US2550771A - Magnetostriction transducer - Google Patents

Description

May 1, 1951 l.. w. cAMP MAGNETOSTRICTION TRANSDUCER Filed July 20. 1948 2 Sheets-Sheet 1 gaas- UUUUUUUU INVENTOR.

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May 1, 1951 w. cAMP MAGNETOSTRICTION TRANsDucER 2 Sheets-Sheet 2 Filed July 20. 1948 .ix-STAD A l l l l l l b-a-a -313 @nl Q C IN V EN TOR.

Patented May 1, 1951 MAGNETOSTRICTION TRANSDUCER Vania Application July 20, 1948, Serial No. 39,770

Claims. (Cl. 171-209) My invention relates broadly to electroacoustic systems and more particularly to an improved method of constructing magnetostriction transducers and to an improved construction of magnetostriction transducer.

One of the objects of my invention is to provide a method of producing magnetostriction transducers by which an increased efficiency is obtained in the production of electro-acoustic energy.

Another object of my invention is to provide an improved design of magnetostriction transducer by which a higher acoustic einciency is obtainable than has heretofore been possible.

A further object of my invention is to provide a construction of lamination for magnetostriction transducers having a high coelncient of electromechanical coupling for a low Q system thereby resulting in higher eiciency of energy conversion.

Still another object of my invention is to provide a construction of lamination for magnetostriction transducers which permits a greater variation in the operating Q.

A still further object of my invention is to provide a construction of dual purpose magnetostriction vibrator which may be operated at a low Q with one end radiating and a high Q with the other end radiating for enabling the same die to be used in the stamping of laminations for :f

two entirely different types of vibrators.

Still another object of my invention is to provide a construction of magnetostriction transducer in which the laminations provide very substantial space for the winding coils and allows the coils to be placed in the optimum position for minimum flux leakage while employing a minimum of material and in a stack of minimum weight. f

Still another object of my invention is to provide a construction of magnetostriction transducer which is slotted in its more massive section for the insertion of a permanent bar magnet.

Still another object of my invention is to pro- Vide a construction of magnetostriction transducer which provides means for magnetically polarizing the laminations thereof or for producing a continuous unidirectional flux through the laminations.

Other and further objects of my invention reside in the production of a magnetostriction transducer with mathematical accuracy on a uniform mass production scale as set forth more fully in the specification hereinafter lfollowing by reference to the accompanying drawings, in which:

Figure 1 is a theoretical diagram explaining the principles of operation of a magnetostriction bar; Fig. 2 illustrates two magnetostriction bars joined at opposite ends to provide a closed magnetic circuit; Fig. 3 is a curve diagram showing the velocity amplitude of a resonance vibrating system as a function of frequency; Fig. 4 illustrates a magnetostriction lamination symmetrical with respect to both its transverse and longitudinal axes; Fig. 5 illustrates a magnetostriction lamination which is unsymmetrical with respect to its transverse central axis and symmetrical with respect to its longitudinal axis, where the displacement node is located at the junction of the constricted section to the solid block; Fig. 6 shows a magnetostriction lamination which is unsymmetrical with respect to its transverse central axis and symmetrical with respect to its longitudinal axis, where the displacement node is located in the constricted section according to my invention; Fig. 'I is a perspective View of a stack of magnetostriction laminations arranged in a unit and embodying my invention; Fig. 8 is a transverse sectional view taken on theline 8--8 of Fig. 6; Fig. 9 is a diagrammatic and schematic View showing the manner of arranging the windings in electrical and mechanical protective relation to the magnetostriction bars of the vibratile unit; and Fig. 10A shows the relative proportions of the lamination embodying my invention to which I have related the curve diagram of Fig. 10B showing the amplitude of theoretical displacement from rest position of any point along the length of the lamination during the compression part of the cycle; and to which I have related the curve diagram of Fig. 10C showing the distribution of the internal stresses in the material of the lamination illustrated in Fig. 10A.

Fig. 1 illustrates laminated bar l of magnetostrictive material with acoil 2 wound about the bar. An alternating current sent through thiscoil 2 will cause the bar to shorten and elongate with the alternating magnetic flux induced in the bar I by the alternating current. If a steady magnetic flux is maintained in the bar l by means of a direct current in the coil, or by an arrangement of permanent magnets, and the al.

ternating flux is superimposed on it, the rod will lengthen and shorten with the same frequency as the alternating current.

