17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China

ULTRASONIC TRANSDUCERS CALIBRATION SYSTEM WITH 3D PROCESSING AUGUR 5.4

Andrey E. BAZULIN, Evgeny G. BAZULIN, Dmitry S. TIKHONOV ,

Аlexey Kh. VOPILKIN SPC “ECHO+”, Moscow, Russia

E-mail: android@echoplus.ru, bazulin@echoplus.ru, dtikh@echoplus.ru, vopilkin@echoplus.ru Web: http://www.echoplus.ru/eng

Abstract This paper describes ultrasonic transducers calibration system AUGUR 5.4 developed

in Scientific and Production Center ECHO+. AUGUR 5.4 is a new generation of calibration systems developed since 1991. The system allows measuring or calculating most part of contact and immersion probes parameters which described in European Standard EN 12688-2. Main system feature is based on use of scanning device with two axes and single hemispherical test block (for contact probes) and small ball-type reflectors (for immersion probes). Single measurement on hemispherical block with pulse-echo technique allows calculating of all pulse parameters (pulse shape, pulse spectrum, sensitivity), probe index, beam axis offset, beam and squint angle, angles of divergence. The principle of full 3D directivity pattern calculation (therefore beam and divergence angles in any plane) based on calculation of multiple-frequency holograms of “omnidirectional source” (FT-SAFT framework). Distance-amplitude curves could be calculated numerically with discrete model of probe’s crystal and set of flat bottom reflectors. Keywords: probe verification, probe calibration, FT-SAFT, ultrasonic transducer, ultrasonic testing, probe parameters

Introduction

The task of ultrasonic probes verification is a necessary stage of probes manufacturing process and ultrasonic non-destructive examination routine. The procedure of probes verification given for example in Russian Standard GOST 23702-90 [1] and European Standard EN 12668-2 [2]. The comparison of both Standards could be was made by Igor N. Ermolov with consultations from H. Wüstenberg [private communications]. Quantity of probes parameters is 69 for GOST and 23 for EN. Else one Standard is the DNV [3] and this one contains demands to some additional probe parameters verification.

The probes calibration systems AUGUR 2.2 [4] and AUGUR 4.4 [5] were developed in SPC “ECHO+” in 1992 and 1996 respectively. The principle of probes parameters measurement based on acquisition of single B-scan from side-drilled hole in rectangular reference block SO-2 with scanning along one axis. This single measurement allows verification of pulse and spectrum parameters, sensitivity, directivity pattern in single plane. For probe index verification additional reference block SO-3 (semi-cylinder block) used. The distance-amplitude curves were calculated numerically taking into account the real probe parameters [6]. The AUGUR 4.4 system used at various Russian enterprises (Steel Works, Russian Railways etc)

The modern probes calibration system AUGUR 5.4 was developed in ECHO+ Center in 2007. Its description is given in next part of paper.

Brief system description

The AUGUR 5.4 system has main parts: system unit with electronic equipment, scanning device with two axes, set of test-blocks and probe holders. See Fig. 1 for details.

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System unit is a single channel flaw detector with pulse generator, gain amplifier, ACD and stepper motor controls. System unit plugs to any PC with USB 2.0 cable. Scanning device has two axes with stepper motors.

System software (running on the PC) has following main functions: • data acquisition with preset and adjustable configurations; • data processing and probe parameters calculation with accordance to selected

probe type and verification methodology; • ultrasonic data and datasheets visualization.

Main system technical characteristics: Pulse generator: shock and bipolar with varying length Pulse length adjusting: 0.1…1 ms Pulse amplitude: 50, 100, 150, 200 V Gain range: -20+56 dB with step 1 dB Maximal input signal amplitude: 10±0.1 V Bandwidth (-3 dB cutoff): 0.5-15 MHz TVG range: at least 40 dB Damping resistance: from 25 to 500 Ohms

Equivalent noise per root bandwidth: 20×10-9 HzV / Minimal scanning step size 0.01 mm The main difference from the previous generation systems is:

• second axis of scanning device allows to acquire 2D ultrasonic data and hence calculate probe index and directivity pattern in any plane;

• the list of measured or calculated parameters is generally in agreement with EN and DNV.

