|In figure 1 the common pulse excitation technique is compared with the pulse compression technique. For convenience a simple simulation is shown. The system consisting of a transmitting probe, the propagation medium (concrete) and the receiving probe is represented by a delay of 250 µs, a damping of 50 dB and addition of 0.25 mV (rms) noise. The transmitted chirp has a bandwidth of 400 kHz and a length of 250 µs. This yields an amplitude gain of AG = 7. In accordance with that a chirp with an amplitude of only 70 V leads to the same signal to noise ratio after correlation as the pulse with amplitude AG * 70Volts ~ 500Volts.
Chirp-signals are best suitable for ultrasonic testing in concrete. They can be generated easily by arbitrary waveform generators. The frequency-spectrum of the chirp can be adapted to the transfer function of the system (figure 2). Consequently, a maximum of ultrasonic energy is brought into the system .
Fig. 1. principle of the pulse compression technique
Fig. 2. linear frequency-modulated chirp signal x(t) and its frequency spectrum X(f)
Broadband transducers are required for chirp signals. The sensitivity of piezoelectric sensors is much lower for broadband signals than for narrowband signals. So the advantage of piezoelectric sensors is diminished in this case. We used an alternative broadband-sensor: the laser interferometer.
The main advantage of optical detection by laser interferometry is the ability of working without contact to the specimen. Thus, laser can be used on rough or awkward shaped surfaces or on areas, which are difficult to reach (bridges etc.). This frequently occurs in civil engineering. Scanning areas on the surface of concrete with a laser interferometric sensor might be an effective method for non-destructive evaluation (NDE). On the other hand laser interferometry has not yet become a practicable tool in NDE, due to the fact that its sensitivity is insufficient in most practical applications. The signal to noise ratio in manufacturers specification cannot be maintained on ordinary scattering surfaces. The surface of concrete is rough and the coherence of laser light will result in interference speckles at random distribution. When optimising the measurement only for one single point on the surface, it is possible to adjust the laser by changing the focus, so that the photo-detector is covered by a bright speckle. In scanning mode normally the photo-detector is partially covered by random distributed bright speckles.
Thus, the effective sensitivity inherently fluctuates. Although the absolute sensitivity of laser interferometer remains the same, its noise level increases significantly, when dark speckle are encountered. This adverse effect can be eliminated almost completely by the random speckle modulation technique suggested in (5). The idea is very simple. Changing the focus by a small amount moves the speckle pattern in an unpredictable way. Doing this slowly enough and observing the interferometer signal amplitude, the ultrasonic measurement can be taken, when the interferometer noise is low. This will be repeated at each measurement point.
We modified a Polytec heterodyne scanning laser vibrometer PSV 100. Instead of changing the focus as described in (5), we let the scanning mirrors vibrate with small amplitudes. The deflection angle is smaller then 0.04°. That is a deflection of less than 0.7 mm at a distance of 1 m between laser vibrometer and surface. It can be assured, that a bright speckle falls on the photodiode for approximately 1 ms by a modulation-frequency of about 50 Hz. This is sufficient to trigger an ultrasonic transmitter and detect the response before the bright speckle disappears. Figure 4 shows selected signals of a measurement. It can be seen, that for bright speckles (large signal amplitude) the noise is low. In the example shown there is enough time to trigger two measurements.
The improvement by using the random speckle modulation is shown in figure 5. We carried out a line-scan (ultrasonic B-scan) over a specimen by moving the laser beam in a range of 50 mm. An ultrasonic transducer is used to generate the pulse having a centre frequency of 100 kHz. Without random speckle modulation some of the A-scans are very noisy, while others gave a good signal. The random speckle modulation increases the average signal to noise ratio by 3 to 5.
Fig. 3. scheme of an ultrasonic laser interferometric measurement including random speckle modulation
The experimental setup is shown in figure 6. It consists of a personal computer, an arbitrary waveform generator followed by a power amplifier, an ultrasonic transducer (Krautkraemer G0,2-R1) and a Polytec laser interferometer with the developed random speckle modulator. The PC controls the process via IEEE 488.2 and performs the cross correlation and data storage. |
The arbitrary waveform generator provides the chirp signal. The waveform can be easily varied for different applications by PC-controlling the generator. The maximum output voltage of the gated power amplifier is 400 Vpp with a signal period of maximum 500 µs and repetition rates lower than 50 Hz. That is quite enough for most applications.
For digitising the received data we use an oscilloscope with a signal averaging option. The sampled data can be read via IEEE- connection into the PC, where the correlation is carried out off -line.
Fig. 6. scheme of the realised measurement system
The scan angle of the laser vibrometer is ( 20° in each direction. The theoretical number of points is 4096 in each direction, this corresponds to 1.6 million points. But it takes approximately 1 to 5 seconds measure time for each point depending on the number of averaged signals. Currently, scan areas up to 100 x 100 points are practicable, which takes less than 3 hours data aquisition. |
Our arrangement is merely an easy test by combining standard devices. The speed can be considerable increased by using fast DSP-boards for digitising and calculating the correlation.
In order to test our method we scanned an area of a concrete specimen, which includes some defined voids in various depths. The test specimen (2000x1500x700mm) has a maximum aggregate size of 16 mm.
Figure 7 shows the plan of the specimen and the scanned area together with the transducer positions. The scanned area was 500x300mm with a raster size of 5 mm. The time duration of the transmitted chirp signal with a frequency range of 30 kHz to 150 kHz was about 500 µs. Five signals were averaged at each point. The measurement took approximately 1.5 s per point.
Fig. 7. concrete specimen; the duct, some artificial defects, the measurement fields and the positions of the ultrasonic probes are indicated; the thickness of the plate is 700 mm
In figures 9 and 10 the results of reconstruction by means of 3D-SAFT are shown. The backwall can be identified in both images without doubt. Figure 9 represents the results of the lower part of the specimen (figure 7). The voids in the depth of 450 mm and 540 mm are clearly recognisable. The backwall exists in the reconstructed data, but its visibility is suppressed in this image to show detected voids by drawing iso-lines in suitable amplitude. In a depth of 450 mm is a void, filled with a rigid foam quader of 50x50x50 mm3. The other void is a plastic-tube with a length of 50 mm and a diameter of 50 mm filled with gravel.|
Figure 10 illustrates another measurement, now in the upper scan area of the test specimen (figure 7). A mortar filled duct in a depth of 250 mm with a gravel filled void behind (350 mm) having a lateral size of 200 mm could be located. The reconstructed backwall seems to have a gap. This can be easy interpreted as a shadowed region because of the duct. Additional measurements involving different positions of the transmitting probe may clarify this effect further.
Fig. 9. 3D-SAFT reconstruction of measured data set, left: 2D-projections, right: 3D-visualisation, in the right figure backwall is made invisible for presentation.
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