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Annular Phased Array Dynamic FocusingMethod for Large Target UltrasonicTestingYue Wu a , Dayong Guo a , Kailiang Que b , Bin Chen b & Jiayi Cheng aa Department of Mechanical Engineering , Tsinghua University ,Beijing , 100084 , Chinab Allrising (Beijing) Technology Co. Ltd. , Beijing , 100085 , ChinaPublished online: 24 Jan 2014.

To cite this article: Yue Wu , Dayong Guo , Kailiang Que , Bin Chen & Jiayi Cheng (2014) AnnularPhased Array Dynamic Focusing Method for Large Target Ultrasonic Testing, Ferroelectrics, 459:1,14-23, DOI: 10.1080/00150193.2013.837740

To link to this article: http://dx.doi.org/10.1080/00150193.2013.837740

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Ferroelectrics, 459:14–23, 2014Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2013.837740

Annular Phased Array Dynamic Focusing Methodfor Large Target Ultrasonic Testing

YUE WU,1,∗ DAYONG GUO,1 KAILIANG QUE,2 BIN CHEN,2

AND JIAYI CHENG1

1Department of Mechanical Engineering, Tsinghua University,Beijing 100084, China2Allrising (Beijing) Technology Co. Ltd., Beijing 100085, China

A novel method called dynamic focusing immersed ultrasonic testing (DFIUT) is pro-posed which utilizes an annular phased array ultrasonic transducer (APAUT) and oneof the distinguished advantages is dynamic focusing. The proposed DFIUT is aimedat solving the focus deviation problem in the detection of large targets. The designmethodology of APAUT and implementation of DFIUT method are presented in thispaper. Furthermore, an annular phased array immersed ultrasonic testing (APAIUT)system is also set up. An experiment over the large molybdenum target using the DFIUTis conducted to verify the effectiveness of the proposed method.

Keywords Non-destructive testing (NDT); annular phased array; dynamic focusing;large target

1. Introduction

Targets for LCD are unceasingly becoming large (typical size of a target is 25000 × 300× 20 (mm)) as LCD becomes larger and larger, while target’s quality becomes more andmore sensitive to the safety of LCD equipment. It is required that flaws in target should beless than 0.3 mm in diameter, such that it is important to do Non-destructive Testing (NDT)for large target.

Immersed ultrasonic testing (IUT) is considered as the most effective NDT method forlarge target. Currently, fixed focus immersed ultrasonic transducer (FFIUT) is widely usedin the detection of large targets. However, during the detection, it’s difficult to keep thetarget absolutely horizontal due to its large size. So the focal point of FFIUT may easilydeviate from the target when the transducer is moved from one end to the other, which maycause a loss in resolution and accuracy, shown schematically in Figure. 1.

Obviously, it is difficult to solve this problem by means of mechanical compensation.APAUT has an advantage of high resolution in the axial direction; while on the otherhand, APAUT can change the focus dynamically by adjusting the delays of each elementexcitation. Therefore, the dynamic focusing can be utilized to keep the focal point in thetarget adaptively. APAUT has been extensively studied previously by various researchers,including the sound field analysis, application in medical use and the measurement offorgings [1–4], but little work has been reported on focus deviation in the detection of

Received December 11, 2012; in final form March 14, 2013.∗Corresponding author. E-mail: wuyue.1618@yahoo.com.cn

[1426]/14

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Annular Phased Array Dynamic Focusing Method [1427]/15

Fi xed Focus

Tr ansducer

Fi xed Focus

Tr ansducer

Focus

Devi at i on

Tar get

θ

Figure 1. Schematic diagram of focus deviation. (Color figure available online.)

targets. In this paper, a novel method named DFIUT using APAUT is proposed, and anexperiment over a molybdenum target is conducted to verify the effectiveness of the method.

2. APAUT Design

APAUT is composed of several annular elements which have the same center. There arethree major issues needed to be considered when designing the APAUT: the number ofelements, the size of each element and the center frequency.

A. Number of Elements

As we know, the more elements the transducer has, the narrower the sound beam will be[5], according to Equation (1).

b = 1.03F/Nk (1)

where b stands for the width of the beam, F is focus length, N is the number of elementsand k is the ratio of element spacing and wavelength.

However, with the number of elements increasing, the system will need more channels,which may increase the cost, uncertainty and complexity of the system. According toprevious research [2], the number of elements is set to be 16.

B. Size of Each Element

Currently, there are two major categories of principles for the design of element size: equalarea and equal width. It is assumed that the total area keeps constant, the number of elementsis 16, the focus is set to be 100 mm. The continuous emitting field of the two principles is

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16/[1428] Y. Wu et al.

40 60 80 100 120 140 160 180 2000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Annular Phased Array Continuous Emitting Field

Axial Position(mm)

No

rmaliz

ed S

ound P

ress

ure

Equal AreaEqual Width

(a) Comparison of equal area and width

40 60 80 100 120 140 160 180 2000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Annular Phased Array Continuous Emitting Field

Axial Position(mm)

No

rmaliz

ed S

ound P

ress

ure

5MHz10MHz2.5MHz

(b) Comparison of different frequency Figure 2. Simulation of continuous emitting field.

