GB2142431A - Improvements relating to electro-mechanical transducers - Google Patents

Abstract

An electro-mechanical transducer assembly for launching/detecting acoustic waves in a liquid in a pipe section (1), with wavelength lambda in the range 3 to 6 times the pipe section diameter, comprises a conducting region of the pipe section (1) of a length about lambda /3 with portions (6) forming an approximation to a polygon, such that it has a resonance mode with the whole circumference of the pipe oscillating radially in phase at the operating frequency. For launching waves, alternating current in the fixed coil (2) induces a corresponding current in the surface of the pipe section (1) which, in conjunction with the magnetic field generated by the magnet system (4,5) generates a radial force that is effectively transferred to the liquid. When used as a detector of waves, the pipe section walls, driven by wave pressure, oscillate radially in the magnetic field generated by the magnet system (4,5) and so generate an electrical current in the pipe wall that induces an alternating emf in the fixed coil (2). For fluid flow measurement two such transducers are coupled in a regenerative loop. <IMAGE>

Description

SPECIFICATION Improvements relating to electro-mechanical transducers Measurement of fluid flow in a pipe by observation of the change in the velocity of sound waves in a moving fluid is a well established technique. In many current ultrasonic flowmeters the acoustic system is designed to minimise the acoustic effects of the containing pipe by the use of relatively short wavelength sound to permit a well defined beam of sound between the transmitter and receivertrans- ducers. This is generally satisfactory where the pipe diameter is large. However there is increasing recognition that where the pipe diameter is small relatively large wavelength sound is preferable to permit an adequate acoustic path length.In this context by relatively large I mean that the wavelength is at least three times greater than the pipe diameter and particularly is three to six times the pipe diameter. For practicable values of pipe wall thickness the effect of the pipe on the velocity of long wave sound is small and changes if any occur slowly. Thus the advantage of relatively large wavelengths is that long waves are guided by the pipe and so permit a long acoustic path length.

Furthermore long acoustic waves in the fluid are not responsive to local variations in flow velocity so that their transit time represents the mean flow velocity.

Various methods and apparatus have been proposed in the past for launching and detecting acoustic waves in a fluid in a pipe and the most popular have been methods which use the piezoelectric effect or magnetostrictive materials. The problem with using these materials for relatively long wavelength signals is that a large quantity of material is required.

In accordance with one aspect of the present invention a method for launching relatively long wavelength acoustic waves in a fluid in a pipe section comprises inducing an alternating current at an operating frequency in an electrically conducting region of the pipe section, the pipe section wall thickness being greater than the electro magnetic skin depth at the operating frequency; and providing a magnetic field normal to the current, the cross sectional shape of the pipe section being such that the pipe wall resonates in a radial direction at the operating frequency.

This method relies on the fact that a conductor carrying an alternating current which flows at right angles to an applied magnetic field will experience an alternating force normal to both the current direction and direction of the magnetic field. In a cylindrical pipe the natural oscillation frequency of the pipe walls will be considerably higher than that required for propagating sound waves of a required wavelength. To deal with this, the cross-sectional shape of the pipe section is altered, for example into a substantially square form. For example, a typical operating frequency is 50 KHz while a typical pipe having a 5 mm internal diameter and a circumference of 1.6 cms will have a resonant frequency of about 370 KHz. The lower resonant frequency is achieved by suitably shaping the pipe cross-section in the electrically conductive region.

In accordance with a second aspect of the present invention, a transducer assembly comprises a pipe section for containing a fluid; and a transducer for launching or detecting relatively long wavelength acoustic waves in the fluid in the pipe section, the pipe section having an electrically conductive region, and the transducer comprising one or more electrically conductive coils positioned close to but acoustically isolated from the electrically conductive region of the pipe section, and a magnet system arranged to generate a magnetic field in the electrically conductive region of the pipe section parallel to the wall of the pipe section and normal to the direction of the coil windings, the arrangement being such that at the operating frequency the thickness of the wall of the pipe section is less than the electromagnetic skin depth and the crosssectional shape of the pipe section is such that the pipe section has a predominantly radial mechanical resonance.

