Ultrasonic transducers

1. 1 Introduction

Transducer is a device which turns energy of 1 form compared to that of another. With reference to ultrasonic transducer the ultrasonic energy is to be converted to electric powered, mechanical, or other energy form. A reversible transducer transforms energy in both guidelines with similar efficiencies.

The transducers can be categorised as follows:

1. Piezoelectric oscillators: Basic principle of piezoelectric result is used and this is reversible. The possible consistency range is from 20 kHz to more than 10 GHz.

2. Magnetostrictive oscillators: Employs the trend of magnetostriction, a reversible form of change. Can be made to operate at mega-hertz and even gigahertz frequencies.

3. Mechanised transducers: Includes whistles and sirens (mechanised oscillators) and radiometers, and are irreversible. Mainly used for high-power applications.

4. Electromagnetic transducers: Applied for high-intensity applications at low frequencies, in the audible range. They have been used for low-intensity work at frequencies of up to 50 kHz and, also as receivers at mega­hertz frequencies.

5. Electrostatic transducers: Used as generators at low intensities with an upper rate of recurrence limit of a few hundred kilo-hertz. Reversible in alteration and used as receivers at frequencies as high as 100 MHz.

6. Miscellaneous transducers: Includes thermal, chemical type, and optical transducers.

Ultrasonic receivers are grouped into two

1. Receivers terminating acoustic beams: The cross-section of the device embraces the whole or a huge proportion of this of the beam and its dimensions prolong from several to a sizable amount of wavelengths. The occurrence of the device materially affects the construction of the acoustic field, to give surge to regular reflections of the beam.

2. Receivers operating as probes: ultrasonic probe receivers are used for mapping out acoustic domains and for measurement of local intensities. the use of probe receivers is fixed to lower frequencies (e. g. in the kilo-hertz range). , as their proportions need to be small enough, not to annoyed the characteristics of the field, ( to be significantly less than about one-tenth of the wavelength).

1. 2 Piezoelectric transducers

1. 2. 1 Basic considerations

Piezoelectric transducers use the piezoelectric effect, uncovered by the Pierre and Jacques Curie in 1880. The effect occurs naturally in certain one crystals with polar axes, (e. g. quartz, tourmaline, lithium sulphate, cadmium sulphide, and zinc oxide. )

When mechanised stress is applied to the floors of piezoelectric crystals, covered with gold or gold, equivalent and complete opposite electric charges will be induced to them and a voltage will be observed. This is the direct piezoelectric impact, and the crystalline axis perpendicular to the layered faces is the relevant polar axis. Whenever a voltage is applied across the electrodes to create an electric field, a converse impact is observed, producing a mechanical strain.

These results are associated with compressions and shears, in quartz, for example, the main polar axes are called the X- and Y axes, there may be three of each. The X-axes are focused at sides of 120 apart, and with matching Y-axis perpendicular to it. The electrodes rest at right sides to a X-axis for X-cut quartz crystals, and are associated with compressions, and Y-cut quartz crystals with shears. The Z-axis, is known as the optic axis and sits perpendicular to the planes comprising the X- and Y-axes. Optic is a non-polar axis for which the piezoelectric impact is not witnessed.

A piezoelectric transducer oscillates at the applied frequency with amplitude of the order of 10-6 times its width, on making use of an alternating voltage across its electrodes, . If, the transducer is excited at one of its resonance frequencies the amplitude is noticeably increased, e. g. to about 10-4 times the width at the essential frequency

Artificially induced piezoelectric transducers are of polycrystalline framework. They are made of large numbers of minute crystallites bonded together, to the mandatory form and size. The final product is by means of a ceramic. Ahead of polarisation, these ceramic transducers do not require to be cut with regards to any particular axis, because they are isotropic. So it is possible to have a shape in virtually any convenient form with the addition of small quantities of other materials, the transducer's properties can be upgraded or changed.

The piezoelectric effect is assessed by the d coefficient, that can be expressed in one or two ways.

(i) If the crystal is subjected to a mechanised stress, at exactly the same time, the electrodes are short-circuited by way of a wire, charges induced by the strain will stream through the cable until the potential difference across the

crystal is reduced to zero. Considering, q is the worthiness of the total charge moving and F the power producing the stress, then d coefficient can be given as

d=q/f coulombs per Newton 3. 1

(ii) When a voltage V is applied across the crystal, which no load is applied e. g. vacuum, a displacement l is produced because of the resultant stress, then volts per metre 3. 2

The electromechanical coupling coefficient is thought as

Both d and k range with heat range and reduce to zero at the Curie temperature Tc.

