Abstract

Analog characteristics of ultrasonic flaw detectors are essential for successfully solving today's inspection tasks. With modern computerization, analog components may not always receive their fair share of attention. The article highlights some important transmitter and receiver elements, e.g., pulser type, damping frequency, bandwidth, filter. Some of the text relating to analogs in the new standard EN 12688-1 "Characterization and verification of ultrasonic examination - Part 1: Instruments" is also reviewed.

Background

Ultrasonic Flaw Detectors are instruments for material testing for detection of certain imperfections inside the material under test. These instruments are designed for an increasingly wide range of applications, e.g., complex material structures and composites. More and more fully computerized systems are being developed, and they offer many advantages, but also rely on human resources for their handling and development. As computerization takes center stage, it seems that less attention is given to analog performances.

Usually the inspection task, in particular the material, dictates the needed instrument specifications. For successful inspection results, the analog performance of an ultrasonic instrument is an essential factor.

The following material characterizations of the work piece are key elements of an effective inspection:

• Sound attenuation (acoustic parameters, grain size, texture, anisotropy, composite)
• Geometry (thickness, shape)
• Type of imperfection (crack, air inclusion, foreign body, size)

With this information an appropriate flaw detector must be selected. The selection is usually based on primary test results - but which came first? The chicken or the egg?

With expert knowledge or modeling software much is possible without using any instrument or ultrasonic probe. But in practice the investigation is usually carried out under laboratory conditions, by means of a wide range of ultrasonic instruments and probes for primary testing.

What test frequency should be used? With the above mentioned material characterizations the very first decision is usually the selection of the type of probe with its main parameters like crystal diameter, sound beam (focus etc.) In particular, a necessary test frequency must be estimated (e.g. 5 MHz) . Therefore it is widely recognized that the applicable test frequency is an important characteristic of the UT equipment. Two electronic devices - the transmitter and receiver - are responsible for this essential step.

The Transmitter

EN 12668-1 Fig. 10: Transmitter pulse parameters to be measured

The transmitter generates a high voltage pulse which can be a spike, square wave (bi-polar or uni-polar), tuned, or a tone burst (See EN 12668-1 Fig. 10 ). The pulse shape delivers the frequency which should be suitable for our inspection. But a pulse does not contains only one frequency; usually a frequency of a certain bandwidth is generated. This phenomenon is known as the Fast Fourier Transformation (FFT)
(See UT A-Z: FFT Text and Figure as well as Bandwidth Text and Figure.

Over the years, manufacturers have not clearly favored a certain pulse shape. The reason for this is obvious - none is ideal for all tasks. But certainly a specific task can be solved best with the adequate pulse shape.

High attenuating material may need a test with narrow band pulses like square waves or tone burst signals, while for high resolution purposes a broadband spike pulser often delivers the best result.

Another reason why manufacturers still prefer the old fashioned spike pulser is its easy adjustment in conjunction with a damping adjustment. If a square wave pulser is applied we have to control the pulse width parameter in relation to the probe frequency. That can cause an "unfriendly" operation of the instrument. Only an expert may be able to operate such an instrument with increased operation variables. Below we will have more comments on this subject (reliability, reproducibility, POD).

In respect to an optimized excidation we should also consider parasitic occurrences; the probe cable can influence the generated frequency band. You can get an ideal echo pulse using a 2 meter cable but you wonder why a short cable which you thought was better, delivers an worse echo containing a long pulse train. The reason is that the cable reflection of the longer cable generated an inverse excitation pulse which helped stop the unwanted pulse train (Laplace transformation delivers the theoretical background). That means the spike pulser together with the cable actually act as a square wave type also and the cable lenght determines the puls width.

It's not really possible to give an all-purpose answer without knowing the specifics of an individual case.

What pulse rise time do we need? The answer to that question relates to the comments on pulse shape. If your application needs a low frequency (1 - 2 MHz) a 50 ns rise time is sufficient. For high resolution (high frequency) applications a 5 ns rise time may be necessary. However, be aware of the mentioned cable problems discussed below.

What transmitter voltage do we need? Usually, as much as possible. However, more voltage means higher costs and also can be a disadvantage for some other reasons. Analog circuits have a saturation effect problem, that means they need a certain recovery time after being loaded with high voltage (blind for a certain time). For high resolution purposes that can be a disadvantage when using too high a voltage. Also, certain probes like PVDF specify a maximum applicable voltage.
For what purpose is a high voltage transmitter useful at all? For high attenuating material or large wall thickness it is recommended. It is not often possible to compensate for a weak transmitter by using too much amplification at the receiver side. Amplifier noise or an electric noisy environment are limitations of too much gain.

What discrete resister value is needed (in Ohms) ? High resolution applications may need low values, but again, it's speculative. It very much depends on other equipment devices like cable, probe, amplifier input. Some probe manufactures have elements already built inside the probe. If you use your instrument for a wide range of applications your damping element should also be wide range. 10 Ohm to 2000 Ohm is excellent.

