WO1989005445A1 - An acoustic emission transducer and an electrical oscillator - Google Patents

An acoustic emission transducer and an electrical oscillator Download PDF

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Publication number
WO1989005445A1
WO1989005445A1 PCT/GB1988/001008 GB8801008W WO8905445A1 WO 1989005445 A1 WO1989005445 A1 WO 1989005445A1 GB 8801008 W GB8801008 W GB 8801008W WO 8905445 A1 WO8905445 A1 WO 8905445A1
Authority
WO
WIPO (PCT)
Prior art keywords
piezoceramic
base member
acoustic emission
transducer assembly
emission transducer
Prior art date
Application number
PCT/GB1988/001008
Other languages
French (fr)
Inventor
Trevor John Holroyd
Michael Sadler
Original Assignee
Rolls-Royce Plc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Rolls-Royce Plc filed Critical Rolls-Royce Plc
Publication of WO1989005445A1 publication Critical patent/WO1989005445A1/en

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/15—Constructional features of resonators consisting of piezoelectric or electrostrictive material
    • H03H9/17—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator
    • H03H9/176—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator consisting of ceramic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
    • B06B1/0603—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a piezoelectric bender, e.g. bimorph
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
    • B06B1/0644—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element
    • B06B1/0651—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element of circular shape
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H11/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties
    • G01H11/06—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties by electric means
    • G01H11/08—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by detecting changes in electric or magnetic properties by electric means using piezoelectric devices
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/15—Constructional features of resonators consisting of piezoelectric or electrostrictive material
    • H03H9/17—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator
    • H03H9/171—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator implemented with thin-film techniques, i.e. of the film bulk acoustic resonator [FBAR] type
    • H03H9/172—Means for mounting on a substrate, i.e. means constituting the material interface confining the waves to a volume
    • H03H9/173—Air-gaps
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H9/00—Networks comprising electromechanical or electro-acoustic devices; Electromechanical resonators
    • H03H9/15—Constructional features of resonators consisting of piezoelectric or electrostrictive material
    • H03H9/17—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator
    • H03H9/171—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator implemented with thin-film techniques, i.e. of the film bulk acoustic resonator [FBAR] type
    • H03H9/172—Means for mounting on a substrate, i.e. means constituting the material interface confining the waves to a volume
    • H03H9/174—Membranes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R17/00—Piezoelectric transducers; Electrostrictive transducers
    • H04R17/04—Gramophone pick-ups using a stylus; Recorders using a stylus
    • H04R17/08—Gramophone pick-ups using a stylus; Recorders using a stylus signals being recorded or played back by vibration of a stylus in two orthogonal directions simultaneously

Definitions

  • 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.
  • the prior art acoustic emission transducer is used in conjunction with amplifying and processing equipment which are remote from and connected to the acoustic emission transducer by electrically conducting cables.
  • the present invention seeks to provide an acoustic emission transducer assembly which is integral with a part of an electrical circuit board.
  • the present invention provides 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 being mounted on the base member, the base member forms a part of a circuit board.
  • a first surface of the base member may have at least one conducting track, the piezoceramic member being mounted on the conducting track.
  • the circuit board may be a printed circuit board, the circuit board has electronic components mounted thereon.
  • the electronic components may form an amplifier.
  • the circuit board may have a hybrid electronic circuit.
  • the hybrid electronic circuit may form an amplifier.
  • the piezoceramic member may have 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 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 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 insulating material.
  • the electrically insulating material may be a ceramic.
  • 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 conducting track.
  • the adhesive may comprise a silver loaded epoxy resin.
  • the piezoceramic member may be formed from lead zirconate titanate.
  • the present invention also seeks to provide an electrical oscillator which is integral with a part of an electrical circuit board.
  • the present invention also provides an electrical oscillator assembly comprising a base member and a piezoceramic member, the piezoceramic member is mounted on the base member, the base member forms a part of a circuit board.
  • the base member may have a first surface, the first surface of the base member has at least one conducting track, the piezoceramic member being mounted on the conducting track.
  • the piezoceramic member may have 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.
  • 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 view in the direction of arrow D in Figure 2.
  • Figure 4 is a cross-sectional view through a second embodiment of an acoustic emission transducer assembly according to the present invention.
  • Figure 5 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
  • the piezoceramic element 14 has an electrically conducting coating or layer
  • 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 impedance 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.
  • 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 40-60 Hz, 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. 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 hindrance to use.
  • 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 allowed to vibrate as a whole while remaining fixed to the base member.
  • acoustic emission transducers are used in conjunction with amplifying and processing equipment which are remote from and connected to the acoustic emission transducer by electrically conducting cables.
  • An acoustic emission transducer 100 is shown in Figures 2,3 and 5, 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 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 5.
  • the electronic components or hybrid electronic circuit form the amplifier components or circuits.
  • the piezoceramic disc 104 is preferably a lead zirconate titanate polycrystal, although other suitable piezoelectric, ferroelectric, electrostrictive or electroacoustic polycrystals or monocrystals may be used.
  • the acoustic emission transducer assembly 100 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 baseplate 102 into the piezoceramic disc 104.
  • the acoustic emission waves cause the piezoceramic disc 104, it is believed, to oscillate, resonate or vibrate as a whole by flexing of the piezoceramic disc 104 while the annular peripheral region
  • the piezoceramic disc 104 remains fixed to the baseplate 102, and the piezoceramic disc 104 has resonant frequencies.
  • the fundamental resonant frequency of the acoustic emission sensor assembly is dependent upon the diameter, the thickness and the mechanical properties of the piezoceramic disc.
  • 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.
  • a second acoustic emission transducer 200 according to the invention is shown in Figure 4 and is substantially the same as the embodiment in Figures 2 and 3 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.
  • 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.
  • the acoustic emission transducer as described is integral with a part of an electrical circuit, because the baseplate of the acoustic emission transducer forms a part of a printed ceramic circuit board, or a part of a ceramic substrate which has a hybrid electronic circuit. This results in a more compact transducer and amplifier arrangement or a more compact transducer, amplifier and processor arrangement.
  • the piezoceramic member has been shown to be mounted by its annular peripheral region onto a conducting track, it may be possible to mount the annular peripheral region of the piezoceramic member directly onto the base member and to provide a lead and solder joints to connect the second surface of the piezoceramic member and the conducting track in a similar manner to the connection to the first surface of the piezoceramic member.

