US2492275A - Galvanometer with resilient damper - Google Patents

Description

iatented Dec. 27, 1949 I GALVANOMETER WITH RESILIENT DAMPER Glenn L. Dimmick, Haddon Heights, N. 1., asai'gnor to Radio Corporation of America, a corporation of Delaware Application October 4, 1946, Serial No. 701,099

This invention relates to sound recording apparatus, and particularly to a modulating device known as a galvanometer, such as is used in the recording of sound on film.

Galvanometers of the general magnetictype of the present invention are known, reference being made to my prior United .States Patents No. 1,936,833 of November 28, 1933 and No. 2,189,- 311 of February 6, 1940. Although galvanometers of the type shown in these patents have been used in commercial sound recording systems, they have been subject to a slight non-linear relationship between the input voltage of the signal and the vibration amplitude of the modulating element such as the mirror. The present galvanometer is of the general overall construction of my prior galvanometers, but the structure and circuit have been modified to obtain an improved overall performance.

Specifically, the new construction of the gaivanometer reduces the odd and even harmonic distortion of the wave shape of the signal, reduces the detrimental effects of hysteresis, and provides a more faithful correspondence between the motion of the mirror and the input signal voltage. These results have been accomplished by improving the magnetic circuit or path, by providing' a special form of armature, and by varying the. type of electrical circuit both within and without the galvanometer.

The principal object of the invention, therefore, is to facilitate the recording of sound on film.

Another object of the invention is to provide an improved system for modulating a light beam.

A further object of the invention is to provide an improved magnetic and electrical light beam modulating galvanometer.

A still further object of the invention is to provide a sound recording galvanometer having a substantially uniform motional response throughout the major portion of the audio range.

Althou h the novel features which are believed to be a characteristic of this invention will be pointed out with particularity in the appended claims; the manner of its organization and the mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying drawings forming a part hereof, in which:

Fig. 1 is aperspective view showing the principal elements of a recording galvanometer embodying the invention;

Fig. 2 is a schematic circuit diagram of and for the galvanometer;

10 Claims. (01. 171-95.)

Fig. 3 is a detail view of the armature used in the galvanometer;

Fig. 4 is a graph illustrating the adjustments made to obtain the desired frequency response:'

and

Figs. 5 and 6 are graphs showing the improvement of the galvanometer of this invention over my prior galvanometer constructions.

Referring now to the drawings, in which the same numerals identify like elements, the galvanometer is provided with magnets 5 and U- shaped pole pieces 0 and I, an armature 9 being clamped between two non-magnetic spacers i0 and Ii and held between the ends l3 and ll of the pole pieces 6 and I. As in my prior con-- bifurcated support 24 which is mounted on the pole pieces 6 and I. This ribbon serves to hold the V-block '20 on the knife edges at the end of the armature. The ribbon 23 is of beryllium copper approximately one mil thick and sixteen mils wide. 20, and, due to rigidity in the lengthwise direction, it prevents the block 20 from executing sidewise or lateral vibration in response to the lateral vibration of the armature. When the armature vibrates laterally, this motion is converted into torsional vibration of the V-block 20, since the ribbon 23 is relatively flexible in torsion.

In my prior constructions, two coils were wound or positioned around the armature in the space within the pole pieces 6 and I. In the present invention, a single coil 26 is provided at this point around the armature, the coil being tapped; In one form, the coil may be wound with 1350 turns of No. 42 wire, with the tap taken off at 675 turns, although 1400 turns may be used with the tap at 700 turns. As shown in Fig. 2, the terminals of the coil are connected to the output of a noise reduction amplifier 3| consisting of a rectifier and direct current amplifier as described in U. S. Patent No. 2,361,451 of October 31, 1944 and U. S. Patent No. 2,419,001 of April 15, 1947, while one terminal of the coil and the tap are connected to a recording am- It passes over the top of the block sheave is the use of a diirerent type of damper. The previous dampercomprised a U-shaped element between the ends ot which and the armature were positioned tungsten loaded rubber elements, as shown in my above mentioned Patent No. 2,189,311. The 'new galvanometer uses, in place of this prior damping element, a lineqasnper as shown at 33. which is of the type ?disclosed in copending application of Edward w. Kellogg. Serial No. 611,890, filed August 21, 1945. This damper comprises a strip of rubber-like resilient material, preferably tungsten loaded neoprene substantially .70 inch in length and .06 inch square or .016 inch in diameter ii round. One end of the damper is rigidly cemented to a disc 34 which is soldered to a .01 inch diameter rod 3! partially inserted into the damper and mounted at the end of the V- block 2,0 and the other end of the damper is free. This line damper is mounted in this manner, since a greater torsional movement is obtained at this point than at any other moving point oi the structure. In the past, dampers have been placed around the armature 9 where the vibration is a very small fraction of a mil which made it difllcult to get sufllcient damping with the small components within the coil. Since the length of the free portion of the armature is approximately .240

inch and the distance between the groove and' strip 23 is approximately .020 inch, the vibrating elements act as a step-up transformer convertvalue of mils to a value of mils.

ing the relatively small torsional vibrations oi the armature about its clamped end into relatively large torsional vibrations of the mirror 2! and block 20 about the center of strip 23. The whole galvanometer is relatively small, so the torsional line must, of necessity, be small so it is a definite advantage to couple the small torsional line to that portion of the vibrating mechanism having the greatest amplitude of torsional vibration. v

The use of this type of damper 33 has not-only reduced the mechanical resonant peak in amplitude, but the resulting characteristic is more uniform over the critical frequency range. That is, as shown in mypatent, this prior type of damper introduced a double humped characteristic, while the present damper produces a smooth continuation of the characteristic. vFurthermore, this damper is relatively unail'ected by temperature This result is illustrated in Fig. 4, as

changes. I indicated by the upper dotted and solid char acteristic curves A and B.