The occurrence response of your transducer will depend on its Q factor. In case the characteristic impedances of transducer and medium are R1 and R2, then Q can be symbolized as where K is a dimensionless constant.

Ceramic transducers have higher d coefficients and electromagnetic coupling coefficients compared to the quartz crystals. But quartz crystals are highly secure.

1. 2. 2. Coupling of Piezo electric transducers

A suitable liquid must be provided to avoid an air gap, for reliable coupling of ultrasound between the transducer and a solid. To generate longitudinal waves at normal temperatures, a film of petrol is usually enough, but, at low conditions a high-vacuum grease can be used to prevent lack of continuity of quality impedance. While working with high temperature ranges, a couplant which does not evaporate, should be chosen.

. For transverse influx propagation, it's important to utilize adhesive such as epoxy resin, to be able to ensure the couplant has enough strength to withstand the use of the shear strains without collapsing. Canada balsam or even nail varnish, on some events provides good coupling for shear waves, depending on the temperatures.

1. 2. 3 Ultrahigh occurrence (u. h. f. ) piezoelectric transducers

An early approach to producing u. h. f. ultrasonics was to place one end of the single-crystal quartz fishing rod in a electromagnetic cavity resonator Ci (see Amount ). The surface was fired up at the required occurrence, and waves were propa­gated across the rod. Initially the method was applied limited to producing ultrasound in single-crystal quartz, due to difficulty of coupling other materials to the free end of the pole. Another electromagnetic cavity resonator C2 at the other end of the fishing rod acted as a receiver. In later stages the free ends of the fishing rod and sturdy specimen was covered with thin film of indium.

1. 2. 4 Piezoelectric sandwich transducers

To generate waves at the frequencies ranging from 40 kHz down to 20 kHz. regularity, for High-intensity applications, with a piezoelectric ceramic, the width should exceed 100 mm.

A ceramic stop of this thickness is both expensive and it is highly absorbent. For this reason, assimilated acoustical energy being converted into heat, brings about a rapid increase of temps and the Curie temperatures is soon reached, with a consequent disappearance of the piezoelectric result. To avoid this sandwiching of the piezoelectric transducers can be employed.

A sandwich transducer includes a comparatively skinny piezoelectric plate located between two thicker steel plates. They have got high compressive strengths and by compressing the sandwich forever using high tensile bolt harm can be averted. (see Physique 3. 7); the transducer is reported to be mechanically biased.

1. 2. 5 Surface wave piezoelectric transducers

Surface waves can be produced by using mode alteration with a longitudinal wave transducer as the primary source, but additionally it is possible to propagate them directly. Surface waves are made by placing a typical longitudinal wave transducer in contact with the advantage of the material and inclined at an perspective of 45 (Fig 3. 4) and are received in same fashion. Another method of generating and acquiring surface waves is by layer two electrodes on the surface of your piezoelectric material and making use of the fascinating voltage at the required regularity across them (see Number 3. 5). This system was used for delay range applications

1. 2. 6 Procedure of piezoelectric transducers

A quartz crystal installed at its nodes, is a perfect one for propagating continuous waves over the narrow frequency strap. Electrical relationships must be made to the electrodes and additional damping triggered by them should be maintained minimal. Nodal mounting is not highly recommended for very skinny transducers and where contact with a good medium has to be maintained. For instances like these, the transducer is organised in position through a light planting season against a good surface. Then your sound surface provides one electrical contact with the transducer electrode and the other is provided by the spring and coil. To obtain maximum efficiency, the impedances of the enjoyable and getting electri­cal circuits should be appropriately matched to the electrical power impedance of the transducer.

