US3116377A

US3116377A - Sub-miniature transducer

Info

Publication number: US3116377A
Application number: US163708A
Authority: US
Inventors: Joachim H Todt
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-01-02
Filing date: 1962-01-02
Publication date: 1963-12-31
Anticipated expiration: 1980-12-31

Description

Dec. 31, 1963 J. H, ToDT 3,116,377

SUB-MINIATURE TRANSDUCER Filed Jan. 2, 1962 INVENTOR.

L/ofQc/L//M A. foar BY WMKQ M o@ /Jfgf/M United States Patent O 3,116,377 SUB-MINIATURE TRANSDUCER Joachim H. Todi, 301 Rice Creek Terrace, Minneapolis, Minn. Filed Jan. 2, 1962, Ser. No. 163,708 Claims. (Cl. 179-115) This invention relates to the iield of transducers and more particularly to the field of sub-miniature transducers.

Manufacturers of yacoustic electrical apparatus such as communications equipment, hearing aids, electronic stethoscopes, and other `acoustic communication apparatus have found that it is most ditiicult to produce or manufacture an acoustic transducer which retains the property of the larger transducer but is of a sub-miniature size. For one thing, hearing aids require microphones and receivers which have very good response characteristics in the acoustic range and the general trend of the manufacturers producing these components has been to make them of a size suflicient to reproduce frequencies in the acoustic range with relatively good high fidelity but their size has restricted their use in certain electrical apparatus. (This is especially true where confined to the space which is in the general shape of the ear canal Where the transducerscould be placed in the ear. llt is generally well known to those skilled in the art that in a transducer such as a loudspeaker where the voice coil is placed upon a stem of the speaker or diaphragm, that there is relatively little distortion resulting from the mov-able conductor in the magnetic eld. However, when compared to a magnetic speaker, that is, one in which the coil is fixed, the efficiency of the moving coil speaker as compared to the magnetic speaker is relatively low. lt is also known that the magnetic speaker produces certain harmonics due to hysteresis in the movable member and therefore it also has some undesirable characteristics.

In a very small transducer which is used as a microphone or receiver, a small amount of distortion can be tolerated for the advantages gained in the extremely small size and ease of using the component in the ioverall electrical apparatus. With the availability of certain metals exhibiting extremely high values of permeabiltiy and eX- tremely low hysteresis losses such as permalloy generally consisting of approximately 80% nickel and 20% iron or in some instances may contain approximately 2% molybdenum with a reduction in the iron content. By the use of the permalloy material, it is possible to form low mass reactance parts which are movable and which are highly suitable to provide a transducer with high efiiciency.

ln the instant invention, the permalloy material is formed around the stem of the transducer diaphragm and is allowed to move or be moved within the magnetic lield created by the magnetic pole pieces surrounding the permalloy material. Where current is placed in the coils, the magnetic iieid is created in the permalloy material causing the diaphragm to produce an `acoustical output sound wave. Where the acoustical sound wave is received by the diaphrgam, movement of the permalloy material in the magnetic iield produces an induced current into the coil `and an output signal may be taken therefrom representative of the sound waves falling upon the diaphragm.

'It is therefore ia general object of the present invention to provide an improvement in sub-miniature transducers which may be used as a micro-phone or receiver.

It is still another object of the present invention to provide a diagram with a movable element which has low mass and exhibits high values of permeability and low values of hysteresis losses.

rice

It is another object of the present invention to provide a permalloy bobbin for use with a diaphragm where the permalloy spool remains in a balanced iiux position where no external signals or forces are applied to the transducer.

[It is another object of the present invention to provide pole pieces cooperating with the permialloy material in which the yair gap therebetween is of a varying dimension for certain applications.

It is yet another object of the present invention to provide a transducer which may be used as a microphone or receiver which has no moving coil parts.

These and other objects and advantages of my invention will more fully appear from the following description, made in connection with the accompanying drawings, wherein like reference characters refer to the same or similar parts throughout the several views, and in which:

PIG. l is a cross sectional view of the sub-miniature transducer taken along lines 1-1 of FIG. 2;

FIG. 2 is atop view of the transducer showing the perforated cover plate;

lFIG. 3 is a sectional View showing a variation in the pole pieces of the instrument to produce a varying air gap; and

FdG. 4 is another alternate construction for the pole pieces to produce a varying air gap.

