CA1118090A - Electromechanical transducers using an electret - Google Patents
Electromechanical transducers using an electretInfo
- Publication number
- CA1118090A CA1118090A CA000306344A CA306344A CA1118090A CA 1118090 A CA1118090 A CA 1118090A CA 000306344 A CA000306344 A CA 000306344A CA 306344 A CA306344 A CA 306344A CA 1118090 A CA1118090 A CA 1118090A
- Authority
- CA
- Canada
- Prior art keywords
- electret
- transducer according
- intermediate layer
- transducer
- layer
- Prior art date
- Legal status (The legal status 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 status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R19/00—Electrostatic transducers
- H04R19/01—Electrostatic transducers characterised by the use of electrets
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
- H02N1/00—Electrostatic generators or motors using a solid moving electrostatic charge carrier
- H02N1/06—Influence generators
- H02N1/08—Influence generators with conductive charge carrier, i.e. capacitor machines
Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to electromechanical transducers comprising an electret between two electricity conducting electrodes. A solid intermediate layer having a different deformability from that of the electret is placed between said electret and an electrode, the material forming said layer having an electrical resistivity higher that 1015 ohms.cm (for instance a silicone elastomer) and said layer having a smooth and continuous face in intimate contact with the whole of the useful area of the charged face of the electret.
The invention relates to electromechanical transducers comprising an electret between two electricity conducting electrodes. A solid intermediate layer having a different deformability from that of the electret is placed between said electret and an electrode, the material forming said layer having an electrical resistivity higher that 1015 ohms.cm (for instance a silicone elastomer) and said layer having a smooth and continuous face in intimate contact with the whole of the useful area of the charged face of the electret.
Description
L8~3~
The invention relates to electromechanical transducers~ i.e. devices for transforming variations of a mecchanical magnitude into variations of an electrical magnitude or conversely, the first being particularly pressure variations, possibly produced by sound or supersonic wave~s or translated into such waves .
It concerns more particularly, among these trans-ducers, those comprising between two electricity conduc-ting electrodes, generally in the form of foils, connec-ted to an outside user circuit, an electret, i.e. a di-electric foil permanently electrically charged on one at least o~ its two faces or in its mass, near said ~ace~
To make possible the deformations or displacements of the charged face of the electrel: in relation to the oppo-site electrode - displacements which are transformed into electrical signals or conversely - known transducers of the kind in ~uestion generally comprise an air space ; between this face and this electrode.
This air space may be reduced to the spaces of very small volume created between the electret and the electrode considered ~or between the electret and a solid intermediate layer interposed between the electret and ` the electrode) because of the natural irregularities of ; the facing faces : this is for example the case in US
patents 3 736 436 and 3 742 152 0 The air space in question may also be larger, spa-cers being then interposed between the facing faces of the electret and the electrode ~
In all cases, it is advisable, during the relative deformations or displacements mentioned of the electret in relation to the considered electrode, to avoid the charged B face of the first from coming into contact with the second-
The invention relates to electromechanical transducers~ i.e. devices for transforming variations of a mecchanical magnitude into variations of an electrical magnitude or conversely, the first being particularly pressure variations, possibly produced by sound or supersonic wave~s or translated into such waves .
It concerns more particularly, among these trans-ducers, those comprising between two electricity conduc-ting electrodes, generally in the form of foils, connec-ted to an outside user circuit, an electret, i.e. a di-electric foil permanently electrically charged on one at least o~ its two faces or in its mass, near said ~ace~
To make possible the deformations or displacements of the charged face of the electrel: in relation to the oppo-site electrode - displacements which are transformed into electrical signals or conversely - known transducers of the kind in ~uestion generally comprise an air space ; between this face and this electrode.
This air space may be reduced to the spaces of very small volume created between the electret and the electrode considered ~or between the electret and a solid intermediate layer interposed between the electret and ` the electrode) because of the natural irregularities of ; the facing faces : this is for example the case in US
patents 3 736 436 and 3 742 152 0 The air space in question may also be larger, spa-cers being then interposed between the facing faces of the electret and the electrode ~
In all cases, it is advisable, during the relative deformations or displacements mentioned of the electret in relation to the considered electrode, to avoid the charged B face of the first from coming into contact with the second-
- 2 -~L8~10 SUCh contact can in fact produce saturation effects causing non linearity of operation of the transducer.
It is also advisable to prevent the mean position of the charged face of the electret in relation to the two electrodes from depending on the difference of pressure between the air contained in the above space and the e~vironment, i.e. in short on the pressure of the environment when the gap considered is sealingly insulated from the environment, as is necessarily the case for underwater applications: on this mean position depends in fact the sensitivity of the transducer .
We may add that the deformations of relatively large magnitude of the electret fatigue this latter -whose thickness is generally very small and of the order of a few microns or of a few tens of microns only - and thus cause rap.id wear thereof .
To remedy these different disadvantages, it has already been proposed to form the spacers, p~ced between the charged face of the electret and the facing electrode, as networks of bars (perforated grids or foils) or studs, the networks being sufficiently dense to reduce the maximum amplitude of the elementary local deformations of the electret between two adjacent elementary supports (bars or studs) to a value less than the height of these : 25 supports (see the article by SESSLER and WEST published on pages 129 to 131 oS volume 12 (2), 196~ of the "Journal of the Audio Engineering Society", New York; US patent 3 612 778; French patent 2 271 733).
But these spacers ensuring a distributed support of the electret are difficult to form and their mnunting is delicateO Mo,eover, they onl~ partially do away with the disadvan-tages pointed out of sensitivity to the outside gCD
pressure and of fatigue due to deformation~
The invention has especially as its aim to provide electro-mechanical transducers of the above ~ind in which the above disadvantages are overcome.
S In US patent 3 736 ~36 mentioned above it was indeed proposed to place between the charged face of the elec-tret and the facing electrode an elastic solid interme-diate layer, but nothing was indicated in this document as to the nature of the elastic material nor the mode of applying said layer against said electret : now elastic materials have in general a relatively low electrical resistivity, i.eO less than 101~ ohms.cm tand even less than 101 ohms.cm for certain neoprenes), so that the exclusive use of a layer formed of such a material would lead to the construction of transducers havin~ a q~lite insufficient use~ul life; all the informa-tion given in the patent in question implies moreover necessarily the existence of small air gaps between the electret and the intermediate layer .
The invention improves transducers of the kind in auestion in which a solid intermediate layer of a material havincJ a different deformability from that of the electret is placed between the charged face of this latter and the ~acing face of the electrode and it is characterised in that on the one hand, the material forming this inter-mediate layer has a high electrical resistivity and in `~ that, on the other hand, this layer has a smooth and conti-nuous face in intimate contact with the whole of the useful area of the charged face or the electret .
By "high electrical resistivi-ty" is meant prefe-rably a resistivity c~reater than 1015 ohmsDcm, this being able to reach and even to exceed 1017 ohmsOcm for certain 3L111 ~Q9~
silicones.
In preferred embodiments recourse is had furthermore to one and/or the other of the following arrangements:
- the intermediate layer is formed from an elastomer, particularly with a silicone bases, this latter being preferably a siloxane such as methylvinyl-silicone or methylvinylphenylsilicone - the elastomer is in the form of a foam with integral skin - the intermediate layer has an aluminum oxide basis.
- when the intermediate layer and the electret are both delimited by parallel flat faces, said intermediate layer is mounted so as to be exposed to the pressure variations generated by the simple touch of a finger, this arrange-ment being advantageously used to construct a multiple key keyboard - the intermediate layer is formed from a rnaterial having a coe~ficient of thermal expansion distinctly different from that of the electret - the electret and the intermediate layer are formed from materials having absorption coefficients distinctly differenl: for certain radiations, causing .
differences of heatin8 and so of deformation of these materials, the assembly of the electrodes, the electret and the intermediate layer being arranged so that the layer and the electret may be reached by such radiations emitted from outside the electrodes - the transducer is associated with a circuit adapted for the treatment of low amplitude electrical signals, such as a MOS type circuit, comprising advantageously an input resistor, an inverter and a feed-back resistor.
In accordance with the present invention, there is provided an electro-mechanical transducer comprising disposed between two electrically conducting electrodes which have connected to the same outside utilization circuit, on the one hand, an electret comprising a dielectric foil permanently electrically charged on at least one of its two faces or adjacent thereto, and - on the other hand, disposed between said at least one face of the electret and the Eacing electrode face, a solid intermediate layer made from a material ha-ving a defo~nability different ~rom that of the electret, the material form- :
ing said intermediate layer having an electrical resisitivity at least greater than 10 ohms/cm and said intermediate layer having a smooth and continuous face in intimate contact with the whole of the useful area of said at least one charged face of the electret.
