This is the sixth chapter of an eleven part article on Ultrasonics by John Drury, the Author of Ultrasonic Flaw Detection for Technicians. This article was first published in INSIGHT magazine throughout 2004/5. The chapters can be downloaded in PDF for you to build into a complete series.

To access the other chapters please use the navigation at the bottom of this page.

J.C. Drury ' BACK TO BASICS - ULTRASONICS'

Chapter6.
PULSE-ECHO FLAW DETECTOR

The ultrasonic flaw detector is required to provide the voltage pulse to activate the probe crystal, to amplify received signals from the probe and to display those signals so that the relative time of arrival and amplitudes of the signal train can be viewed and interpreted. In order to display the very short intervals of time involved in testing metals, the early pulse echo systems used a cathode ray tube (CRT) as the display module. More recently, equipment manufacturers have turned to digital technology and used LCD panels for the display. The result has been the manufacture of much smaller and lighter ultrasonic equipment. Ultrasonic sets in the early 1960’s used thermionic valves (vacuum tubes) and weighed 25 to 30 Kg (50 – 60 lbs). From the late 1960’s, transistor technology and smaller CRT’s meant that the flaw detectors became smaller and lighter weighing between 5 and 10 Kg (10 – 20 lbs). In the new millennium, the weight has come down to around 3 Kg. Figures 1 to 3 show the progression.

Circa 1960

Fig. 1

Circa 1980 Fig. 2	Circa 2000 Fig. 3

Figure 4 is a block diagram of a typical analogue flaw detector showing the main components and the controls associated with each component.

Fig 4

The clock or ‘timer’ is the heart of the flaw detector. It feeds an electrical pulse to the Pulse Generator and simultaneously to the Timebase Generator. This timer pulse causes the pulse generator to send a short, high voltage pulse to the crystal and at the same time triggers the timebase generator to begin to sweep the electron beam in the CRT tube from left to right between the ‘X’ plates at a constant speed.

As soon as the high voltage pulse at the transmitter crystal is cut of, the crystal starts to vibrate and an ultrasonic pulse propagates into the test piece. While this sound pulse travels through the material, the CRT sweep continues to track the time as it moves towards the right hand side of the display. Reflections from internal surfaces arrive at the receiver crystal, generate a voltage in the crystal and this voltage is amplified and passed to the ‘Y’ plates where it causes a vertical deflection of the electron beam proportional to the amplitude of the received signal.

When the electron beam reaches the extreme right hand side of the CRT it flies back to the left hand side and waits for the next trigger pulse from the clock. This whole sequence of events takes place so quickly that we wouldn’t be able to see the trace. The clock repeats the sequence many times a second and the result is a flicker free trace that increases in brightness the more times we repeat the process each second. The number of trigger pulses per second is known as the ‘Pulse Repetition Frequency’ (PRF), or ‘Pulse Repetition Rate’ (PRR). It is important that we allow enough time between pulses for all the multiple echoes within the specimen to die away or we will the tail end of these echoes showing as ‘Ghosts’ at confusing positions on the timebase. For this reason the PRF is controlled by the Depth Range Coarse control in the timebase generator circuit. However, some flaw detectors have an additional manual control that the operator can use. Ghost echoes are most likely to be encountered when testing fine-grained light alloy forgings that have very low attenuation of sound.

The voltages developed in the receiver crystal are very small and need to be amplified. The ‘Amplifier’ circuit needs to be tuned to accept the frequency of the ultrasonic pulse and this can be by way of switched bands for example, 1-3Mhz, 3-7MHz, 7-10MHz & 10-15MHz, or it could be a wideband amplifier with the range 1-15MHz. If the former, the set will have a ‘Frequency’ selector switch that should be switched to the appropriate band for the probe in use just as you would use the tuning dial to select the desired radio programme.

The ‘Gain’ or ‘Sensitivity’ control allows the amplification to be increased or decreased depending on the strength of the received signals much like the volume control on a radio. The Gain control is usually calibrated in decibels (dB) and is sometimes called the ‘Attenuator’. Strictly speaking, an attenuator should be calibrated such that increasing the dB reduces the signal amplitude, but this is seldom the case over recent years. The ‘Bel’ is a unit that is commonly used in electronics to compare the ratio between two power or voltage values and is a logarithmic unit so that large ratios can be expressed concisely. The intensity of sound in a received pulse is a measure of the power or energy in that pulse, and that mechanical energy is converted into electrical energy by the piezoelectric crystal. If the power increases from P 1 to P 2, then the gain can be expressed as: -

However, the Bel is too large a unit for the values we shall encounter in ultrasonics and so we use a unit of one tenth of a Bel or decibel. The equation then becomes: -

The CRT measures voltage and electrical power is proportional to the square of the voltage: -

And, removing the brackets: -

The height of a signal on the CRT is proportional to the voltage applied to the ‘Y’ plates and so we can change the equation so that it is in terms of signal height: -

Example 10

Calculate the gain ratio in dB between a signal that is 60% full screen height and one that is only 30% full screen height.

When we measure depth or thickness from the timebase, we use the left hand flank of the signal on the screen. Sometimes surface roughness, material grain size, or electronic ‘noise’ create noise signals (grass) that obscure the point where the flank meets the timebase and it is difficult to make the correct reading. In these circumstances, we can use the ‘Suppression’ or ‘Reject’ control to remove the grass, much as we use a tone control on a radio to cut out ‘hiss’. Because this control can also cut out small relevant signals and make the gain non linear, a warning light comes on when the control is in use.

