US3363227A

US3363227A - Electroacoustic transducer with improved electromagnetic drive

Info

Publication number: US3363227A
Application number: US524013A
Authority: US
Inventors: Jr Frank Massa
Original assignee: Dynamics Corp of America
Current assignee: MASSA DONALD P COHASSET; Dynamics Corp of America; Massa Products Corp
Priority date: 1966-02-01
Filing date: 1966-02-01
Publication date: 1968-01-09
Anticipated expiration: 1985-01-09
Also published as: US3441903A

Description

Jan. 9, 1968 ss JR 3,363,227

ELECTROACOUSTIC TRANSDUCER WITH IMPROVED ELECTROMAGNETIC DRIVE Filed Feb. 1, 1966 2 Sheets-Sheet 1 FIG! INVENTOR liigflw MAssA, JR.

ORNEY Jan. 9, 1968 F. MASSA, JR 3,363,227

ELECTROAGOUSTIC TRANSDUCER WITH IMPROVED ELECTROMAGNETIC DRIVE Filed Feb. 1, 1966 2 Sheets-Sheet 2 FIGZ IN'VENTOR RANK MASSA,JR.

TTORNEY United States Patent OflFice 3,363,227 Patented Jan. 9, 1968 Mass.

Filed Feb. 1, 1966, Ser. No. 524,013 8 Claims. (Cl. 340--8) This invention is concerned with an improved electromagnetic transducer, and, more particularly, with an improved electromagnetic design which is particularly advantageous for efficient operation of electroacoustic transducers operating in the mid and high audible frequency regions.

It is well known that electromagnetic circuits which must operate efficiently at the higher frequencies must have magnetically-conducting elements which employ thin laminations in order to reduce the eddy-current losses in the magnetic core. The lamination thickness must be particularly reduced if a high operating flux density is required in the magnetic circuit. The eddy-current losses are proportional to t f B where t is the lamination thickness, is the frequency, and B is the peak value of the alternating flux density. It can be seen readily from the above relationship that as the frequency and flux density increase in a magnetic circuit, the thickness of lamination must be reduced in order to hold the eddy-current losses to reasonable levels so that the efiiciency of the electroacoustic transducer can be kept at an acceptable level.

The conventional method of laminating a magnetic circ'uit consists in employing stacks of thin magnetic stampings which are held together by a bonding cement or by bolts or clamps. For electromagnetic circuits operating at the lower audio frequencies the lamination thickness may be of the order of and the problem of stacking fiat laminations of such thickness presents no difiiculty in the handling and assembling of the stampings. When laminations have to be used in which the thickness is reduced to the order of only a few thousandths of an inch or less the cost of handling many thousands of paper-thin laminations becomes very high. My invention solves this problem with a new design of an electromagnetic transducer for eflicient high-power operation, which employs large quantities of thin magnetic lamination material without the necessity of handling individual thin stampings.

It is a primary object of my invention to produce a laminated electromagnetic circuit for use in operating an electroacoustic transducer which eliminates the need for stacking and handling large quantities of very thin stampings such as are required in conventional designs.

It is another object of my invention to increase the electromagnetic elficiency of an electromagnetic trans ducer by employing a design of magnetic structure and associated current coils such that 100% of the conductor used in the coils is actively linked with the magnetic circuit.

A still further object of my invention is to improve the method of assembling an electromagnetic circuit employing the permanent magnets for polarization to achieve accurate control and maintenance of a uniform air gap even when the dimensions of the structure over which the air gap is to be maintained are very large.

Another object of my invention is to produce an improved electromagnetic transducer employing an inertial type of electromagnetic drive whereby 100% of the electrical and magnetic materials are closely coupled for creating increased electromagnetic driving forces whereby the efficiency of the transducer is increased over conventional designs.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, as well as its advantages thereof, will best be understood from the following description of several embodiments thereof when read in connection with the accompanying drawings in which:

FIG. 1 is a cross-sectional view taken in a plane containing the center line of one type of transducer illustrating the embodiment of my invention;

FIG. 2 is a sectional view taken along line IIII of FIG. 1, and

FIG. 3 is a view taken along the line IIIIII of FIG. 1.