The invention comprises, apart from the principal - 5a -Q~3~
arrangement, certain other arrangements which are prefe-rably used at the same time and which will be more explicetly discussed hereafter.
In what follows, some pre~erred embodiments of the invention will be described with reference to the accompanying drawings in a way which is of course in no wise limiting.
Fig. 1 of these drawings shows schematically a known electr~-mechanical transducerO
Figs. 2 and 3 show schematically an electro-mechanical transducer constructed according to the invention, respectively for a normal condition and for a contracted condition of its intermediate layer .
Fig. 4 shows schematically anotn~r composite transducer constructed accordincl to the invention 0 Fig. 5 shows with exploded parts, a control keyboard using a plurality of transducers according to the invention ~
And Fig. 6 is the diagram of a user circuit associated9 according to the invention, with one of the a~ove transducers .
Generally, the transducers to which the invention relates comprise, placed between two electricity conduc-ting elements 1 and 2 forming electrodes and connected to an outside user circuit 3, an electret 49 i.e~ a dielectric foil carrying permanently along at least one F o its two faces or adjacent thereto, electrical charges:
these charges have been assumed positive in the embodi-ments illustrated and so have been shown in the drawinys with the ~ sign, but they could also be negative or even be formed by di~oles directed parallel to each otherO
In ~nown transducers of the type considered, the . .
dlsplacements or deformations of the charged face ~ of the electret with respect to the opposite electrode 2 cause voltage variations at the terminals of the elec-trodes and conversely, which allows in particular supersonic waves or sound waves to be transformed into electrical signals or conversely, or even to transform mechanical shocks or pressures into electrical cont:rol signals .
Such relative displacements or deformations of ~ace ~ with respect to the facing face of electrode 2 are generally made possible by the presence there-between 0c a free gap 5 relatively narrow and filled with air, this gap being created by placing spacers between said two faces.
With the purpose of preventing, for the reasons outlined above, the considered displacements or defor-mations of face ~ from having a large amplitude, it has already been proposed to use as spacers relatively dense networks of bars (grids) or ri~id studs which distribute the support æones of face F all along this latter: such multiple spacers, ~lhich have been shown diagrammati-cally by studs 6 in Fig~ 1, are relatively costly to manufacture and difficult to position; moreover, if they reduce the disadvantages mentioned above, they do not overcome them .
In particular, the thic.~ness of the assembly formed by the electret 4 and electrode 1 applied there-against, the electrode being formed particularly by metallization, is often very small, being generally less than 100 microns, so that9 even if the openings of the meshes of the supporting networks considered are only of the order of a mm, relatively accentuated local -- 7 _ defo~mations of said assembly can still be observed, as can be seen from Fig. 1, when a pressure P is applied theretoO
It will moreover be noted that the portion of face ~ in contact with spacer elements 6 is immobilised and so does not participate in the operation of the trans-ducer .
To remedy such disadvantages9 in accordance with the invention, there is placed between face P and the facing electrode 2 a layer 7 (Fig. 2) presenting :
- a deformability different from that o~ electret under the influence of certain outside parameters applicable to said layer, parameters whose variations it is desired to use with the help of the transducer considered ~
- a high electrical resistiviky, preferably greater than 1015 ohms.cm, being able to reach advantageously and even to exceed 10 7 ohms.cm, - and a smooth and continuous face in intimate contact with the whole of the useful area of face F .
By "intimate contact" is meant a surface to sur face adhesion without any in~ermediate layer or trace of air, which is advantageously obtained :
- by giving to the smooth face of the intermediate layer an excellent surface condition (without unevennesses greater than 10 microns), this layer being for example manufactured to tnis end by moulding, - and by juxtaposirg this layer and the electret in a vacuum, In a first particularly advantageous embodiment shown in Figs. 2 and 3,said layer 7 is elastic and formed from an elastomer more ~lexible or softer than the electret~
3Q'9~
As can be seen in Fig. 2, this layer fills completely the space between face F and electrode 2, which ~as occu-pied in known constructions by air and the gridO
But, because of its deformability, said layer in no way opposes the relative displacements of face F in relation to electrode 2 ~
Thus it is that, if a tightening pressure S
t~ig. 3) is applied between the two electrodes, thick-ness c of this layer 7 is reduced whereas thic~cness e of the electret remains constant, which brings interface F
nearer of electrode 2 parallel to itself. The assembly then elastically assumes its initial condition (Fig. 2) as soon as pressure S is removed .
It can be shown that, in such conditions, the sensivity of the transducer, i.e. the ratio between the electrical voltage V created between electrode 1 and 2 and pressure S is inversely proportional to Young's modulus E for intermediate layer 7 .
It can also be shown that, if we call p the permittivity of the electret and ~'that of the layer 7, the sensivity of the transducer is maximum when the products pc and p'e are eaual and that, if the electret is practically indeformable, said sensivity is practi-cally independent of the static outside pressure when the first of these products,pc, is very great with respect to the second p'e.
Again, it can be shown that, if pressure S varies sinusoidally at ~requency f the response of the transdu-cer becomes constant when this frequency f exceeds a certain threshold and that, to reduce this threshold, it is advantageous to use an impedance adaptation circuit having high inp~t resistance (particularly of the order g _ 9~
of 1 gighom), a low input capacity and a low output im-pedance, which may be easily achieved by means of field effect transistors.
The electret is preferably formed from a fluo-rinated polymer. It may have a substituted polyolefin basis such as : polyethylene, polypropylene, polytetra-fluoroethylene, a polycarbonate, or a halogenated poly-hydrocarbon such as fluorinated propylene polyethylene, a copolymer o hexafluoropropylene and tetrafluoro-ethylene, a polyvinylidene fluoride, a compound contai-ning at least 95% polychlorotrifluoroe-thylene, a poly-chlorotrifluoroethylene and polytetrafluoroethylene based compound, a polyvinylidene fluoride and polytetra-fluoroethylene based compound .
It can also be formed as a thin layer from an inorganic compound such as aluminium oxide (electrical resistivity : 1016 ohmsOcm).
Its thickness may be between a few hundred angs-trom and a few centimeters.
Its superficial charge density may be between 10-11 C/cm2 and 10~4 C/cm ~
The elastomer forming layer 7 may be a silicone based compound of a thickness between a few thousand angstrom and a few centimeters.
This compound is preferably a silanol polyether, more often called siloxane, such as for example methyl-vinylsilicone - or methylvinylsiloxane -(named VMQ
according to standard ASTM.D 1418) or methylvinylphenyl-silicone - or methylvinyl.phenylsiloxane (named PVMQ).
It may be in compact and solid form or on the contrary in the form of a foam : in this latter case, layer 7 in auestion is formed with an integral skin, -- 1~ --9~) i.e. with a continuous smooth surface so as to ensure the above-mentioned intimate contact with all points of the area of the facing face ~ of the electret .
According to one variation, intermediate layer 7 may be formed by a part of the electret itself whose deformability has been modified by an appropriate treatment of any desirable nature, mechanical, chemical, electrical or using radiation (visible light, electron beam, X or gamma rays...0).
Electrodes 1 and 2 may be formed in any desirable way, i.g~ by vacuum or electrochemical metallisation of the face opposite the electret or of layer 7. These elec-trodes could also be formed by simple foils or metal ribbons laid on the face considered of the electret or Of layer 7 .
It is to be noted that t:he assembly may be given a tubular shape, one of the twc) electrodes being a wire or rod which extends along the axis of the tube and being surrounded by concentric coaxial tubes forming successively the intermediate layer, the electret and the second electrode .
Purely by way of example and of course in no wise limiting the invention, it can be pointed out that, for a transducer of the kind shown in ~igsO 2 and 3 and comprising :
- an electret 4 formed by a fluorinated polypropylene polyethylene foil of 1 mm thickness whose face ~ has a surface charge of 10 9 C/cm , - an intermediate layer 7 formed by a silicon elastomer foil 1 mm thick, - and electrodes 1 ancl 2 deposited by vacuum metallization on the external faces of the two foils applied one against 9~
the other, the sensitivity of this transducer in an open circuit is of the order of lmV/pascal .
