US1833070A - Electromechanical translating device - Google Patents

Description

Nov. 24, 1931. s. B. cRoUsE ET Al.

ELECTROMECHANICAL TRANSLATING DEVICE Filed April l5, 1929 2 Sheets-Sheet 1 V' //fl /A//f/TOHS GfoRGE M0055, V/croR 1 0.5 00o.

NOV. 24, 1931. G B, CROUSE ET AL 1,833,070

ELEGTROMECHANICAL TRANSLATING DEVICE Filed .April 1.5, 1929 2 Sheets-Sheet 2 Fig. 7.

Fig. 6.

latented Nov. 24, 1931 UNITED STAT ES PATEN T OFFICE GEORGE B. CROUSE, OF NEW YORK, AND VICTOR L. OSGOOD', OE BROOKLYN, NEW YORK,

ASSIGNORS TO CONN ER CROUSE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK ELECT'ROMECHANICAL TRANSLATING 1315.170' ICE Application filed April 15,

This invention relates to electro-mechani cal translating devices for converting mechanical vibrations into electrical oscillations, or the reverse, and more specifically,

to devices commonly known as electro-mag- Lil) nctic phonograph pickups for converting the mechanical motion of a` needle ruiming in a record groove into electrical energy.

The devices at present employed for this purpose comprise a U shaped permanent magnet, having two soft iron U shaped pole shoes, mounted with the open ends of the Us facing, but not touching each other. A soft iron armature is mounted on soft rubber bearings whichare located between one pair of adjacent faces of the pole shoes. The armature extends upward between the other adj ac-ent faces of the pole shoes, so that rotation of the armature in its bearings willr cause a variation of magnetic flux from thepermamagnet in the armature.

he needle which tracks in the record groove is rigidly attached' to the lower end the armature whereby the armature is caused to vibrate about the axis of the bearings in accordancey with thev variations im- -pressed in the record groove. A coil of wire surrounds the armature between the legs of the pole shoes so that a variation in flux passing through the armature generates a voltage in this coil. The coil is usually connected to a vacuum tube amplifier and thence to a reproducer.

No m re satisfactory material has been found for the bearings of this mechanism than soft rubber. Furthermore, in order that the natural periods of the vibrating system may be suppressed and also that the armature may be held in a central position between the pole shoes against the upsetting magnetic pull, it has been found necessary to employ soft rubber blocks bearing against the armature on either side. The use of rubber for these two purposes has proved in practice to be very unsatisfactory, due to the change in the physical properties of rubber with age, causing sticking of the pivots, variations in adjustment, and ultimately, instability of the armature and sticking against the pole shoes. Furthermore, since the ad- 1929. Serial No. 355,259.

justment in these devices depends on the rubber, it is necessary to employ fairly large air gaps between the pole shoes and the armature, which makes necessary the use of expensive cobalt steel permanent magnets. In

addition, the entire structure requires a high accuracy in manufacture of the parts, and in assembly and adjustment if satisfactory sensitivity and tone quality are to be obtained.

The present invention employs the same general principle of translating mechanical into electrical oscillations, `It differs, how ever, from the previous devices in the employment of a vibratory system which is substantially purely reactive. We use the term purely reactive in a sense exactly analogous to its ordinary use in electrical parlance, to designate a dynamical system in which the dissipative losses in the system are negligible compared to the reactions ofmass and spring compliance, as distinguished for instance froin the dynamical system of the previous devices described in which there are large dissipative losses in the rubber, which forms a part of the system. We dene the vibratory system as including those, and only those parts, which participate in the normal motion of the armature. We prefer to employ a stiff spring pivot in place of the rubber bearings, and of means for suppressing the vnatural periods of the system, comprising a metal brake shoe operating against the moving armature in place of rubber damping blocks. llhile spring pivots have been employed in mechanical vibrating systems before-for instance, for the support of balanced armatures in certain types of loud speaking telephone devices-their employe ment in electro-magnetic pickups was con sidered impracticable because the spring pivot on the one hand must be very stiff to carry `the weight of the pickup and to resist the shocks incident to norma-l handling whereas, on the other hand, in order to obtain satisfactory reproduction of the low frequencies, a very flexible pivot was required. We have found that we may employa pivot of great mechanical stiffness, which is amply strong to sustain the weight of the pickup, while at the same time obtaining great freedom of movement of the armature by employing the upsetting magnetic pull on the armature to offset the stiffness of the pivot to normal motion. As will be explained in detail later, we prefer to desi gn the mechanica-l structure so that saturation effects will prevent instability of the armature when abnormally displaced from its mean position. We have found that it is possible to embody these ideas in a device which is extremely simple, rugged and sensitive.

