures.

5.2.1 Experimental setup

Target structure

600

t = 20

Unit: mm

Dual PZT

300

15cm ruler

(a) (b)

Fig. 14. A roof structure at Korea Advanced Institute of Science and Technology in South Korea: (a) overview

of the target structure and (b) the inspected spot.

Fig. 14 shows the target structure, which is a steel base plate (600 mm× 300 mm× 20

mm) where two inclined steel columns supporting a building roof are welded. A packaged

dual PZT, same with the one used in the previous chapter (Fig. 7(a)), was installed near the

right edge of the steel base plate. All measurement devices and parameters were identical to

the ones used in the previous admittance measurement test in the bridge structures. Similarly

- 34 -

with the previous bridge structure test, 6 V-excitation was also applied as well as 1 V-

excitation.

5.2.2 Admittance measurement results

(a)

(b)

Fig. 15. Admittance measurement results from the field building structure test: (a) Normalized conductance

signals at 1 V-excitation and (b) normalized conductance signals at 6 V-excitation.

Fig. 15(a) indicates the normalized conductance signals from the base plate of in-

clined column structure in the frequency range of 10 to 30 kHz, when 1 V of excitation was

applied. Only the dual PZT technique succeeded to obtain clear resonance peaks. The nor-

malized conductance signals obtained from the impedance analyzer and the self-sensing cir-

cuit is shown to be very noisy, which is difficult to distinguish the resonant modes of the host

10 15 20 25 30-1

-0.5

0

0.5

1

1.5

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Circuit Analyzer Dual

10 15 20 25 30-1

-0.5

0

0.5

1

1.5

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Circuit Dual

- 35 -

structure and the background noise. When the excitation voltage was increased by 6 V, the

dual PZT technique shows more distinct conductance signal, whereas the self-sensing circuit

technique failed to obtain conductance signal as shown in Fig. 15(b).

These signals indicate that the host structure is very stiff, since all the conventional

admittance measurement techniques failed to obtain the admittance signals. The response of

the host structure may be very small compared to overall output signals measured by both the

impedance analyzer and self-sensing circuit, thus the actuator PZT cannot effectively meas-

ure the small response of the host structure under the excitation-induced strain. However, it is

noted that the dual PZT obtained very clear admittance signals even in the unfavorable host

structure.

SNRs, CCs and RMSDs are computed using Eq. (18) to (20) to compare the quantita-

tive measurement performance. Fig. 16(a) shows the SNR values of each measurement tech-

nique, and only the dual PZT technique indicates dominantly high SNR values. Note that the

SNR of the dual PZT increases when the excitation voltage is enhanced, which is analogous

observation with the one of the previous bridge structure test. Next, CCs and RMSDs of each

measurement technique at 1 V-excitation are computed and shown in Fig. 16(b) and (c) to

compare the repeatability. Similarly with SNRs, only the dual PZT technique shows domi-

nantly high CC and low RMSD values.

The admittance measurement results from the full-scale building structures show that

the dual PZT can obtain clear admittance signal from massive and stiff host structures where

even the conventional impedance analyzer and self-sensing circuit failed to do so. It is possi-

ble to measure the accurate admittance signals showing high SNR and good repeatability us-

ing the dual PZT which can separately excite the host structure and measure the response

from it.

- 36 -

Analyzer Circuit Dual Circuit Dual-10

0

10

20

30

SN

R [d

B]

1 V Excitation 6 V Excitation

(a)

(b)

(c)

Fig. 16. Quantitative indicators of measurement performance: (a) Signal-to-noise ratios (SNRs), (b) cross-

correlation coefficients (CCs) and root mean square deviations (RMSDs).

2 4 6 8 100.2

0.4

0.6

0.8

1

Case No.

CC

Circuit Analyzer Dual

2 4 6 8 100

0.5

1

1.5

Case No.

RM

SD

Circuit Analyzer Dual

- 37 -

CHAPTER 6

APPLICATION OF DUAL PZT IMPEDANCE TECHNIQUE TO

STRUCTURAL DAMAGE DETECTION

In this chapter, a set of tests were performed to validate the applicability of the pro-

posed dual PZT technique in multiple bolt-loosening detection in metallic structures. Due to

a difficulty in simulating damage to in-service real-field structures, a laboratory-size steel H-

beam bolted with a steel gusset plate was used for the target structure. In a similar fashion

with the previous chapters, the damage detection performance of the proposed dual PZT

technique is compared with the one of the impedance analyzer and self-sensing circuit.