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Annular Phased Array Dynamic Focusing Method [1429]/17

figured out using Equation (2) [2]. The results are shown in Fig. 2(a).

p =N∑

j=1

2A sin

(kR2j − R2j−1

2

)ei(k

R2j +R2j−12 + π

2 −φj ), φj = kR2j + R2j−1

2,

R2j−1 =√

a22j−1 + z2, R2j =

√a2

2j + z2 (2)

In Fig. 2(a), it is apparent that APAUT with equal area has a better sound fielddistribution (main lobe strengthened, grating lobe suppressed) than that with equal width.In addition, the elements will have better impedance consistency with equal area, whichmakes it easier to match the emitting and receiving circuit. Above all, equal area principleis chosen to design the APAUT.

C. Center Frequency

The center frequency is significant in detecting resolution. Generally speaking, with thefrequency increasing, the sound beam will be narrower and resolution will increase, whileon the other hand, the attenuation will be stronger. In order to choose the center frequencyappropriately, different frequencies, i.e., 2.5 MHz, 5 MHz and 10 MHz, are tested insimulation, and the focus is set at a constant, i.e., 100 mm. The results are presented inFig. 2(b).

As shown in Fig. 2(b), the beam width becomes narrower and the side lobe is effectivelysuppressed with frequency increasing. However, there is a grating lobe whose amplitude isalmost the same as the main lobe, which may cause an artefact of image. For 2.5 MHz, theenergy of main lobe is much weaker than 5 MHz and the sound beam is too wide to havean appropriate longitudinal resolution. Thus, 5 MHz is chosen as the center frequency ofthe APAUT, considering both the grating lobe effect and resolution.

In summary, a 16-channel APAUT with a center frequency of 5 MHz is designed,and the specific dimensions for each element are listed in Table 1. Figure 3 shows theappearance of the APAUT.

Figure 3. Appearance of the APAUT. (Color figure available online.)

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Tabl

e1

APA

UT

’sdi

men

sion

sw

itheq

uala

rea

(equ

alw

idth

)

No.

12

34

56

78

Inne

rra

dius

(mm

)2.

00(2

.00)

5.89

(3.4

4)8.

22(4

.88)

10.1

2(6.

32)

11.7

9(7.

76)

13.3

1(9.

20)

14.7

2(10

.64)

16.0

4(12

.08)

Out

erra

dius

(mm

)5.

39(2

.94)

7.72

(4.3

8)9.

62(5

.82)

11.2

9(7.

26)

12.8

1(8.

70)

14.2

2(10

.14)

15.5

4(11

.58)

16.8

0(13

.02)

No.

910

1112

1314

1516

Inne

rra

dius

(mm

)17

.30(

13.5

2)18

.51(

14.9

6)19

.67(

16.4

0)20

.80(

17.8

4)21

.89(

19.2

8)22

.96(

20.7

2)23

.99(

22.1

6)25

.01(

23.6

0)O

uter

radi

us(m

m)

18.0

1(14

.46)

19.1

7(15

.90)

20.3

0(17

.34)

21.3

9(18

.78)

22.4

6(20

.22)

23.4

9(21

.66)

24.5

1(23

.10)

25.5

1(24

.54)

18/[1430]

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Annular Phased Array Dynamic Focusing Method [1431]/19

40 60 80 100 120 140 1600.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Sound Pressure Distribution of APAUT in Axial Direction

No

rma

lize

d P

ress

ure

Axial Direction(mm)

F=80mmF=100mmF=120mm

(a) Axial direction

-5 -4 -3 -2 -1 0 1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Sound Pressure Distribution of APAUT in Lateral Direction

No

rma

lize

d P

ress

ure

Lateral Postion(mm)

F=80mmF=100mmF=120mm

(b) Lateral directionFigure 4. Sound pressure distribution of APAUT.x

An experiment for sound pressure of the APAUT is conducted over a scanning systemwith a hydrophone. A negative square wave is used for the excitation. The width of pulse is100 ns. The results of the axial and lateral sound pressure distribution are shown in Fig. 4.In Fig. 4(a), we can see that there is side lobe besides the main lobe, which increases withfocus changing from 80 mm to 120 mm. This issue will be studied in future research.

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20/[1432] Y. Wu et al.

