IE45000B1 - Ultrasonic transducer - Google Patents

Abstract

1526565 Surface wave transducers TI (GROUP SERVICES) Ltd 24 June 1977 [24 June 1976] 26254/76 Heading H3U A surface wave transducer comprises a plurality of generally-parallel conductors 1 located close to the surface of a conducting body 2, so that an alternating current supply of particular frequency connected to the conductors produces surface waves in the body having twice that frequency. As shown, the conductors are part of a single strip and their total extent is not more than half the surface wavelength, see Fig. 2. Alternatively the conductors may be arranged in two groups (Fig. 4, not shown) each of extent #/2 and separated by #. In modifications the conductors may be part of a coil, wire, or printed circuit, and may surround body 2 (Fig. 5, not shown). The conductors may be curved to produce focusing of the surface waves. Two spaced transducers may be successively energized to produce in-phase waves forming a pulse CB, Fig. 8 (not shown).

Description

This invention relates to the generation of surface acoustic waves in electrically conducting bodies.

In non-conducting bodies, surface waves are usually generated by piezo-electric transducers but where, a body is electrically conducting it is known to use electromagnetic methods, relying on the Lorenz forces produced by the reaction between eddy currents induced in the surface and the body and an applied steady magnetic field. The eddy currents are induced by a coil placed close to the surface with -its axis parallel to the surface and fed with a current of the appropriate frequency.

The present invention is based on an appreciation of the fact that-an alternating current flowing In a conductor close to, and parallel to, the surface of a 1 conducting body induces eddy currents in the body and generates its own alternating magnetic field which interacts with the eddy currents to produce Lorenz forces.

-This phenomenon therefore offers the possibility of providing an electromagnetic surface acoustic wave trans20 ducer whose Operation in generating surface waves does not depend on the presence of an externally applied steady magnetic field.

According to the invention a transducer comprising a plurality of parallel conductors arranged in a common plane Is located close to the surface of an electrically conducting body so that said common plane of the parallel conductors lies substantially parallel to the surface of •‘3000 - 3 the body, and an alternating current supply of half the frequency of the required surface acoustic waves is connected to the conductors so that the current flowing therein generates surface acoustic waves in the body which propagate substantially normal to the length of the conductors, the parallel conductors occupying a width of substantially half a wavelength or less of the surface acoustic waves as measured in the direction of propagation.

The frequency of the generated surface acoustic waves is double that of the supply energising the transducer, this feature being directly attributable to the fact that the magnetic field effective in generating the waves is the alternating magnetic field produced by the energising current, rather than an externally applied steady magnetic field, this alternating magnetic field always being in phase with the energising current. As a result of this frequency doubling effect, the invention allows ready discrimination between signals corresponding to the energising current and signals corresponding to the surface acoustic waves in a detecting transducer.

Preferably the transducer comprises a coiled conductor with successive turns forming said parallel conductors.

The conductor may be a substantially flat, elongate coil lying in said common plane so that the elongate portions form said parallel conductors. All of these elongate portions may be closely grouped together within an overall width of half a wavelength, or the conductor may be coiled about a central space so that the elongate portions are grouped on either side of said space, each group lying within an overall width of half a wavelength or less and the centres of the two groups being spaced apart by substantially a whole wavelength. - 4 480 00 Alternatively, the conductor may be doiled about the electrically conducting body so that successive turns lie in a common annular plane about the body, the width of the coil measured in said plane normal to the conductors being substantially half a wavelength or less of the surface acoustic waves.

The invention will now be described by way of example with reference to the accompanying drawings in which:Figure 1 is a schematic plan view of a transducer used according to one embodiment of the invention.

Figure 2 is a section through the transducer of Figure 1, Figure 3 Is a schematic plan view of a transducer used according to a second embodiment of the invention, Figure 4-is g. section through the transducer of Figure 3, Figure 5 Is a schematic view of a transducer used according to a third embodiment of the invention, Figure 6 is a block diagram of a power source suitable for energising the transducers of Figures 1 to 5, Figure 7 is a block diagram of an arrangement for energising two transducers so as to obtain amplification of the propogated waves, and Figure 8 is a schematic diagram illustrating the amplification achieved by the arrangement of Figure 7.

The transducer illustrated in Figures 1 and 2 comprises a conductor in the form of a flat copper strip 1 that is coiled, on its edge, into an elongate coil with the elongate portions substantially straight and parallel to one another and closely spaced together so as to occupy an overall width of substantially half a wavelength or less of the surface acoustic waves that the transducer •ί as ο ο ο is to generate in a conducting body 2 (Figure 2}, In use, the coil is placed close to the surface of the body 2 so that it lies in a plane parallel with the surface, and an alternating electrical supply of the frequency of the waves to be generated, is connected to the coil.