For the simplest typeof motion of the bar of Fig. 1, the center remains dxed and each half moves outward from thecenter. The distance;

which a point on the bar will move from its rest position increases with the distance of this point from the center. When the driving frequency and the length of the rod have the relation:

where f is the frequency, L the length of the rod, and c is the Velocity of sound in the rod, there is a resonance phenomenon and the motion of the rod is much greater thanrat "other fre'- quencies. Under these circumstances, the ,bar is called a half wave vibratorgbecausethe lengthof the bar corresponds to onehalfqthe length-,of a sound wave in the material at this frequency.

The vibrating system of Fig. l has a Very poor path for the magnetic uX since halfthepaths length is in air. This can be remedies -byyjoining two of these bars with end pieces as shown material in a transverse plane 'through' the bars, equidistantfrom.the-,.ends, remains fixed.A The barsfaresymmetrical with respectto this plane and I thetwo `endsof the bars have the `sameiam.- plitude of motion. Ifthissystem is to be used tD ..seI.1d 0ut '.an. acoustic signal in ,onedirection oneendiis ,placediin Contact Withthe medium (water in underwatersignalling)4 .and ithe other endlsi. isolated'from the k'medium .by an interveningmaterial which will not .permitfthe soundrto for a simple bar, one 'can `choose 'a' proper length for a desired frequency of operation, knowing @the velocity of soundin the'material used. For thel vibrator of fFig.; 21the relationship between frequency and length 4is Isomewhat more complicated,y but -the'resonant frequency is still determined by the physical dimensions and-the physi-A cal properties of the 'material used; Thereare a number of vdifferent magnetostrictive f materials,

but ilthe f same rprinciples rof design are lapplicable to all of them.

vSince these -vibrators are'to `be -usedin thev vicinity of rresonance for efficient -energy convei sion,another Very important point is the rateatwhichlthe'magnitude of maximum-velocity of vibrationdrops oifrom its peakifat the resonantfrequency, as the driving frequency :Varies from the :resonant frequency. To make this point clear consider the curve of. Fig. 3 Whichshows how the maximumvelocityof the radiating face changesY with frequency when` the vibrator is' being drivenA by'a lconstantforce.` The power radiated intothe f medium vis directly proportional to the-square'of this velocityr If "the'points zf1 andlfzrepresent.

frequencies :at -whichl the vvibrator :is gde'livering only one half :the 4poweridelivered at the maxi# l mum, f0.1 then the quantity:

denes a quantity generally known as the mechanical "Q of the vibrator. If this quantity is large, the velocity falls off rapidly away from the resonance frequency, and if small, slowly. The "Q, then, of a transducer is a factor of merit. For most applications, it must be small. The mechanical Q of a magnetostrictive transducer is readily controlled by the lamination design. It is determined by the expression:

where M is a quantity determined by the dimensionsof the lamination, and is called the equivavlentmass, Wu=21rfu, and R is the resistance to motion, internal and external, which absorbs energy from the system. The external resistance is offered by-.the medium and gives rise to useful energyabsorption. The internal resistance leads to a wasteful absorption Within the lamination and the bonding material. It can be held to a minimum by a proper design.

The 4mechanical Q ofa transducer is, ina' sense, a measure of its loading, and, like anyk other electromechanical device, there isa Ubest' load for maximum efficiency. This best load is ar function of another very important quantity the coefcient ofYV electromechanical coupling. For magnetostrictive materials,'this quantity ranges from `0:1 to 0.3, depending'upon the materiaLits degree of magnetization, and its structural form.' If 'it Vis desired to operatefa transducer at a low Q, any improvement in the coefficientof 1electromechanical coupling, increases the efficiency of energy conversion and isan improvement'of major importance.

