WO1989005199A1

WO1989005199A1 - An acoustic emission transducer and an electrical oscillator

Info

Publication number: WO1989005199A1
Application number: PCT/GB1988/001007
Authority: WO
Inventors: Trevor John Holroyd; Michael Sadler
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1987-12-05
Filing date: 1988-11-17
Publication date: 1989-06-15
Anticipated expiration: 1990-06-05
Also published as: GB8728509D0; WO1989005445A1; ES2010038A6; CN1033503A; AU2726288A; AU2781689A; CN1033729A; ES2010039A6

Abstract

A problem with prior art acoustic emission transducers is that for operation at lower frequencies, i.e. for resonance at 40-60KHz, the size of the piezoceramic element has to be relatively large to obtain a resonance of the acoustic emission waves within the piezoceramic element. This requires the use of relatively expensive and difficult or complex manufacturing processes. An acoustic emission transducer assembly (40) as provided by the invention can be used at the relatively low frequencies desired, is relatively small and is cheap and easy to manufacture. The acoustic emission transducer assembly (40) comprises a brass base disc (42), a piezoceramic disc (44) and a brass case (46). The base disc (42) and the piezoceramic disc (44) are arranged coaxially, and the annular peripheral region (54) only of the piezoceramic disc (44) is mounted on an annular projection (48) which extends axially from the base disc (42). The annular projection (48) is provided with apertures (52) to prevent compressions in a chamber (56) damping vibrations of the piezoceramic disc (44). An electrical oscillator assembly is also provided by the same construction.

Description

AN ACOUSTIC EMISSION TRANSDUCER AND AN ELECTRICAL OSCILLATOR The present invention relates to an acoustic emission transducer assembly, and to an electrical oscillator.

A prior art acoustic emission transducer comprises a piezoceramic element which is mounted on a baseplate. The piezoceramic element is enclosed by a casing which is secured to the baseplate.

A problem associated with .this type of acoustic emission transducer is that for operation at lower frequencies, for example an acoustic emission transducer with a resonance at 40-60 KHz, the size of the piezoceramic element has to be relatively large to obtain a resonance of the acoustic emission waves within the piezoceramic element at the desired frequency. This would require the use of a single relatively large piezoceramic element which will be.relatively difficult to manufacture^', or the use of a plurality of piezoceramic elements bonded together. Both these methods would require greater expense and would require a more difficult or more complex manufacturing process. The present invention seeks to provide an acoustic emission transducer assembly for use at relatively low frequency operation which is relatively small and is relatively inexpensive and easier to manufacture.

Accordingly the present invention provides an acoustic emission transducer assembly comprising a piezoceramic member mounted on a base member, the base member being adapted for acoustic coupling to a surface of a component, the piezoceramic member having a peripheral region, the piezoceramic member being mounted on the base member by the whole of the peripheral region only whereby the piezoceramic member is caused to vibrate as a whole while remaining fixed to the base member.

The piezoceramic member may be a piezoceramic disc. The base member may be a base disc, the piezoceramic disc and the base disc may be arranged coaxially. The base disc may have an annular projection extending axially from a first surface of the base disc, the piezoceramic disc being mounted on the annular projection by the annular peripheral region. The base member may have a depression formed on a first surface of the base member, the piezoceramic member being aligned with and arranged to lie over the depression and the peripheral region of the piezoceramic member being mounted on the first surface. At least one aperture may interconnect a chamber formed between a first surface of the piezoceramic disc and the base disc with the pressure acting on a second surface of the piezoceramic disc to at least reduce compressions of the air in the chamber. The at least one aperture may extend through the annular projection.

The" at least one aperture may be formed by a groove on the annular projection and the piezoceramic disc.

The base member may have an aperture extending therethrough, the piezoceramic member being aligned with and arranged to lie over the aperture and the peripheral region of the piezoceramic member being mounted on the first surface.

The base member may be formed from an electrically conducting material or an electrically insulating material.

The electrically insulating material may be a ceramic.

A second surface of the base member may have an electrical insulation layer. The electrical insulation layer may comprise glass fibres in an epoxy resin.

The piezoceramic member may be mounted on the base member by an adhesive. The adhesive may form an electrical contact between the piezoceramic member and the base member. The adhesive may comprise a silver loaded epoxy resin. The piezoceramic member may be formed from lead zirconate titanate.

The base member may be formed from brass. A case may be secured to the base member to enclose the piezoceramic member to give mechanical and electro¬ magnetic protection. The case may be formed from brass, stainless steel or aluminium.

