Surface transducer

Electrochemical immunosensors based on screen-printed electrodes have recently been applied to the detection of environmental pollutants such as PCBs, PAHs, pesticides [17-20] and of important molecules in clinical and food field [21-23]. In this case, the screen-printed electrodes are both the solid-phase for the immunoassay and the electrochemical transducers antibody or antigen molecules are directly immobilised at the sensor surface (transducer) and one of these species is enzyme-labelled in order to generate an electroactive product which can be detected at the screen-printed electrode surface. 

Whereas most fixed-cell instruments are power-compensation instruments (because it is possible to place heaters on the base of cells that are not removable), batch-cell instruments are available as either power-compensation or heat-flux designs. One design of a heat-flux, batch-cell instrument is the micro-DSC in (Setaram). The instrument consists of a calorimetric block into which two channels are machined. One channel holds the sample cell, the other holds the reference cell. At the bottom of each channel, between the cell and the block, is a plane-surfaced transducer. The transducers provide a thermal pathway between the cells and the block and are used to maintain the cells at a temperature identical to that of the block. The electrical signal produced by the transducer on the sample side is proportional to the heat evolved or absorbed by the sample. The temperature of the calorimetric block is maintained by a precisely thermostated circulating liquid. The liquid is raised in temperature by a separate heater and is cooled by a supply of circulating water. The precise control of the temperature of the circulating liquid allows scan rates of just 0.001°C min-1 to be attained and ensures that the calorimetric block is insulated from the surrounding environment. 

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. 

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... 

The transducer with orthogonal coils was presented in [1], [2], [3], It includes two coils, one parallel to the examined surface and the other perpendicular to the first one (see fig. 1)... 

Let us consider a conductive material of conductivity o in which a long, very narrow discontinuity was machined under the examined material surface The surface examination is accomplished with a transducer with orthogonal coils, the coil parallel to the inspected surface serving as emission coil, and the coil perpendicular to the surface being the reception coil. 

Figure 2 a) A deep sub-surface crack in a half-space conductor b) the utilized transducer... 

Let us consider a conductive material with the conductivity a, in which have been machined relatively small discontinuities shaped like dead hole. The surface has been inspected with a transducer with orthogonal coils connected in an impedance bridge [11]. 

One notes by V(x, y, z) the bi-dimensional Fourier transform of the voltage u x,y,z) when the transducer is scanning the inspected surface at a height z=const. 

I. Grimberg R., Plavanescu, R., Andreescu, Savin, A. 1989, Transducer for Surface Control by Eddy Currents, Romanian Patent Application, No. 996JO. 

The transducer is modelled by a distribution of particle velocity source over the radiating surface Tr as... 

Moving the transducer along the concrete surface one is able to directly compare the data from different positions simultaneously reeieving information on intensity and phases of the signal and their relative changes. A Pentium-PC, included in tlie testing-system,... 

As reported before [Ref. 1], there are some essential parameters that influence the results of the testing, such as the thickness of the expired specimen, the quality and coarse grain of the built-in concrete, and the properties of the specimen-surface for the transducer s coupling. At the onset of testings none of tlrese parameters were available. As a result, we had to carry out preliminary investigations in order to prove the applicability of our testing-technique "in situ". 

These two transducer pairs are activated alternating. For this purpose an ultrasonic instrument is combined with a two channel multiplexer. Figure 8 presents a modified standard instrument USN52 which also implies a modified software. This system performs four measurements per second - alternating the velocity and the thickness are determined. The probe can be scanned over the surface and in every position both, the velocity and the wall thickness are indicated Using the serial interface of the instrument finally a two-dimensional map of velocity or thickness can be generated. 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

Fig. 2 shows the CFRP-sandwich specimen and the transducer mounted on the scanner. Fig. 23 presents a C-scan of the specimen as first interesting result. Only the defects visible from the outside are indicated. The distance between transducer and specimen was smaller than the focal length, so that the angle of incidence at the edge of the sound beam converts the longitudinal waves to Rayleigh-waves in the specimen. These waves provide a very sharp image of the surface. This method opens the possibility for a non-contact acoustic microscope. 

In UltraSIM/UlSim the ultrasonic sound propagation from a virtual ultrasonic transducer can be simulated in ray tracing mode in any isotropic and homogeneous 3D geometry, including possible mode conversions phenomenons, etc. The CAD geometry for the simulation is a 3D NURBS surface model of the test object. It can be created in ROBCAD or imported from another 3D CAD system. 

The virtual transducer can be placed in a specific location on the test object surface, it can be moved along a path (e.g. a robot scanning path generated off-line or a path resulting from a real inspection sequence) or it can be moved along the surface, dynamically updating the ultrasonic sound propagation in the material. 

This report presents the results of investigations aimed at the creation of the surface wave transducer for the automated control. The basic attention is drawn to the analysis of the position of the front meniscus of the contact liquid when the surface waves excite through the slot gap and to the development of system for acoustic contact creation. 

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