TABLE OF CONTENTS

Introduction The Digitization Technique Data Reduction Conclusion Literature

Introduction

Recently, quasi-gate-free digital ultrasonic instruments were developed for in-service inspection of NPP (Nuclear Power Plants) to improve the inspection results and decrease test and evaluation time.
The signal recognition and the data reduction for inspection visualization are important issues in the application of these new systems.

In particular an optimization of:

• Digitization of the analog signal without missing essential information.
• Sampling rate during generation of the row data as RF signal data in such a way so that defect analysis is possible, for instance with Synthetic Aperture Focusing Technique (SAFT), Time of Flight Diffraction Technique (TOFD) or with ultrasonic holography.
• Number of pixels in the evaluation range or pixel width to ensure a reliable comparison with the test results of a previous in-service inspection as well as description of possible defects.
• Determining the Amplituden Laufzeit Ortskurven (ALOK = Amplitude Time Locus Curves) parameters i and k.

Digital ultrasonic instruments have been in use for some time as components of NPP inspection. So far, guidelines and standards have not contained characterizations of signal recognition or evaluation, as long as the Digitization has been applied for gate (monitor) techniques only. That means there is no documentation concerning the Digitization of ultrasonic echo signals in DIN 54119 (08/1981) "Ultrasonic Testing - Terms (Ultraschallprüfung - Begriffe)" and DIN 25450 (09/1990) "Ultrasonic Test Systems for Manual Testing (Ultraschallprüfsysteme für die manuelle Prüfung)".

Only in DIN 25435-1 is it mentioned that the resolution must be equal to or greater than 1 dB if analog to digital conversion is applied (DIN 25435-1 [11/1987] "Wiederkehrende Prüfungen der Komponenten des Primärkreises von Leichtwasserreaktoren - Mechanisierte Ultraschallprüfung"). For our interpretation a limit of the quantifying error is determined and in particular the applicable echo dynamic range is limited (e.g., for a 8 Bit A/D converter approx. 20 dB).

In ultrasonic testing procedures for defect description certain methods are used which cannot be detailed within the framework of a guideline stipulation. Nevertheless, standards of inspection procedures and instruments must be applied for defect detection, to ensure repeatability and comparison with previous inspections. This must also be a consideration when digital ultrasonic instruments are used.

Therefore, in new or modified ultrasonic testing standards it is necessary to include and define applicable terms, as well to define the characterization of ultrasonic instruments, such as A/D converters, sampling error, sampling frequency, data recognition and reduction, evaluation procedure, etc.

The Digitization Technique

Fig 1: Sampling of an analog signal at the times nTa Fig 2:The Shannonsches' sampling theorem: The signal can be reconstructed if fa > 2fm is applied. (fm upper frequency limit) Fig 3: a). Sample and hold, b). Amplitude curve of the hold device c.) Sampling error Fig 4: Sampling rate fa as function of the sampling error - F and the upper frequency limit fm Fig 5: Sampling error for an 2 MHz shear wave probe Fig 6: ALOK - Algorithm: Determination of the paramenters i and k: Probapility of detection, registration level, Wave mode, Probe frequency and bandwidth, Material Fig 7: Digitization and pixel generated A-scan display Fig 8: Digitization and pixel generated RF signal

The most important Digitization parameter of analog signals is the sampling rate (Fig 1) which determines the sampling error and finally the record quality of the analog signal. If the quantifying error can be described, usually with measurement instruments, in relation to the signal amplitude as an "relative quantifying error" so the sample error is the same either for small or large signals. (The quantifying error can exceed 1 dB for very small signals, since it is negligible for large signals) The sampling error (loss of information), is at least as high as the Digitization frequency.

The evaluation of large amounts of data during the search process results in a considerable slowdown of the inspection process, without an improvement in information quality. The Shannonsche sampling theorem (also called Nyquist criteria) delivers an excellent solution /1/. The Shannonsche sampling theorem states that the curve of an analog signal of bandwidth of a certain frequency can be described if the sampling rate is twice that of the signal's upper frequency limit f_m (Fig 2) (ideally the description can be complete).

Figure 2 shows that the spectrum X(f) of the sampled signal is periodic. The spectral period duration is equal to the sample period T_a = 1/f_a. The overlap of the partial spectrums (higher degree folding products) can be prevented if the Nyquist criteria f_a ( 2f_m is applied. The complete reconstruction of the measured signal would be possible with an ideal square wave low path filter, which delivers the value 1 for the sampling point nT_a and delivers the value 0 for all other sampling points. A Sample and Hold is in practice an applied unit which confirms the Shannonsche sampling theorem (Fig 3a).