The damping is a very 'magic' feature. It is a resistor in parallel to the transmitter output. Earlier analog instruments provided the function as an analog potentiometer with an adjustment range of 10 Ohm to 2000 Ohm. Today we find digital settings in increments of 4 values, (e.g., 33, 50, 75, 500), and sometimes even only one value is available. The reason for this simplification may be that the older inexpensive potentiometer function is today more difficult to provide by means of a digital setting.

How this feature really works is often only partially known, even for experts. Basically it is known that the damping adjusts the pulser shape, (the generated frequency spectrum), and it eliminates probe cable reflections by means of reducing visible echo trains. In general the operator appreciates this setting for tuning the A-scan display until a clear echo picture is visible. The EN 12688-1 does not contains any characterization of the damping device under transmitter paragraph 6.4; it is mentioned in passing under the receiver paragraph 6.5. Also the device is highly stressed by the high voltage pulse and its life may be limited. EN 12688-1 should include it in the test procedure of verification of required instrument performance.

Manufacturer's Technical Specification

The EN 12668-1 describes what manufacturer's technical specification for ultrasonic instruments shall contain. The information is listed in 6.2 to 6.5 for General attributes, Display, Transmitter, Amplifier and attenuator and Digital ultrasonic instruments (Logarithmic Amplifiers in Annex A).

The Amplifier

What receiver frequency range do we need? As we mentioned already in the discussion of transmitter frequency, for the amplifier we must consider the bandwidth, in addition to a nominal center frequency.

An amplifier (sometimes called receiver) is an electronic device which usually provides a constant gain for signals from low to high frequency, thus called broadband. Basically electronic devices deliver a lower and upper frequency limit, that is equal to the operation range of the flaw detector, e.g. 1 MHz - 15 MHz. The limits are defined at the 3 dB drop points of the receiver amplitude curve.

For simple general purpose applications we may say that we are now roughly able to operate all probe frequencies within this bandwidth. But in actual practice we must consider a few other important factors. The quoted probe frequency does not mean that the echo signals shows this frequency. Usually the received echo signal is lower than the probe frequency (see also Fig 3). That frequency drop is caused by the material attenuation and if a water path is applied, the frequency is shifted by this as well; clearly noticeable when dealing with high frequencies above 25 MHz.

The impulse answer of the pathing device is another reason why the amplifier should provide a lower frequency limit than that of the probe, however, important only for high resolution. A shock wave echo of for instance 5 MHz can keep its optimum shape only if the lower bandwidth of the pathing device is about 3 times less than the pulse center frequency. In that case the amplifier should have a lower frequency limit of at least 1.5 MHz.

Why is a filter used? So far we have looked at the broadband curve, but for certain applications we will need to path the echo signal through a filter. A filter is a device which reduces the broadband curve of the amplifier and is part of the amplifier unit.
The EN 12668-1 quotes the following filter definations:

Center frequency: f0 = fu x fl

and

Bandwidth: f = fu - fl

wherein is

fu = higher frequeny fl = lower frequency (3 dB points)

The received signal that follows an ultrasonic pulse generation usually shows not only a clear echo signal reflected from the defect, it also contains a lot of noise. In practice the term noise is not specific. It is used for several occurrences:

• Pulser reverberations
• Probe noise (delay line, crystal, backing)
• Material noise (grain, texture, composite)
• Electronic amplifier noise
• Electro-magnetic interferences

The instrument operator often does not need, or want to know, what kind of noise. She or he optimizes the instrument by turning the filter settings until a clear echo picture is visible. A similar procedure has already been described for the pulser damping adjustment. Unfortunately, filter and damping influence each other. That means the operator needs to adjust all possible combinations. Old fashioned analog instruments controlled these two functions with a potentiometer knob for the damping and a switch knob for the filter setting. Even the potentiometer allows virtually any value; the operator was easily able to find iteratively the optimum adjustment for filter and damping. With digital instruments the values of both parameters need discrete settings.

We may philosophize over the question of whether these two 'knobs' are useful at all. Investigations of Probability of detections (POD) see human error as an important factor. Advocates of the use of fewer system variables (adjustments), (e.g., damping) may say "A limited but reliable and reproducible adjustment is better than a to greater but less controllable one". Those concerns obviously need to be considered if the system provides even more adjustment variables, such as pulser voltage or pulse width in conjunction with square wave pulser.

Conclusion

Inspection of Simple general purpose applications might succeed even if analog characteristics are not optimized. Also the overall instrument performance usually delivers enough reserve to compensate those problems of mismatched characterizations. However, in extreme cases, when ultrasonic testing is working at limits, we must be aware of the need to optimize, especially the analog variables of an ultrasonic instruments.

Author

Rolf Diederichs
studied Communication Engineering at the Engineering University in Köln and has 20 years working experience in the field of computerized Ultrasonic Systems, specifically in the area of Nondestructive Testing and Process Control. He worked in Development, Engineering, After Sales, Applications, Sales, Product Marketing for Ultrasonic equipment and automatic testing machines. He is a member of the DGZfP and ASNT. The areas of expertise included the development of pulser & receiver, active electronics in probes and arrays, fast data acquisition, manual and automated wall thickness measurement and UT in plastic extrusion.
Today he is publisher and editor of NDTnet.
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