GB2388269A

GB2388269A - Magnetostrictive acoustic actuator with grease

Info

Publication number: GB2388269A
Application number: GB0209975A
Authority: GB
Inventors: William John Metheringham
Original assignee: Newlands Technology Ltd
Current assignee: Newlands Technology Ltd
Priority date: 2002-05-01
Filing date: 2002-05-01
Publication date: 2003-11-05
Also published as: GB0209975D0

Abstract

Giant magnetostrictive material (GMM) 4 is located within the centre 3 of a coil support 2. A biasing field is produced in the GMM element 4 by high strength permanent magnets 6. Non magnetic (eg ceramic) spacers 5 may be located between the ends of the GMM element 4 and the permanent magnets 6. An electromagnetic coil 10, connected to an audio frequency signal source, produces a magnetic field causing a change in length in the magnetostrictive element, which in turn causes a pusher 7 to move. A layer of lubricant (eg petroleum grease) is provided on the interfaces between the ends of the GMM element 4, the permanent magnet 6, the non-magnetic spacers 5 and the pusher 7. The lubricant holds the component of the actuator together during manufacture, acts as a shock absorber and decreases acoustic mismatch of impedance. The actuator couples to a surface (eg desktop, window) to act as a loudspeaker.

Description

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( 2388269

MAGNETOSTRICTIVE ACTUATOR

Field of the Invention

This invention relates to a magnetostrictive actuator and especially to such an actuator for use an acoustic sounder. for example for the reproduction of music.

Background to the Invention

It is known to produce an acoustic actuator, for example for coupling acoustic waves into a surface such as a desk top or window to cause the surface to act as a loud-

speaker. A typical actuator comprises a rod or bar of a giant magnetostrictive material (GMM) located between permanent magnets and surrounded by an electromagnetic coil connected to an audio frequency signal source. A pre-load may be imposed on the GMM bar by means of a spring or the like urging the components together.

It has now been found that by providing a film of a lubricant, preferably a grease, between the components improves the sound output and life of the actuator.

Summary of the Invention

According to the invention, there is provided a magnetostrictive actuator, com-

prising a GMM element, at least one permanent magnet at one end thereof for produc-

ing a biasing field in the element, an electromagnetic coil arranged to produce in the

element a magnetic field proportional to a signal input to the coil, thereby causing pro-

portional changes in length in the element, and a pusher caused to move in response to changes in length of the element, characterized in that the or each interface between an end of the element and the permanent magnet and between the other end of the ele-

ment and the pusher is provided with a layer of a lubricant therebetween.

Preferably, a second permanent magnet is located between the said other end of the element and the pusher. A non-magnetic spacer may be located between the element and the or each permanent magnet.

The lubricant is preferably in the form of a gel or a semi-solid such as a grease.

A thin layer such as a film will typically be sufficient.

The electromagnetic coil is preferably connected to an audio frequency signal source.

. . ( - 2 The actuator of the invention may form part of a self-contained device for in-

ducing acoustic waves into a panel such that the panel radiates audible sound, the device comprising a casing containing a power source, an audio frequency signal input, and an amplifier connected to the power source and to the signal input and providing an output signal to the actuator, which is adapted to couple acoustically with the panel when the device is brought into contact therewith, thereby producing an amplified audio output from the panel. Such a device is disclosed and claimed in general in PCT/GB02/01111.

The addition of grease to an actuator offers several benefits: firstly, the grease acts as a shock absorber during the manufacturing assembly process; secondly the grease makes the stack of components (i.e. GMM rod, magnets, spacers and pusher) hold together better - this is especially applicable to the use of non-magnetic (typically ceramic) spacers therefore making manufacturing easier; and finally there is a de-

crease in the acoustic mismatch of impedance with the addition of grease.

Without the addition of the grease to the actuator, it was found that up to 50% of the actuators had undergone chipping/breakage of the GMM during manufacture.

Tests indicate that adding the grease to the actuator eliminates this problem.

It has been found that actuators with grease improve significantly over time.

One possible reasoning for this is that the grease aids the peening process and smoothes out the two surfaces. It is interesting to note that when the actuator is dis-

mantied then there is no evidence of grease between the two smooth surfaces.

The use of grease is preferable to liquids such as oil, which is less convenient for mass production as it is messy and difficult to apply just to a specific area. Oil also does not aid keeping the stack together in production.

Brief Descriptions of the Drawing

The figure is a schematic cross-section through a magnetostrictive actuator showing the addition of grease in accordance with the invention.

Detailed Description of the Illustrated Embodiment

The actuator comprises a body 1 having located therein a non-magnetic coil support 2 through the centre of which passes a tubular space 3 in which are received two short lengths of GMM 4 together constituting a bar at the outer ends of which are

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- 3 located two ceramic spacers 5. The spacers 5 in turn abut high strength disc magnets 6 mounted in the opposed ends of the support 2. The lower of the magnets 6 (it will be appreciated that "lower" refers merely to the drawing orientation and does not suggest that the device necessarily has only one orientation) rests against a pusher member 7, the whole assembly being subjected to a compressive force, sufficient to provide the desired pre-load on the GMM, by means of a spring washer 8 or the like. The support also carries the electromagnetic coil 10 surrounding the GEM bar and connected to an audio frequency signal source.

Between each component interface in the structure a thin layer of petroleum grease is applied before assembly, assisting in holding the components together as the device is assembled, and producing improved performance of the device as hereinbe-

fore described.

Claims

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1. A magnetostrictive actuator, comprising a GEM element, at least one permanent magnet at one end thereof for producing a biasing field in the element, an

electromagnetic coil arranged to produce in the element a magnetic field proportional

to a signal input to the coil, thereby causing proportional changes in length in the ele ment, and a pusher caused to move in response to changes in length of the element, characterized in that the or each interface between an end of the element and the per manent magnet and between the other end of the element and the pusher is provided with a layer of a lubricant therebetween.

2. An actuator according to Claim 1, wherein a second permanent magnet

is located between the said other end of the element and the pusher.

3. An actuator according to Claim 1 or 2, wherein a non-magnetic spacer is located between the element and the or each permanent magnet.

4. An actuator according to Claim 1, 2 or 3, wherein the lubricant is in the form of a gel or a semi-solid.

5. An actuator according to Claim 4, wherein the lubricant is a petroleum grease.

6. An actuator according to any preceding claim, wherein the electromag netic coil is connected to an audio frequency signal source.

7. A self-contained device for inducing acoustic waves into a panel such that the panel radiates audible sound, the device comprising a casing containing a power source, an audio frequency signal input, and an amplifier connected to the power source and to the signal input and providing an output signal to an actuator in accor dance with any preceding claim adapted to couple acoustically with the panel when the device is brought into contact therewith, thereby producing an amplified audio output from the panel.

8. A magnetostrictive actuator, substantially as described with reference to the drawing.