In regards to PllG. l, a sub-miniature transducer lil has an outer cylindrical housing 11 which is generally made of some non-toxic material such as stainless steel which supplies a .rigid support for the elements contained within the housing. For certain applications, the housing will be used in such a manner that it comes in contact with the human body and therefore it is highly desirous that the material lforming housing 11 be of a non-toxic nature. A rear cover 12 which for some applications may also be perforated, is generally formed of the same non-toxic material as housing 11. Rear cover 12 is circular in nature and is sprung slightly so that the center portion is situated outwardly from a diametrical line between the edges of the cover. At the very edges of the cover 12 where it joins housing 11 it will be seen that housing 11 is rolled over or bent to provide a crimped edge 13 in order to hold rear cover 12 in place. Situated opposite rear cover 12 at the other end of cylindrical housing 11 is a perforated cover plate 114 which forms the front cover plate of the transducer. Front cover plate 14 contains a plurality of perforations 15 which admit sound pressure waves to the inside of the transducer in one instance and where used as `a receiver, sound is transmitted outwardly through perforations 115. Front lcover 14 is also held in place by cylindrical housing 11 by crimping the end portions 16 of housing 11 to keep front cover 14 in place. =Front cover 14 is also formed of the same material generally as housing 11 and rear cover 12 and for most applications is flexed or bent outwardly at its center to provide a bowed appearance.

Situated within housing 11 and directly underneath front cover plate 14 is a cone shaped thin plastic diaphragm 17 which is in the form of a speaker cone and has a concentric cylindrical stern l1S extending outwardly and away from the center of the diaphragm. ln other words, stem portion 18 extends rearwardly toward rear cover 12. Situated around the edge of diaphragm 17 are several corrugated rings 2li which provide the cone shaped diaphragm with greater flexibility. e Generally speaking, the diaphragm may be formed of a plastic material and one such material which is found to be excellent material for such use is Mylar. Another diaphragm 2.1 is located directly in front of rear cover 12 and also is formed of the same material as that generally used for diaphragm 17. Diaphragm 21 also contains a plurality of corrugated rings 22 which are situated near the outer diameter of the diaphragm in the same manner as that for diaphragm 17. The outer edges of

diaphragms

17 and 21 extend generally to the outer edges of housing 11 and conform to the same general circular shape. Since it is also desirable to keep the stern portion 18 moving concentrically about a central axis of the transducer, a small tip 23 which is a rearward portion and extension of stem 18, is fitted into a hole for perforation 24 which is formed in the center of the rear centering diaphragm which is sometimes known as the spider as used with loudspeaker assemblies. With the arrangement of

diaphragms

21 and 17 as shown Iand described, it will be seen that an axial movement maybe created by diaphragm 17, stem 18, and diaphragm 21 and the radial movement of the moving components should be extremely small or be non-existent.

A sleeve member or bobbin 25 is formed from a strip of highly magnetic permeability material such as permalloy by `wrapping multiple lay'ers of the thin material around stem 13. In other words, for all practical purposes, the stem or bobbin 25| is formed of concentric rings of the thin permalloy material and is wound in such a fashion that the edges of the strip near the front and rear portions are diametrically opposite each other and are substantially parallel to each other. It should be kept in mind that the material is extremely thin and most of the magnetic materials of this type are generally of a thickness approximating onefourth to one-eighth of a mil (0.25)(-3 to 0,125 l03 inches). It will also be seen that by the use of such thin material, that a relatively low mass sleeve or bobbin 25 can be created. In fact, where the unit is designed to such dimensions that it may be fitted within the ear canal, it will be seen that the parts within transducer 10 are extremely small. Opposite the forward edge of bobbin 25 in a radial direction is a washer shaped magnetic pole piece 26 which forms an airvgap 27 with bobbin 25. Another washer shaped magnetic pole piece 30 is located rearwardly and axially away from washer shaped magnetic pole piece 26 to form an air gap 31 -with bobbin 25. Centrally located between washer shaped pole pieces 26 and 301 is another washer shaped pole piece 32 nwhich forms an air gap 33 with sleeve member or bobbin 25. As will be seen later, pole piece 32 actually forms a return magnetic path. A pair of ring shaped