The above proposed structure has, in relation to S those already known, numerous advantages and in parti-cular the following :
- during operation of the transducer, the electret suffers no deformation since it is the intermediate layer 7 which is deformed : this phenomenon is favourable to the long life of the electret and so of the transducer~
- as pointed out above, the sensitivity of the transducer can be made independant of the static pressure of the environment in which it is placed, which can be advanta-geous for a certain number of applications and in parti-cular for underwater detection,- the char~3ed face F of the electret, since it is inti-m2tely coated over its whole length with layer 7, is peLfectly pro~ected from all outside agents which might damage it ~dust, ~amp, electrical ions etc...), - even when the transducer is subjected to a high pressure, no point of the charged face F of its electret can come into contact with the electrode opposite, since it is separated from this latter by layer 7 ~
- since no local deformation is required of the electret9 this latter may ke provided in great thickness, which improves its sensitivity, - the abcence of any sap or spacer between the charyed face of the electret and the electrode opposite allows a great flexibility in the manufacture o~ the laminated assembly composed of the electret, the 1ecible layer and the two electrodes plat~d intimately one against the other~ this assembly being readily marketable in the form g~
of composite foils, ribbons or tubes of large size or even deformed into a shape other than flat or cylindrical by stamping or in any other desirable way, the shape adopted being able particularly to be a bul~ing surface, centred for exemple on a sound source with a view to interference free recording , - the presence of an elastomer layer in the transducer damps considerably the resonance freauency of this latter, which enlarges its pass-band, in particular towards the high freauencies and avoids the use of external damping means, the materials forming the elec:tret and the intermediate layer have an acoustic impedanc:e close to that of water, so that the transducers considered are perfectly adapted to the emission or the reception o~ acoustic waves in a~ueous or similar environments (underwater detection, medical analysis...), these treatments being carried out practically without loss of energy ancl without reflection ("transparent" ant:anna) : thus the acoustic impedance of water is 1.5 x 106 kg~m2s and that of a transducer ac-cording to the invent:ion is of the order of 1.8 x 106 k~/m2s whereas that of air layer transducers is only 3.104 kg/m s and that of piezo-electric ceramics reaches 28.106 kg/m s.
With respect to the elementary assembly shown in FigsO 2 and 3, numerous variations may be envisaged .
Thus, in the variation shown in Fig~ ~, two assemblies of the kind shown in Fig~ 2 are stacked head to foot alongside one another, electrode 2 being then disposed in the centre of the stack and being common to both stacked assembli.es: in other words we find stacked successively on each face of this electrode 2, an elastic 8~9~
layer (71~ 72)~ an electret (41~ 42) and a second electrode (11, 12), the order of stacking of the elas~
tomer layer and the electret being reversible ~
By interconnecting the ~wo second electrodes, we obtain an electrical shield for the transducer .
With such a connection, the transducer obtained is practically insensitive to its flexions about an axis passing through the mean plane of the central electrode, any local mechanical or electrical modification affecting one side of this electrode due to such a flexion being automatically c~mpensated for by an inverse local modi-fication of the other side of this electrode.
Of course, the type of stacking described with reference to Fig. 4 can be generalised .
There can also be used a plurality of elementary transducers of the kind described above, forming for ~xample a sort of mosaic and interconnected in series and/or parallel in a way known per se .
In Fig. 5, there is shown a particularly advanta-geous application of the above-described structures having elastic intermediate layers.
It refers to a multi~le control keyboard whose keys are actuated simply by brushing with the fingers .
A keyboard of this type has already been proposed ~S in US patent 3 668 417, but in this patent, the displace-ments of the charged face of the electret are made possible by the presence of an air layer along this face and not by the elasticity of an intermediate layer .
In the present case, the keyboard has a plurality of elastomer transducers of the kind described with references to FigsO 2 and 3 disposed respectively op-posite as many apertures 10 in an outer rigid cover 11 - 14 ~
, 3Q9~
Here, electrode 2, elastic layer 7 and electret ~ are all three formed from continuous foils common to all the transducers, and it is electrode 1 which alone is divi-ded into independent conducting areas each connected by a conductor 12 to an outside user circuit (nos shown)O
In the case of this figure 5 it has been assumed that ~hes~ areds 12 are formed by metallization of an extra insulating foil 13 itself brought against the face of electret 4 opposite the charged face F thereof .
Moreover, this foil 13 is itself covered on the outside by a conducting layer 14, which may be electri-cally connected to electrode 2 50 as to form a screening for the assembly O
By brushing with the finger one of the conducting areas of electrode 2, which appears through one of the apertures 10, the piece of elastic foil 7 is compressed, which brings said area 2 of the portion opposite face F
closer to electret 4 : this bringing nearer generates a potential difference between this area 2 and the other conducting area 1 opposite, which potential difference may be used for any purposes desired .
In such cases, as well as more generally in all cases where it is desired to use the sensor for detecting the overshooting of a pressure threshold (push-button, 25 passage detecting door-mat, vehicle counting~) it is - advantageous to associate this sensox with a semi-conduc-tor circuit adapted for the treatment of low amplitude electrical signals such as generated by such a sensor :
such a circuit is for example an MOS circuit, preferably of the CMOS type, having the advantages of high input impedance, low consumption, good compatibility with the digital electronic systems and low cost.
~ 15 -1)9~
Such a circuit has been shown for example in figure 6; it comprises essentially a resistor R1 determining the input impedance of the circuit, an inverter 8, constructed particularly by means of a NAN~ gate, and a feed-back resistor R2. ~rom an analog signal generated by the transducer this circuit allows a logic signal to be obtained compatible with the user circuit properly speaking to which it is fed: said circuit has been designated by the reference 9 in the drawing. ~he choice of the two resistors R1 and R2 determines the swîtching threshold of inverter 8 and so the sensivity of the device .
Instead of control by brushing with a inger or by direct touch, the control pressures may be exerted on the different transducers by means of push-buttons and/or dead point overshoot resilient mechanism.s .
As an example, push-buttons could be used, each associated with an elastic rocking mechanism gauged so as to project a piece or a part of a deformable piece on to the area of electrode 2 corresponding to this button as soon as this latter has been pressed beyond a certain threshold. Such a formula allows trigger pressure thres-holds to be determined accurately at will .
Generally, the structures described above lend 25. themselves particularly well to applications bringing into play a pressure variation, because ot the elasticity of the intermediate layer, which allows a rapid and reliable tomatic return of each transducer to its rest position as soon as the pressure exerted thereon is removed O
Thus, with the structures in question, very compact and sensitive pressure sensors can be constructed usable in particular for recording sounds (sound sensors) or else _ 16 -for medical applications (occular displacements, blood pressure, intercranial pressure, respiratory flow sen-sors..OO).
Said structures can also be used for measuring pressures in difficult conditions (shock waves... ) consi-dering the excellent mechanical and electrical protection of the electret which remains permanently in intimate contact with the elastomer ~
These structures can also be used at relatively high temperatu~es if, for the electret and for the elas~
tomer, materials are chosen having a good resistance to the temperature, the elastomer being for example a silanol polyether and the electret a halogenated poly-hydrocarbon ba~sed polymer .
One particular use of such transducers under high pressure is the production of sparks in a gas for the purpose, for example, of lighting a combustible mixture :
if in fact a high pressure is e~erted on a transducer of the above kind whose electrodes are connected to those of a capacitor, this latter can be charged up to the break-down v~ltage of a gas present between these latter elec-trodes, said breakdown being favoured by these latter having a pointed shapeO
The presence of an elastic layer in the transduc~rs considered may also be used for damping e~fects, e.g. for applications using safety or end stops .
Instead o being more deformable than the electret 9 the intermediate layer could be selected less deformable than this electret, which could be envisaged in the case of a particularly hard intermediate layer .
This would *or example be the case for a fluorinated propylene polyethylene (sometimes ~nown under its commercial 9a~
name TE~LON l~EP) electret laid on a fused glass support, made from used quartz or silica which forms then the intermediate layer ~
In other variations, the difference between the relative deformabilities of the electret and the inter-mediate layer is not manifested directly by application o a mechanical stress ~pressure or shock), but indirectly by temperature variations or even more indirectly by irradiation O
Thus, if intermediate layer 7 tFig. 2) has a thermal expansion coefficient very different from that of electret 4, the heating of t:he assembly causes as before a relative displacement of charged fac~ ~ of the electret in relation to one of the electrodes, such a lS displacement being capable of c:reating an electrical voltage at the terminals of ~hese electrod~s. Thus~ with the transducers considered, temperature detectors can be constructed capable o~ automatically triggering o~f a visual or sound alarm as soon as said temperature exceeds a predetermined threshold.
~y way of example, we can take up again the example which has just been given since the respective thermal expansion coefficients of fluorinated propylene poly-etl~ylene and fused quartz are 9.10 5 cm/cm/degree and 25- 5~5.10-7 Cm/cm/degree~
An electret made from polyethylene (expansion coefficient 17.10 5 cm/cm/degree) or from polyvinylidene chloride ~19.10 5) plated on a cristalline alumina support, a substance whose expansion coefficient is close to that of quartz,could also be used, this support forming the intermediate layer O
Or again recourse could be had to an electret o*
9~
the above kind made from fluorinated propylene poly-ethylene plated on a polyimide support texpansion coefficient 2.10 5 cm/cm/degree) : laminates of these ~ two polymers are distributed under the name KAPTON
type Fal~ ~ ~s sufficient to charge the desired por-tion thereof in order to make them capable of forming transducers according to the invention .
