GB2026284A - Seismic transducer - Google Patents

Abstract

A seismic transducer comprises a piezoelectric bender ceramic element (2) mounted as a cantilevered beam so as to bend with seismic vibrations and derive voltages in accordance therewith. <IMAGE>

Description

SPECIFICATION Seismic transducer The invention relates to a transducer for providing an electrical output in accordance with seismic vibra tions.

In seismic work the forces are extremely small and transducers have, hitherto, employed complex mechanical constructions to derive outputs of useful amplitude.

The present invention seeks to provide an inexpensive transducer of simple construction which gives adequate sensitivity in seismic applications.

According to the invention there is provided a seismic transducer comprising a bender piezoelectric ceramic element mounted as a cantilvered beam, having one end fixed and the other end free to allow bending in response to seismic vibrations, the element having electrodes from which potentials representative of the degree of bending can be derived.

Preferably the bender element is a bimorph, having two superimposed piezoelectric layers. However, other numbers of layers may be employed. A metal stiffening layer, which may also act as an electrode, may be bonded between the or each adjacent pair of piezoelectric layers.

Preferably the transducer is designed to produce adequate responses in the range 0 to 100 Hz and the natural resonance frequency of the bender element is significantly higher than 100 hz-typically 500 Hz or so.

Mass loading, namely the fixing of a passive local or distributed mass to the element, may be employed to tune the natural resonance frequency but mass loading, while perhaps increasing sensitivity, often reduces linearity of the response over the working band-width.

It is found that the invention can provide, in some embodiments, a transducer which is sufficiently sensitive at seismic frequencies as not to require mass loading.

The invention will further be described with refer encetotheaccompanying drawing, which is a perspective view of a transducer in accordance with one embodiment of the invention.

Referring to the drawing the transducer comprises a solid mount 1 which, in use, is fixed firmly with respect to the ground. Mounted by one end in the mount lisa ceramic piezoelectric bimorph element 2. The element is mounted in cantilever fashion, the other end 3 of the element being free.

Element 2 comprises two superimposed layers 4,5 of ceramic piezoelectric material which are bonded together. Surface metal electrodes are provided on the top and bottom of the element, the top electrode being shown at 6. Leads 7 connect the electrodes to an amplifier (not shown). The piezoelectric polarization of the layers 4 and 5 is such that when the element bends, and one layer is compressed while the other is extended, the electrodes receive potentials in accordance with the degree of bending. The potential derived at the upper electrode 6 with respect to the junction between the layers 4,5 is of opposite polarity to the potential derived at the lower electrode with respect to the junction.

Mechanical stops 8 are provided to limit excessive movements which might fracture the element.

In the example illustrated, the free length of the cantilever is 0.84", the width of the cantilever is 0.125", and the thickness of the cantilever is 0.019".

No mass loading is provided and the cantilever has a natural resonance frequency of about 500 Hz.

The transducer may be used to respond to seismic vibrations in the range 0-10Q Hz. Alternatively, useful outputs can be derived in response to shock excitation relying on the resonance of the element.

1. Aseismictransducercornprising a bender piezoelectric ceramic element mounted as a cantil veredheam, having one end fixers and the other end free taallow bending in response seismic vibrations, the element having electrodes from which potentials representative of the degree of bending can be derived.

2. A seismic transducer as claimed in Claim 1 wherein the bender element has a plurality of superimposed piezoelectric layers.

3. A seismic transducer as claimed in Claim 2 wherein the bender element is a bimorph, there being two piezoelectric layers.

4. A seismic transducer as claimed in Claim 2 or Claim 3 wherein a metal stiffening layer, which also acts as an electrode, is bonded between the or each adjacent pair of piezoelectric layers.

5. A seismic transducer as claimed in any of the preceding claims wherein the natural resonance frequency of the bender element is substantially higher than 100 Hz.

6. A seismic transducer as claimed in Claim 5 wherein the natural resonance frequency of the bender element is approximately 500 Hz.

7. Aseismictransduceras claimed in any of the preceding claims wherein the bender element is mass-loaded 8. A seismic transducer as claimed in any of the preceding claims wherein mechanical stops are provided to limit excessive movements of the element 9. A seismic transducer substantially as hereinbefore described with reference to the accompanying drawings.