The present invention also seeks to provide an electrical oscillator which is relatively small and is relatively inexpensive and easier to manufacture.

The present invention also provides an electrical oscillator assembly comprising a piezoceramic member mounted on a base member, the piezoceramic member having a peripheral region, the piezoceramic member being mounted on the base member by the whole of the peripheral region only whereby the piezoceramic member is allowed to vibrate as a whole while" remaining fixed to the base member.

The present invention will be more fully described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a cross-sectional view through a prior art acoustic emission transducer assembly.

Figure 2 is a cross-sectional view through an acoustic emission transducer assembly according to the ^~ present invention.

Figure 3 is a graph of voltage versus frequency response for the acoustic emission transducer assembly according to the present invention.

Figure 4A is a graph of voltage versus frequency response for a prior art type acoustic emission transducer assembly.

Figures 4B to 4D show graphs of voltage versus frequency response for three acoustic emission transducer assemblies according to the present invention. Figure 5 is a cross-sectional view through a second embodiment of an acoustic emission transducer assembly according to the present invention. Figure 6 is a view in the direction of arrow D in Figure 5.

Figure 7 is a cross-sectional view through a third embodiment of an acoustic emission transducer assembly according to the present invention.

Figure 8 is a perspective view of an electronic circuit board with an acoustic emission transducer according to the present invention.

A prior art acoustic emission transducer assembly 10 is shown in Figure 1, and comprises a baseplate 12, a piezoceramic element 14 and a case 16. The piezoceramic element 14 is mounted on a first surface of the baseplate 12 by an adhesive, glue or cement. The piezoceramic element 14 has an electrically conducting coating or layer 22 on its surface remote from the baseplate 12, and an electrically conducting coating or layer 20 on its surface adjacent the baseplate 12. The side surfaces of the piezoceramic element 14 are provided with an electrical insulating coating or layer 24. The whole of the surface of the piezoceramic element 14 adjacent the baseplate 12 is bonded to the baseplate 12. The case 16 is secured to the baseplate 12, and encloses the piezoceramic element 14 to provide mechanical and electromagnetic protection, the case 16 is secured to the baseplate 12 for example by a screwthread arrangement or other suitable means.

The electrically conducting coating 20 on the piezoceramic element 14 adjacent the baseplate 12 is electrically connected to the case 16 to form an electrical earth connection. The electrically conducting coating 22 on the piezoceramic element 14 remote from the baseplate 12 is electrically connected to an amplifier

(not shown) via a signal lead 28 which is electrically connected to the conducting coating 22 by a joint 26.. The signal lead 28 forms a part of a coaxial cable 30. The baseplate 12 as shown in the embodiment is brass as this provides a good acoustic impeldance match with the piezoceramic element 14. An electrical insulator 18 is provided on a second surface of the baseplate 12, and the insulator 18 may be a ceramic or epoxy resin coating. The case 16 may be aluminium, stainless steel or other conducting material. The baseplate may be an insulating material, i.e. a ceramic, in which case an insulator will not be required, but an electrical connection between the conducting coating 20 and the case 16 is required.

In operation the acoustic emission transducer assembly 10 is placed on a surface of a component for sensing the acoustic emission waves, secondary acoustic emission waves or stress waves, within the component. Acoustic emission waves, secondary acoustic emission waves or stress waves, generated in the component are transmitted through the baseplate 12 into the piezoceramic element 14. The acoustic emission waves cause oscillations or vibrations within the piezoceramic element 14 which produce electrical signals by the piezoelectric or ferroelectric effect. The electric signals are then amplified and processed to give details of the acoustic emission activity of the component.

A problem with this type of acoustic emission transducer assembly is that if the piezoceramic element is designed to have a resonance in the range of 40-60KHz, for relatively low frequency operation, the size of the piezoceramic element must increase to obtain a resonance of the acoustic emission waves within the piezoceramic element at the desired frequency. As an example for resonance at 60KHz the piezoceramic element has a thickness of approximately 30 mm. As discussed previously it is relatively difficult to prepolarise relatively large piezoceramic elements, and the use of a plurality of piezoceramic elements bonded together is more complex.

Furthermore the resulting transducer inevitably has relatively large physical dimensions which can be a hinderance to use.