The last feature that we need to consider in the amplifier circuit is the one that controls the degree of rectification and smoothing of the pulse. The received signals are, of course, a few cycles of alternating voltage. We can display these as they are – ‘Unrectified’ – but it is not so easy to measure amplitude directly from the screen. It is more usual to display these signals as ‘Rectified’ and smoothed signals in which the negative half cycles are inverted and the signal envelope smoothed out. On some equipment, we may also have the choice to only display the ‘Positive’, or ‘Negative’ half cycles and this may give a sharper flank to the signal. Figure 5 illustrates the four conditions, but unsmoothed to illustrate the principle.

The ‘Timebase’ circuit controls the sweep speed and delay functions. The sweep speed will determines the thickness range that can be displayed on the CRT. A high sweep speed (fast timebase) may only allow a return path from a 10mm thickness in the test piece and at the other extreme, a low sweep speed (slow timebase) may allow a return path from 5 metres or more. Two controls achieve the desired thickness range, the ‘Coarse Depth’ or ‘Range’ control switches the range in steps (10mm, 50mm, 100mm, 500mm, 1m & 5m for example) and the ‘Fine Depth’ or ‘Range’ control is a continuously variable control that allows fine adjustment during calibration to allow for the specific material velocity. The fine depth range control is sometimes labelled ‘Material’ or ‘Velocity’.

There are times when we don’t want the timebase generator to begin the sweep when the crystal is pulsed. For instance, when we are carrying out an immersion test we want the timebase to start when the sound enters the specimen so that the left hand end of the timebase represents the top surface of the test piece. Another example might be when we are testing a long shaft and we want to look in more detail at, say, the last 200mm of the shaft. In either case, we can delay the start of the sweep with the ‘Delay’ control.

The last component to consider is the display module, the CRT. The image created by the electron beam (the trace) must be displayed so that the baseline is aligned with the graticule, extends beyond the left and right hand ends of the graticule, is bright enough to see in the test environment and is in focus. There are four controls for these functions, the ‘X-shift’ and ‘Y-shift’ controls position the trace, the ‘Brightness’ control can be adjusted for indoor or outdoor viewing, and the ‘Focus’ control sharpens the trace. On many flaw detectors, only the focus and brightness controls are provided for operator adjustment.

Digital flaw detectors provide the same PRF, Amplifier and Timebase functions but these are usually controlled using a combination of menu selection and so called ‘Smart Knobs’ through the controlling central processing unit (CPU). Figure 6 is a representative block diagram for a digital instrument.

One of the real advantages of the digital instruments is the facility to store calibrations for a number of inspection procedures and probes, to store whole traces complete with the calibration data for each trace and to create databases to store thickness readings. Because the instruments are based on computer technology, it is possible to connect the flaw detector to a PC through a serial cable and download stored data, for reporting purposes.

The LCD display also has advantages over the CRT. It consumes less power than the CRT; it can be backlit for viewing in low light conditions and at the same time is easy to see without backlighting in daylight. In difficult conditions, the trace can be ‘Frozen’ so that the operator can move to a more comfortable position before reading the timebase.

Fig 6

Many flaw detectors, both analogue and digital, have gating circuits that allow signals to be monitored by the instrument and the output used to trigger audible or visual alarms, or to be connected to chart recorders or computers. The monitor gates may be displayed in one of two ways. The timebase may be raised over the gate distance as shown in figure 7, or a separate ‘bar’ may be used as shown in figure 8.

There are four main functions controlling the gate, these are: -

Gate Start

Gate Width

Gate Level or Threshold

Gate sense (Rising or Falling Signal)

The gate ‘start’ control positions the left hand edge of the gate, the first depth that you want to start monitoring. The gate ‘width’ control then allows you to set the right hand edge of the gate, the last depth that you want to monitor. Any signal within that depth range is said to be ‘in the gate’. You may only want signals exceeding a predetermined amplitude to ‘trigger’ the gate alarm and you do this using the gate ‘level’ or ‘threshold’ control. For those gates that look like figure 6.7, you set a signal in the gate at the desired amplitude, and adjust the ‘threshold’ until the alarm just triggers. For those gates that look like figure 6.8, you simply adjust the gate ‘level’ control until the gate is at the desired screen height. For some inspections, such as when you are using the ‘through transmission’ technique, you may wish to monitor for a decrease in signal amplitude. The gate sense can be changed using the ‘sense’ control. When ‘falling signal’ has been selected, the alarm does not trigger as long as there is a signal in the gate that exceeds the threshold level. Instead, the alarm operates as soon as the signal drops below the gate threshold.

Some flaw detectors have more than one gate. Two gates can be used in several ways; one can monitor backwall echo amplitude (falling signal) and one can be used to monitor part of the timebase for discontinuities (rising signal). The two gates can be used to monitor consecutive backwall echoes and the difference (gate 1 minus gate 2) can be output as the thickness of the object. The ‘menu’ of a digital flaw detector may allow you the choice to monitor either signal amplitude or ‘time of flight’ (depth). This is also possible with some analogue flaw detectors by the appropriate pin selection on the output connecting lead. Generally, the voltage range for the output signal is about 0V to 5V; this means that the vertical or horizontal (amplitude or timebase) scales of the display will be proportional to the output range. If monitoring and recording amplitude, for example, a full screen echo height will output 5V and a half screen height signal will output 2.5V.

Reference: -

‘Ultrasonic Flaw Detection for Technicians’ - Third Edition, June 2004 by J. C. Drury