The instantaneous current flow and magnetic field produced are shown in Figures 1 and 2, respectively, and the Lorenz forces produced by the interaction between the eddy currents induced in the body and the magnetic field are shown by the vertical arrows in Figure 2. Because the alternating current and alternating magnetic field are always in phase, the resulting Lorenz forces are always uni-directional. Thus the Lorenz forces due to the current flowing in the straight sections of the coil on both sides of its major axis reinforce one another. Furthermore, the frequency of the generated surface waves is double that of the supply energising the transducer because each half cycle of the energising supply produces the same peak amplitude effect of the Lorenz forces in the body. Theoretically, the surface waves generated should have the form of a rectified sine wave because of this, but the material hysteresis and elasticity of the conducting material smooth this wave form.

Typically, the transducer might have ten turns and be spaced at a distance of 0.5 m.m. from the surface of the body. The alternating current might have a frequency of 250 kHz and generate a surface wave of frequency 500 kHz and wavelength 6.0 m.m.

The fact that the frequency of the generated surface wave is doubled as compared with the alternating current energising the transducer, is advantageous as it allows discrimination between signals corresponding to the j ,1 3 0 0 0 - 6 energising current and generated surface wave in a transducer set up to detect the surface wave, for example, in an ultrasonic flaw detection system.

Another embodiment of the invention is illustrated in Figures 3 and 4 which is similar to that of Figures 1 and 2 except that the conductor 1 is coiled about a central space so that the elongate portions on either side of the space each form a group of parallel conductors within an overall width of substantially half a wavelength or less and the centres of the two groups are spaced apart by substantially a full wavelength. Figure 4 shows the instantaneous magnetic field produced and the Lorenz forces produced by the interaction between the eddy currents induced in the body and the magnetic field. Each of the two groups of parallel conductors has a similar, although smaller, effect to the whole transducer in Figure 2, and because they are spaced a wavelength apart the waves produced by each are in phase with those produced by the other and an amplification effect is obtained. Further, because the two parts of the coil are a wavelength apart, distortion caused by field cancellation is avoided at the centre of the coil, and field cancellation as the coil is moved away from the surface of the conducting body is decreased. Thus, this transducer is preferred for working at larger surface clearances, although on the other hand its band width is reduced because of the one wavelength spacing of its two parts.

In alternative embodiments of the invention, the conductor 1 might be made from wire Or etched on a printed circuit board.

In another embodiment of the invention, either of the transducers of Figures 1 to 4 might be modified by ίβ 0 0 0 curving the straight portions of the conductor 1 in their own plane so as to focus the generated surface waves to a point at the centre of curvature.

Another embodiment of the invention is illustrated in Figure 5 in which the transducer comprises a conductor 1 coiled about an elongate body 2 in which the surface waves are to be generated, successive turns of the coil being close to and parallel to the surface of the body and the turns occupying a width along the axis of the body equal to substantially half a wavelength or less of the waves.

Preferably, the alternating power source, used to energise the aforesaid transducer is an r.f. pulse circuit such as shown in Figure 6, it being easier to provide a pulse circuit to produce a large alternating current than to provide a source supplying a large steady current.

The illustrated r.f. pulse circuit comprises a charging circuit 3 which pharijes a capacitor 4 in a tuned circuit 5, and a thyristor 6 which is fired by a trigger circuit 7 and serves to discharge the capacitor through the transducer 1. Typically, the pulse current might be 80 amps rms.

Amplification of the generated surface waves can be achieved by providing two or more similar transducers at spaced positions along the path of propagation and energising successive transducers at successive instants of time so that the surface wave generated by one transducer reaches the next transducer as the latter is energised and generates a surface wave in phase with the firstmentioned wave.

A surface wave generation arrangement comprising two transducers 1', 1 is shown in Figure 7, in which I s 0 o 0 - 8 each transducer is connected to a respective r.f. pulse t circuit 8', 8 powered through a respective driver circuit 9', 9 and triggered by a common trigger pulse generator 10 which is connected directly to the driver circuit 91 of the first transducer 2’ in the path of propagation and via a variable delay circuit 11 to the driver circuit 9 of the second transducer 1 in the path of propagation.

Figure 8 illustrates how surface wave pulses from each transducer 1', 1” in Figure 7 reinforce one another.

At time t , the first transducer 1' is energised and generates a wave pulse that splits into two wave pulses Al and B propagating in opposite directions, as shown at the later time t£ in Figure 8. At time t^, when the pulse B reaches the second transducer 1 the latter is energised and generates a wave pulse in phase with pulse B so as to amplify the latter and produce a pulse A2 that travels in the opposite direction to pulse B, as shown at the later time t^ in Figure 8.

Claims (24)

1. CLAIMS:1. A method of generating surface acoustic waves in an electrically conducting body using a transducer comprising a plurality of parallel conductors arranged in a common plane, the transducer being located close to the surface of the body so that said common plane of the parallel conductors lies substantially parallel to the surface of the body, and an alternating current supply of half the frequency of the required surface acoustic waves being connected to the conductors so that the current flowing therein generates surface acoustic waves in the body which propagate substantially normal to the length of the conductors, the parallel conductors occupying a width of substantially half a wavelength or less of the surface acoustic waves as measured in the direction of propagation.