For pulsed wave operation it is vital that the pulses are held sufficiently short to avoid their overlapping. No fixed waves are to be produced in the medium. To produce very short pulses and where a narrow frequency band is not needed, transducer material, such as a ceramic can be used. The transducer is backed by a stop of a materials having a very high acoustic absorption coefficient and of sufficiently large electro-mechanical conductivity to provide contact with that transducer surface. An assortment of tungsten powder and Aroldite is utilized for this function. A high direct voltage (typically from 300 V to 600 V) of instantaneous length of time is applied regularly to the transducer electrodes at the required pulse repetition occurrence. At each electric powered impulse, the transducer experience a high first strain and it oscillates over about several cycles, the amplitude lowering quickly. . Thus, for a transducer operating at a consistency of 6 MHz to create pulses each of three wavelengths, the pulse length is about only 0. 5Ој for propagation into most metals. The relationship between pulse-length (PL) in secs and the rate of recurrence bandwidth can be given as:

PL= 1. 3/ Consistency Bandwidth 3. 4

1. 3 Magnetostrictive transducers

Magnetostrictive transducers are made of ferromagnetic materials, which can simply be magnetised and exhibits magnetostriction or the Joule impact. When a club or rod of one of the materials is put in a magnetic field, it suffers a change long, either an increase or decrease, depending on mother nature of the materials and the strength of the field, immaterial of the hallmark of pressure. Hence, when the direction of magnetic field is reversed, there is absolutely no change in the sense of any risk of strain. Figure 3. 11 shows the relationship between mechanical stress and the magnitude of the field strength for a few ferromagnetic materials. The graph imples, the variation is not a linear one, generally. Nickel is found to be the most satisfactory materials for magnetostrictive transducers, having an electromechanical coupling coefficient of 31 per cent and a Curie temperatures of 358C. Permendur, an alloy, has an increased Curie point (about 900C) and low electromechanical coupling coefficient.

Though ferrites (non metals) comes with an benefit of being poor conductors rather than being heated up by eddy currents, and show magnetostrictive effect aren't often used as transducers due to their poor mechanical properties.

There is a converse magnetostrictive result, when a mechanical stress put on a ferromagnetic rod lying down in a magnetic field gives rise to an alteration in the magnetic flux density. That is known as the Villari impact.

Magnetostrictive transducers are in the varieties of rods surrounded by coil windings (see Amount 3. 7). An alternating magnetic field of the same consistency is induced by an alternating electric current through the coil ; giving climb to longitudinal oscillations of the fishing rod.

These oscillations take place at a twice the rate of recurrence of the field and undertake the proper execution of unsmooth, rectified alternating current, resulting in unwanted frequencies. As in the case of ceramic transducers. This drawback is prevented by polarisation, as with ceramic transducers. It is not possible to obtain a high polarising field by long term magnetisation, and a reliable immediate field of suited magnitude is provided by transferring a direct current through another coil wound round the transducer. So, the oscilla­tions appear about various other point instead of occurring about the foundation of the curve. In the event the amplitude of the applied alternating field is low for changes to occur along the linear portion of the curve, and, is less than the value of the polarising field, then sinusoidal oscillations happen at the applied rate of recurrence.

The resonance rate of recurrence inversely proportional with the distance of the transducer fishing rod. The regularity is increased by lowering the space, but, simultaneously, there is a intensity is decreased for a rod of given cross-sectional proportions, which results from the decrease in size of the vibrating mass. So, at frequencies more than 100 kHz, the outcome from this kind of transducer becomes vanishingly small.

The considerable leakage of magnetic flux is discovered, which really is a drawback of using rod-shaped oscillators. Transducers designed to form closed magnetic circuits are used for high-intensity applications The window-type transducer is clamped nodally, and the vibrations produced are longitudinal. In ring-type transducer, vibrations are in a radial manner, and therefore ultrasonic energy is targeted at the centre resulting in high acoustic strength.

Absorption of ultrasound by induction of Eddy currents and Hystersis results increased amount of eating. Though there are a variety of ferromagnetic materials with low hysteresis loss, their magnetostrictive properties are poor. The losses scheduled to eddy current can be reduced by using laminated stacks consisting of alternating linens of the metallic and of some insulating materials such as mica. Because the rise in heat may result in lack of magnetostrictive properties, it's important to cool the transducer during its operation.