• the hemispherical of semi-cylinder test block could be used as single test block for contact probes verification;

• immersion and focusing probes were added in range of probes types available for verification;

• adjustable damping resistances, generator pulse amplitude, increased gain diapason and bandwidth, allows to simulate the particular type of ultrasonic equipment and expand the range of probes types available for verification.

• system could be plugged to any PC with USB 2.0 connector.

System passed trials as measuring instrument in Russian Federation [7]. The system verification routine is mostly according with European Standard EN 12668-1 [8].

The next part contains brief description of probes verification methodology.

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Fig. 1 AUGUR 5.4 system. A. Common system view. B. System with hemispherical test block. C. System with immersion tank. D. Example of transducer datasheet.

Probes verification methodology

Main feature of system is based on use of scanning device with two scanning axes and single hemispherical test block (110 millimeters in diameter) for contact probes and small ball-type reflectors for immersion probes. Single measurement on hemispherical block with pulse-echo technique allows calculating of all pulse parameters (pulse shape, pulse spectrum, sensitivity), probe index, beam axis offset, beam and squint angle, angles of divergence.

The principle of full 3D directivity pattern calculation (therefore beam and divergence angles in any plane) based on calculation of multiple-frequency holograms of “omnidirectional source” measurements where “omnidirectional source” is an inner spherical surface of test block or small ball-type reflector (FT-SAFT framework). The directivity pattern calculation algorithm is as follows:

1. With use of Fourier transform in time domain multiple-frequency holograms calculation.

2. With use of 2D Fourier transform in spatial domain holograms spatial spectrum calculation.

3. With use of wave number c

fk

π2= at each frequencyf spatial spectrum to

directivity pattern conversion. 4. The impulse directivity pattern calculation as a sum of directivity patterns at

frequencies ( )maxmin , fff ∈ , where ( )maxmin , ff is a probe bandwidth.

See Fig. 3 for example of directivity pattern calculation.

A

B C

D

PC

Scanning device

Immersion tank

System unit

Test-blocks

Probe holders

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All probe parameters which are able to be verified by AUGUR 5.4 are enumerated in Table 1.

Table 1. List of probe parameters

Parameter (EN terminology) Measurement or calculation methodology

Pulse shape Recording the echo pulse with maximal amplitude from bottom surface of semi-cylinder or hemispherical test-block (contact probes) or flat plexiglas target (immersion probes)

Pulse duration Calculation with -20 dB drop (any other drop level is available certainly)

Pulse spectrum Calculation the DFT of pulse

Bandwidth, relative bandwidth Calculation the lf and uf for a -6 dB drop

Center frequency Calculation as ul fff ×=0

Pulse-echo sensitivity Calculation as ratio of echo pulse amplitude from bottom surface to transmitter pulse amplitude

Cross-talk damping of dual crystal probe

Direct measurement of echo pulse from top and bottom surfaces of test-block

Distance-amplitude curve (DAC)

Calculation the set of curves with applying probe and flat reflectors discrete model taking into account real probe parameters. Direct measurement also could be applied using additional test blocks

Noise curve Recording the pulse from the probe when probe is clean from coupling liquid

Probe index, beam axis offset Calculation from transducer coordinates relative to center of semi-cylinder or hemispherical test-block when the echo pulse has a maximum amplitude

Directivity pattern (beam angle and angles of divergence) in two orthogonal planes.

Calculation from spectrum of holograms measured from bottom surface of semi-cylinder or hemispherical test-block (contact probes) or ball-type reflector (immersion probes) Single frequency or impulse directivity pattern could be calculated

Effective plate size Calculation by equations from [2] or [9]

Focal distance, nearfield Calculation by distance-amplitude curve

Focal width Calculation by probe 3D acoustic field reconstruction by angular spectrum method [10], see Fig. 4

Focal length Calculation by DAC as -6 dB drop

Amplitude uniformity along the wide plate*

The acoustic field from point-source is equivalent to data measured by pulse-echo at the previous stages of verification

Plate angle, plate center coordinates**

Calculation of this additional parameters is useful for further coherent treatment of data acquired within this probe

Time of flight in prism** Calculation is useful for measurement of wear plate thickness (or prism material velocity)

* According to DNV-2000 ** GOST 23702-90

The impedance measurement could be completed with special electronic equipment. The time for common parameters set verification (impulse parameters, sensitivity, probe

index and directivity pattern in plane of incidence, noise curve, DAC) could be competed for two

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or three minutes including probe set up, measurements, calculation and datasheet printing. Verification of full parameters set with 2D scanning and manual analysis of some characteristics could be completed for about twenty or thirty minutes.