It is also possible for intermediate layer 7, with regard to certain radiations, to have a power o~ absorp-tion very different from that of the electret and to be capable of trans*orming these radiations into heat, if those latter are not alrèady heat radiations (infrared).
In such a case, if the transducer is arrange~ so that the intermediate layer and the electret are hoth exposed to the radiations considered, one of these two elements is hea-ted more than the other and so is more expanded than the other, even if their thermal expansion coefficients are e~ual, which results again in a relative displacement of interface F in relation to the electrodesO
The same result could also be obtained, even ~ith identical coefficients o~ absorption for the two elements with regard to the radiations considered, if the assembly is arran~ed so that only one of these elements can be exposed to such radiations or if one of them is much more exposed to said radiations than the other .
As e~amples the following can be mentioned :
- an electret of the above type made from fluorinated propylene polyethylene practically transparent in the visi-ble spectrum, plated on a ruby support-forming the above intermediate layer - (chromium doped alumina with a weight concentration of impurities of 10 to 10 ~), a suhstance which absor~s to a very high degree the blue and transforms Tra c/~ ~a r/f~' - 1~ -L8(~
it partly into heat, partly into red light ;
an electret of the same type as previously who~se response is practically flat up to the wave length ~ of 3.8 microns, plated on a TETRASIL glass obtained from silicon halogenide, which also has a very flat response, except for radiations whose wave-length is close to 2.75 microns, which it transforms into heat, - conversely, an electret, still of the previous type, very adsorbent in the wave-length range between 7.5 and 9.5 microns, plated on a suppo~t such as usually used i n infrared spectographs for their transparency to these rays1 particularly made from potassium bromide KBr, sodium chloride NaCl, silver chloride AgCl or caesium iodide CsI , The transducers constructed according to these three examples allow detection or measuring to be carried out on radiations respectively blue, having a wave-len~th eaual to 2.75 microns and infrared (infra-red imagery).
In these different cases, it is of course adYisable that at least one of the electrodes is transparent to the radiations used; it is advantageously formed by metallization of a very thin layer, e.g. of gold with a thickness of a few angstroms, which is possible for the contact resistance of such a layer can be kept relatively low (10 oh~s) much less than the input impedance of the user circuit .
Following which and whatever the embodiment adopted, there is finally provided a transducer whose constitution, operation and advantages follow sufficiently from what has gone before 0 As is evident and as it follows moreover already ~q G/e ~c~ r ~ _ 20--~8~
from what has gone before, the invention is in no wise limited to those of its embodiments and modes of appli-cation which have been more~specially considered; it embraces, on the contrary,all variations thereof, particularly those where the transducers considered are used for mechanical signal emission purposes (acoustic or other) rather than for reception or detection purposes of such signals as most of those which have been more particularly described above .
It is also advisable to prevent the mean position of the charged face of the electret in relation to the two electrodes from depending on the difference of pressure between the air contained in the above space and the e~vironment, i.e. in short on the pressure of the environment when the gap considered is sealingly insulated from the environment, as is necessarily the case for underwater applications: on this mean position depends in fact the sensitivity of the transducer .
We may add that the deformations of relatively large magnitude of the electret fatigue this latter -whose thickness is generally very small and of the order of a few microns or of a few tens of microns only - and thus cause rap.id wear thereof .
To remedy these different disadvantages, it has already been proposed to form the spacers, p~ced between the charged face of the electret and the facing electrode, as networks of bars (perforated grids or foils) or studs, the networks being sufficiently dense to reduce the maximum amplitude of the elementary local deformations of the electret between two adjacent elementary supports (bars or studs) to a value less than the height of these : 25 supports (see the article by SESSLER and WEST published on pages 129 to 131 oS volume 12 (2), 196~ of the "Journal of the Audio Engineering Society", New York; US patent 3 612 778; French patent 2 271 733).
But these spacers ensuring a distributed support of the electret are difficult to form and their mnunting is delicateO Mo,eover, they onl~ partially do away with the disadvan-tages pointed out of sensitivity to the outside gCD
pressure and of fatigue due to deformation~
The invention has especially as its aim to provide electro-mechanical transducers of the above ~ind in which the above disadvantages are overcome.
S In US patent 3 736 ~36 mentioned above it was indeed proposed to place between the charged face of the elec-tret and the facing electrode an elastic solid interme-diate layer, but nothing was indicated in this document as to the nature of the elastic material nor the mode of applying said layer against said electret : now elastic materials have in general a relatively low electrical resistivity, i.eO less than 101~ ohms.cm tand even less than 101 ohms.cm for certain neoprenes), so that the exclusive use of a layer formed of such a material would lead to the construction of transducers havin~ a q~lite insufficient use~ul life; all the informa-tion given in the patent in question implies moreover necessarily the existence of small air gaps between the electret and the intermediate layer .
The invention improves transducers of the kind in auestion in which a solid intermediate layer of a material havincJ a different deformability from that of the electret is placed between the charged face of this latter and the ~acing face of the electrode and it is characterised in that on the one hand, the material forming this inter-mediate layer has a high electrical resistivity and in `~ that, on the other hand, this layer has a smooth and conti-nuous face in intimate contact with the whole of the useful area of the charged face or the electret .
By "high electrical resistivi-ty" is meant prefe-rably a resistivity c~reater than 1015 ohmsDcm, this being able to reach and even to exceed 1017 ohmsOcm for certain 3L111 ~Q9~
silicones.
In preferred embodiments recourse is had furthermore to one and/or the other of the following arrangements:
- the intermediate layer is formed from an elastomer, particularly with a silicone bases, this latter being preferably a siloxane such as methylvinyl-silicone or methylvinylphenylsilicone - the elastomer is in the form of a foam with integral skin - the intermediate layer has an aluminum oxide basis.
- when the intermediate layer and the electret are both delimited by parallel flat faces, said intermediate layer is mounted so as to be exposed to the pressure variations generated by the simple touch of a finger, this arrange-ment being advantageously used to construct a multiple key keyboard - the intermediate layer is formed from a rnaterial having a coe~ficient of thermal expansion distinctly different from that of the electret - the electret and the intermediate layer are formed from materials having absorption coefficients distinctly differenl: for certain radiations, causing .
differences of heatin8 and so of deformation of these materials, the assembly of the electrodes, the electret and the intermediate layer being arranged so that the layer and the electret may be reached by such radiations emitted from outside the electrodes - the transducer is associated with a circuit adapted for the treatment of low amplitude electrical signals, such as a MOS type circuit, comprising advantageously an input resistor, an inverter and a feed-back resistor.
In accordance with the present invention, there is provided an electro-mechanical transducer comprising disposed between two electrically conducting electrodes which have connected to the same outside utilization circuit, on the one hand, an electret comprising a dielectric foil permanently electrically charged on at least one of its two faces or adjacent thereto, and - on the other hand, disposed between said at least one face of the electret and the Eacing electrode face, a solid intermediate layer made from a material ha-ving a defo~nability different ~rom that of the electret, the material form- :
ing said intermediate layer having an electrical resisitivity at least greater than 10 ohms/cm and said intermediate layer having a smooth and continuous face in intimate contact with the whole of the useful area of said at least one charged face of the electret.
The invention comprises, apart from the principal - 5a -Q~3~
arrangement, certain other arrangements which are prefe-rably used at the same time and which will be more explicetly discussed hereafter.
In what follows, some pre~erred embodiments of the invention will be described with reference to the accompanying drawings in a way which is of course in no wise limiting.
Fig. 1 of these drawings shows schematically a known electr~-mechanical transducerO
Figs. 2 and 3 show schematically an electro-mechanical transducer constructed according to the invention, respectively for a normal condition and for a contracted condition of its intermediate layer .
Fig. 4 shows schematically anotn~r composite transducer constructed accordincl to the invention 0 Fig. 5 shows with exploded parts, a control keyboard using a plurality of transducers according to the invention ~
And Fig. 6 is the diagram of a user circuit associated9 according to the invention, with one of the a~ove transducers .
Generally, the transducers to which the invention relates comprise, placed between two electricity conduc-ting elements 1 and 2 forming electrodes and connected to an outside user circuit 3, an electret 49 i.e~ a dielectric foil carrying permanently along at least one F o its two faces or adjacent thereto, electrical charges:
these charges have been assumed positive in the embodi-ments illustrated and so have been shown in the drawinys with the ~ sign, but they could also be negative or even be formed by di~oles directed parallel to each otherO
In ~nown transducers of the type considered, the . .
dlsplacements or deformations of the charged face ~ of the electret with respect to the opposite electrode 2 cause voltage variations at the terminals of the elec-trodes and conversely, which allows in particular supersonic waves or sound waves to be transformed into electrical signals or conversely, or even to transform mechanical shocks or pressures into electrical cont:rol signals .