Referring to the drawings, reference numeral 10 generally designates an electroacoustic transducer employing an electromagnetic drive system constructed in accordance with this invention. The transducer 10 comprises rigid circular plate 11 having a pair of parallel plane surfaces. Both sides of plate -11 are counter-bored, as illustrated, to provide two recessed sections with an intervening annular web portion 12. A hole is shown through the center of web portion 12, the purpose of which is only to eliminate unnecessary weight from the vibrating structure. Two tightly-wound

annular assemblies

13 and 14 of thin magnetically-conducting ribbon are securely bonded to the opposite faces of the web member 12 such that plate member 11 becomes an integrated rigid structure in combination with

magnetic assemblies

13 and 14.

Magnetic assemblies

13 and 14 are substantially identical in the construction as illustrated in FIG. 1. Each assembly is so formed as to provide an E-shaped magnetic cross section.

One method which I have found very satisfactory for preparing each of the assemblies -13 and 14 is to employ a circular piece of rigid tubing 15 as a form which is attached to a lathe or other similar coil-winding equipment. A roll of continuous thin magnetic ribbon or tape, illustrated in cross-section by the reference character 16, is wound tightly over the form 15 using an adhesive such as epoxy between successive layers as the thin magnetic ribbon or tape 16 is wound over the form 15. When the desired thickness of the wound magnetic tape is achieved, the full Width tape 16 is replaced by a narrower magnetic tape 17 and the winding continued until the desired thickness of the narrow tape 17 is built up, as illustrated. Next, a ring-shaped form is placed adjacent to the wound narrow section 17 to fill the space between the narrow ta e 17 and the full width tape 16 to provide a continuous surface for continuing the winding of the wide magnetic ribbon or tape 16 to provide a center portion 18 of the magnetic E-shaped assembly which is shown in cross-section in FIG. 1. After the center section is built up to the required amount, a narrow ribbon or tape 19 is used to continue the formation of the second annular slot in the E- shaped cross-section, and, finally, a second ring-shaped form is inserted to provide a continuous surface for winding a final section 20 of wide ribbon to complete the fabrication of the magnetic assembly as shown in FIG. 1.

By followng the process just outlined, a composite circular magnetic element is fabricated which, upon removal of the two ring-shaped forms, becomes a rigid circular annular laminated assembly with two circumferential slots, as illustrated by the E-shaped cross-sectional view of the

assemblies

13 and 14 in FIG. 1. By employing this fabrication procedure, I am able to use a continuous strip of very thin magnetic material to produce large-size magnetic assemblies for efiicient high-frequency operation with low production cost as compared with the alternative method of using E-shaped stampings for the magnetic structure. Another advantage of my circular ring-shaped magnetic assembly is that is creates a continuous circular slot in the magnetic structure within which

circular coils

21 and 22 may be assembled. In this design, the

currentcarrying coils

21 and 22 are completely surrounded by active magnetic material, which means that 100% of the winding is effective in generating electromagnetic forces. In the conventional assembly employing flat stacks of E- shaped laminations, rectangular-shaped coils are dropped in the parallel slots which are formed by the flat laminations and the ends of the coils remain outside the magnetic structure, and serve no useful electromagnetic function. The over-hanging portions of the rectangular coils add unnecessary moving mass to the vibrating system and also add unnecessary electrical resistance to the coils, which increases the electrical losses during the operation of the transducer.

In order to obtain the highest possible amount of actlve electrical conductor in the two circular slots of

magnetic assemblies

13 and 14, I have chosen to wind the coils with insulated copper ribbon in much the same manner as I have described in connection with the fabrication of the laminated magnetic structures. By employing thin insulated copper ribbon for the

coils

21 and 22, they can be wound to very accurate tolerances, and the copper will occupy practically the full cross-sectional area of the circular slot when it is assembled in place. In winding the coils, the first and last turn may be folded at 90 so that the ends of the copper ribbon will project out through the open ends of the slots. These free coil ends may be again folded over and cemented in place into radially under-cut recesses in the faces of the magnetic ring assemblies 13 and 14 so that the coil ends may be brought out into the center opening of the plate member 11 without projecting into the air gap as the leads run by over the face of the magnetic assembly. The physical details for recessing a small radial slot in the faces of the

magnetic assemblies

13 and 14 are not shown since they are not important in connection with the main purpose of this invention. Rather than complicate the illustrations and figures, the

coils

21 and 22 are schematically shown connected together in series by the

electrical conductors

23 and 24, and the complete inter-connected system of coils is in turn connected by means of

electrical conductors

25 and 26 to the insulated electrical terminals 28. Electrical terminals 28 are sealed into the counter-bored region of a housing 30 as shown. A rubber-covered underwater cable 31 containing two insulated conductors 32 is molded to a metallic flanged member 33 which contains a machined groove with a conventional O-ring 34 to effect an underwater seal when flanged member 33 is bolted in position to the housing 30 by means of bolts 35. The conductors 32 are soldered to the electrical terminals 28 to establish electrical connection from an external power source into the

current coils

21 and 22 for operating the transducer.