An acoustic emission transducer assembly 40 according to the present invention is shown in Figure 2, and comprises a base disc 42, a piezoceramic disc 44 and a case 46. The piezoceramic disc 44 is mounted on the base disc 42, and the base disc 42 and the piezoceramic disc 44 are arranged coaxially. The base disc 42 has an annular projection 48 extending axially from a first surface of the base disc 42, and an electrical insulator 50 is provided on a second surface of the base disc 42. The electrical insulator 50 comprises a thin layer of glass fibres coated in epoxy resin. The piezoceramic disc 44 has an annular peripheral region 54 on a first surface 45, and the piezoceramic disc 44 is mounted on the annular projection 48 of the base disc 42 by the annular peripheral region 54 only. The piezoceramic disc 44 is bonded on the base disc 42 by an adhesive, glue or a cement which forms an electrical contact between the piezoceramic disc 44 and the base disc 42, for example a silver loaded epoxy resin or other suitable means.

A chamber 56 is formed between the first surface 45 of the piezoceramic disc 44 and the base disc 42, and the annular projection 48 is provided with at least one vent aperture 52 which extends therethrough to interconnect the chamber 56 with the pressure acting on a second surface 47 of the piezoceramic disc 44 to equalise the pressures acting on the first and second surfaces 45,47 to prevent compressions of the air in the chamber 56 damping vibrations of the piezoceramic disc 44. A notch or groove could be provided on the rim of the annular projection 48 to form a vent aperture with the piezoceramic disc 44. The case 46 is secured to the base disc 42, and encloses the piezoceramic disc 44, the case 46 is secured to the base disc 42 for example by a screwthread arrangement or other suitable means, to provide mechanical and electromagnetic protection for the piezoceramic disc 44.

The piezoceramic disc 44 has an electrically conducting coating or layer 58 on its second surface 47 and an electrically conducting coating or layer 60 on its first surface 45. The coating 60 on the piezoceramic disc 44 is electrically connected to the case 46 in the example by the glue and the base disc 42 to provide an electrical earth connection.

A signal lead 62 is electrically connected to the conducting coating 58 on the second surface 47 of the piezoceramic disc 44 by a solder joint 64, and the signal lead 62 is electrically connected to an amplifier (not shown) via a coaxial cable 66. The solder joint 64 is made as close to the edge of the piezoceramic disc 44 as is possible, ^" so that the solder joint does not damp vibrations of the piezoceramic disc 44.

The base disc 42 is preferably brass to provide a good acoustic impedance match between the base disc and the piezoceramic disc, but other suitable materials could be used, for example epoxies or ceramics. The case 46, as mentioned, provides mechanical and electromagnetic protection for the piezoceramic disc 44, and the case may comprise any suitable material which has these qualities, for example brass, aluminium or stainless steel.

The piezoceramic disc 44 is preferably a lead zirconate titanate polycrystal, although other suitable piezoelectric, ferroelectric, electrostrictive or electroacoustic polycrystals or monocrystals may be used.

In operation the acoustic emission transducer assembly 40 is placed on a "surface of a component for sensing the acoustic emission waves, secondary acoustic emission waves or stress waves, within the component. The acoustic emission waves, secondary acoustic emission waves or stress waves generated in the component are transmitted through the brass base disc 42 into the piezoceramic disc 44. The acoustic emission waves cause the piezoceramic disc 44, it is believed, to oscillate, resonate or vibrate as a whole by flexing of the piezoceramic disc 44 while the annular peripheral region 54 of the piezoceramic disc 44 remains fixed to the base disc 42, and the piezoceramic disc 44 has resonant frequencies.

The fundamental resonant frequency of the acoustic emission sensor assembly 40 is dependent upon the diameter, the thickness and the mechanical properties of the piezoceramic disc 44. The desired fundamental resonant frequency can be selected, for any particular piezoceramic material, by varying the diameter of the piezoceramic disc, by varying the area of the annular mounting at the peripheral region of the piezoceramic disc, by varying the thickness of the piezoceramic disc or by a combination of the three.

Figure 3 shows the voltage output against frequency for the acoustic emission transducer assembly 40 in response to the detection of the breaking of a pencil lead at a remote location on a solid metal surface. The fundamental resonant frequency is denoted by spike A on the graph, and this corresponds in this example to a frequency of 41.5KHZ. A second resonant frequency is denoted by spike B on the graph, this corresponds to 119.6KHZ, and a third resonant frequency is denoted by spike C on the graph and this corresponds to 451.7 KHz.