By using speed transformer, an elevated intensity, distributed more than a smaller area, can also be obtained with both pole and windowpane types of transducers. This contains a tapered coupling fishing rod and provides a rise in the value of the particle velocity at the end remote from the transducer. For maximum efficiency, the transformer is designed to resonate by which makes it one wavelength long and assisting it at a nodal point, i. e. far away of your quarter-wavelength from the transducer. The diagram illustrates the use of the velocity transformer to the development of the ultrasonic drill

Magnetostrictive oscillators being reversible can be used as receivers. A good example of a magnetostrictive probe device involves a nickel rod performed vertically in a liquid in which ultrasound is radiated within an upward path. The fishing rod is con­tained in a cheap tube so that only the free end is exposed to the waves that happen to be then transmitted along its span. An up-to-date is induced by the Villari effect in the pick-up coil positioned near the upper end of the pole. Another coil hauling a direct current provides the polarising field. The forming of fixed waves is avoided by inserting an absorbent material near the top of the rod.

Nickel film transducers are used for producing and obtaining ultrasound of very high frequencies which range from 100 MHz to 100 GHz in solids. A slim film of nickel, of thickness corresponding to one half-wavelength at the resonant regularity, is transferred on the end-surface of the specimen into which audio is to be passed. The fishing rod is located using its plated end inside a microwave electromagnetic cavity resonator, excited at the mandatory frequency. The device may consist of a similar film coated on the contrary surface of the specimen and also located in a cavity resonator. Instead a single nickel film can become both source and receiver, using reflection method. No coupling materials is required no special technique is essential for covering the nickel film.

1. 4. Mechanical Transducers

Mechanical ultrasonic generators are being used for high-intensity propagation in liquids and gases at frequencies as high as about 25 kHz. They are present mainly in the kinds of whistles and sirens. They can be powerful and less costly than piezoelectric and magnetostrictive transducers, but with limited range of applications.

Ultrasonic whistles are of two types, the cavity resonator, used mainly for gases, and the wedge resonator, useful for both gases and fluids. .

1. 4. 1. Cavity Resonators

Galton whistle (see Figure 3. 12) involves a cylinder terminated by the end-surface of any piston which is often changed to provide resonance at the mandatory frequency The substance, flows via an annular slit at broadband and attacks the rim of the tube where vortices appear and produce edge-tones. The regularity of the edge-tones is determined by the velocity of the substance which is often adjusted before cavity resonates. For air, at a occurrence of 20 kHz, important resonance occurs for a cavity amount of roughly 4 mm.

The second type of cavity resonator is the Hartmann generator, similar in design to the Galton whistle, except that the annular slit is replaced by way of a conical nozzle (see Body 3. 13). The liquid is pressured through the nozzle and emerges at a supersonic speed to produce great shock waves, which cause the cavity to be thrilled at a high power. Resonance is achieved by adjusting the liquid velocity.

1. 4. 2. Wedge Resonator

The wedge resonator involves a rectangular dish with wedge-shaped ends, mounted on nodal holds and placed in a liquid jet stream. (Amount 3. 14). The wedge is established into flexural vibrations having an strength comparable your achieved by the Hartmann generator. Functioning frequencies are of the order of 20 kHz.

Sirens also are being used for generating high-energy ultrasound in liquids. The siren involves a rotor disc with lots of identical openings spaced evenly round the circumference of your circle just a little smaller than the disk. The rotor changes concentrically in front of a similar disk (the stator), which is retained at slumber whilst liquid jets are aimed through the openings. The frequency of the emitted ultrasound is equal to the regularity of interruption of the jet movement, as the holes move relatively one to the other, and is computed as the product of the amount of holes in the rotor and the acceleration of revolution. The firmness emitted by the siren is not really a 100 % pure one but this is unimportant for the applications for which it is used. One good thing about this instrument is the fact by changing the speed of rotation the consistency can be mixed in a continuous manner.

The use of mechanical receivers has been limited to measurements of intensities in fluids and gases. Both principal types of mechanical receivers are the Rayleigh disc and the radiometer.

The Rayleigh disk includes a thin round disk suspended vertically in the ultrasonic field by means of a torsion fibre. First the disc is positioned, with its airplane areas parallel with the path of propagation. Inside the occurrence of ultrasound, the sound waves exert a few on the disc, which rotates until taken to rest in a steady position as a result associated with an opposing few exerted by the suspension. The position of rotation necessary to reach the point out of equilibrium depends on the the acoustic power.

A radiometer is a tool which measures straight the pressure of radiation, a amount which is proportional to the acoustic strength. The easiest form of radiometer is a tiny solid sphere suspended in the sound field. It really is deflected horizontally in the direction of propagation when the ultrasound exists. The device is calibrated by subjecting it to known substance stresses and then measuring the effect­ing displacements.