Such relative displacements or deformations of ~ace ~ with respect to the facing face of electrode 2 are generally made possible by the presence there-between 0c a free gap 5 relatively narrow and filled with air, this gap being created by placing spacers between said two faces.
With the purpose of preventing, for the reasons outlined above, the considered displacements or defor-mations of face ~ from having a large amplitude, it has already been proposed to use as spacers relatively dense networks of bars (grids) or ri~id studs which distribute the support æones of face F all along this latter: such multiple spacers, ~lhich have been shown diagrammati-cally by studs 6 in Fig~ 1, are relatively costly to manufacture and difficult to position; moreover, if they reduce the disadvantages mentioned above, they do not overcome them .
In particular, the thic.~ness of the assembly formed by the electret 4 and electrode 1 applied there-against, the electrode being formed particularly by metallization, is often very small, being generally less than 100 microns, so that9 even if the openings of the meshes of the supporting networks considered are only of the order of a mm, relatively accentuated local -- 7 _ defo~mations of said assembly can still be observed, as can be seen from Fig. 1, when a pressure P is applied theretoO
It will moreover be noted that the portion of face ~ in contact with spacer elements 6 is immobilised and so does not participate in the operation of the trans-ducer .
To remedy such disadvantages9 in accordance with the invention, there is placed between face P and the facing electrode 2 a layer 7 (Fig. 2) presenting :
- a deformability different from that o~ electret under the influence of certain outside parameters applicable to said layer, parameters whose variations it is desired to use with the help of the transducer considered ~
- a high electrical resistiviky, preferably greater than 1015 ohms.cm, being able to reach advantageously and even to exceed 10 7 ohms.cm, - and a smooth and continuous face in intimate contact with the whole of the useful area of face F .
By "intimate contact" is meant a surface to sur face adhesion without any in~ermediate layer or trace of air, which is advantageously obtained :
- by giving to the smooth face of the intermediate layer an excellent surface condition (without unevennesses greater than 10 microns), this layer being for example manufactured to tnis end by moulding, - and by juxtaposirg this layer and the electret in a vacuum, In a first particularly advantageous embodiment shown in Figs. 2 and 3,said layer 7 is elastic and formed from an elastomer more ~lexible or softer than the electret~
3Q'9~
As can be seen in Fig. 2, this layer fills completely the space between face F and electrode 2, which ~as occu-pied in known constructions by air and the gridO
But, because of its deformability, said layer in no way opposes the relative displacements of face F in relation to electrode 2 ~
Thus it is that, if a tightening pressure S
t~ig. 3) is applied between the two electrodes, thick-ness c of this layer 7 is reduced whereas thic~cness e of the electret remains constant, which brings interface F
nearer of electrode 2 parallel to itself. The assembly then elastically assumes its initial condition (Fig. 2) as soon as pressure S is removed .
It can be shown that, in such conditions, the sensivity of the transducer, i.e. the ratio between the electrical voltage V created between electrode 1 and 2 and pressure S is inversely proportional to Young's modulus E for intermediate layer 7 .
It can also be shown that, if we call p the permittivity of the electret and ~'that of the layer 7, the sensivity of the transducer is maximum when the products pc and p'e are eaual and that, if the electret is practically indeformable, said sensivity is practi-cally independent of the static outside pressure when the first of these products,pc, is very great with respect to the second p'e.
Again, it can be shown that, if pressure S varies sinusoidally at ~requency f the response of the transdu-cer becomes constant when this frequency f exceeds a certain threshold and that, to reduce this threshold, it is advantageous to use an impedance adaptation circuit having high inp~t resistance (particularly of the order g _ 9~
of 1 gighom), a low input capacity and a low output im-pedance, which may be easily achieved by means of field effect transistors.
The electret is preferably formed from a fluo-rinated polymer. It may have a substituted polyolefin basis such as : polyethylene, polypropylene, polytetra-fluoroethylene, a polycarbonate, or a halogenated poly-hydrocarbon such as fluorinated propylene polyethylene, a copolymer o hexafluoropropylene and tetrafluoro-ethylene, a polyvinylidene fluoride, a compound contai-ning at least 95% polychlorotrifluoroe-thylene, a poly-chlorotrifluoroethylene and polytetrafluoroethylene based compound, a polyvinylidene fluoride and polytetra-fluoroethylene based compound .
It can also be formed as a thin layer from an inorganic compound such as aluminium oxide (electrical resistivity : 1016 ohmsOcm).
Its thickness may be between a few hundred angs-trom and a few centimeters.
Its superficial charge density may be between 10-11 C/cm2 and 10~4 C/cm ~
The elastomer forming layer 7 may be a silicone based compound of a thickness between a few thousand angstrom and a few centimeters.
This compound is preferably a silanol polyether, more often called siloxane, such as for example methyl-vinylsilicone - or methylvinylsiloxane -(named VMQ
according to standard ASTM.D 1418) or methylvinylphenyl-silicone - or methylvinyl.phenylsiloxane (named PVMQ).
It may be in compact and solid form or on the contrary in the form of a foam : in this latter case, layer 7 in auestion is formed with an integral skin, -- 1~ --9~) i.e. with a continuous smooth surface so as to ensure the above-mentioned intimate contact with all points of the area of the facing face ~ of the electret .
According to one variation, intermediate layer 7 may be formed by a part of the electret itself whose deformability has been modified by an appropriate treatment of any desirable nature, mechanical, chemical, electrical or using radiation (visible light, electron beam, X or gamma rays...0).
Electrodes 1 and 2 may be formed in any desirable way, i.g~ by vacuum or electrochemical metallisation of the face opposite the electret or of layer 7. These elec-trodes could also be formed by simple foils or metal ribbons laid on the face considered of the electret or Of layer 7 .
It is to be noted that t:he assembly may be given a tubular shape, one of the twc) electrodes being a wire or rod which extends along the axis of the tube and being surrounded by concentric coaxial tubes forming successively the intermediate layer, the electret and the second electrode .
Purely by way of example and of course in no wise limiting the invention, it can be pointed out that, for a transducer of the kind shown in ~igsO 2 and 3 and comprising :
- an electret 4 formed by a fluorinated polypropylene polyethylene foil of 1 mm thickness whose face ~ has a surface charge of 10 9 C/cm , - an intermediate layer 7 formed by a silicon elastomer foil 1 mm thick, - and electrodes 1 ancl 2 deposited by vacuum metallization on the external faces of the two foils applied one against 9~
the other, the sensitivity of this transducer in an open circuit is of the order of lmV/pascal .
The above proposed structure has, in relation to S those already known, numerous advantages and in parti-cular the following :
- during operation of the transducer, the electret suffers no deformation since it is the intermediate layer 7 which is deformed : this phenomenon is favourable to the long life of the electret and so of the transducer~
- as pointed out above, the sensitivity of the transducer can be made independant of the static pressure of the environment in which it is placed, which can be advanta-geous for a certain number of applications and in parti-cular for underwater detection,- the char~3ed face F of the electret, since it is inti-m2tely coated over its whole length with layer 7, is peLfectly pro~ected from all outside agents which might damage it ~dust, ~amp, electrical ions etc...), - even when the transducer is subjected to a high pressure, no point of the charged face F of its electret can come into contact with the electrode opposite, since it is separated from this latter by layer 7 ~
- since no local deformation is required of the electret9 this latter may ke provided in great thickness, which improves its sensitivity, - the abcence of any sap or spacer between the charyed face of the electret and the electrode opposite allows a great flexibility in the manufacture o~ the laminated assembly composed of the electret, the 1ecible layer and the two electrodes plat~d intimately one against the other~ this assembly being readily marketable in the form g~
of composite foils, ribbons or tubes of large size or even deformed into a shape other than flat or cylindrical by stamping or in any other desirable way, the shape adopted being able particularly to be a bul~ing surface, centred for exemple on a sound source with a view to interference free recording , - the presence of an elastomer layer in the transducer damps considerably the resonance freauency of this latter, which enlarges its pass-band, in particular towards the high freauencies and avoids the use of external damping means, the materials forming the elec:tret and the intermediate layer have an acoustic impedanc:e close to that of water, so that the transducers considered are perfectly adapted to the emission or the reception o~ acoustic waves in a~ueous or similar environments (underwater detection, medical analysis...), these treatments being carried out practically without loss of energy ancl without reflection ("transparent" ant:anna) : thus the acoustic impedance of water is 1.5 x 106 kg~m2s and that of a transducer ac-cording to the invent:ion is of the order of 1.8 x 106 k~/m2s whereas that of air layer transducers is only 3.104 kg/m s and that of piezo-electric ceramics reaches 28.106 kg/m s.