An acoustic emission transducer assembly which has been tested which had a fundamental resonance frequency of 41.5KHZ^* had an external diameter of 10 mm and a thickness of 5 mm. The piezoceramic disc had a diameter of 5 mm and a thickness of 1/4 mm and the annular peripheral region of the piezoceramic disc which is mounted on the annular projection is 1/4 mm wide. Figures 4B to 4D show the voltage output against frequency for three acoustic emission transducer assemblies according to the present invention in response to the detection of the breaking of a pencil lead at a remote location on a solid metal surface. Figure 4B is for an acoustic emission transducer in which the piezoceramic disc had a diameter of 5mm, a thickness of 1/4 mm and the annular peripheral region of the piezoceramic disc which is mounted on the annular projection was 1/4 mm wide. Figures 4C and 4D also have a piezoceramic disc which has a diameter of 5mm and a thickness of 1/4 mm, but the annular peripheral region of the piezoceramic disc which is mounted on the annular projection was 3/8 mm and 1/2 mm respectively. From Figure 4B to 4D it can be seen that the variation of the area of mounting of the piezoceramic disc at its peripheral region varies the frequency of the fundamental resonant frequency i.e. increasing the area of the mounting increases the fundamental resonant frequency.

Figure 4A shows the voltage output against frequency for an acoustic emission transducer assembly of the prior art type in response to the detection of the breaking of a pencil lead at a remote location on a solid metal surface. The acoustic emission transducer assembly comprised a piezoceramic disc having a diameter of 5mm, a thickness of l/4mm, but the whole of the piezoceramic disc was mounted on the baseplate. It can be seen in Figure 4A that a fundamental resonant frequency at the relatively low frequencies i.e. 40 -60kHz is not obtained.

The acoustic emission transducer assembly 40 and the piezoceramic disc 44 are preferably operated at the fundamental resonant. frequency of the piezoceramic disc 44 although^' it is possible to operate at other frequencies for example the second or third resonant frequencies.

The acoustic emission transducer assembly as described is of relatively small size, and low profile compared to the prior art, is relatively easy and cheap to manufacture and allows use at relatively low operational frequencies.

Another acoustic emission transducer 100 according to the invention is shown in Figure 5,6 and 8, and comprises a baseplate 102 and a piezoceramic disc 104. The baseplate 102 has a generally circular depression or groove 118 formed on a first surface of the baseplate, and a first conducting track 110 and a second conducting track 112 are formed on the first surface of the baseplate 102. The first conducting track 110 extends completely around the periphery of the circular depression or groove 118 on the first surface of the baseplate 102, and the first conducting track 110 is electrically connected to electrical earth. The second conducting track 112 is electrically connected to an amplifier (not shown) .

The piezoceramic disc 104 has an electrically conducting coating or layer 108 on its first surface and an electrically conducting coating or layer 106 on its second^" surface and has an annular peripheral region 122.

The piezoceramic disc 104 is mounted on the baseplate 102 so that the piezoceramic disc 104 is aligned with and lies over the circular depression or groove 118 and the whole of the annular peripheral region 122 of the piezoceramic disc 104 rests on the first conducting track 110. The piezoceramic disc 104 is bonded onto the baseplate 102 by a glue or a cement which forms an electrical contact between the electrically conducting coating 108 on the piezoceramic disc 104 and the first conducting track 110, for example a silver loaded epoxy resin. The conducting coating 106 on the piezoceramic disc 104 is electrically connected to the second conducting track 112 by a signal lead 114 and solder joints 115,116.

A chamber 124 is formed between the first surface of the piezoceramic disc 104 and the circular depression 118 on the baseplate 102, and the baseplate 102 is provided with at least one vent aperture 126 which extends through the baseplate 102 to interconnect the chamber 124 with the pressure acting on the second surface of the piezoceramic disc 104.

The baseplate 102 is an insulating material, i.e. ceramic in this example, and provides a good acoustic impedance match between the baseplate 102 and the piezoceramic disc 104. The baseplate 102 in this example forms a part of a printed ceramic circuit board which has electronic components mounted thereon or forms part of a ceramic substrate which has a hybrid electronic circuit or thick film hybrid electronic circuit, as shown in Figure 8. The electronic components or hybrid electronic circuit form the amplifier components or circuits.

A third acoustic emission transducer 200 according to the invention is shown in Figure 7 and is substantially the same as the embodiment in Figures 5 and 6 and comprises a baseplate 102 and a piezoceramic disc 104. The major difference between the embodiments is that in this embodiment the baseplate 102 has an aperture 218 which extends therethrough. The first conducting track 110 and the second conducting track 112 are formed on a first surface of the baseplate 102. The first conducting track 110 again extends completely around the periphery of the aperture 218 on the first surface of the baseplate 102, and the first conducting track is electrically connected to earth.