The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Seismic transducer The invention relates to a transducer for providing an electrical output in accordance with seismic vibra tions. In seismic work the forces are extremely small and transducers have, hitherto, employed complex mechanical constructions to derive outputs of useful amplitude. The present invention seeks to provide an inexpensive transducer of simple construction which gives adequate sensitivity in seismic applications. According to the invention there is provided a seismic transducer comprising a bender piezoelectric ceramic element mounted as a cantilvered beam, having one end fixed and the other end free to allow bending in response to seismic vibrations, the element having electrodes from which potentials representative of the degree of bending can be derived. Preferably the bender element is a bimorph, having two superimposed piezoelectric layers. However, other numbers of layers may be employed. A metal stiffening layer, which may also act as an electrode, may be bonded between the or each adjacent pair of piezoelectric layers. Preferably the transducer is designed to produce adequate responses in the range 0 to 100 Hz and the natural resonance frequency of the bender element is significantly higher than 100 hz-typically 500 Hz or so. Mass loading, namely the fixing of a passive local or distributed mass to the element, may be employed to tune the natural resonance frequency but mass loading, while perhaps increasing sensitivity, often reduces linearity of the response over the working band-width. It is found that the invention can provide, in some embodiments, a transducer which is sufficiently sensitive at seismic frequencies as not to require mass loading. The invention will further be described with refer encetotheaccompanying drawing, which is a perspective view of a transducer in accordance with one embodiment of the invention. Referring to the drawing the transducer comprises a solid mount 1 which, in use, is fixed firmly with respect to the ground. Mounted by one end in the mount lisa ceramic piezoelectric bimorph element 2. The element is mounted in cantilever fashion, the other end 3 of the element being free. Element 2 comprises two superimposed layers 4,5 of ceramic piezoelectric material which are bonded together. Surface metal electrodes are provided on the top and bottom of the element, the top electrode being shown at 6. Leads 7 connect the electrodes to an amplifier (not shown). The piezoelectric polarization of the layers 4 and 5 is such that when the element bends, and one layer is compressed while the other is extended, the electrodes receive potentials in accordance with the degree of bending. The potential derived at the upper electrode 6 with respect to the junction between the layers 4,5 is of opposite polarity to the potential derived at the lower electrode with respect to the junction. Mechanical stops 8 are provided to limit excessive movements which might fracture the element. In the example illustrated, the free length of the cantilever is 0.84", the width of the cantilever is 0.125", and the thickness of the cantilever is 0.019". No mass loading is provided and the cantilever has a natural resonance frequency of about 500 Hz. The transducer may be used to respond to seismic vibrations in the range 0-10Q Hz. Alternatively, useful outputs can be derived in response to shock excitation relying on the resonance of the element. CLAIMS

1. Aseismictransducercornprising a bender piezoelectric ceramic element mounted as a cantil veredheam, having one end fixers and the other end free taallow bending in response seismic vibrations, the element having electrodes from which potentials representative of the degree of bending can be derived.

2. A seismic transducer as claimed in Claim 1 wherein the bender element has a plurality of superimposed piezoelectric layers.

3. A seismic transducer as claimed in Claim 2 wherein the bender element is a bimorph, there being two piezoelectric layers.

4. A seismic transducer as claimed in Claim 2 or Claim 3 wherein a metal stiffening layer, which also acts as an electrode, is bonded between the or each adjacent pair of piezoelectric layers.

5. A seismic transducer as claimed in any of the preceding claims wherein the natural resonance frequency of the bender element is substantially higher than 100 Hz.

6. A seismic transducer as claimed in Claim 5 wherein the natural resonance frequency of the bender element is approximately 500 Hz.

7. Aseismictransduceras claimed in any of the preceding claims wherein the bender element is mass-loaded

8. A seismic transducer as claimed in any of the preceding claims wherein mechanical stops are provided to limit excessive movements of the element

9. A seismic transducer substantially as hereinbefore described with reference to the accompanying drawings.

The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.