magnets

34 and 35 concentrically encircle the stem and sleeve member and are axially arranged between

pole pieces

26, 32, and 30. In other words, 'magnet 34 as sandwiched between

pole pieces

26 and 32 and magnet 35 is sandwiched between pole piece 32 and pole piece 30. For this particular embodiment, ring magnets 134- and 35 are formed adjacent to the internal edge of housing 11 and extend radially inwardly toward stem 18 approximately one-third of the radial distance to the center of stem 18. The magnetic eld which is created about

ring magnets

34 and 35 is such that the same magnetic poles of

ring magnets

34 and 35 are oriented towards each other. For example, the north pole N of ring magnet 34- is oriented towards the front of the transducer 1@ and the north pole N of ring magnet 35 is oriented toward the rear of the transducer. Thus, it will be seen that a north pole is created adjacent to air gap 27, 1a south pole is created adjacent to air gap 33, `and .an-other north pole is created adjacent to air gap 311.

Situated radially between

pole pieces

26 and 32 is an electrical coil 36 which is formed of a number of turns of wire and is formed concentrically around sleeve member 25. Coil 36 is llocated radially between ring magnet 34 and the `annular edge of

pole pieces

26 and 32 which form

lair gaps

27 and 33 respectively. In a similar manner, another electrical coil 37 is for-med of a multiple number of turns of wire concentrically about sleeve member 25 and is located axially between poile `pieces 32 and 30. Coil 37 is located radialily between ring magnet 35 and the annular edge of pole pieces 32 and 30 forming

air gaps

33 and 31 respectively. For certain applications it will be desirable to provide the proper insula tion for

coils

36 and 37 and the insulation is designated as numeral 38 for

coil

36 and 39 for coil 37. For some applications the insulation upon the conductors themselves may form sufficient insulating qualities to dispense with the insulation around the multiple turn coils. The coils are internally connected to each other (although not shown) and `a pair of output conductors 411 and 41 are connected to coils 36 and 37 respectively. In order to provide a tight tting relationship between

pole pieces

26, 32, 3), and

magnets

34 and 35, a pair of

spacers

42 and 43 in the form of annular rings are used to hold the yvarious elements in place. Ring 4-2 is situated near the internal edge of housing .11 and bears axially against the outer edge of diaphragm 17 and the outer radial edge of washer shaped pole piece 26. In like manner, annular ring 43 is situated adjacent the inside wall of housing 11 and bears axially against rear diaphragm 21 and against the pole piece 3ft' at its furthest radial extension `or outer edge. Gener-ally speaking,

spacers

42 and 43 are formed of some plastic material and it has been found that nylon serves as a very good annular ring spacer. Magnets 34 and 3S which are preferably made of an elongated single domain iron are then held in close contact with the washer shaped pole pieces which are generally formed of silicon iron, heavy gauge permalloy or other material having similar characteristics toA provide a highly eicient magnetic field path. It should also be noted that bob-bin 25 has a longitudinal dink equal to the center-to-center distance of the faces of pole pieces 26 and 30 forming the annular air gaps. In this position, the transducer is in a rest position with no external forces being applied.

With the diaphragm 17 at rest, the flux pattern would be from the face of pole piece 3i) at annular gap 31 through the permalloy sleeve member 25 and back to air gap 33 to the pole piece 32. Also, a magnetic path would be formed from the face of the annular air gap 27 through the permalloy sleeve member 25 and back to pole piece 32 through air gap 33. It should be remembered that an annular air gap is formed at each of the openings in the washer shaped pole pieces. Since the flux field establishes itself within the permalloy sleeve member 25, the sleeve member or bobbin is attracted between the two north pole positions or centrally as just previously described. Diaphragm 21 therefore acts largely in a centering capacity to keep the stem 18 and sleeve member 25 axially centered within the air gaps.