As will be áppreciated, the transducer assembly can be used both for launching acoustic waves into the fluid and for detecting acoustic waves already present in the fluid. To launch acoustic waves the transducer assembly will operate in accordance with the method desrjbed above in accordance with the first aspect of the invention. When acoustic waves are already present in the fluid, these will cause radial movement of the electrically conductive region of the pipe section which will interact with the magnetic field to induce a current in the or each electrical coil which can be sensed.

One of the main advantages of this invention is that a section of pipe in situ can provide the electrically conductive region and the transducer can be positioned around the pipe section. Alternatively, the transducer assembly including the pipe section can be provided as a unit which is inserted into another length of pipe.

Typically, the pipe section will be made from a metal such as copper but in some cases the electrically conductive region may be formed by a radially outer portion of the pipe wall while a radially inner portion may comprise a different material for example to provide a chemically inert surface adjacent the fluid. An example of such a material is stainless steel.

If the skin depth region is ferro magnetic as well as conducting it is possible to gain some advantage by reducing the size of the magnets of the magnet system.

In one example, an electrically conductive coil is mounted circumferentially about the pipe section, the magnet system generating, in use, a magnetic field extending in a direction substantially parallel with the axis of the pipe section. This provides a reasonably simple construction but may require disassembly of a pipe in order to mount the transducer assembly.

In some cases, a plurality of conductive coils may be mounted about the pipe section, adjacent coils having opposite winding directions and the magnet system providing magnetic fields associated with each coil, the polarising field alternating in sign in correspondence with the coils. In this way, the driving force can be made to spread over as great an axial distance as required and it is quite possible to make the driving force alternate at any axial pitch desired such as 2 cms.

In another example, an active region of the or each coil is provided in a direction substantially parallel with the axis of the pipe section, and the magnet system is arranged to generate a magnetic field extending in the circmferential direction. This arrangement does not require disassembly of a pipe length and thus does not require a length of pipe to be drained prior to mounting the assembly.

As has been previously mentioned, the method and apparatus in accordance with the invention is particularly useful in a fluid flow measuring system.

Some examples of methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which Figure 7A is a iongitudinal section through a first example of a transducer assembly; Figure 1B is a section taken on the line 1-1 in Figure 1A; Figure 1C is a section taken on the line 2-2 in Figure 1A; Figure2A is a plan of a second example of a transducer assembly; Figure 2B is a section taken on the line 3-3 in Figure 2A; Figure 3 is a view similar to Figure 2A but of a third example: and, Figures 4 and 5 illustrate schematically two geometrical tuning techniques.

The electro-mechanical transducer assembly illustrated in Figures 1A-1 C comprises a copper pipe section 1 through which a liquid flows in use. The pipe section 1 has an internal diameter of 5 mm and a circumference of 1.6 cm. Around the circmference of the pipe section 1 is mounted an electrical coil 2 having a number of windings connected to a source of alternating current (not shown). The electrical coil 2 is bonded to a relatively rigid frame 3 which has a mechanical resonant frequency remote from the operating frequency of the assembly. A magnet system having four shaped North pole pieces 4 and four correspondingly shaped South pole pieces 5 is arranged about the coil 2 so that a magnetic field extends between the pole pieces in a direction parallel with the axis of the pipe section 1.

The pipe section 1 itself has been shaped using conventional forming tools into a substantially four sided shape with curved corners as may be seen in Figures 1 B and 1 C. This is because the natural resonant frequency of the original (circular) pipe was about 370 kHz whereas an operating frequency of 50 kHz is required. The length of the pipe section 1 which has been so shaped will typically be ;n the order of one third of the wavelength to be propagated through the fluid. The coil 2 is shorter than this but several coils with alternating winding direction in conjunction with magnets of opposite polarities could also be used. It will be seen in Figure 1C that each pair of pole pieces 4, 5 is associated with a respective side of the pipe section 1.