Fig. 2 Common view of B-scan from hemispherical block.

Fig. 3 Example of 3D directivity pattern calculation from 2D angular spectrum of multifrequency

holograms (left side). At the right side two slices of directivity pattern shown.

Fig. 4 Example of acoustical field measurement and calculation. A – measurement of field of pitch-

and-catch probe with plate length 30 mm, the reflector is bottom of hemispherical test block. B – calculation of immerse probe field from single measurement in the near field. One can see the near field zone and far

field zone structure, focal distance.

A B

X

Y

X

Z

Focal distance width and length

Amplitude uniformity along the plate

-140 -120 -100 -80 -60 -40 -20 0 20 40 600

5

10

15

20

25

30

35

40

45Диаграмма направленности в дополнительной плоскости (максимальная) на частоте 1.7969 МГц

угол, град

-100 -80 -60 -40 -20 0 20 40 60 80 1000

5

10

15

20

25

30

35

40

45Диаграмма направленности в основной плоскости (максимальная) на частоте 1.7969 МГц

угол, град

kx, рад/мм

kу, рад/мм

Растровое изображение спектра двумерной голограммы на частоте 1.7969 МГц

-10 -5 0 5 10

-10

-8

-6

-4

-2

0

2

4

6

8

10

10

20

30

40

50

60

Directivity pattern in plane of incidence

Directivity pattern in perpendicular plane

Probe index

Impulse shape

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Summary and discussion

The simple and compact system for probes verification was developed. The list of probe parameters which available for verification is in agreement with EN 12688-2.

One of the systems is working on Vyksa Steel Works to complete standards of probes calibration routine in international project NordStream.

References

[1] GOST 23702-90 (Russian Federation standard). Ultrasonic transducers. Testing methodology. 1990.

[2] EUROPEAN STANDARD. EN 12668-2:2001. Non-destructive testing. Characterization and verification of ultrasonic examination equipment – Part 2: Probes.

[3] Offshore standard DNV-OS-F101, Det Norske Veritas, 2000. [4] Badalyan V.G., Bazulin E.G., Bychkov I.V, Vopilkin A.Kh., Kaplun S.M., Lomakin

A.V., Pentjuk M.V., Ruben E.A., Tikhonov D.S., Stern A.M. – A computerized system for testing and certifying AUGUR 2.2 ultrasonic nondestructive test transducers, Russ. j. nondestruct. test., 1993, vol. 29, no2, pp116-122.

[5] Probe calibration system AUGUR 4.4, The Federal Agency of the Russian Federation on Technical Regulating and Metrology certificate RU.C.34.003.A №14077.

[6] Antipin V.E., Gusarov V.R., Perlatov V.G. Role of distributed transfer functions for electroacoustic transducers in the metrological security of an ultrasonic inspection. Sov. J. Nondestr. Test. (Engl. Transl.) ; Vol/Issue: 24:8; Translated from Defektoskopiya; 24: No. 8, 44-49 (Aug 1988)

[7] Probe calibration system AUGUR 5.4, The Federal Agency of the Russian Federation on Technical Regulating and Metrology certificate RU.C.27.003.A №30200.

[8] EUROPEAN STANDARD. EN 12668-1:2001. Non-destructive testing. Characterization and verification of ultrasonic examination equipment – Part 1: Instruments.

[9] Nondestructive testing hand-book in 7 volumes. Chief editor V.V. Kluev. Vol. 3. Ultrasonic testing, I.N. Ermolov, Ju.V. Lange, Mashinostroenie, 2004. – p864 (in Russian).

[10] Ermert, H, Karg, R. Multifrequency acoustical holography. IEEE Transactions on Sonics and Ultrasonics. 1979, Vol. 26, Issue 4, pp. 279-285.