The curve of such a Sample and Hold is depicted in Fig 3b. The amplitude deviations of the recorded analog signal from the original signal are as high or as small as the sampling rate chosen in relation to the upper frequency limit. Those deviations are in practice efficiently described with the term relative sampling error (- F) (further called sampling error) (Fig 3c).

Figure 4 shows a graphic presentation of the sample frequency f_a as a function of the sampling error ( - F) and the upper frequency limit f_m shows that a sampling frequency 4 f_m > f_a > 2 f_m describes an analog signal incorrectly, however, based on the Shannonsche sampling theorem the frequency was determined as being sufficient. The reconstruction shows amplitude errors of 10 % to 33 % of the actual signal height. For achieving sampling errors less than 10% it is necessary to apply a sample frequency higher than 4 times the upper signal frequency limit.

The determination of sampling frequency for sampling errors of 10% and 5% is shown in Figure 5, an example of the Siemens probe TTK-2 MHz. Considering a 3,35 MHz upper frequency limit f_m and a sampling error of 5% ( f_a approx.6 f_m ) a 20 MHz sampling rate of the ultrasonic signal can be determined.

The determination of the upper frequency limit f_m is not commonly used in actual practice. It is more accurate and easier to determine the probe center frequency /2/. Also if the spectrum is unknown the manufacturer delivers the value of the probe center frequency.

The reconstruction of an analog signal can be performed by using linear interpolation, as long as the sample rate is 10 times higher than the nominal or center frequency of the probe /3/. For a sinus signal and a sampling error of 5 % a sample frequency of 20 MHz can be also determined for the above mentioned example. The example shows the same result for sample frequency whether upper frequency limit or the nominal or center frequency of the probe is used.

The sampling error is the most important value of digital instruments; the necessary sample rate is based on that. Considering the state of the art of in-service inspection techniques of NPP, a sampling error of 5 % of the ultrasonic echo signal can be achieved.

Data Reduction

Excluding special methods for failure analysis, where all positive and negative amplitude values must be digitized, in in-service inspection of NPP the number of samples can be reduced without missing important information.

The data reduction techniques must be verified on reference bodies containing artificial reflectors. A high data reduction rate can be achieved with the proven ALOK - Algorithm. This method is based on the determination of the half wave maximums of the RF signal. The precise determination of the half wave maximum is achieved by using a high sampling rate (usually 50 MHz).

The ALOK - Algorithm recognizes only those amplitude and time of flight values, which lie within a time gate, and are greater or equal to the "i" preceding half wave value maximums and are higher than "k" successive half wave maximums. The measurement resolution of this technique can be adjusted with the "reduction parameter" i and k.

Figure 6 shows when using high i- and k- values neighboring echo maximums can not be detected. To evaluate a defect indication (e.g. difference between one large and many small indications) it is usually necessary that the axial and lateral sound resolution be in the range of a wave length . In other words, for a sound area of 2 at least one measurement point must be taken. The parameters i and k can be calculated:

(i + k +1) / 4 < 2 => i + k < 7

This result is confirmed by most of ALOK applied to inspections and correlated with the results of the guidelines for determination of parameters i an k published by Fraunhofer Institute for NDT (IZFP) (i and k with the amplitude interval: Amplitude maximum - 6 dB) /4/. The ALOK method loses the signal phase information (e.g., phase shift at crack tips, not for i = 0, k = 0) and based on the amplitude and time of flight values (raw data) it is not possible to generate a correct A-scan display later. Since the relevant maximums and time of flight values are memorized together as pair values, a characterization of indications on hand of the ALOK method is clear and reproducible.

Another method to reduce the number of measurement values is possible with the "Pixel method" of the ultrasonic signal. The pixel method memorizes or displays the amplitude value, usually the maximum value, within a time gate (pixel width). Pixel method values are:

• the analog A-scan (RF, rectified or filtered signal) after the Digitization (Fig 7)
• the RF signal after the Digitization (Fig 8)

The pixel method of the A-scan is an pragmatic solution to storage of UT signals in the field of automated testing /5/. Since for this kind of data reduction no high sample rate is necessary (e.g., . 10 MHz for nominal probe frequency of 1-4 MHz), the amount of data is small. This method has been applied with the transducer array technique for many in-service NPP inspections and has been proven as a search technique. The pixel width in the sound beam axis was in the range of a wave length. This resolution should also be kept for future inspections. By means of rectification and filtering of the RF signals the amplitude data (without the phase information) can be precisely determined. But the related time of flight values depend on the adjusted pixel width. For failure analysis, the transducer array technique uses beam swiveling (analysis of multiple angle of incidences).