In use, an alternating electrical current is passed through the coil 3 having a frequency in the order 50 KHz. This current induces an "image" current in the adjacent portions 6 of the pipe section 1 and this image current interacts with the four magnetic fields so that the portions 6 are caused to flex together alternately radially inwardly and radially outwardly thus imparting acoustic waves into the fluid contained in the pipe section. An equal and opposite alternating radial force is generated on the coil 2 so that the reaction to the driving force on the walls of the pipe section 1 is taken by the fixed coil 2 and not the magnet system. The transducer induces two wave systems in the fluid, one in each direction along the pipe section 1 whose pressure waves sum to the electromagnetic force but whose axial particle velocity waves cancel in the transducer region.

The thickness of the wall of the pipe section 1 is chosen so that at the operating frequency of the alternating current the resulting alternating magnetic field generated bsoil 2 cannot penetrate through the wall oft he pipe section 1 due to the skin effect and the image current only flows in the outer surface of the pipe section 1.

It will be appreciated that the system for launching acoustic waves can be completely reversed so that the transducer assembly also acts as an efficient detector of waves. When the transducer assembly is used as a detector, acoustic waves in the pipe section 1 in the frequency band to which the transducer assembly is tuned cause the portions 6 of the pipe section 1 to oscillate radially. Since the wall of the pipe section 1 is thick compared with the electromagnetic skin depth an electric current isinduced in the pipe surface which in turn induces an alternating emf in the fixed coil 2. This can then be detected by conventional detection equipment.

The example shown in Figures 2A and 2B differs from that shown in Figure 1 in that the magnet system comprises two pairs of shaped pole pieces 7, 8 providing North and South poles positioned so that magnetic flux extends around the pipe section 1 in a circumferential direction. In this example, a pair of coils 9 are positioned so that their active regions 10 extend in a direction parallel with the axis of the pipe section 1, each active region being adjacent a respective side of the pipe section. The coils 9 are mounted on relatively rigid frames 11. In this example, an electrical current induced in the wall of the pipe section 1 by the active regions 10 of the coils 9 flow in the axial direction but will be at right angles to the direction of the adjacent magnetic field and again this will cause radial movement of the wall of the pipe section 1.

The examples shown in Figures 1 and 2 represent arrangements of the assembly at two extremes and various intermediate arrangements could be used providing the induced current is normal to the adjacent magnetic field. An example of such an intermediate arrangement is shown in Figure 3.

Since the wall of the pipe section 1 is normal to the magnetic field then regardless of the position of the coil 9 the induced electrical current will always be at right angles to the magnetic field and thus radial movement of the wall of the pipe section 1 will still occur.

As has previously been explained, the pipe section 1 will have a radial resonant frequency which can be varied by suitably shaping the wa;l of the pipe section. The transducer assembly is efficient over a narrow band of frequencies in the vicinity of the radial resonant frequency of the transducer pipe section 1. To select from the finite band available an operating frequency defined in terms of the wavelength in the fluid it is possible to use an array of transducers 12 positioned along the pipe section and arranged to cause alternating polarity of pipe wall motion (as seen in Figure 4). Alternate transducers 12 are spaced apart by a distance corresponding to the wavelength x being propagated and in use they effectively force the generation of a wavelength determined by their spacing.

Alternatively, or additionally as shown in Figure 5, an outer sleeve 13 may be provided around the pipe section 1, the sleeve 13 having a fluid port 14. A regular array of apertures 15 are provided in a portion of the pipe section 1 within the sleeve 13 the end 16 of which is closed to reflect acoustic waves impinging on the end. The holes 15 are positioned in relation to the transducers 12 and the end of the pipe section 16 so that they dissipate unwanted wavelengths but not the desired wavelength X. In addition, the holes 15 provide fluid communication between the sleeve 13 and the pipe section 1.