Assuming a current is applied to

coils

36 and 37, it can be further assumed that the permalloy bobbin is magnetized longitudinally as just described so that a north pole N is formed at the end of bobbin 25 near diaphragm 17. A south pole would then be formed at the end of sleeve member 25 nearest diaphragm 21. Since like poles repel each other and unlike poles attract each other, the permalloy bobbin isforcced downwardly towards the rear of the transducer and speaker diaphragm 17 which is formed as a part of stem 18 or connected to stern 18 is drawn inwardly. The operation of the sleeve member may be described in a slightly different manner arriving at the same results by assuming that current passes through

coils

36 and 37 in such a manner that the magnetic field created by the magnet 35 is increased at air gap 31 and the magnetic field created by magnet 34 is decreased at air gap 27. It is well recognized that a magnetic eld surrounds any conductor carrying current and therefore it is simply a matter of adding magnetic lines of flux in one part of the magnetic circuit and decreasing the number of lines of flux in another portion of the magnetic circuit by adding lines of flux which are traveling in an opposite direction. Thus there is an unbalanced condition created and the member lying in the magnetic field is thus carried along with the unbalanced magnetic field. In fact, in this particular manner of operation, a true magnetic push pull circuit or balanced bridge is formed.

It can be seen that by applying sound pressure Waves causing an axial movement of diaphragm 17 and bobbin 25 which is attached thereto, a current will be created in

coils

36 and 37 by the changing magnetic lines of force moving with the permalloy material. rlhus the transducer may receive electrical signals and provide acoustic output signals or may receive sound pressure signals upon the diaphragm 17 and provide electrical signals as an output.

Because the sound receiving or transmitting element is symmetrically suspended and includes magnetic elements forming a symmetrical magnetic field around the diaphragm, the highest possible efiiciency is obtained from the transducer and the problem of creating harmonics from a stiff spring suspension are substantially eliminated. As was pointed out earlier, diaphragm 21 is used largely to center stem 18 and bobbin 25 and therefore diaphragm 21 creates no spring pressure against stem 18 and therefore does not preload diaphragm 17.

If there are existing forces created by diaphragm 17 and diaphragm 21 upon the axial position of stem 18 and hence permalloy sleeve member 25, correction for such forces may be accomplished by changing the air gap between pole piece 26 and sleeve member 25. The annular air gap 27a is tapered so that as sleeve member 25 moves forwardly towards the front cover 14, the air gap would be reduced linearly and would have a greater effect upon the magnetic attraction and repulsion between pole piece 26 and sleeve member 25. It may be found desirable in other instances to form the annular air gap so that it is shaped like a spherical section and air gap 2711 of FIG. 4 shows such an air gap. Air gap 27h provides a non-linear changing air gap and dependent upon the type of forces exerted upon diaphragm 17 and diaphragm 21, it will be seen that a proper shape may be found for the air gap to correct for any spring forces introduced by the diaphragms or non-linearity of the associated amplifier. The other air gaps, such as 33 and 31 remain unchanged and are equal and parallel to each other.

While diaphragm 21 is preferably made of plastic, it can be made of metal but preferably should be made of a non-magnetic metal. Metal produces any current losses which are undesirable in the operation of the transducer. It should also be kept in mind that since coils 36 and 37 are generally of a higher impedance, it will be found that they are more easily matched for the output impedances of the amplifying apparatus which is generally used with the transducer.

It is also apparent that the problem of impedance matching between the transducer and its associated amplier is substantially eliminated. That is, when a conventional winding is employed upon a moving diaphragm, the number of turns of the coil must remain low to produce a low mass coil, but the low number of turns also produces a coil of very low impedance which must be matched to a high input impedance of the amplifier and this is generally done with an impedance matching transformer. However', in the instant invention, because the number of turns of the coil can be considerably higher due to its stationary position within the transducer and non-existing requirement of low mass as found with the moving coil described above, the coil has a much higher impedance and substantially eliminates the impedance mismatch between transducer and amplifier, thus eliminating the requirement for the impedancematching transformer. Elimination of such a device reduces the weight of the overall apparatus used with the transducer, and in apparatus such as hearing aids eliminates a rather difficult design problem in attempting to produce a small sized component in addition to eliminating the weight of the component.

Upon examination of the disclosure it will be found that a new and novel transducer has been described wherein there is no moving voice coil or moving electrical conductors within the transducer. Further, it will be seen that by the use of a sleeve member made of permalloy that a new and novel transducer has been described that has a low moving mass. For certain applications the air gaps may be varied to produce improved results and the transducer has been described as operating both as a microphone and a receiver in the acoustical range.

it will, of course, be understood that various changes may be made in the form, details, arrangement and proportions of the parts Without departing from the scope of my invention which consists of the matter shown and described herein and set forth in the appended claims.