An important feature of my invention is that I achieve very high electromagnetic efficiency by the design which has been described. By using two electromagnetic assemblies, one on each side of plate 11, I effectively double the electromagnetic drive forces for operating the transducer which is desirable for high-power underwater operation.

To complete the operating electromagnetic circuit for the illustrated transducer, I prepare a pair of

magnetic assemblies

37 and 38 to mate with the

assemblies

13 and 14 previously described. In preparing the

magnetic assemblies

37 and 38, I advantageously employ a rigid tubular member 39 to serve as a form in the same manner as member 15 is employed in the

assemblies

13 and 14. Over the form 39 a tight continuous winding of thin magnetic ribbon is rigidly bonded to build up a section 40 having a thickness which is identical to the corresponding thickness of the mating section 16 or the

magnetic assembly

13 or 14. At this point the winding of the thin magnetic ribbon is in terrupted and a number of permanent magnets 41, shown both in FIGS. 1 and 2 and having a cylindrical contour, are bonded to the outer periphery of the fabricated section of the wound magnetic tape. The magnetic ribbon is then bonded to an outer surface portion 42 of the magnets 41, and the winding is continued until the desired thickness of magnetic material is achieved to provide a section 43 corresponding to the section 18. At this point the winding is again interrupted and a second set of mag nets 44 is bonded into position, as illustrated in FIGS. 1 and 2. The cylindrical contour of the surfaces of the magnets 44 corresponds to the diameter of the built-up magnetic winding. After assembling the magnets 14 into position the magnetic ribbon is tightly bonded and wound over the outer surface 45 of the magnet 44 until the thickness of the winding is built up to the desired amount to provide a section 46 mating with the section 20 to complete the total

magnetic assemblies

37 and 33.

It is noted that the wound coil assemblies can have shapes other than the illustrated cylindrical shape, such as oval, square, rectangular or other polygonal shapes, in which case the magnets should have corresponding shapes. It is also noted that an electromagnet assembly can be used in place of a permanent magnet assembly in which case the

assemblies

37 and 38 have constructions similar to that of the

assemblies

13 and 14.

In the transducer which I have used to illustrate one application of my invention, the operation of the electromagnetic transducer results from the inertial alternating forces generated between a massive spring-suspended base member which contains a portion of the electromagnetic circuit and the driven portion of the magnetic circuit which is attached to the vibratory sound radiating portion of the transducer. Having prepared the lamination and

permanent magnet assemblies

37 and 38 as rigid composite cylindrical annular units, I securely bond the units onto planar recessed

surfaces

47 and 48 of

massive base members

49 and 50. The

base members

49 and 50 are fabricated of a non-magnetic material such as bronze so as not to magnetically short-circuit the

permanent magnets

41 and 44. Before bonding the

magnetic assemblies

37 and 38 to the recessed surfaces 47 and 48 of

parts

49 and 50, I prepare the end surfaces of the

magnetic assemblies

37 and 38 by grinding or by other suitable means to make them perfect planes. The same surface preparation is preferably employed for the assembled

magnetic structures

13 and 14 and prior to bonding their surfaces into the recessed sections of plate 11.

To secure the desired operation of the transducer illustrated in FIG. 1, a plurality of peripherally-mounted

spring members

51 and 52 attached by means of bolts 53 and 54 to the prepared outer peripheral surface portions of

base members

49 and 50, as shown. The

spring members

51 and 52 preferably have intermediate portions of sufficient flexibility to permit resilient movement of the ends thereof toward and away from each other. A layer of epoxy cement may be advantageously employed between the faces of the

spring members

51 and 52 and the surfaces of

base members

49 and 50 at the time of bolting the springs in place with bolts 53 and 54. The opposite faces of the

springs

51 and 52 are secured to the outer periphery of the plate member 11 by means of

nuts

55 and 56 that are secured to the

studs

57 and 58 which are assembled in properlylocated tapped holes in plate 11.