With respect to the elementary assembly shown in FigsO 2 and 3, numerous variations may be envisaged .
Thus, in the variation shown in Fig~ ~, two assemblies of the kind shown in Fig~ 2 are stacked head to foot alongside one another, electrode 2 being then disposed in the centre of the stack and being common to both stacked assembli.es: in other words we find stacked successively on each face of this electrode 2, an elastic 8~9~
layer (71~ 72)~ an electret (41~ 42) and a second electrode (11, 12), the order of stacking of the elas~
tomer layer and the electret being reversible ~
By interconnecting the ~wo second electrodes, we obtain an electrical shield for the transducer .
With such a connection, the transducer obtained is practically insensitive to its flexions about an axis passing through the mean plane of the central electrode, any local mechanical or electrical modification affecting one side of this electrode due to such a flexion being automatically c~mpensated for by an inverse local modi-fication of the other side of this electrode.
Of course, the type of stacking described with reference to Fig. 4 can be generalised .
There can also be used a plurality of elementary transducers of the kind described above, forming for ~xample a sort of mosaic and interconnected in series and/or parallel in a way known per se .
In Fig. 5, there is shown a particularly advanta-geous application of the above-described structures having elastic intermediate layers.
It refers to a multi~le control keyboard whose keys are actuated simply by brushing with the fingers .
A keyboard of this type has already been proposed ~S in US patent 3 668 417, but in this patent, the displace-ments of the charged face of the electret are made possible by the presence of an air layer along this face and not by the elasticity of an intermediate layer .
In the present case, the keyboard has a plurality of elastomer transducers of the kind described with references to FigsO 2 and 3 disposed respectively op-posite as many apertures 10 in an outer rigid cover 11 - 14 ~
, 3Q9~
Here, electrode 2, elastic layer 7 and electret ~ are all three formed from continuous foils common to all the transducers, and it is electrode 1 which alone is divi-ded into independent conducting areas each connected by a conductor 12 to an outside user circuit (nos shown)O
In the case of this figure 5 it has been assumed that ~hes~ areds 12 are formed by metallization of an extra insulating foil 13 itself brought against the face of electret 4 opposite the charged face F thereof .
Moreover, this foil 13 is itself covered on the outside by a conducting layer 14, which may be electri-cally connected to electrode 2 50 as to form a screening for the assembly O
By brushing with the finger one of the conducting areas of electrode 2, which appears through one of the apertures 10, the piece of elastic foil 7 is compressed, which brings said area 2 of the portion opposite face F
closer to electret 4 : this bringing nearer generates a potential difference between this area 2 and the other conducting area 1 opposite, which potential difference may be used for any purposes desired .
In such cases, as well as more generally in all cases where it is desired to use the sensor for detecting the overshooting of a pressure threshold (push-button, 25 passage detecting door-mat, vehicle counting~) it is - advantageous to associate this sensox with a semi-conduc-tor circuit adapted for the treatment of low amplitude electrical signals such as generated by such a sensor :
such a circuit is for example an MOS circuit, preferably of the CMOS type, having the advantages of high input impedance, low consumption, good compatibility with the digital electronic systems and low cost.
~ 15 -1)9~
Such a circuit has been shown for example in figure 6; it comprises essentially a resistor R1 determining the input impedance of the circuit, an inverter 8, constructed particularly by means of a NAN~ gate, and a feed-back resistor R2. ~rom an analog signal generated by the transducer this circuit allows a logic signal to be obtained compatible with the user circuit properly speaking to which it is fed: said circuit has been designated by the reference 9 in the drawing. ~he choice of the two resistors R1 and R2 determines the swîtching threshold of inverter 8 and so the sensivity of the device .
Instead of control by brushing with a inger or by direct touch, the control pressures may be exerted on the different transducers by means of push-buttons and/or dead point overshoot resilient mechanism.s .
As an example, push-buttons could be used, each associated with an elastic rocking mechanism gauged so as to project a piece or a part of a deformable piece on to the area of electrode 2 corresponding to this button as soon as this latter has been pressed beyond a certain threshold. Such a formula allows trigger pressure thres-holds to be determined accurately at will .
Generally, the structures described above lend 25. themselves particularly well to applications bringing into play a pressure variation, because ot the elasticity of the intermediate layer, which allows a rapid and reliable tomatic return of each transducer to its rest position as soon as the pressure exerted thereon is removed O
Thus, with the structures in question, very compact and sensitive pressure sensors can be constructed usable in particular for recording sounds (sound sensors) or else _ 16 -for medical applications (occular displacements, blood pressure, intercranial pressure, respiratory flow sen-sors..OO).
Said structures can also be used for measuring pressures in difficult conditions (shock waves... ) consi-dering the excellent mechanical and electrical protection of the electret which remains permanently in intimate contact with the elastomer ~
These structures can also be used at relatively high temperatu~es if, for the electret and for the elas~
tomer, materials are chosen having a good resistance to the temperature, the elastomer being for example a silanol polyether and the electret a halogenated poly-hydrocarbon ba~sed polymer .
One particular use of such transducers under high pressure is the production of sparks in a gas for the purpose, for example, of lighting a combustible mixture :
if in fact a high pressure is e~erted on a transducer of the above kind whose electrodes are connected to those of a capacitor, this latter can be charged up to the break-down v~ltage of a gas present between these latter elec-trodes, said breakdown being favoured by these latter having a pointed shapeO
The presence of an elastic layer in the transduc~rs considered may also be used for damping e~fects, e.g. for applications using safety or end stops .
Instead o being more deformable than the electret 9 the intermediate layer could be selected less deformable than this electret, which could be envisaged in the case of a particularly hard intermediate layer .
This would *or example be the case for a fluorinated propylene polyethylene (sometimes ~nown under its commercial 9a~
name TE~LON l~EP) electret laid on a fused glass support, made from used quartz or silica which forms then the intermediate layer ~
In other variations, the difference between the relative deformabilities of the electret and the inter-mediate layer is not manifested directly by application o a mechanical stress ~pressure or shock), but indirectly by temperature variations or even more indirectly by irradiation O
Thus, if intermediate layer 7 tFig. 2) has a thermal expansion coefficient very different from that of electret 4, the heating of t:he assembly causes as before a relative displacement of charged fac~ ~ of the electret in relation to one of the electrodes, such a lS displacement being capable of c:reating an electrical voltage at the terminals of ~hese electrod~s. Thus~ with the transducers considered, temperature detectors can be constructed capable o~ automatically triggering o~f a visual or sound alarm as soon as said temperature exceeds a predetermined threshold.
~y way of example, we can take up again the example which has just been given since the respective thermal expansion coefficients of fluorinated propylene poly-etl~ylene and fused quartz are 9.10 5 cm/cm/degree and 25- 5~5.10-7 Cm/cm/degree~
An electret made from polyethylene (expansion coefficient 17.10 5 cm/cm/degree) or from polyvinylidene chloride ~19.10 5) plated on a cristalline alumina support, a substance whose expansion coefficient is close to that of quartz,could also be used, this support forming the intermediate layer O
Or again recourse could be had to an electret o*
9~
the above kind made from fluorinated propylene poly-ethylene plated on a polyimide support texpansion coefficient 2.10 5 cm/cm/degree) : laminates of these ~ two polymers are distributed under the name KAPTON
type Fal~ ~ ~s sufficient to charge the desired por-tion thereof in order to make them capable of forming transducers according to the invention .
It is also possible for intermediate layer 7, with regard to certain radiations, to have a power o~ absorp-tion very different from that of the electret and to be capable of trans*orming these radiations into heat, if those latter are not alrèady heat radiations (infrared).
In such a case, if the transducer is arrange~ so that the intermediate layer and the electret are hoth exposed to the radiations considered, one of these two elements is hea-ted more than the other and so is more expanded than the other, even if their thermal expansion coefficients are e~ual, which results again in a relative displacement of interface F in relation to the electrodesO
The same result could also be obtained, even ~ith identical coefficients o~ absorption for the two elements with regard to the radiations considered, if the assembly is arran~ed so that only one of these elements can be exposed to such radiations or if one of them is much more exposed to said radiations than the other .