What is claimed is:

1. A sub-miniature transducer for changing energy from one form into another comprising:

(a) a diaphragm having corrugated outer edges and a stem extending outwardly therefrom;

(b) 1a bobbin of multiple thin layers of high magnetic permeability material attached to said stem of said diaphragm;

(c) a pair of magnet members aligned parallel to said bobbin, said magnet members being separated lalong the longitudinal axis of said stern and having pole pieces formed on each end thereof adjacent to said bobbin to form air gaps therebetween;

(d) a pair of electrical coils disposed between said pole pieces for carrying a changing current associated with a changing magnetic field, said magnetic field cooperating with movement of said bobbin attached to said diaphragm;

(e) and a housing having said diaphragm attached thereto about its outer edges and having said pole pieces fixedly held in operable position therewithin, said housing containing perforations adjacent the diaphragm.

2. A sub-miniature transducer for changing energy from one form into another comprising:

(b) a bobbin of multiple thin layers of high magnetic permeability material attached to said stem of said diaphragm;

(c) a pair of magnets aligned parallel to said stem and bobbin, said magnets being separated along the longitudinal axis of said bobbin and each having a magnetic field;

(d) magnetic pole pieces disposed on each end of said magnets and in spaced relation to said stem and sleeve to form air gaps therebetween;

(e) a pair of electrical coils disposed between said three pole pieces for carrying a changing current associated with a changing magnetic field, said magnetic field cooperating with movement of said bobbin attached to said diaphragm through said stem;

(f) and a housing having said diaphragm attached thereto about its outer edges and having said magnetic pole pieces fixedly held in operable position therewithin, said housing containing perforations adjacent `the diaphragm.

3. A sub-miniature transducer for changing energy from one form into another comprising:

(a) a circular thin plastic diaphragm having concentric corrugated rings about the outer edges thereof and a concentric cylindrical stem extending out- Wardly therefrom;

(b) a sleeve member of multiple thin layers of high magnetic permeability material fixedly wound around said cylindrical stem of said diaphragm;

(c) a pair of ring shaped magnets concentrically encircling said stern and sleeve member, said magnets being separated along the axial axis of said sleeve Iand each having a magnetic field;

(d) washer shaped magnetic pole pieces disposed on each end of, and between, said ring shaped magnets for concentrically encircling said stem and sleeve to form air gaps therebetween;

(e) a pair of electrical coils of multiple turns concentrically formed around said stem and sleeve member, and disposed between said three pole pieces for carrying a changing current associated ywith a changing magnetic field, said magnetic field cooperating with movement of said sleeve member attached to said diaphragm through said stem;

(f) and a housing having said diaphragm attached thereto about its outer edges and having said magnetic pole pieces fixedly held in operable position therewithin, said housing containing perforations adjacent the diaphragm.

4. A sub-miniature transducer for changing energy from one form into another comprising:

(a) a cone shaped thin plastic diaphragm having concentric corrugated rings about the outer edges thereof and a concentric cylindrical stem extending outwardly therefrom;

(b) a sleeve member of multiple thin layers of high magnetic permeability material having a composition of approximately 80% nickel and 20% iron, said sleeve being fixedly wound around said cylindrical stem of said diaphragm;

(c) a pair of ring shaped magnets concentrically encircling said stem and sleeve member, said magnets being separated along the axial axis of said sleeve and each having a magnetic field;

(d) three washer shaped magnetic pole pieces one of which is disposed on each end of, and between, said ring shaped magnets, all three of which concentrically encircle said stem and sleeve to form an air gap therebetween;

(e) a pair of electrical coils concentrically formed around said stem and sleeve member, and disposed longitudinally between Said three pole pieces and radially inside said pair of magnets for carrying a changing current therein associated with a changing magnetic'field, said magnetic eld cooperating with movement of said sleeve member attached to said diaphragm through said stem;

(f) and a housing having said diaphragm attached thereto about its outer edges and having said magnetic pole pieces iixedly held in operable position therewithin, said housing containing perforations adjacent the diaphragm.

5. The invention as set forth in claim 2 wherein: said bobbin has a longitudinal length equ'al to the center-tocenter distance of the faces of said greater disposed pole pieces forming said vair gaps, said length measured parallel to said longitudinal taxis.