6.1 Experimental setup

Fig. 17 shows a steel H-beam (H 300 mm×300 mm×10/15 mm) on which a steel gus-

set plate (300 mm × 160 mm × 15 mm) bolted with twelve bolts is arranged. A dual PZT,

identical to the one used in the lab-scale test (Fig. 7(a)), was installed on the center of the top

flange of the H-beam. The overall configuration of the data acquisition system and the meas-

urement parameters were identical to the ones for the previous admittance measurement test

in the building structure. Damage was introduced by loosening the bolts sequentially as

summarized in Table 3. A series of bolt-loosening composed of seven cases were introduced,

and three admittance signals (ten time-averaged) were obtained at each case. As similar with

the previous tests, not only 1 V excitation applied to all measurement techniques, but 6 V-

excitation was also applied to the dual PZT technique and self-sensing circuit measurement.

- 38 -

Unit: mm

tf = 15

tw = 10

tg = 6

150

150

160 150 240

300

Dual PZT

Bolt #1Bolt #2Bolt #3...

Bolt #8...

Bolt #12

Fig 17. A steel H-beam where a steel gusset plate tightened with twelve bolts is arranged.

Table 3. Bolt-loosening scenario for the damage detection test.

Case Damage description Case Damage description

1 Intact 5 6 bolts loosened (#1 ~ 6)

2 1 bolts loosened (#1) 6 8 bolts loosened (#1 ~ 8)

3 2 bolts loosened (#1, 2) 7 12 bolts loosened (#1 ~ 12)

4 4 bolts loosened (#1 ~ 4)

6.2 Experimental results

Figure 18(a) to (e) show the normalized conductance signals obtained at intact, four

bolts loosened and twelve bolts loosened cases in the frequency range of 43 to 47 kHz. Simi-

larly with the previous admittance measurement test in the bridge structure, the dual PZT and

impedance analyzer succeeded to acquire resonance peaks from the host structure, but the

self-sensing circuit failed to do. Note that the normalized conductance signals obtained from

the dual PZT and impedance analyzer indicate that the signals are changed after introducing a

series of bolt-loosening damage.

- 39 -

(a)

(b)

(c)

(d)

43 43.5 44 44.5 45 45.5 46 46.5 47

0

0.5

1

1.5

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Baseline Intact 4-bolts loose 12-bolts loose

43 43.5 44 44.5 45 45.5 46 46.5 47

-0.5

0

0.5

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Baseline Intact 4-bolts loose 12-bolts loose

43 43.5 44 44.5 45 45.5 46 46.5 47-0.5

0

0.5

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Baseline Intact 4-bolts loose 12-bolts loose

43 43.5 44 44.5 45 45.5 46 46.5 47-0.5

0

0.5

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Baseline Intact 4-bolts loose 12-bolts loose

- 40 -

(e)

Fig. 18. Normalized conductance signals obtained from (a) the impedance analyzer (at 1 V-excitation), (b) self-

sensing circuit (at 1 V-excitation), (c) dual PZT (at 1 V-excitation), (d) self-sensing circuit (at 6 V-excitation)

and (e) dual PZT (at 6 V-excitation).

To quantitatively alarm the damage severity, a damage index (𝐷𝐷𝐼𝐼) is introduced as:

𝐷𝐷𝐼𝐼 = 1 − 𝐶𝐶𝐶𝐶𝑠𝑠𝑎𝑎𝑥𝑥 (21)

where, 𝐶𝐶𝐶𝐶𝑠𝑠𝑎𝑎𝑥𝑥 is the maximum cross-correlation coefficient [27]. Figure 19 shows the 𝐷𝐷𝐼𝐼

values alarming the damage severity. The 𝐷𝐷𝐼𝐼 corresponding to the test number 1 represents

the baseline, and the other 𝐷𝐷𝐼𝐼 values indicate the cases computed using the raw baseline

conductance signal and each test conductance signals in various damage cases. Dual PZT

technique together with the impedance analyzer shows gradual increment in 𝐷𝐷𝐼𝐼 values as

the number of loosened bolts increases, meaning that successful damage detection and se-

verity alarming is possible. However, in the self-sensing circuits for 1 V-excitation, false

alarming happened even in the intact cases, which may result from the inferior signal-to-

noise level. In addition, damage alarming was failed in the self-sensing circuit technique in 6

V-excitation, which can be answered by that the large excitation-induced PZT strain handi-

caps the sensing ability of the PZT. In results, it is found that the dual PZT technique success-

43 43.5 44 44.5 45 45.5 46 46.5 47-0.5

0

0.5

1

Frequency (kHz)

Nor

mal

ized

con

duct

ance

Baseline Intact 4-bolts loose 12-bolts loose

- 41 -

fully detects multiple bolt-loosening damage even in low excitation voltage (1 V), without

any external low-noise high-gain preamplifiers which exist in the impedance analyzer.

2 4 6 8 10 12 14 16 18 20 220

0.5

1

Test No.