Abstract

An acoustic emission transducer assembly (100) comprises a baseplate (102), and a piezoceramic member (104). The piezoceramic member (104) is mounted on the baseplate (102), and the baseplate (102) is adapted for acoustic coupling to surface of a component. The baseplate (102) forms a part of a circuit board. The circuit board may be a printed ceramic circuit board with electronic components mounted thereon or a hybrid electronic circuit. The acoustic emission transducer assembly (100) is integral with a part of an electrical circuit. The piezoceramic member (104) is arranged to lie over a circular depression formed on a surface of the baseplate (102). The piezoceramic member (104) has an annular peripheral region (122) by which it is secured to the baseplate (102). An electrical oscillator is also provided which comprises a piezoceramic member which is mounted on a baseplate. The baseplate again forms a part of a circuit board.

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.
The prior art acoustic emission transducer is used in conjunction with amplifying and processing equipment which are remote from and connected to the acoustic emission transducer by electrically conducting cables.
The present invention seeks to provide an acoustic emission transducer assembly which is integral with a part of an electrical circuit board.
Accordingly the present invention provides 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 being mounted on the base member, the base member forms a part of a circuit board. A first surface of the base member may have at least one conducting track, the piezoceramic member being mounted on the conducting track.
The circuit board may be a printed circuit board, the circuit board has electronic components mounted thereon. The electronic components may form an amplifier.
The circuit board may have a hybrid electronic circuit. The hybrid electronic circuit may form an amplifier. The piezoceramic member may have 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 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 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 insulating material.
The electrically insulating material may be a ceramic.
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 conducting track. The adhesive may comprise a silver loaded epoxy resin.
The piezoceramic member may be formed from lead zirconate titanate. The present invention also seeks to provide an electrical oscillator which is integral with a part of an electrical circuit board.
Accordingly the present invention also provides an electrical oscillator assembly comprising a base member and a piezoceramic member, the piezoceramic member is mounted on the base member, the base member forms a part of a circuit board.
The base member may have a first surface, the first surface of the base member has at least one conducting track, the piezoceramic member being mounted on the conducting track. The piezoceramic member may have 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 view in the direction of arrow D in Figure 2.
Figure 4 is a cross-sectional view through a second embodiment of an acoustic emission transducer assembly according to the present invention.
Figure 5 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 impedance 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 40-60 Hz, 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. 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 hindrance to use.
Our copending patent application No , our ref. 1856 which also claims priority from UK Patent Application No. 8728509 filed on the 5 December 1987, discloses an acoustic emission transducer assembly which 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. The above mentioned copending patent application discloses 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 allowed to vibrate as a whole while remaining fixed to the base member.
As mentioned previously prior art acoustic emission transducers are used in conjunction with amplifying and processing equipment which are remote from and connected to the acoustic emission transducer by electrically conducting cables.
An acoustic emission transducer 100 according to the present invention is shown in Figures 2,3 and 5, 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 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 5. The electronic components or hybrid electronic circuit form the amplifier components or circuits.
The piezoceramic disc 104 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 100 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 baseplate 102 into the piezoceramic disc 104. The acoustic emission waves cause the piezoceramic disc 104, it is believed, to oscillate, resonate or vibrate as a whole by flexing of the piezoceramic disc 104 while the annular peripheral region
122 of the piezoceramic disc 104 remains fixed to the baseplate 102, and the piezoceramic disc 104 has resonant frequencies.
The fundamental resonant frequency of the acoustic emission sensor assembly is dependent upon the diameter, the thickness and the mechanical properties of the piezoceramic disc. 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.
An acoustic emission transducer assembly which has been tested which had a fundamental resonance frequency of 8
41.5KHz comprises a piezoceramic disc which has 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 baseplate is 1/4 mm wide. A second acoustic emission transducer 200 according to the invention is shown in Figure 4 and is substantially the same as the embodiment in Figures 2 and 3 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.