rllherefore, among the objects of this invention are the production of an electro-magnetic phonograph pickup of high sensitivity and quality of reproduction, which shall be free from variations of adjustment in service, and the reduction of manufacturing cost of such devices. Another object of this invention is the substitution of low priced tungsten magnet steels for the high priced cobalt steels in phonograph pickups. A further object is the elimination of soft rubber and similar impermanent materials in the construction of these devices. It is a further object of the invention to provide a manually operable adjustment for relatively suppressing the higher frequencies, whereby the tone quality of reproduction may be varied by the user.

The manner in which these and other objects of the invention are attained will become clear from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a front elevation of one embodiment of the invention,

Fig. 2 is a vertical sectional view center-line of Fig. 1,

Fig. 3 is an enlarged view of the armature along the of the device,

Fig. 4 is an enlarged view of the damping member of the device,

Fig. 5 is a diagram. representing electrical analogs of the dynamic system of Fig. 1,

Fig. 6 is a front elevation of another embodiment of the invention, and

Fig. 7 is a diagram representing electrical analogs of the dynamic system of Fig. 6.

Referring to Figures 1 and 2 which represent respectively a front and side vertical section of one embodiment of the invention, the numeral 1 indicates a permanent magnet, having the soft iron pole shoes 2 and 3 mounted against its pole faces on the back plate 4. The armature structure is a soft iron T shaped member 5 carrying the needle, 6 clamped to the armature by means of the locking screwr 7 at its lower end. The spring pivot is formed by nicking the armature as indicated at 5. The armature is rigidly mounted at the top on the pole shoes Q and 3 by means of the brass screws 8 and 9, and spaced away from the pole shoes by means of the bronze member 10 to form magnetic air gaps 11 and 12 at the upper end between the armature and thepole shoes. The armature is located in assembly so that small and substantially equal air gaps 13 and 14 are formed between the armature and the pole shoes. It will be seen that the structure so far described forms a magnetic bridge, whose degree of balance will be determined by the relative lengths ofthe lower air gaps 13 and 14. Therefore, the magnitude and direction of the magnetic fiux flowing through the armature will be varied by variations in thc position of the lower end of the armature as controlled by the needle. These flux variations are converted into electrical potential by means of the coil 15 wound in the spool 16 surrounding the armature.

The member 10 in addition to acting as a spacer for the upper air gaps, is formed with a spring projection 10 which bears lightly against the side of the armature and presses snugly against one pole face, this pressure being maintained by another spring projection 10 on the side of the projection 10. The purpose of this spring is to damp the natural periods of the mechanical system, motion of the spring in the direction of normal motion of the armature being prevented by its abutment against the pole face. The tension of this spring against this armature may be made adjustable by obvious means not shown, and then serves to selectively suppress the higher frequencies; the greater the pressure, the greater the suppression of the higher frequencies, and the lower the frequency acted upon.

The back plate 4 is attached to an arm 17 by mounting screws 18 and 19 for mounting the pickup adjacent to the turntable in the usual manner. The terminals 22 and 23 of the coil 15 may be connected to an audio amplifier and loud speaker in the usual. way by means of the terminal block 20, the insulated cable 21 and the coil terminal leads Q2 and 23. A drawn metal cover, not shown, is used for protecting the parts of the pickup from dust and handling,

lt will be noted that none of these parts require to be held to accurate machine dimensions with the single exception of the depth of the nick in the armature 5. The only requirement is that the faces of the pole shoes shall be square with each other. The nal dimeusioning of the air gaps .13 and 14 is accomplished in assembly by means of a jig. The saving in manufacturing cost resulting from these facts will be apparent.