2. = A method as claimed in claim 1 in which the transducer comprises a coiled conductor with successive turns forming said parallel conductors.

3. A method as claimed in claim 2 in which the conductor is an elongate coil lying in said common plane with the elongate portions forming said parallel conductors.

4. A method as claimed in claim 3 in which all of said elongate portions are closely spaced together within an overall width of half a wavelength of the surface acoustic waves.

5. A method as claimed in claim 3 in which the conductor is coiled about a central space so that the elongate portions are grouped on either side of said space, each group lying within an overall width of half a wavelength or less and the centres of the two groups being spaced apart by substantially a whole wavelength. - 10 Λ® θ 0 θ

6. A method as claimed in any one of the preceding claims in which the parallel conductors are formed of strip metal arranged in a plane normal to said common plane of the conductors.

7. A method as claimed in any one of claims 3 to 6, in which said elongate portions are straight.

8. A method as claimed in any one of claims 3 to 6 in which said elongate portions are curved and have a common centre of curvature to which the surface acoustic waves are focussed. '

9. A method as claimed in claim 2 in which the conductor is coiled about the body so that successive turns lie In a common annular plane about the body, the turns lying within a width of half a wavelength of the surface acoustic waves as measured in said annular plane normal to the turns.

10. A method as claimed in any one of the preceding claims in which the alternating current supply is a pulsed alternating supply.

11. A method as claimed in claim 10 comprising providing two or more similar transducers at spaced positions along the path of propagation of the surface acoustic waves in the body, and energising each successive transducer at a successive instant of time when the surface acoustic waves reach it from the preceding transducer so that the surface acoustic waves generated by the successive transducer are in phase with those generated by the preceding transducer.

12. Means for generating surface acoustic waves in an electrically conducting body comprising a transducer having a plurality of parallel conductors arranged in a common plane and located close to the surface of the body 4 3 0 0 0 so that said common plane of the parallel conductors lies substantially parallel to the surface of the body, and an alternating current supply of half the frequency of the required surface acoustic waves connected to the conductors 5 so that the current flowing therein generates surface acoustic waves in the body which propagate substantially normal to the length of the conductors, the parallel conductors occupying a width of substantially half a wavelength or less of the surface acoustic waves as measured 10 in the direction of propagation.

13. Means as claimed in claim 12 in which the transducer comprises a coiled conductor with successive turns joining said parallel conductors.

14. Means as claimed in claim 13 in which the 15. Conductor is an elongate coil lying in said common plane with the elongate portions forming said parallel conductors.

15. Means as claimed in claim 14 in whieh all of said elongate portions are closely spaced together within 20 an overall width of half a wavelength of the surface acoustic waves.

16. Means is claimed in claim 14 in whieh the conductor is coiled about a central space so that the elongate portions are grouped on either side of said 25 space, each group lying within an overall width of half a wavelength or less and the centres of the two groups being spaced apart by substantially a whole wavelength.

17. Means as claimed in any one of claims 12 to 16 in which the parallel conductors are formed of strip 30 metal arranged in a plane normal to said common plane of the conductors.

18. Means as claimed in any one of claims 14 to 17 in which said elongate portions are straight.

19. Means as claimed in any one of claims 14 to 17 in which said elongate portions are curved and have a 5 common centre of curvature to which the surface acoustic waves are focussed.

20. Means as claimed in claim 13 in which the conductor is coiled about a central axis so that successive turns lie in a common annular plane adapted to surround 10 the body, the turns lying within a width of half a wavelength of the surface acoustic waves as measured in said annular plane normal to the turns.

21. Means as claimed in any one of the preceding claims in which the alternating current supply is a 15 pulsed alternating supply.

22. Means as claimed in claim 21 comprising two or more similar transducers at spaced positions along the path of propagation of the surface acoustic waves in the body, and means controlling energisation of each success20 ive transducer at a successive instant of time when the surface acoustic waves reach it from the preceding transducer so that the surface acoustic waves generated by the successive transducer are in phase with those generated by the preceding transducer. 25

23. A method of generating surface acoustic waves in an electrically conducting body substantially as herein described with reference to Figures 1 and 2, Figures 3 and 4 or Figure 5, either alone or in combination with Figures 6 and/or Figures 7 and 8 of the accompanying 30 drawings.

24. Means for generating surface acoustic waves in an electrically conducting body substantially as herein dSOOO - 13 described with reference to Figures 1 and 2, Figures 3 and 4 or Figure 5, either alone or in combination with Figures 6 and/or Figures 7 and 8 of the accompanying drawings. F. R. KELLY & CO.,