Dam

age

Inde

x

Analyzer (1V) Circuit (1V) Circuit (6V) Dual (1V) Dual (6V)Intact 1 bolt

loosened2 bolts

loosened4 bolts

loosened6 bolts

loosened8 bolts

loosened12 bolts

loosened

Fig. 19. Damage index values calculated for all measurement techniques.

- 42 -

CHAPTER 7

CONCLUSION

This study proposed an EM impedance measurement technique using a dual PZT for

continuous monitoring of large scale structures. The dual PZT can measure the small re-

sponse of massive and stiff structures by separating the actuation and sensing PZTs, so that

the excitation-induced PZT strain cannot interrupt the sensing performance of the PZT. In

this study, a theoretical model of the dual PZT admittance measurement was formulated first,

and the model was verified by a numerical simulation using ABAQUS/CAE 6.11 and a lab-

scale test using an aluminum beam specimen. Then, the feasibility of the proposed technique

is validated in the anchor structure of the extra-dosed bridge and the inclined column struc-

ture supporting a building roof. The theoretical formulation, numerical simulation and lab-

scale test showed that the dual PZT admittance shares identical resonant frequencies with the

conventional admittance, and the dual PZT admittance does not contain the passive admit-

tance due to the separate excitation and sensing. Also, it is proved that the separate excitation

and sensing of the dual PZT provides the superior admittance measurement performance in

massive and stiff structures by two actual field admittance measurement tests in the bridge

and building structures. In an unfavorable host structure, only the dual PZT succeeded to ob-

tain the admittance signals with high SNR and good repeatability, whereas the conventional

impedance analyzer and the self-sensing circuit failed to do so. Considering the low cost

comparable to the self-sensing circuits and faster measurement speed than the conventional

impedance analyzers, the dual PZT impedance measurement technique is expected to be a

much better application of the continuous health monitoring of large scale structures.

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요 약 문

대형 구조물의 건전성 모니터링을 위한 동심원형

이중 압전센서 기반의 기전 임피던스 계측

본 연구에서는 대형 구조물의 건전성 모니터링을 위한 새로운 기전 임피던스

계측 기법을 제안한다. 압전센서와 대상 구조물간의 기계-전기적 결합을 이용하는

기전 임피던스 기반의 손상진단 기법은, 압전센서 근처 국부 손상에 매우 민감한

것으로 알려져 있다. 일반적으로 기전 임피던스 신호는 상용 임피던스 계측기를

이용하여 계측되어 왔고, 최근 저가의 자가 센싱회로를 통해서도 계측이 가능해 졌다.

임피던스 계측기는 정확한 임피던스를 측정할 수 있지만, 상당히 고가이고, 장비의

부피가 크며, 느린 계측 속도 때문에 실제 현장에서 실시간 계측이 힘들다는 단점이

있다. 자가 센싱회로의 경우, 저가의 소형 측정장비를 이용하고 실시간 측정이

가능하다는 장점이 있으나, 기저 저항체 또는 커패시터를 이용하여 간접적으로 응답을

계측하기 때문에, 구조물로부터 계측된 응답의 높은 신호대 잡음 비를 기대하기

어렵다. 여기서, 두 계측기법 모두 교량, 원전 구조물과 같은 대형 구조물로부터 기전

임피던스 신호를 획득하는 데 한계가 있다. 본 연구에서는 동심원형의 이중

압전센서를 이용하여, 저가의 소형 계측 장비를 이용하면서 높은 신호대 잡음 성능을

가지는 기전 임피던스 계측 기법을 제안한다. 우선, 이중 압전센서 임피던스의 이론적

모델을 정립하였고, 수치해석 및 실험실 규모의 임피던스 계측실험을 통해 정립된

모델을 검증하였다. 검증된 이중 압전센서 임피던스 계측기법을 실규모 교량 및 건축

구조물에 적용을 하였고, 대형 구조물에서 제안하는 이중 압전센서 임피던스 계측

기법만이 높은 신호대 잡음비 및 반복률을 보이며 기전 임피던스 신호 획득에

성공하는 것을 확인하였다.

핵심용어: 이중 압전센서, 기전임피던스, 대형 구조물, 분리된 가진과 측정, 높은

신호대 잡음비 및 반복률

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감사의 글

KAIST 에서의 석사과정 2 년을 마무리 하면서 도움을 주신 많은 분들께 감사를

표하고자 합니다. 먼저, 부족한 점을 채워주시고 끊임없는 관심과 가르침으로 새로운

분야에 대한 두려움을 극복할 수 있도록 도와주신 지도교수님이신 손 훈 교수님께 깊

은 감사의 뜻을 전합니다. 그리고 바쁜 일정 중에서도 시간을 내어 학위논문 심사에

참여 해주시고 많은 가르침을 주신 정형조 교수님, 홍정욱 교수님, 그리고 경북대학교

노용래 교수님께도 감사의 말씀을 전합니다. 연구를 진행하면서 난관에 닥칠 때 마다

건설적인 조언으로 많은 도움을 준 공저자 형진이와 연구실 최고 선배 윤규형에게도

감사 드립니다. 또한 본 연구를 수행할 수 있도록 지원해 주신 한국연구재단 (2010-

0017456)과 국토해양부 (U-City 석박사 지원사업)에 깊은 감사의 뜻을 전합니다.