Three types of useful laminationY constructions areshown in Figs. 4, 5 and 6. For all of these constructions, thedimension b is limited by a stiffness requirement for the -radiating face, andl it may well be the same for all of them. -In'discussing each construction there will be used the convention that S-1-/S2=m At, resonance, the standing wave systemlfor the fundamental form of the constructionof'Fig. 4 is given by:

cos kb' sin ka sin 2&6 sin 2k@ p-is the density'of the Amagnetostrictivematerial.

For this'system M, and therefore Q can-be'.

Varied only by varying the ratio:

S1/S2=m=cotlca cot-kb An increase of this ratio beyond thewali'ie '6--is' not practical because of the stiffness requirementv at Ythe radiating face.

At Aresonance `the standing Wave :system for the fundamental form of the construction shown in Fig. 5 is given by:

b; y=A cos kx and its equivalent mass per unit area of radiating surface is given by:

- sin 2kb M:p/2 b+ sin 21m cos g sin Mb-l-a-rv) and its equivalent mass per unit area of radiating surface:

m=S1/S2=cot Ica cot Ich.

sin2 kb sin2 kg sin2 ka sin2 cL sin2 kb sin2 ka Second: by the variation of m=S1/S2=cot Icq cot kL Though this second method appears to be the same as the rst, it is quite diierent. In the expression m=cot ka cot Ich, vb is fixed by a stinness requirement and a is the only variable. But in the practical use of the expression L never becomes small enough to be restricted by this requirement, so that there are two variables in this expression. Third: this lamination may be used with either end as the radiating faceoperating with a low Q with the less massive end acting as the radiator, and with a high Q where the more massive end acts as the radiator. While it appears that the system of Fig. could be used in the same way, it cannot in practice because with its massive end in water, it operates with a Q that is too high.

The parallel lines I4 of Fig. 6 indicate a modication of the lamination consisting of a slot of width C cut from this section. Into this slot may be placed a permanent magnet H to provide a constant polarization for the metal. When the 6 slot is present, this section is designed according to y cot Icq cot kL=n=S3/S2 and the expression for M must be changed slightly to account for the loss in mass. The polarities for the lamination and the distribution of the magnetic flux through the lamination have been indicated by arrows. The form of polarization shown in Fig. 6 is merely illustrative of one method of eiecting polarization.

The construction of Fig. 6 covers any unsymmetrical lamination consisting of two sections as joined at the point =b|a, which point is a displacement node in the vibrator. The one section being designed according to the formula:

cos kb cos lca=m and the other section designed according to the formula:

cos Icq cos lcL=n where m and n may or may not be equal, but q- I-b, and Lia. It will be observed that the design of Fig. 4 requires that both sections be designed alike, while that of Fig. 5 requires that the back section be constructed according to the formula My invention covers an innite number of constructions all having the common characteristic of a displacement node in the constricted section, and an unsyrnrnetrical arrangement of mass about this section. K

Fig. 6 shows an embodiment of my invention in which thelamination 3 may be slotted at C to receive the flat magnet H. The laminations are stacked to form the magnetostriction transducer unit represented in perspective view in Fig. 7 where the front face of the stack at l@ and the rear face of the stack at l5 join by the two sections and 5 at opposite sides of the longitudinal slot l2. Windings l!) and Il are Wound on thelegs 4 and 5 of the stack as indicated. The polarities and the direction of the magnetic flux have been indicated by arrows. As heretofore noted the slot and the magnetic insert may or may not be included. Fig. 8 is a transverse sectional view online 8--8 of Fig. 6 and illustrates more particularly the manner in which flat magnet H extends between the rear ends of thelaminations 3.

In Fig. 9 I have schematically and diagrammatically shown the manner of protecting the exciting windings lil and il from the sections li and 5 of the magnetostriction unit. The windings lil and ll are wound upon theconstricted sections 4 and 5 but they are protected from contact with the sections d and 5 by air-cell rubber pads 6, l, 8 and 9, which cushion the windings I0,

and l I and prevent Contact of the wires with the sharp edges of the stack. The exciting flux is superimposed on the unidirectional polarizing ux provided by the permanent magnetizing device H. The polarities indicated are selected. for illustraf tive purposes only and it will be understood that the polarities may be reversed without adverse effect to the operation.