In order to explain in detail the advantages to be gained by the invention, as well as to indicate the method to be employed in proportioning the parts, it is necessary to discuss the details of the dynamics of the above construction.

The most convenient method of doing this is by means of electrical analogs in which Electric current corresponds to mechanical velocity.

with the compliance of the pivots', is to form an anti-resonant circuit as indicated in the equivalent diagram of Figure 7 at 26 and 27.

The element may be so proportioned as to bring the natural period of this anti-resonant circuit at the frequency of resonance of the system as a whole, and thus suppress the free oscillation. The advantage of this method is that there is no dissipation of energy at frequencies other than the resonant frequency. The simplicity of the moving system which we employ makes possible the use of this method. In the previous devices, the use of yielding rubber pivots and other elements leads to very complex equivalent diagrams with a number of natural periods requiring suppression.

Returning to the spring damper, we have formed no adequate theory to account for the behavior ofthe spring l0. It appears that the damping applied by this member is very much greater at the high frequencies and indeed will completely suppress the up*- per frequencies if the pressure is sufficiently increased. At light pressures it will completely suppress the natural frequency of the moving system which will in most practical embodiments occur at frequencies around 2000 cycles Unless special precautions are taken to prevent the effect, a torsional resonant period of the armature for rotation about an axis parallel with the longitudinal axis of the needle will be present. This resonant frequency will disturb the shape of the response curve in a complicated manner. Both this period and the period in the direction of normal motion are effectively suppressed by the spring damper.

Sensitivity is a function not only of the dynamic properties of the system which have just been discussed, but also of the constant if in the equation above. This constant is primarily controlled by the length of the air 13 and 14. The use of the stiff spring pivot in cooperation with the magnetic effect as discussed above, makes possible the use of very small air gaps, resulting in a large value of K and, therefore, a high sensitivity.

xWhile certain preferred embodiments of the inyenti on have been shown and described, it will be understood that it may be embodied in other forms and that various changes may be made in its structural details without de parting from the principle of the invention as defined in the appended claims.

We claim:

l. In an electro-mechanical translating device, the combination of a magnet provided with pole pieces having inner and outer legs with air gaps between them, a coil located between the legs of the pole pieces and an armature structure having a relatively fixed part associated with the inner leg and a vibratory part passing through the air gaps and the coil and connected to the fixed part by a pivot located adjacent to the inner air Gap.

b 2. In an electro-mechanical translating device, the combination of a magnet provided P with pole-pieces havinga pair of spaced air gaps between them, a coil located between the air gaps, an armature structure consisting of a vibratory part passing through the air gaps and the coil and a fixed part to which the vibrating part is connected by a spring pivot, a non-magnetic element providing a gap between the fixed part and one of the pole pieces and having a portion coacting with the vibratory part for damping its natural pe riod of vibration and non-magnetic means for attaching the fixed part to the pole-piece with the element between them.

3. In an electro-mechanical translating device, the combination of a magnet provided with pole-pieces having a pair of spaced air gaps between them, a coil located between the air gaps, and an armature structure consisting of a vibratory part passing through the air gaps and the coil and a fixed part to which the vibrating part is connected by a spring pivot, said vibratory part and pole pieces being so proportioned that for relatively large displacements of the vibratory part the magnetic saturation limits the flux across the gaps.

4. In an electro-magnetic pickup for use with a phonograph record, the combination of a magnetic structure, a coil and a substantially purely reactive vibratory system consisting solely of a single spring pivoted stylus carrying arm coacting with the structure and the coil for translating a band of audio frequencies and preventing abnormal translation of any frequency within `the band.

5. In an'electro-magnetic pickup for use with a phonograph record, the combination of a magnetic structure provided with pole pieces, a member of magnetic material physically connected to the structure but magnetically separated therefrom, a coil adjacent. the pole pieces, an armature passing through the coil and between the pole pieces and connected to the member by a portion of reduced size forming a spring pivot, and a stylus attached to the armature and adapted to coact with the record.

In testimony whereof we aliix our signatures.

GEORGE B. CROUSE. VICTOR L. OSGOOI).