기대와 두려움을 안고 시작한 대학원 생활은 생각과는 다르게 순탄하지만은 않

았고, 제 인생에서 넘어야 할 첫 번째 관문이었습니다. 항상 저를 믿고 응원해주는 가

족과 민정이가 있었기에 무사히 첫 관문을 넘어 이 자리에 있을 수 있었습니다. 또한

즐겁게 연구실 생활을 할 수 있도록 도와준 김준희 박사님, 윤규형, 기영이형, 민구형,

진열이, 현석이, 형진이, 낙현이, 지민이, 병진이, 준우, 수영이, Truong, Peipei 그리고

선혜누나와 현미누나까지 모든 SSS Lab 멤버들에게 깊은 감사의 말씀을 전하고 싶습

니다. 그리고 학교생활에 활력소를 불어 넣어준 건설 및 환경공학과 선후배님들과 직

원분들, HYKACE 선후배님들 그리고 SOS 멤버들에게도 감사의 뜻을 전합니다.

앞서 거론한 많은 분들의 도움과 관심이 없었더라면 저는 이 글을 쓸 수 없었

을 것입니다. 제 자신이 여전히 많이 부족함을 알기에 초심을 잃지 않고, 현실에 만족

하지 않으며, 부단히 노력하여 앞만보고 나아가겠습니다. 도움을 주신 모든 분들께 이

감사의 글을 바칩니다.

2012년 12월 KAIST에서 송호민 올림.

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CURRICULUM VITAE

Homin Song

Research Assistant

Department of Civil and Environmental Engineering

Korea Advanced Institute of Science and Technology

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

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

Email: shm3313@kaist.ac.kr

RESEARCH INTERESTS

Impedance-based structural health monitoring techniques Guided wave-based structural health monitoring techniques Wireless active sensing Pattern recognition

EDUCATION

2011 – 2013

M.S., Civil and Environmental Engineering Korea Advanced Institute of Science and Technology (KAIST), KOREA.

2005 – 2011 B.S., Civil and Environmental Engineering Hanyang University (HYU), KOREA.

JOURNAL PUBLICATIONS

1. Homin Song, Hyung Jin Lim and Hoon Sohn, Electromechanical impedance measurement us-ing a dual piezoelectric transducer for continuous monitoring of large structures, in preparation for Journal of Sound and Vibration, 2012.

- 49 -

CONFERENCE PROCEEDINGS

1. Homin Song, Hyung Jin Lim and Hoon Sohn, Development of a dual PZT based electrome-chanical impedance measurement technique for structural health monitoring, The 38th Korean Society of Civil Engineers Annual Conference, Gwangju, Korea, Oct 24-26, 2012.

2. Yun Kyu An, Homin Song, Hyun Jun Park, Hoon Sohn and Chung Bang Yun, Remote guided wave imaging using wireless PZT excitation and laser vibrometer scanning for local bridge monitoring, The 6th International Conference on Bridge Maintenance, Safety and Management, Lake Como, Italy, July 8-12, 2012.

3. Hyung Jin Lim, Homin Song and Hoon Sohn, Development of dual PZT based impedance measurement techniques for large-scale structures, The 6th European Workshop on Structural Health Monitoring, Dresden, Germany, July 3-6, 2012.

PARTICIPANT PROJECTS

2012 – Present

Development of smart sensing technology for equipment abnormality detec-tion (Research Assistant): Hyundai-Kia Automobile (Funded: 80,750,000 KRW (80,750 USD) for 06/01/12-05/31/13)

2011 – 2012

Ubiquitous and Ecology City Development (Research Assistant): Korea Min-istry of Land, Transportation and Maritime Affairs (Funded: 209,695,000 KRW (209,695 USD) out of total 27,594,803,000 KRW (27,594,803 USD) for 11/14/08-04/30/13)

2011

Development of Smart Steel Members for Online Steel Structure Monitoring (Research Assistant): POSCO Corporation (Funded: 41,000,000 KRW (41,000 USD) for 01/05/11 to 12/31/11)

HORNORS & AWARDS

2012 Excellent Paper Award The 38th Korean Society of Civil Engineers (KSCE) Annual Conference

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