In Figs. 10A, 10B and 10C I have more clearly illustrated the theory of operation of the magnetostriction unit by illustrating a typical lamination in Fig. 10A with the curves of Figs. 10B and 10C related thereto. rlhe curve of Fig. 10B illustrates the theoretical displacement of any point from a rest position along the length of the lamination during the compression part of a cycleaand progressively-:along:the length :of the lamination and with respect to the portions thereof designated in- FiglOA. InlFig. 10C I have illustrated by a curve diagram the theoretical internal Vstresses in -the material of Y'the'larnination cor-responding to the conditions of Fig; B and the structure of Fig. 10A.

My 'invention has'particular application to underwater sound equipment. A' number of the stacks constructed in accordance with my invention may be used to build up .arrays having large driving surfaces. It is possible to support these arrays with vthe stacks directly in the water. Al ternately, the stack may be-'enclosedy and have the drawing faces lthereof cemented to a rubber diaphragm through which vibrations are propagated or received. My-inventionfalso khas application wherever` vibratory motion is to be produced in any body or fluid.

I have found the magnetostriction unit of my invention highly practical and e'iiicient in operation and while 'Ihave described myinvention :in certain of its preferred embodiments I realize that -modiiications 'may bemade and Idesire it to `beunderstood that no limitations upon my -invention are intendedV other thanfmay-be imposed by the scope of the appended claims.

What I claim as new 'and desire to secure by Letters Patent of the United States is as follows:

1; In a magnetostrictionl oscillator1`system-- a magnetostriction transducer element comprising a' stack of laminations each having a vibratory face portion and aconstricted portion-'connected therewith and constructed according to the equation:

cot v'Ich"cotfccr--fnz in which lc=constant for the properties of -theparticular materialiof the laminations=21r/l\ Where A=wave length of vibration'in the material of the lamination h=length of the vibrating 'face' portion of the laminations azlength ofthe constricted portion ofthe laminations m=ratio of the cross-sectional varea 'of section 'b' with respect to the cross-sectional "area of section d.v

2. In a magnetostriction oscillator system, .an

unsymmetrical magnetostriction transducer element comprisinga stack of. laminations each having a vibratory face portion and -a .constricted portion consisting of two sections joined .at .the point rc=b+a Where forward Vibratory face portion-along Vthe lengthl ofthe lamination 3.In a magnetostricti'on. oscillator system including an unsymmetrical magnetostrictionf transducer element comprising a stack of laminations each consisting of a light vibratory forward face portion, a more massive Vibratory rearward end portion and a restricted leg section joining the two end portions, the lamination being designed as two equivalent quarter-wave sections according to the equations cot kb cot lca=m=S/S2 for the forward quarter-wave section and cot Icq cot cL=n=S3/Sz for the rearward quarter-wave section vin'iwhich lc=a constant pertinent to the properties of the material in the laminations b=length of forward vibrating end portionof the lamination a=a length of restricted portion adjacent 1to ZJ-v S1=width of forward Vibrating face portion S2=width of restricted portion S3=width of massive end portion q=a length of restricted portion adjacent the` rearward massive end portion such that a+q=entire length of restricted portion L=1ength of rearward section of lamination,

whereby the vibrational node for the fundamental mode of vibration is located within therestricted leg section 4. In a magnetostriction oscillator system as set .forth inclaim 3 in which the magnetostriction transducer element comprises a stack of laminations each of which consists of two or more coplanar sections `joined "laterally adjacent to each other to form a singleunit.

5. A magnetostriction oscillator comprising a magnetostriction element formed by a stack-of laminations, each end of said laminations hav-V ing an acoustically Vibratory end face vand van adjacent solid `end section, a constricted portion extending between said solid end sections, a'slot extending longitudinally between said solid end" sections and within said constricted portion, the displacement node of the laminations lying-in the constricted portion of the stack; one of said end sections having a central slot therein disposed in alignment with the slot inv saidconstricted portion and a flat permanent magnetic member in said last mentioned slot for establishing a uni' directional magnetic eld through Vsaid laminations.

LEON W. CAMP.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PTENTS Number Name Date 2,076,330 Wood et al Apr. 6, 1937 2,190,666 Kallmeyer Feb. 20, i940l 2,411,911 Turner Dec. 3, 1946

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