The acoustic emission transducer as described is integral with a part of an electrical circuit, because the baseplate of the acoustic emission transducer forms a part of a printed ceramic circuit board, or a part of a ceramic substrate which has a hybrid electronic circuit. This results in a more compact transducer and amplifier arrangement or a more compact transducer, amplifier and processor arrangement.
Although the piezoceramic member has been shown to be mounted by its annular peripheral region onto a conducting track, it may be possible to mount the annular peripheral region of the piezoceramic member directly onto the base member and to provide a lead and solder joints to connect the second surface of the piezoceramic member and the conducting track in a similar manner to the connection to the first surface of 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 being mounted on the base member, the base member forms a part of a circuit board.
2. 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 at least one conducting track, the piezoceramic member being mounted on the conducting track.
3. An acoustic emission transducer assembly as claimed in claim 1 or claim 2 in which the circuit board is a printed circuit board, the circuit board has electronic components mounted thereon.
4. An acoustic emission transducer assembly as claimed in claim 1 or claim 2 in which the circuit board has a hybrid electronic circuit.
5. An acoustic emission transducer assembly as claimed in claim 3 in which the electronic components form an amplifier.
6. An acoustic emission transducer assembly as claimed in claim 4 in which the hybrid electronic circuit forms an amplifier.
7. An acoustic emission transducer assembly as claimed in claim 1 in which the piezoceramic member has 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.
8. An acoustic emission transducer assembly as claimed in claim 7 in which the base member has an aperture extending therethrough, the piezoceramic member being aligned with and arranged to lie over the aperture, the peripheral region of the piezoceramic member being mounted on the conducting track.
9. An acoustic emission transducer assembly as claimed in claim 7 in which the base member has a depression formed on the first surface of the base member, the piezoceramic member being aligned with and arranged to lie over the depression, the peripheral region of the piezoceramic member being mounted on the conducting track.
10. An acoustic emission transducer assembly as claimed in claim 9 in which the piezoceramic member has a first surface and a second surface, a chamber is formed between the first surface of the piezoceramic member and the base member, at least one aperture interconnects the chamber with any pressure acting on the second surface of the piezoceramic member to at least reduce compressions of air in the chamber.
11. An acoustic emission transducer assembly as claimed in claim 1 in which the piezoceramic member is a piezoceramic disc.
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 to the base member by an adhesive.
15. An acoustic emission transducer assembly as claimed in claim 2 in which an adhesive forms an electrical contact between the piezoceramic member and the conducting track.
16. An acoustic emission transducer assembly as claimed in claim 15 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. 12
18. An electrical oscillator assembly comprising a base member and a piezoceramic member, the piezoceramic member is mounted on the base member, the base member forms a part of a circuit board.
5 19. An electrical oscillator assembly as claimed in claim 18 in which the base member has a first surface, the first surface of the base member has at least one conducting track, the piezoceramic member being mounted on the conducting track.
10 20. An electrical oscillator assembly as claimed in claim 18 or claim 19 in which the piezoceramic member has 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 5 as a whole while remaining fixed to the base member.
21. 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 being mounted on the
20 base member, the base member being formed from an insulating material, the base member forms a part of a circuit board, the base member has at least one conducting track, the piezoceramic member being mounted on the conducting track, the piezoceramic member having a
25 peripheral region, the piezoceramic member being mounted on the base member by the whole of the peripheral region only.
PCT/GB1988/001008 1987-12-05 1988-11-17 An acoustic emission transducer and an electrical oscillator WO1989005445A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8728509 1987-12-05
GB878728509A GB8728509D0 (en) 1987-12-05 1987-12-05 Acoustic emission transducer

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WO1989005445A1 true WO1989005445A1 (en) 1989-06-15

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PCT/GB1988/001008 WO1989005445A1 (en) 1987-12-05 1988-11-17 An acoustic emission transducer and an electrical oscillator

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PCT/GB1988/001007 WO1989005199A1 (en) 1987-12-05 1988-11-17 An acoustic emission transducer and an electrical oscillator

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AU (2) AU2781689A (en)
ES (2) ES2010039A6 (en)
GB (1) GB8728509D0 (en)
WO (2) WO1989005199A1 (en)

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Also Published As

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AU2781689A (en) 1989-07-05
ES2010038A6 (en) 1989-10-16
ES2010039A6 (en) 1989-10-16
CN1033503A (en) 1989-06-21
WO1989005199A1 (en) 1989-06-15
CN1033729A (en) 1989-07-05
GB8728509D0 (en) 1988-01-13
AU2726288A (en) 1989-07-05

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