As e~amples the following can be mentioned :
- an electret of the above type made from fluorinated propylene polyethylene practically transparent in the visi-ble spectrum, plated on a ruby support-forming the above intermediate layer - (chromium doped alumina with a weight concentration of impurities of 10 to 10 ~), a suhstance which absor~s to a very high degree the blue and transforms Tra c/~ ~a r/f~' - 1~ -L8(~
it partly into heat, partly into red light ;
an electret of the same type as previously who~se response is practically flat up to the wave length ~ of 3.8 microns, plated on a TETRASIL glass obtained from silicon halogenide, which also has a very flat response, except for radiations whose wave-length is close to 2.75 microns, which it transforms into heat, - conversely, an electret, still of the previous type, very adsorbent in the wave-length range between 7.5 and 9.5 microns, plated on a suppo~t such as usually used i n infrared spectographs for their transparency to these rays1 particularly made from potassium bromide KBr, sodium chloride NaCl, silver chloride AgCl or caesium iodide CsI , The transducers constructed according to these three examples allow detection or measuring to be carried out on radiations respectively blue, having a wave-len~th eaual to 2.75 microns and infrared (infra-red imagery).
In these different cases, it is of course adYisable that at least one of the electrodes is transparent to the radiations used; it is advantageously formed by metallization of a very thin layer, e.g. of gold with a thickness of a few angstroms, which is possible for the contact resistance of such a layer can be kept relatively low (10 oh~s) much less than the input impedance of the user circuit .
Following which and whatever the embodiment adopted, there is finally provided a transducer whose constitution, operation and advantages follow sufficiently from what has gone before 0 As is evident and as it follows moreover already ~q G/e ~c~ r ~ _ 20--~8~
from what has gone before, the invention is in no wise limited to those of its embodiments and modes of appli-cation which have been more~specially considered; it embraces, on the contrary,all variations thereof, particularly those where the transducers considered are used for mechanical signal emission purposes (acoustic or other) rather than for reception or detection purposes of such signals as most of those which have been more particularly described above .
Claims (16)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An electro-mechanical transducer comprising disposed between two electrically conducting electrodes which have connected to the same outside utilization circuit, on the one hand, an electret comprising a dielectric foil permanently electrically charged on at least one of its two faces or adjacent thereto, and on the other hand, disposed between said at least one face of -the electret and the facing electrode face, a solid intermediate layer made from a material having a deformability different from that of the electret, the material forming said intermediate layer having an electrical resisitivity at least greater than 1015 ohms/cm and said intermediate layer having a smooth and continuous face in intimate contact with the whole of the useful area of said at least one charged face of the electret.
2. A transducer according to claim 1, characterized in that the inter-mediate layer is formed from an elastomer.
3. A transducer according to claim 2, characterized in that the elasto-mer comprises a silicone-based composition.
4. A transducer according to claim 2, characterized in that the elasto-mer is in the form of a foam with an integral skin.
5. A transducer according to claim 3 wherein said elastomer comprises methylvinylsiloxane.
6. A transducer according to claim 3 wherein said elastomer comprises methylvinylphenylsiloxane,
7. A transducer according to claim 1, characterized in that the inter-mediate layer comprises an aluminum oxide-based composition.
8. A transducer according to claim 1, characterized in that the elec-trodes, the electret and the intermediate layer are coaxial and concentric.
9. A transducer according to claim 1, for which the intermediate layer and the electret both include parallel flat faces, said transducer being arranged so as to expose its intermediate layer to pressure variations generat-ed by the simple touch of a finger.
10. A transducer according to claim 9, wherein said transducer is asso-ciated with several other transducers of the same type, each actuatable through an aperture in a rigid cover, all the apertures in said cover being aligned with each other so as to form a multiple control keyboard.
11. A transducer according to claim 1, characterized in that the inter-mediate layer is formed from a material having a thermal expansion coefficient distinctly different from that of the electret.
12. A transducer according to claim 1, characterized in that the electret and the intermediate layer are formed from materials having absorption coeffi-cients distinctly different for certain radiations, thereby resulting in dif-ferences in heating and thus in the deformation of these materials, the assem-bly of the electrodes, the electret and the intermediate layer being arranged so that the layer and the electret may be reached by such radiations emitted from outside the electrodes.
13. A transducer according to claim 1, further comprising, stacked successively on each of the two faces of a common central electrode, an elec-tret, an intermediate layer made of material which deforms differently than the last-mentioned electret and which has an electrical resistivity greater than 1015 ohms/cm and a second electrode, the order of stacking of the layer and the electret being reversible.
14. A transducer according to claim 1 wherein the transducer is associat-ed with an electrical circuit of the MOS type.
15. A transducer according to claim 14, characterized in that the MOS
type electrical circuit comprises a resistor determining the input impedance, an inverter and a feed back resistor, the output of this circuit being applied to a utilization circuit.
type electrical circuit comprises a resistor determining the input impedance, an inverter and a feed back resistor, the output of this circuit being applied to a utilization circuit.
16. A transducer according to claim 1, wherein the transducer is associated with a dead point overshoot resilient control mechanism allowing the trigger pressure threshold to be accurately determined.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR7720524A FR2397120A1 (en) | 1977-07-04 | 1977-07-04 | IMPROVEMENTS IN ELECTROMECHANICAL TRANSDUCERS |
| FR7720524 | 1977-07-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1118090A true CA1118090A (en) | 1982-02-09 |
Family
ID=9192950
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000306344A Expired CA1118090A (en) | 1977-07-04 | 1978-06-28 | Electromechanical transducers using an electret |
Country Status (11)
| Country | Link |
|---|---|
| US (1) | US4250415A (en) |
| JP (1) | JPS5414696A (en) |
| BE (1) | BE868729A (en) |
| CA (1) | CA1118090A (en) |
| CH (1) | CH624523A5 (en) |
| DE (1) | DE2829225A1 (en) |
| FR (1) | FR2397120A1 (en) |
| GB (1) | GB2013032B (en) |
| IT (1) | IT1103644B (en) |
| NL (1) | NL7807245A (en) |
| SE (1) | SE7807402L (en) |
Families Citing this family (61)
| Publication number | Priority date | Publication date | Assignee | Title |
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| FR2472901A1 (en) * | 1979-12-28 | 1981-07-03 | Thomson Csf | BIMORPHIC TRANSDUCER IN POLYMER MATERIAL |
| GB2072458A (en) * | 1980-03-03 | 1981-09-30 | Tape Developments Ltd C | Electroacoustic transducers |
| US4441038A (en) * | 1980-06-30 | 1984-04-03 | Tokyo Shibaura Denki Kabushiki Kaisha | Electret device |
| US4443711A (en) * | 1980-06-30 | 1984-04-17 | Tokyo Shibaura Denki Kabushiki Kaisha | Electret device |
| US4370182A (en) * | 1981-03-16 | 1983-01-25 | Gte Products Corporation | Method of making tape transducer |
| US4382196A (en) * | 1981-03-16 | 1983-05-03 | Gte Products Corporation | Tape transducer |
| DE3127406C2 (en) * | 1981-07-10 | 1986-03-27 | Siemens Ag, 1000 Berlin Und 8000 Muenchen | Modulation device for optical signal transmission |