In order to very accurately produce a uniform and stable air gap over the entire surface of the mating magnetic elements, I perform the following operations:

After rigidly bonding the

magnetic assemblies

13 and 14 into the recessed portions of plate 11, I grind the exposed ends of the magnetic lamination assembly in a plane with the peripheral face of plate 11. In other words, after the magnetic assemblies are secured in place, plate 11 is carefully ground so that its opposite faces are in perfect parallel planes, including the ends of the magnetic laminations. In the next step of my process to secure a uniform air gap, 1 permanently attach the

spring members

51 and 52 to the peripheral faces of

base members

49 and 50 by means of the bolts 53 and 54, as shown. I next grind the free unmounted faces of the

spring members

51 and 52 so that they are in an exact plane with the exposed ends of the

magnetic lamination assemblies

37 and 38. At this point I will have two mating magnetic assemblies comprising the inertial portion of the transducer containing the

massive base members

49 and 50 and the

spring members

51 and 52 and the driven portion of the transducer including the plate member 11 whose opposite parallel surfaces lie in an exact planes with the free ends of the respective magnetic assemblies. The next step is to assemble the

studs

57 and 58 into the peripheral tapped holes in plate 11. Then a number of

shims

59 and 60 having clearance holes to fit over the studs are placed between the surfaces of the plate 11 and the faces of the

springs

51 and 52. The

shims

59 and 60 will exactly determine the magnitude of the air gap which will result, and due to the placement of the shims over the entire peripheral surface of the assembled structures, the air gap will be extremely uniform to result in perfect operation of the electromechanical vibrating structure. To complete the transducer assembly, I simply attach the hemispherically-shaped housing structure 30 and 'a similar structure 62 to the outer peripheral surfaces of plate 11 by means of

bolts

63 and 64. Tapped holes are placed about the periphery of plate 11 as illustrated in FIG. 3, in order to receive the

bolts

63 and 64. O-ring grooves are provided in the end faces of the

housing members

30 and 62, and O-rings 65 are used to complete the water-tight seal when the housing structures are attached. Having completed the assembly of the transducer, its operation is clearly obvious from the electromagnetic forces which are generated in the air gaps when alternating current passes through the

coils

21 and 22.

My invention has chosen a balanced armature type of electromagnetic transducer, and the polarity of the current and magnets is such that at any given instant a force of attraction is developed at one air gap surface while a corresponding force of repulsion is developed at the opposite air gap. In this manner the magnetic forces combine so that effectively the operating area on each side of the plate adds to contribute to the total electromagnetic driving force and thus twice the driving force may be generated prior to saturation as would be possible if only one side of the plate were equipped with a magnetic drive. It is, of course, possible to eliminate one of the two magnetic assemblies and still take advantage of the teachings of my invention. By eliminating one of the two symmetrical electromagnetic structures, the power handling capacity of the transducer will be lowered but all the remaining advantages which have been described in connection with the fabrication of the thin lamination assembly and the adjustment of the uniform air gap as well as the use of 100% active coil will be realized. Also, I have shown two concentric sections of permanent magnet in the arrangement illustrated. In a smaller transducer I could cut the E-shaped crosssectional magnetic structure shown in FIG. 1 along its center line and use only one coil and one permanent magnet and still achieve all the advantages of the teachings of my invention. A single magnet and a single coil surrounded by the adjacent laminated magnetic structure forms the minimum active electromagnetic structure that will operate completely as an electromechanical transducer. The use of successive concentric elements in the assembly merely increases the total effective area of the electromagnetic structure.

It should be obvious to any one skilled in the art that the current in coil 22 should flow in the opposite sense to the current in coil 21 in order that the magnetic forces will be additive. It is also obvious that the common magnetic poles of the permanent magnets should face toward the center line of the E-shaped assembly, as illustrated by the N-S polarity symbols in FIGS. 1 and 2.

While there have been shown and described several specific illustrative embodiments of the present invention, it will, of course, be understood that various modifications and alternative constructions may be made Without departing from the true spirit and scope of the invention. Therefore, the appended claims are intended to cover all such modifications and alternative constructions as fall within their true spirit and scope.