A higher resolution can be achieved by pixelizing the digital RF signal. With a high sample rate (f_a > 10 f_n ; f_n = nominal frequency) and a pixel width < /2 the phase of the ultrasonic signal is determined. If not otherwise stated, (see above), the pixel resolution of the RF signal in the beam axis should also have a width of < .

Conclusion

The current situation of ultrasonic testing standards in general and in particular for in-service inspection of NPP clearly shows that digital ultrasonic instruments must be addressed. Obviously, the European draft standards have taken the first steps. For example, in the prEN 12668-1 (12/1996) "Characterization and verification of ultrasonic examination equipment - Part 1: Instruments" under item. 3.6 "Definitions" the sampling error is defined and Pkt. 6.6 "Digital Ultrasonic Instrument" adds that the data sheet, especially of digital instruments, (principles of the analog to digital conversion, sampling rate, number of pixels used to display the A-scan, etc.), should apply.

Under item 8.7 "Group 1 tests - Digital Ultrasonic Instrument" additional performances are mentioned. The draft standard points out that "conventions are still developing".

In light of the recently modified standard DIN 25435 -1 (Wiederkehrende Prüfungen der Komponenten des Primärkreises von Leichtwasserreaktoren. Mechanisierte Ultraschallprüfung), and with DIN 25450 -2 (Ultraschallprüfsysteme. Mechanisierte Prüfung) still in development, this is the ideal time to determine characterizations and performance standards of Digital Ultrasonic Instruments.
We recommend the use of the following items:

DIN 25435-1
Pkt. 5.2.3 Ultrasonic Instrument

• The sample rate of the analog signal must be at least six times higher than the upper frequency limit of the transducer. If the upper frequency limit is unknown or cannot be determined the sample rate must be 10 times higher than the nominal frequency (manufacturer's data) of the ultrasonic transducer. (The above- mentioned values are based on a relative sampling error of 5 %. For a relative sampling error of 10% the sample rate must be four times higher than the nominal frequency) Data reduction of the inspection
• If a pixel generated A- or RF image is applied, the pixel width must not exceed a wave length of the ultrasonic signal.
• If the ALOK method is applied, the sum of parameters i and k must not exceed the value 7.
• The software version must be noted and the evaluation algorithm must be described.

DIN 25450-2
Item 3 Definitions and Symbols

• Sample rate
• sample error (relative; absolute)
• Pixel, pixel width, resolution
• Response time
Item 4.2 Ultrasonic Instrument
• Principles of the analog to digital conversion, sampling rate, number of pixels used to display the A-scan, influence of the pulse repetition frequency or the measurement range of the sample rate, amplitude dynamic (compression), response time (refresh rate) of the screen, software version. Measurement, Reliability: definitions need to be included. (see above)

This article is seen as the first step for a clarification of definitions and performances of digital Ultrasonic Instruments. The aim is to ensure the quality of inspection and the reproducibility of inspection for in-service inspection of NNP as well to achieve a reduction in inspection and evaluation time.

Literature

1. Tränkler, H.-R.: Meßtechnik und Meßsignalverarbeitung, Technisches Messen tm. 53. Jahrgang, Heft 1-3, 1986
2. DIN 25450 (09/1990) Ultraschallprüfsysteme für manuelle Prüfung
3. Felderhof, R.: Elektrische und elektronische Meßtechnik. Carl Hanser Verlag, München Wien, 6. Auflage, 1993
4. Barbian, O. A., Grohs, B., Licht, R.: Signalanhebung durch Entstörung von Lauf- zeit-Meßwerten aus Ultraschallprüfungen von ferritischen und austenitischen Werkstoffen - ALOK (Amplitude-Lauzeit-Ortskurve) - Teil 1. Materialprüfung 23 (1981) Nr. 11 S. 379-383
5. Bertus, N., Wüstenberg, H., Montag, H.-J., Schenk, G.: Die Benutzung pixelierter digitalisierter A-Bilder als Grundlage für kompakte automatische Ultraschall- Prüfsysteme. DGZfP-Jahrestagung 25-27. September 1989, Kiel, S 537-544

Authors:

G. Csapo, T. Just
Technischer Überwachungs-Verein Nord, e. V., Hamburg
The paper was presented on the DGZfP annual NDT confernce in Dresden in May '97