Table 2Comparison of APAUT and regular focused transducer

APAUTOlympus V308-5 MHz

Focused Transducer

Width of Focus (–3 dB) (mm) 1.56 mm (F = 100 mm) 1.57 mm (F = 100 mm)Focus Depth (–3 dB) (mm) 28.8 mm (in water)

6.9 mm (in aluminum)59.5 mm (in water)14.3 mm (in aluminum)

From Fig. 4(b), the width of focus (−3 dB) is 1.56 mm (F = 100 mm). From Fig. 4(a),focus depth lw (F = 100 mm) is 28.8 mm in water. Considering the difference of soundspeed, focus depth in metal, e.g. aluminum, is 6.9 mm, figured out from Equation (3)

la = lw × Vwater

Valmuninum

(3)

Table 2 is a comparison between the APAUT and the Olympus V308 5 MHz focusedtransducer in the width and depth of focus. The width of focus (−3dB) for APAUT isapproximately the same as the focused transducer, but the depth is almost half of thefocused transducer’s. So the lateral resolution for the APAUT and the Olympus V3085 MHz focused transducer is nearly the same, but the APAUT has a better longitudinalresolution than the focused transducer.

3. Description of DFIUT Method

In the DFIUT method, the emitting wave and the first surface echo are used. A threshold,covering the surface echo, is used to capture the peak time, shown in Fig. 5. From the timedelay t, the distance from the transducer to the surface of target is obtained, which can beutilized to dynamically keep the focal point exactly in the target.

Figure 5. Diagrammatic sketch of distance calculation.

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Annular Phased Array Dynamic Focusing Method [1433]/21

Figure 6. Diagram and appearance of the experimental system. (Color figure available online.)

4. Brief Description of the Experimental System

Figure 6(a) is the diagram of the experimental system. The transmitting and receivingboards are both composed of 16 channels, with an accuracy of delays no more than 2 ns.The negative pulse is 150 volts, and the repetition frequency can be up to 4 KHz. Thesampling speed of the receiving board is 100 MHz.

The system uses a 4-dimensional (including x, y, z and an axis of rotation) scanningframe as the mechanical part so that the APAUT can follow the scanning path accurately.The range is 4000 × 2000 × 600 (mm) with a step of 0.01 mm.

5. Experiments and Results

Two groups of experiments, i.e., resolution verification and target testing verification, areconducted to verify the effectiveness of the proposed method:

A. Resolution Verification

In this experiment, an aluminum specimen with 7 flat-bottom holes is detected using theAPAIUT system. The 0.3 mm flat-bottom hole in the aluminum specimen can be detectedclearly, as seen in Fig. 7(b). So the detecting resolution of the APAIUT system can reach0.3 mm in diameter, which meets the requirements in the detection of targets.

(a) Aluminum specimen (b) Amplitude c-scan imaging

Figure 7. Comparison of aluminum specimen and c-scan result.

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Figure 8. Molybdenum target. (Color figure available online.)

Figure 9. Amplitude C-scan imaging results with and without dynamic focusing.

B. Target Testing Verification

A molybdenum target (2400×270×18 (mm)) is tested in this experiment, as shown inFig. 8. In order to verify the effectiveness of DFIUT, the target is inclined by a slightangle intentionally. The distance from the transducer to the left end of the target is 60 mm,and the right end is 75 mm. Then the inclining angle can be calculated as 0.36◦ refer toFig. 1.

Figures 9(a,b) are the amplitude C-scan imaging results with and without dynamicfocusing. By comparing (a) with (b), defect A and B can be detected clearly by bothmethods. However, the echoes from defect C and D in (b) are apparently weaker than thatof (a). The explanation for this is that defect A and B are on the left side of the target, closeto the left end and they are still in focus depth even if detected with a fixed focus transducer,but defect C and D are on the right side of the target, far from the left side, making themdeviate from focus depth. With dynamic focusing, the defects will be in the focal planethroughout the detection, which will make the detecting resolution consistent even if thedistance varies.

6. Conclusion

In this paper, a new method named DFIUT for large target is proposed to reduce the detectingresolution loss caused by the focus deviation. IPAUT system including an APAUT is setup and the large molybdenum target (2400 mm ∗ 270 mm ∗ 18 mm) is tested with/without

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Annular Phased Array Dynamic Focusing Method [1435]/23

the proposed DFIUT method. The experimental results show that DFIUT is useful for thelarge target detecting and can maintain the detecting resolution while the distance betweenthe probe and the specimen surface changes for the reason that the flatness of both the largetarget and mechanical system might vary.

References

1. F. Dupenloup, J. Y. Chapelon, D. J. Cathignol, and O. A. Sapozhnikov, Reduction of the GratingLobes of Annular Arrays Used in Focused Ultrasound Surgery. IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control. 43(6), 991–998 (1996).

2. Puxiang Lai, Bixing Zhang, and Chenghao Wang, Radiation and reflection acoustical fields ofan annular phased array. ACTA ACUSTICA. 32(3), May, 212–220 (2007).

3. Xia-lin Mo, Application of Annular Array Ultrasonic Testing Probe to the Inspection of Forgings.Nondestructive Testing. 28(2), 81–83 (2006).

4. Ling Yang and Qing-yu Ma, Harmonic Influence of Annular Phased Array. Technical Acoustics.30(6), 33–34 (2011).

5. Ke-ren Shi and Yu-min Guo, Phased Array Ultrasonic Imaging and Testing. Beijing: HigherEducation Press; (2010).

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