- tionality Electric potential corresponds to mechanical force Electric inductance corresponds to mechanical mass.

Electric capacitance corresponds to mechanical compliance.

Electric resistance corresponds to mechanical resistance.

The moving parts of the device shown in Figures 1 and 2 may then be represented by the equivalent diagram of Figure 5. In this figure G is a constant current generator, whose output represents the velocity imparted to the point of the needle G, and C.L is the compliance of the needle between the point and the lower end of the armature. Ll represents the mass of the entire pickup and a portion of the arm. T is a transforn'ier, representing the fact that the length of arm, from pivot to needle point is different than the length of arm from pivot to the lower `gaps 13 and 14.

L.: represents the mass efl'ect of the armature, C the compliance of the pivot, and B1 all mechanical losses` including the friction of the damper 10. N is a quantity which has no equivalent in electrical circuits, but which may be treated as a negative capacity; it represents the effect of the magnetic fiux on the armature when the armature is displaced from a central position in the gaps 13 and 14.

All of the above quantities must be evaluated in terms of their equivalent effect as some one point in the dynamic system, preferably at the center of the air gaps 13 and 14.

Zm represents the impedance refiected into the mechanical s ystem, of the electrical system to wl ich the coil 15 is connected. It is evaluated in terms of the electrical impedance by means of the following expression where S-:the magneto-motive-force available between the poles of the permanent magnet 1.

T=the number of turns on the coil 15.

ZC=impedance of electrical circuit into which coil 15 is connected, and including the iron losses in the armature and pole shoes.

lf2-a factor determined by the properties of the magnetic bridge system formed by the magnet (l), t-hc gaps 11, 12, 13 and 14, and the armature it is the constant of proporbetween the velocity of the armature, and the resulting rate of change of flux therein.

The proportions of the device will be determined by the following considerations.

First, the dynamic properties of the moving system. represented by the equivalent diagram of Figure 3, must be such as to transmit the maximum motion at all frequencies f'ithiu the desired range, from the needle f' point to the point of the armature between the gaps 13 and 14. The quantity C1 is fixed by the commercially established design of needles. L1 is ordinarily so large that it may be neglected except at the very lowest frequencies. Experiment indicates that the quantity Z,n is small compared to the other impedances of the system, and may be neglected in considering the motion. The factors L2, R1 and the combined effect of (l, and N are, however, under some control, and should obviously be made as small as possible. Our invention makes possible the substantial reduction of all of these quantities over those employed in the previous devices. The comparative simplicity of the armature 5 makes possible the reduction of effective mass L2. In Order to obtain sufficient ruggedness of the pivot, the impedance of C2 must be made large, but we have discovered that this may be effectively offset, in the direction of useful motion, by the employment of the quantity N, the magnetic pull on the armatiu'c. This magnetic effect is substantially a straight line function throughout the normal range of motion of the armature, but for large` displacements, increases more rapidly, causing the armature to become unstable and to stick against the pole shoes. In order to prevent this, the armature and pole shoes are so proportioned that, for large displacements of the armature, magnet-ic saturation in the pole shoes and armature limits the flux across the gap, and thus limits the upsetting pull.

A further advantage of the form of armature employed is that the frictional losses, represented by R1, may be reduced to a minimum. The spring pivot employed is substantially frictionless. 1t will be seen, from the diagram, Figure 3, that the moving systcm will have, for vibration. in the normal direction, only one natural period, and it is perfectly possible to design the armature, so that this period lies at a frequency outside the range of frequencies to be transmitted, and, therefore, no frictional damping is required.

Should it be found desirable, in a given design, to place the natural period within the useful frequency range, the free oscillation which would cause an abnormal translation of energy at this frequency may be suppressed by the spring as described, or by the use of a mechanical filter element, which may be constructed as indicated in Figure (i.

In this figure which shows a front view of a pickup mechanism substantially identical with that shown in Figures 1 and 2, identical parts are identically numbered with the previous figures. The difference between this construction and that previously described lies in the fact that the upper bar or fixed part of the armature 5 is spring pivoted at the two points, 24 and 25. The effect of the mass of the armature comprised between the three spring pivots in conjunction

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