The piezoceramic disc 104 is mounted on the baseplate 102 so that the piezoceramic disc 104 is aligned with and lies over the aperture 218 and the whole of the peripheral region 122 of the piezoceramic disc 104 rests on and is bonded onto the first track 110, the axis of the piezoceramic disc 104 and aperture 218 being coaxial. The aperture 118 allows the pressure acting on the first and second surfaces of the piezoceramic disc 104 to be equalised.

The acoustic emission transducer assemblies can be used either as sensors of acoustic emission waves as described, or can be used to generate and transmit stress waves or simulated acoustic emission waves into a component.

Although the acoustic emission transducers have been described with the conducting coating on the first surface of the piezoceramic disc being connected to electrical earth, and the conducting coating on the second surface of the piezoceramic disc being connected by a signal lead to an amplifier, it would function equally well if these connections are reversed or if the conducting coatings on the first and second surfaces of the piezoceramic disc are connected to the first and second inputs of a differential type amplifier.

The mounting of a piezoceramic member onto a base member only by the peripheral region of the piezoceramic member may be used in an electrical oscillator, it may provide the resonant frequency of the oscillator replacing a tuned electrical circuit, i.e. an electrical circuit containing inductances and capacitances, or it may be electrically coupled to a tuned electrical circuit which has almost the same resonant frequency as the piezoceramic member.

Claims

Claims : -

1. An acoustic emission transducer assembly comprising a base member and a piezoceramic member, the base member being adapted for acoustic coupling to a surface of a component, the piezoceramic member having a peripheral region, the piezoceramic member being mounted on the base member by the whole of the peripheral region only whereby the piezoceramic member is allowed to vibrate as a whole while remaining fixed to the base member.

2. An acoustic emission transducer as claimed in claim 1 in which the piezoceramic member is a piezoceramic disc.

3. An acoustic emission transducer as claimed in claim 2 in which the base member is a base disc, the piezoceramic disc and the base disc being arranged coaxially.

4. An acoustic emission transducer assembly as claimed in claim 3 in which the base disc ha*s an annular projection extending axially from a first surface of the base disc, the piezoceramic disc being mounted on the annular projection by the annular peripheral region.

5. An acoustic emission transducer assembly as claimed in claim 1 in which the base member has a first surface, the first surface of the base member has a depression formed therein, the piezoceramic member being aligned with and arranged to lie over the depression and the peripheral region of the piezoceramic member being mounted on the first surface.

6. An acoustic emission transducer assembly as claimed in claim 4 or claim 5 in which the piezoceramic member has a first surface and a second surface, a chamber is formed between the first surface of the piezoceramic disc and the base disc, at least one aperture interconnects the chamber with any pressure acting on the second surface of the piezoceramic disc to at least reduce compressions of the air in the chamber.

7. An acoustic emission transducer assembly as claimed in claim 6 in which the at least one aperture extends through the annular projection.

8. An acoustic emission transducer assembly as claimed in claim 6 in which the at least one aperture is formed by a groove on the annular projection and the piezoceramic disc.

9. An acoustic emission transducer assembly as claimed in claim 1, in which the base member has a first surface, an aperture extending through the base member, the piezoceramic member being aligned with and arranged to lie over the aperture and the peripheral region of the piezoceramic member being mounted on the first surface.

10. An acoustic emission transducer assembly as claimed in claim 1 in which the base member is formed from an electrically conducting material.

11. An acoustic emission transducer assembly as claimed in claim 10 in which the base member has a second surface, the second surface of the base member has an electrical insulation layer.

12. An acoustic emission transducer assembly as claimed in claim 1 in which the base member is formed from an electrically insulating material.

13. An acoustic emission transducer assembly as claimed in claim 12 in which the electrically insulating material is a ceramic.

14. An acoustic emission transducer assembly as claimed in claim 1 in which the piezoceramic member is bonded on the base member by an adhesive.

15. An acoustic emission transducer assembly as claimed in claim 14 in which the adhesive forms an electrical contact between the piezoceramic member and the base member.

16. An acoustic emission transducer assembly as claimed in claim 14 in which the adhesive comprises a silver loaded epoxy resin.

17. An acoustic emission transducer assembly as claimed in claim 1 in which the piezoceramic member is formed from lead zirconate titanate.

18. An acoustic emission transducer assembly as claimed in claim 1 in which a case is secured to the base member to enclose the piezoceramic member.

19. An electrical oscillator assembly comprising a piezoceramic member mounted on a base member, the piezoceramic member having a peripheral region, the piezoceramic member being mounted on the base member by the whole of the peripheral region only whereby the piezoceramic member is allowed to vibrate as a whole while remaining fixed to the base member.