I claim as my invention:

1. In combination in an electromagnetic transducer, two pairs of rigid unitary structures each comprising a first tightly-wound coil assembly of thin magnetically conducting strip material, a second tightly-wound coil assembly of thin magnetically conducting strip material having the same general shape as said first coil assembly and having an internal surface in substantially uniformly spaced facing relation to the outside surface of said first coil assembly t-o define a clearance space, spacing means substantially filling said clearance space, and bonding means between said inside and outside surfaces of said coil assemblies and said spacing means, opposite end surfaces of said coil assemblies of each rigid unitary structure being machined to lie in parallel planes, a rigid circular plate having oppositely facing parallel surface portions bonded to end surfaces of coil assemblies of one pair of said rigid unitary structures, a pair of base members having planar surfaces bonded to end surfaces of coil assemblies of the other pair of said rigid-unitary structures, spring members supporting said base members from opposite sides of said rigid circular plate with end surfaces of the coil assemblies of said one pair of rigid unitary structures in spaced facing relation to end surfaces of the coil assemblies of said other pair of rigid unitary structures, a pair of cupshaped housing members secured on opposite sides of said rigid circular plate, permanent magnet mean-s forming said spacing means of one pair of said rigid unitary structures, wound coils of insulated conductors forming said spacing means of the other pair of said rigid unitary structures, and terminal means for connecting said wound coils to an external Circuit.

2. In combination in an electromagnetic transducer, a first tightly-wound coil assembly of thin magneticallyconducting strip material, a second tightly-wound coil assembly of thin magnetically-conducting strip material having the same general shape as the first coil assembly and having an internal surface in substantially uniform ly spaced facing relation to the outside surface of said first coil assembly to define a clearance space, rigid spacing means substantially filling said clearance space, said spacing means being in the form of permanent magnetic means, and bonding means between said inside and outside surfaces of said coil assemblies and said spacing means whereby said spacing means and said coil assemblies are consolidated into a rigid unitary structure.

'3. In an electromagnetic transducer a-s defined in claim 2, said coil assemblies being of generally cylindrical shape, and said permanent magnet means comprising a plurality of permanent magnets each shaped as a section of a cylindrical shell.

4. In combination in an electromagnetic transducer, a first tightly-wound coil assembly of thin magneticallyconducting strip material, a second tightly-wound coil assembly of thin magnetically-conducting strip material having the same generel shape as the first coil assembly and having an internal surface in substantially uniform spaced facing relation to the outside surface of said first coil assembly to define a clearance space, rigid spacing means substantially filling said clearance space, a third tightlywound coil assembly of thin magnetically conducting strip material having the same general shape as said first and second coil assemblies and having an internal surface in substantially uniformly spaced facing relation to the outside surface of said second coil assembly to define a second clearance space, second spacing means substantially filling said clearance space, and bonding means between said inside and outside surfaces of each of said coil assemblies and both of said spacing means, whereby both of said spacing means and said three coil assembles are consolidated into said rigid unitary structure.

5. In combination in an electromagnetic transducer, a first tightly-wound coil assembly of thing magneticallyconducting strip material, a second tightly-wound coil assembly of thin magnetically-conducting strip material having the same general shape as the first coil assembly, and having a internal surface in substantially uniformly spaced facing relation to the outside surface of said first coil assembly to define a clearance space, rigid spacing means substantially filling said clearance space, bonding means between said inside and outside surfaces of said coil assemblies and said spacing means whereby said spacing means and said coil assemblies are consolidated into a rigid unitary structure, said spacing means having magnetic field inducing means for inducing magnetic fields of opposite poles in said first and second coil assemblies forming a first rigid unitary structure and comprising in addition: third and fourth coil assemblies having sizes and shapes respectively the same as those of said first and second coil assemblies, second spacing means between said third and fourth coil assemblies including a wound coil of an insulated conductor, bonding means between inside and outside surfaces of said third and fourth coil assemblies and said second spacing means whereby said second spacing means and said third and fourth coil assemblies are consolidated into a second rigid unitary structure, and supporting means for supporting said unitary structures with coplanar faces of said third and fourth coil assemblies in spaced facing relation to coplanar faces of said first and second coil assemblies while permitting relative movement of said rigid unitary structures towards and away from each other.

6. In an electromagnetic transducer as defined in claim 5, said supporting means including spring members se cured to one of said unitary structures and having end surface portions in the same plane as the coplanar faces of said assemblies thereof, means secured on the other of said unitary structures and having surface portions in the same plane as the coplanar faces of said assemblies thereof, and shims between said surface portions for providing certain spacing between said coplanar faces of said first and second coil assembly in an unstressed condition of said spring members.