| US5234551A (en) * | 1981-09-24 | 1993-08-10 | Dutt William H | Extended nip press belt having an interwoven base fabric and an impervious impregnant |
| US4509527A (en) * | 1983-04-08 | 1985-04-09 | Timex Medical Products Corporation | Cardio-respiration transducer |
| US4524247A (en) * | 1983-07-07 | 1985-06-18 | At&T Bell Laboratories | Integrated electroacoustic transducer with built-in bias |
| DE3333083A1 (en) * | 1983-09-14 | 1985-04-04 | Metev Gesellschaft für Entwicklung und Verwertung med. Bedarfs GmbH & Co KG, 4840 Rheda-Wiedenbrück | ELECTRICALLY CHARGED LAYING MATERIAL AND METHOD FOR THE PRODUCTION THEREOF |
| GB2151434B (en) * | 1983-12-02 | 1987-10-14 | Raytheon Co | Multi-layered polymer transducer |
| US4805157A (en) * | 1983-12-02 | 1989-02-14 | Raytheon Company | Multi-layered polymer hydrophone array |
| US4567382A (en) * | 1984-04-10 | 1986-01-28 | Microtel B.V. | Electret transducer and a method for manufacturing an assembly of backplate, electret foil and diaphragm plate |
| CA1277415C (en) * | 1986-04-11 | 1990-12-04 | Lorne A. Whitehead | Elastomer membrane enhanced electrostatic transducer |
| US4808849A (en) * | 1987-02-03 | 1989-02-28 | Canadian Patents And Development Limited/Societe Canadienne Des Brevets Et D'exploitation Limitee | Phyllosilicate electrets and a method for their manufacture |
| FR2632737B1 (en) * | 1988-06-10 | 1992-12-24 | Thomson Csf | METHOD FOR OBTAINING A NON-LINEAR ELECTROOPTICAL EFFECT GENERATOR, STRUCTURE OBTAINED AND APPLICATIONS |
| US4939784A (en) * | 1988-09-19 | 1990-07-03 | Bruney Paul F | Loudspeaker structure |
| US5206914A (en) * | 1990-01-05 | 1993-04-27 | Koss Corporation | Electrostatic acoustic transducer having extremely thin diaphragm substrate |
| DE4429035A1 (en) * | 1994-08-16 | 1996-02-22 | Gore W L & Ass Gmbh | Movement sensor for patient fluids for life signs |
| DE4429033A1 (en) * | 1994-08-16 | 1996-02-22 | Gore W L & Ass Gmbh | Patient movement sensor with high sensitivity |
| US5835027A (en) * | 1996-11-07 | 1998-11-10 | Tyburski; Robert M. | Residual charge effect traffic sensor |
| US5907280A (en) * | 1997-04-28 | 1999-05-25 | Dkl International, Inc. | Static electrification assisted dielectrokinesis detection of plastics and other materials |
| KR100792304B1 (en) * | 1999-11-25 | 2008-01-07 | 내츄럴 컬러 카리 키르자바이넨 오와이 | Electromechanic film and acoustic element |
| FI108204B (en) * | 1999-11-25 | 2001-11-30 | Kari Johannes Kirjavainen | A film for converting energies |
| US20020080684A1 (en) * | 2000-11-16 | 2002-06-27 | Dimitri Donskoy | Large aperture vibration and acoustic sensor |
| FI118369B (en) * | 2000-12-19 | 2007-10-15 | Emfitech Oy | Electromechanical converter and method of manufacturing an electromechanical converter |
| DE10222123A1 (en) * | 2002-05-17 | 2003-11-27 | Aircabin Gmbh | Aircraft luggage locker has gas spring offload unit locked closed by handle sensor controlled electrically actuated valve |
| ATE367642T1 (en) * | 2002-05-22 | 2007-08-15 | Hannu Olkkonen | ELECTRICAL CONVERTER |
| AU2003238881A1 (en) * | 2002-06-07 | 2003-12-22 | California Institute Of Technology | Method and resulting device for fabricating electret materials on bulk substrates |
| EP1512216A2 (en) * | 2002-06-07 | 2005-03-09 | California Institute Of Technology | Electret generator apparatus and method |
| DE102007007957A1 (en) * | 2007-02-17 | 2008-08-21 | Lyttron Technology Gmbh | Loudspeaker constructed of foils |
| TWI343756B (en) * | 2009-08-10 | 2011-06-11 | Ind Tech Res Inst | Flat loudspeaker structure |
| JP5446879B2 (en) * | 2008-02-22 | 2014-03-19 | 旭硝子株式会社 | Electret and electrostatic induction type conversion element |
| KR20110002458A (en) * | 2008-03-27 | 2011-01-07 | 아사히 가라스 가부시키가이샤 | Electret and electrostatic induction conversion device |
| CN101981456B (en) * | 2008-03-31 | 2013-11-06 | 旭硝子株式会社 | Acceleration sensor device and sensor network system |
| EP2266795B1 (en) * | 2008-04-17 | 2015-07-01 | Asahi Glass Company, Limited | Electret and electrostatic induction conversion device |
| JP5527211B2 (en) * | 2008-09-19 | 2014-06-18 | 旭硝子株式会社 | Electret, electrostatic induction conversion element, and method for manufacturing electret |
| EP2400573A1 (en) * | 2010-06-23 | 2011-12-28 | Bayer MaterialScience AG | Electromechanical converter, method for manufacture and use of same |
| TWM395976U (en) * | 2010-06-30 | 2011-01-01 | Tsung-Hung Wu | Electret electroacoustic transducer |
| JP5559229B2 (en) * | 2012-03-23 | 2014-07-23 | 東海ゴム工業株式会社 | Transducer |
| DE102012202422A1 (en) * | 2012-02-16 | 2013-08-22 | Robert Bosch Gmbh | Transducer array |
| US9161113B1 (en) | 2012-02-17 | 2015-10-13 | Elvin Fenton | Transparent lens microphone |
| US9980054B2 (en) | 2012-02-17 | 2018-05-22 | Acoustic Vision, Llc | Stereophonic focused hearing |
| JP5691085B2 (en) * | 2012-07-25 | 2015-04-01 | 株式会社ビスキャス | Vibration generator and power generation method using the same |
| JP5691080B2 (en) * | 2012-07-25 | 2015-04-01 | 株式会社ビスキャス | Vibrating power generator, manufacturing method thereof and power generation method |
| JP6097550B2 (en) * | 2012-12-18 | 2017-03-15 | 古河電気工業株式会社 | Laminated power generator |
| JP6133051B2 (en) * | 2012-12-18 | 2017-05-24 | 古河電気工業株式会社 | Laminated power generator, power generation method of laminated power generator |
| JP6133050B2 (en) * | 2012-12-18 | 2017-05-24 | 古河電気工業株式会社 | Stacked power generator |
| JP6133118B2 (en) * | 2013-04-24 | 2017-05-24 | 古河電気工業株式会社 | Vibration generator |
| CN104242723B (en) * | 2013-06-13 | 2019-06-04 | 北京纳米能源与系统研究所 | Single electrode friction nanometer power generator, electricity-generating method and from driving follow-up mechanism |
| JP6278619B2 (en) * | 2013-06-14 | 2018-02-14 | 古河電気工業株式会社 | Vibration generator |
| JP6247928B2 (en) * | 2013-12-25 | 2017-12-13 | 古河電気工業株式会社 | Vibration generator |
| JP6494597B2 (en) * | 2014-03-26 | 2019-04-03 | シチズン時計株式会社 | Static induction generator |
| JP6699119B2 (en) | 2015-01-22 | 2020-05-27 | 株式会社リコー | Element and power generator |
| US10248241B2 (en) | 2015-05-07 | 2019-04-02 | Ricoh Company, Ltd. | Digital signage system |
| US10305020B2 (en) | 2015-05-15 | 2019-05-28 | Ricoh Company, Ltd. | Element and electric generator |
| JP6645161B2 (en) * | 2015-12-11 | 2020-02-14 | 株式会社リコー | Power generator |
| CN110869531B (en) * | 2017-07-13 | 2022-06-03 | 日本制铁株式会社 | Grain-oriented electromagnetic steel sheet and method for producing grain-oriented electromagnetic steel sheet |
| JP2019106440A (en) * | 2017-12-12 | 2019-06-27 | 日東電工株式会社 | Electret film |
| JP2022147757A (en) | 2021-03-23 | 2022-10-06 | 株式会社リコー | element |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3967027A (en) * | 1969-12-19 | 1976-06-29 | Kureha Kagaku Kogyo Kabushiki Kaisha | Stable electret retaining a high surface potential and method of making the same |
| US3736436A (en) * | 1971-11-04 | 1973-05-29 | Mc Donnell Douglas Corp | Electret pressure transducer |
-
1977
- 1977-07-04 FR FR7720524A patent/FR2397120A1/en active Granted
-
1978
- 1978-06-19 CH CH664178A patent/CH624523A5/fr not_active IP Right Cessation
- 1978-06-28 CA CA000306344A patent/CA1118090A/en not_active Expired
- 1978-06-29 US US05/920,158 patent/US4250415A/en not_active Expired - Lifetime
- 1978-06-30 SE SE7807402A patent/SE7807402L/en unknown
- 1978-06-30 IT IT09516/78A patent/IT1103644B/en active
- 1978-07-03 DE DE19782829225 patent/DE2829225A1/en not_active Withdrawn
- 1978-07-04 BE BE189069A patent/BE868729A/en not_active IP Right Cessation
- 1978-07-04 GB GB7828814A patent/GB2013032B/en not_active Expired
- 1978-07-04 JP JP8059678A patent/JPS5414696A/en active Pending
- 1978-07-04 NL NL7807245A patent/NL7807245A/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5414696A (en) | 1979-02-03 |
| BE868729A (en) | 1979-01-04 |
| GB2013032B (en) | 1982-03-17 |
| IT1103644B (en) | 1985-10-14 |
| FR2397120B1 (en) | 1982-12-31 |
| SE7807402L (en) | 1979-01-05 |
| NL7807245A (en) | 1979-01-08 |
| GB2013032A (en) | 1979-08-01 |
| CH624523A5 (en) | 1981-07-31 |
| FR2397120A1 (en) | 1979-02-02 |
| IT7809516A0 (en) | 1978-06-30 |
| US4250415A (en) | 1981-02-10 |
| DE2829225A1 (en) | 1979-01-18 |
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