6. The invention as set forth in cl-aim 2 including: a centering diaphragm having corrugated outer edges attached to `said housing and disposed opposite said diaphragm having said stern for engaging said stern so that said lair gaps are substantially equal as said bobbin moves with respect to said pole pieces.

7. The invention as set forth in claim 2 wherein: the faces of at least one of said pole pieces which are farthest disposed and form said air gaps with said bobbin are tapered towards the stem of said diaphragm so that the farthest disposed sides of said pole pieces form a smaller air gap than the closest disposed sides of said pole pieces.

8. The invention as set forth in claim 3 wherein: the faces of the hole of at least one of said washer shaped magnetic pole pieces which are farthest disposed and form said air gaps are spherical sections where the farthest disposed sides of said pole pieces form a smaller air gap than the closest disposed sides of said pole pieces.

n o 9. A sub-miniature transducer for changing energy from one form to another comprising:

(a) a cone shaped diaphragm responsive to audio sound pressures having concentric corrugated rings about the outer edges thereof and a concentric cylindrical stem extending outwardly therefrom;

(b) a sleeve member of multiple thin layers of high magnetic permeability material ixedly wound around said cylindrical stem of said diaphragm;

(d) washer shaped magnetic pole pieces disposed on each end of said ring shaped magnets for engagement therewith, said pole pieces concentrically encircling said sleeve to form air gaps therebetween;

(e) a pair of electrical coils of multiple turns having output terminals, said coils being concentrically formed around said stem and sleeve member and being disposed between said pole pieces for producing a changing current created by a changing magnetic field, said magnetic field developed by movement of said diaphragm due to said sound pressures thereby causing said sleeve member attached thereto to move with respect to said pole pieces, and increase or decrease said magnetic field of said ring shaped magnets;

(f) and a housing having said diaphragm attached thereto about its outer edges and having said magnetic pole pieces fixedly held in operable position therewithin, said housing containing perforations adjacent the diaphragm to admit said audio sound pressures.

l0. A sub-miniature transducer for changing energy from one form into another comprising:

(a) a cone shaped diaphragm having concentric currugated rings about the outer edges thereof and a concentric cylindrical stem extending outwardly therefrom;

(d) washer shaped magnetic pole pieces disposed on each end of said ring shaped magnets for engagement therewith, said pole pieces concentrically en- `cirieling said stem and sleeve to form air gaps therebetween;

(e) a pair of electrical coils of multiple turns responsive to electrical signals representative of audio sound waves, said coils being concentrically formed around said stem and sleeve member and being disposed between said three pole pieces for receiving a changing current to create a changing magnetic field, said magnetic field causing movement of said sleeve member and said diaphragm attached thereto to produce audio sound pressures;

(f) and a housing having said diaphragm attached thereto about its outer edges and having said magnetic pole pieces ixedly held in operable position therewithin, said housing containing perforations adjacent the diaphragm to transmit said audio sound pressures.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A SUB-MINIATURE TRANSDUCER FOR CHANGING ENERGY FROM ONE FORM INTO ANOTHER COMPRISING: (A) A DIAPHRAGM HAVING CORRUGATED OUTER EDGES AND A STEM EXTENDING OUTWARDLY THEREFROM; (B) A BOBBIN OF MULTIPLE THIN LAYERS OF HIGH MAGNETIC PERMEABILITY MATERIAL ATTACHED TO SAID STEM OF SAID DIAPHRAGM; (C) A PAIR OF MAGNET MEMBERS ALIGNED PARALLEL TO SAID BOBBIN, SAID MAGNET MEMBERS BEING SEPARATED ALONG THE LONGITUDINAL AXIS OF SAID STEM AND HAVING POLE PIECES FORMED ON EACH END THEREOF ADJACENT TO SAID BOBBIN TO FORM AIR GAPS THEREBETWEEN; (D) A PAIR OF ELECTRICAL COILS DISPOSED BETWEEN SAID POLE PIECES FOR CARRYING A CHANGING CURRENT ASSOCIATED WITH A CHANGING MAGNETIC FIELD, SAID MAGNETIC FIELD COOPERATING WITH MOVEMENT OF SAID BOBBIN ATTACHED TO SAID DIAPHRAGM;