7. A method of fabricating an electromagnetic transducer comprising the steps of:

(a) tightly winding a thin strip of magneticallyconducting material to provide a first coil assembly;

(b) tightly winding around the first coil assembly a thin strip of magnetically-conducting material of a width narrower than that of the first coil assembly to provide a second coil assembly of narrower width than the first coil assembly;

(c) tightly winding around the second coil assembly a thin strip of magnetically-conducting material of a Width equal to that of the first coil assembly to provide a third coil assembly equal in width to the first coil assembly;

a slot being formed between the first and third coil as- 8 semblies which is bounded on one end by the second coil assembly,

(d) placing a coiled insulated electrical conductor within the slot to form a first rigid unitary structure,

(e) tightly winding a thin strip of magnetically-conducting material to provide a fourth coil assembly;

(f) tightly winding a thin strip of magnetically-conducting material of a Width equal to that of the fourth coil assembly to provide a fifth coil assembly equal in width to the fourth coil assembly but having an inner surface of greater dimensions than the outer surface of the fourth coil assembly so that when the fourth coil assembly is placed within the fifth coil assembly a clearance space between the fourth and fifth coil assemblies is defined;

(g) placing solid spacing means in the clearance space to provide a second rigid unitary structure;

(h) preparing a pair of support members having plane surface areas;

(i) machining end surfaces of the coil assemblies of the rigid unitary structures into coplanar relation; (j) bonding the machined end surfaces of the coil assemblies to the plane surface areas of the support members;

(k) machining opposite end surfaces of the coil assemblies into coplanar relation; and

(1) connecting the support members to support the opposite end surfaces of the coil assemblies of the rigid unitary structures into spaced facing relation.

8. A method of fabricating an electromagnetic transducer as described in claim 7 wherein the step of connecting the support members includes:

(a) securing end faces of spring members to one of the support members;

(b) machining free end faces of the spring members into coplanar relation with the opposite end surfaces of the coil assemblies secured to the support member having the spring members;

(c) machining end faces of the other support member into coplanar relation with the opposite end surfaces of the other coil assemblies;

(d) attaching the spring member free end faces to the other support member; and

(e) placing shims under the free end faces of the spring members to obtain the proper spacing between the rigid unitary structures.

References Cited UNITED STATES PATENTS 2,160,007 5/1939 Turner 34011 X 2,958,078 10/1960 Hickman et a1. 340-8 X 2,962,679 11/1960 Stratton 33623 X 3,219,969 11/1965 Snavely 340-8 3,225,326 12/1965 Nlassa 340-8 3.230502 l/1966 Chervenak 340--8 X 3,260,990 7/1966 Massa 340-l2 RODNEY D. BENNETT, Primary Examiner.

B. L. RIBANDO, Assistant Examiner.

Claims

1. IN COMBINATION IN AN ELECTROMAGNETIC TRANSDUCER, TWO PAIRS OF RIGID UNITARY STRUCTURES EACH COMPRISING A FIRST TIGHTLY-WOUND COIL ASSEMBLY OF THIN MAGNETICALLY CONDUCTING STRIP MATERIAL, A SECOND TIGHTLY-WOUND COIL ASSEMBLY OF THIN MAGNETICALLY CONDUCTING STRIP MATERIAL HAVING THE SAME GENERAL SHAPE AS SAID FIRST COIL ASSEMBLY AND HAVING AN INTERNAL SURFACE IN SUBSTANTIALLY UNIFORMLY SPACED FACING RELATION TO THE OUTSIDE SURFACE OF SAID FIRST COIL ASSEMBLY TO DEFINE A CLEARANCE SPACE, SPACING MEANS SUBSTANTIALLY FILLING SAID CLEARANCE SPACED, AND BONDING MEANS BETWEEN SAID INSIDE AND OUTSIDE SURFACES OF SAID COIL ASSEMBLIES AND SAID SPACING MEANS, OPPOSITE END SURFACES OF SAID COIL ASSEMBLIES OF EACH RIGID UNITARY STRUCTURE BEING MACHINED TO LIE IN PARALLEL PLANES, A RIGID CIRCULAR PLATE HAVING OPPOSITELY FACING PARALLEL SURFACE PORTIONS BONDED TO END SURFACES OF COIL ASSEMBLIES OF ONE PAIR OF SAID RIGID UNITARY STRUCTURES, A PAIR OF BASE MEMBERS HAVING PLANAR SURFACES BONDED TO END SURFACES OF COIL ASSEMBLIES OF THE OTHER PAIR OF SAID RIGID-UNITARY STRUCTURES, SPRING MEMBERS SUPPORTING SAID BASE MEMBERS FROM OPPOSITE SIDES OF SAID RIGID CIRCULAR PLATE WITH END SURFACES OF THE COIL ASSEMBLIES OF SAID ONE PAIR OF RIGID UNITARY STRUCTURES IN SPACED