The torsion balance radiometer is created for waves going in a horizontal path and the common balance type for vertically aimed waves(Fig 3. 15 a and Fig 3. 15b)

1. 5 Electromagentic Transducers

A light-weight electromagnetic transducers have been used for low-intensity ultrasonic strategy­ments in poorly doing solids and fluids. However the method requires continuous application of a steady magnetic field m which really is a major disadvantage

1. 5. 1. Giacomini's method:

A pub of poorly performing solid is coated with a slender conducting remove of negligible mass over opposite halves of top of the and lower surfaces and the end-face. It is reinforced horizontally at the nodal positions by electrically doing cables, and the layered end is put through a horizontal magnetic field at right angles to the axis. When an alternating electric current is transferred through the executing strip, the club vibrates longitudinally, in accordance with Fleming's left-hand guideline of electromagnetism.

Because electromagnetic transducers are reversible, vibrations in the club are found by the performing remove which, in the presence of a steady magnetic field, will have induced in it an alternating e. m. f. relative to Fleming's right-hand guideline of electromagnetism. This e. m. f. is related to the acoustic power. Thus the device can be utilized as both a transmitter and a device of ultrasound.

1. 5. 2. Filipczynski's Method:

An aluminium film by means of a continuing and winding narrow remove is evaporated to a perspex block to give a coil of negligible mass. The block is then immersed in the liquid and located inside a gap between the pole-pieces of any long lasting magnet which supplies a dependable magnetic field of high strength. Ultrasonic waves pass from the water into the stop, giving surge to oscillations of the aluminium coil which induce in it an e. m. f. related to the power in the block.

1. 6 Electrostatic transducers

An electrostatic transducer is composed essentially of two parallel plates of any conducting material put close to each other to form a power capacitor. One dish is set and the other is absolve to vibrate in a course at right perspectives to the top of plates. A higher resistance is positioned in series with the capacitor and stable charges on the plates managed by a direct potential difference of several hundred volts (Fig 3. 18).

For operation as a transmitter, a signal at the required frequency, is fed to the plates, outcome voltage of amplitude not exceeding the immediate potential difference. The periodic variant of the charges induces vibrations of the movable dish.

For use as a recipient, the movable plate is placed ready to get the sound waves and its own consequent vibrations give rise to periodic variations of the electric capacitance of the transducer, producing an alternating current which flows through the high amount of resistance; the resulting alternating voltage proportional the depth of the received audio.

The electrostatic transducer in the form of the condenser microphone has long been used at audible frequencies. Diaphragm being light, inertial effects are negligible and the sensitivity remains frequent over a broad frequency range. It could be used for gases and liquids as both a device and a trans­mitter at frequencies as high as about 300 kHz.

1. 7 Miscellaneous Transducer

Other ways of generating and getting ultrasound involve the uses of thermal, chemical substance, and optical devices. The chemical substance changes seen in materials irradiated with ultrasound, can be used as a means of detec­tion. Additionally it is possible to generate ultrasonic waves in a clear medium by the crossing of two laser beams from a common source.

There are lots of applications which use thermal transducers. One thermal type of transmitter is the spark-gap generator, which radiates ultrasound because of this of periodic tempera­ture changes taking place whenever a high alternating voltage of confirmed regularity is discharged across a distance in a circuit.

The hot-wire microphone, is a receiving thermal transducer, comprising a thin line, created from platinum and heated to just underneath redness. When sensible waves punch the line, it cools down by an amount directly dependent on the intensity. That is indicated by way of a decrease in its electrical resistance. The hot-wire microphone has been used efficiently for gases at frequencies of up to 600 kHz.

Ultrasonic intensities can even be measured from the rise in tempera­ture within the beam, as shown in Figure 3. 19. Heat produced by the ultrasound is absorbed by the water in the thermally covered flask. Enlargement of the liquid ends up with a growth in the amount of the liquid in the graduated capillary pipe, calibrated by offering a measured amount of warmth from the home heating coil. The waves transmitted through the water are finally soaked up by the wine glass wool placed by the end of the vessel. Acoustic capabilities of from 50 mW to 30 W can be assessed to an correctness of better than 10 % with this device.

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