Any of the examples described could be used in a conventional fluid flow measuring system. For example, a pair of transducer assemblies can be positioned a suitable distance apart along the pipe line, typically 100 wavelengths, and can be used together with established measuring techniques to measure the net velocity of sound in each direction between the-transducer assemblies by measuring the phase change between transmitted and received continuous wave signals of a well defined frequency.

Alternatively, the two transducer assemblies can be used in a regenerative loop to form an oscillator whose frequency is determined by the acoustic time delay between the transducer assemblies. Measurement of this frequency with respect to a well defined clock then permits great precision to be achieved combined with valuable simplicity.

Claims (3)

1. A method for launching relatively long wavelength acoustic waves in a fluid in a pipe section, the method comprising inducing an alternating current at an operating frequency in an electrically conducting region of the pipe section, the pipe section wall thickness being greater than the electro magnetic skin depth at the operating frequency; and providing a magnetic field normal to the current, the cross sectional shape of the pipe section being such that the pipe wall resonates in a radial direction at the operating frequency.

2. A method for launching relatively long wavelength acoustic waves in a fluid in a pipe section substantially as hereinbefore described with reference to the accompanying drawings.

3. Atransducer assembly comprising a pipe section for containing a fluid; and a transducer for launching or detecting relatively long wavelength acoustic waves in the fluid in the pipe section, the pipe section having an electrically conductive region, and the transducer comprising one or more c: :lectrically conductive coils positioned close to but acoustically isolated from the electrically conductive region of the pipe section, and a magnet system arranged to generate a magnetic field in the electrically conductive region of the pipe section parallel to the wall of the pipe section and normal to the direction of the coil windings, the arrangement being such that at the operating frequency the thickness of the wall of the pipe section is greater than the electromagnetic skin depth and the crosssectional shape of the pipe section is such that the pipe section has a predominantly radial mechanical resonance.

3. A transducer assembly comprising a pipe section for containing a fluid; and a transducer for launching or detecting relatively long wavelength acoustic waves in the fluid in the pipe section, the pipe section having an electrically conductive region, and the transducer comprising one or more electrically conductive coils positioned close to but acoustically isolated from the electrically conductive region of the pipe section, and a magnet system arranged to generate a magnetic field in the electrically conductive region of the pipe section parallel to the wall of the pipe section and normal to the direction of the coil windings, the arrangement being such that at the operating frequency the thickness of the wall of the pipe section is less than the electromagnetic skin depth and the crosssectional shape of the pipe section is such that the pipe section has a predominantly radial mechanical resonance.

4. An assembly according to claim 3, wherein the cross-sectional shape of the pipe section lies between a circle and a polygon.

5. An assembly according to claim 3 or claim 4, wherein the electrically conductive region is formed by a radially outer portion of the pipe wall.

6. An assembly according to any of claims 3 to 5, wherein an electrically conductive coil is mounted circumferentially about the pipe section, the magnet system generating, in use, a magnetic field extending in a direction substantially parallel with the axis of the pipe section.

7. An assembly according to any of claims 3 to 5, wherein an active region of the or each coil is provided in a direction substantially parallel with the axis of the pipe section, and the magnet system is arranged to generate a magnetic field extending in the circumferential direction.

8. Atransducer assembly substantially as hereinbefore described with reference to any of the accompanying drawings.

9. Atransducer system comprising a plurality of transducer assemblies according to any of claims 3 to 8, the transducer assemblies being positioned along a pipe line at positions spaced apart by a distance substantially equal to half a wavelength to be transmitted, adjacent assemblies being arranged to cause alternating polarity of pipe wall motion.

10. A fluid flow measuring system incorporating a pair of transducer assemblies according to any of claims 3 to 8, the assemblies being connected in a regenerative loop whose oscillation frequency is inversely proportional to the time delay between the transducers.

Amendments to the claims have been filed, and have the following effect: New or textually amended claims have been filed as follows: