Transducers

A transducer converts the biochemical signal to an electronic signal. The biochemical transducer or biocomponent gives the biosensor selectivity or specificity. The transducer of an electrical device responds in such a way that a signal can be electronically amplified and displayed. The physical transducers vary from electrochemical, spectroscopic, thermal, piezoelectric and surface acoustic wave technology [24, 25]. The most common electrochemical transducers being utilised are based on amperometric and potentiometric techniques [26-28]. 

Examples of enzymes used for biosensors include glucose oxidase for glucose sensors, alcohol oxidase for ethanol sensors, lactate oxidase for lactate sensors and urease for urea sensors. A typical enzyme reaction, as described by equation 5.2 might involve the transfer of an electron, a pH change, hydrolysis, esterification or bond cleavage. The type of enzymatic reaction that occurs determines the type of transducer that is used. 

Enzymatic bioreceptors have an advantage insofar as the enzyme regenerates itself after reaction. The enzyme is then available for further reaction with the sample. Thus, the response output is directly related to concentration changes in the sample. Antibodies on the other hand can only be used for a one-time measurement. They have to be disposed after reaction or the antigens have to be washed off with suitable reagents. 

The bioreceptor does not have to be an enzyme or antibody, virtually any compound that exhibits molecular recognition for an analyte is suitable. This could be a piece of DNA, a cell, a microorganism, an organelle or a plant or even mammalian tissue. Enzymes and antibodies are used most often, as they are relatively simple to incorporate into a device. This is more difficult with tissue slices and biological cells as they must be supplied with nutrients and have waste fluids removed in order to keep them alive. 

The transducer is essentially the detector of the biosensor. It is the component that responds to molecular recognition and converts this response to an output that can be amplified, stored or displayed. 

Optical detection of sample spectra or light scattering using optical fibres 

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Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

The analog AE signals (from max. 12 transducers) are simultaneously acquired from the field. 

A double time correlation among the 3 binary temporal sequences of a triplet of transducers are calculated. A maximum of 8 different but overlapping triplets can be predefined. 

The X coordinate (coordinate along the transducer alignment) of the sources that have overcome the screening test is calculated. 

The AEBIL system manages up to 12 transducers simultaneously and the maximum configuration can be summarized as follows (fig. 3) ... 

The first 3 items of the above list (waveguides, transducers and preamplifiers) are located at or near to the component(s) to be monitored. The other items must be installed in the control room area, mounted into a single instrumentation rack (fig. 4). 

REAL SIGNALS ACQUISITION or AE SIGNALS EROM THE FIELD (MAXIMUMll TRANSDUCERS)... 

TIME CORRELATION TEMPORAL SEQUENCES OF A TRIPLET OF TRANSDUCERS (B DIFFERENT BUT OVERLAPPING... 

Fig.5 Example of histogram presentatioa The lower picture is a schematic representation of the monitored component with Uie position of the transducers (1-4). The higher window is the total AE counts vs linear location representation of the located AE sources...

Transducers twelve 375 kHz resonant transducers have been used, with a 350 kHz cutoff frequency high pass filter section and a 40 dB preamplifier. 

Current cumulative number of AE events on each predefined transducer triplet ... 

The technique presented above has been extensively evaluated experimentally using ultrasonic data acquired from a test block made of cast stainless steel with cotirse material structure. Here we briefly present selected results obtained using two pressure wave transducers, with refraction angles of 45° and 0°. The -lOdB frequency ranges of the transducers were 1.4-2.8 MHz and 0.7-1.4 MHz, respectively. The ultrasonic response signals were sampled at a rate of 40 MHz, with a resolution of 8 bits, prior to computer processing. 

The 45° transducer was used to inspect side drilled holes, with their centres located 40 mm below the surface. Due to the coarse material structure the echoes from the holes were totally masked by clutter. An example of an ultrasonic response signal, emanating from a hole with a diameter of 8 mm, is shown in the left part of Figure 3. Scanning the surface above the 8 mm and 10 mm holes resulted in the B-scan image shown in the upper part of Figure 4. 

Figure 4 Two side-drilled holes at sound path 50 mm, before and after signal processing. The 10 mm hole is located at transducer position 25 mm and the 8 mm hole at 75 mm.

By employing this technique, the frequency range best suited for a particular material can be automatically estimated and utilized for inspection, without the need to employ a tailor-made transducer. Consequently, a single wide-band transducer can be used to get near-optimal inspection results for a wide range of materials. 

A resonance in the layered stracture occurs when echoes between two boundaries travel back and forth due to differences in acoustic impedances at the boundaries. For multi-layer structures a number of resonances can be observed depending on their geometry and condition. For each particular defect-free structure and given transducer we obtain a characteristic resonance pattern, an ultrasonic signature, which can be used as a reference. 

The austenitic and, hence, anisotropic V-butt weld is embedded in isotropic steel it has a width of 10 mm at its baseline and a height of 30 mm. If a notch is present, it has a width of 1 mm and a height of 15 mm it is located at the right-hand side of the V-butt weld. The simulated transducer is a commercial 45°-shear wave probe (MWB45-2E). The parameters varied during the simulations are ... 

Simulations of that kind result in a wide variety of A-scans and wavefront snapshots. The first screening of this material reveals, that the simulations in which the transducer is coupling partly to the V-butt weld and partly to the steel exhibit quite a number of pulses in the A-scans because the coupling at the interface of the weld results — due to the anisotropic behavior of the weld — in a complicated splitting of the transmitted wavefront. The different parts of the splitted wavefront are reflected and diffracted by the backwall, the interface, and — if present — by the notch and, therefore, many small signals are received by the transducer, which can only be separated and interpreted with great difficultie.s. 

Only the simulations in which the transducer is coupling either to the V-butt weld or to the surrounding steel can be analyzed in a simple and intuitive way, which means that the different pulses in the A-scan signals can be related uniquely to the reflection or diffraction of the wavefront at the weld, the backwall, and/or the notch. 

Apart from the well-known notch base corner reflection for isotropic welds, the anisotropic case results in a second corner reflection for transducer positions well above the weld. 

The diffraction of the incident 45°-S V -transducer-pulse at the interface between the isotropic steel and the anisotropic weld may result in two transmitted qSV-wavefronts, a particular phenomenon to be explained with pertinent slowness diagrams. 

Second corner reflection The first corner reflection appears as usual when the transducer is coupled to the probe at a certain distance from the V-butt weld. The second corner reflection appears if the transducer is positioned well above the V-hutt weld. If the weld is made of isotropic material the wavefront will miss (pass) the notch without causing any reflection or diffraction (see Fig. 3(a)) for this particular transducer position. In the anisotropic case, the direction of the phase velocity vector will differ from the 45° direction in the isotropic case. Moreover, the direction of the group velocity vector will no longer be the same as the direction of the phase velocity vector (see Fig. 3(b), 3(c)). This can be explained by comparing the corresponding slowness and group velocity diagrams. 

Splitting into two quasi shear waves If the transducer is coupling to the isotropic steel the incident shear wave may split into two independent quasi shear vertical wave-... 

We present a novel method, called VIGRAL, to size and position the reflecting surface of a flaw. The method operates on standard B-scan recorded with traditional transducers, to extract Time-of-Flight (ToF) information which is then back-projected to reconstruct the reflecting surface of the flaw and characterize its radiation pattern. The VIGRAL method locates and sizes flaws to within k/2, and differentiates between flat and volumetric defects. 

The VIGRAL approach represents the reflecting surface of a defect as an ensemble of virtual point sources. At every measurement point of the B-Scan, the detecting transducer responds... 

There are different possibilities to address the above set of equations which can be solved provided 2in > 3i, and provided the measured ToF information varies between measurement points. For the purpose of the present work we have taken two simplifying assumptions (a) one virtual source predominates at each measurement point, m and (b) each virtual source predominates at more than one measurement point. Note that assumption (b) ensures the condition 2m > 3i that is necessary to obtain solutions for Equations (2) and (3). These assumptions are justified by considering the defect surface as an acoustic secondary field source. At each measurement point the transducer predominantly receives signals from an... 

Defectoscope AUGUR)) is a complex device including Notebook PC with additional electronic boards, small-size automatic two-coordinate scanner, set of piezoelectric transducers, up to 25 m communication line (Fig. 1). 

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