. As mentioned above, one of the disadvantages.

of prior galvanometers' was in the' introduc- Y 'tion' of odd harmonic distortion of the. wave shape.

'Ihishasbeendue chiefly to magnetic. saturation of the armature. ".To avoid this condition, the

armature has been made wider to increase the flux carrying capacity thereof. Indoing the Stiffness of the armature was increased in directproportion to the increase in width but this unby a novel modification oi the magnetic path. This includes increasing the length of the air aps in the magnetic path which increases the permissible tolerances in armature placement, spacer thickness and the non-uniformity of magnetic material. It is well known that the presence of direct current flux increases the amount of even harmonic distortion in an iron path. This detrimental effect has been greatly reduced by increasing the air gap length to dilute the effect of the iron. This also reduces the efiect of hysteresis on the vibrating system thus permitting the mirror to return to its zero or biased position with a greater degree of precision. Thus, in the present construction, the air gap at the clamping end of the armature has been increased from 10 mils to 50 mils by increasing the thickness of the non-magnetic spacers l0 and II. The front air gaps at the points I3 and I4 have been also increased from their former The total amplitude of vibration of the armature 9 is approximately 0.5 mil, this deflection occupying approximately only of the air-gap length provided. The'lengthening of these two air gaps increased the amount of air reluctance included in the magnetic circuit by the ratio of 3 to 1. The amount of the reduction of both even and odd harmonic distortions by this new construction is illustrated in Figs. 5 and 6, under both biased and unbiased conditions as described. In Figs. 5 and 6, the solid lines represent the distortion of the galvanometer of the invention, while the dotted lines represent the distortion of my prior galvanometer structures.

The fact that the air gap reluctance was increased 3 to 1 meant that, in order to obtain the same amount of alternating current flux in the armature, the magnetomotive force in the coil had to be increased by 3 to 1. If the coil were left the same as before, and the current stepped up 3 to 1, thenecessary flux could be obtained, but the heat developed in the coil would go up nine times, and this would, of course, overheat the coil. To avoid this difliculty, the amount of copper in the modulating coil was increased, thus making it more eflicient, but also increasing its the ratio of resistance to inductance to the value required for the proper frequency response, it

was necessary to add 500 ohmsoi resistance in seri'es with the galvanometer, but the resistancev was placed outside the galvanometer so that most 7 of the heat developed did not heat the galvanometer. This resistance is shown at 37 in Fig. 2, I

the condenser 38 being for the purpose of preventing biascurrent from feeding into the recordviding pole pieces which provided higher flux densities in the air gap. This was accomplished by using two percent vanadium permendur in the form of .011 properly annealed.

, The final shape of the armature is as shown in Fig. 3, wherein the clamped portion is shown at I! and the vibrating section is shown-at [5,- the Even harmonic distortion-has been" caused ing amplifier, itsvalue being large, such as 10 mi., to provide a good frequency'response down to cycles. 4

As mentioned above, the prior galvan'ometers had both a modulation coil and a bias coil, while the new galvanorneter has a single coil, which must producethree times the imagnetomotive force to make up for the wider air gap. The

singlecoihhowever. can serve for both m'odulation and noise reduction, inasmuch as the noise dotted linesrepresenting'the width of the arma-I tures in myprior galvanometersreduction current is maximum when the'signal current is minimum, and vice-vcrs a.- However, in order to produce a uniform frequency response:

characteristic as shown by curve C in Fig. 4, it

isn'ecessary'that the desired ratio of inductance I V J to resistancev be maintainedb "I'he ordinatesofcurve D in Fig; 1 represent the current which is obtained through the proper inductance-resist a m-1s ance combination when constant voltage is applied. The ordinates 'of'curve'B represent the amplitude at each'frequency. when a constant current is applied to the galvanometer. Thus. in order to produce the resulting uniform characteristic shown by curve C, theordinates must be equalto the products of theordinates of curve B and curve D. The proper ratio of inductance to resistance was obtained by. using the resistor as shown at 31 which, in a .galyanometer having the number of turns referred to above, had a value of 500 ohms.

To explain further the relationship between curves B, C, and D. the curve marked shows the relationship between amplitude and frequency when constant voltage is applied to the electrical network associated with the, galvanometer. This is the way the galvanometerwould be used in practice, and it is, therefore, desirable for the curve to be as nearly smooth and fiat as possible when operated under normal conditions. Curve D represents the variation of current in the galvanometer with frequency when constant voltage of the galvanometer, especially in the mid-range from 2000 to 5000 cycles. The capacitor alone can be made to neutralize the inductance of the coil at any desired frequency and, therefore, permit the frequency characteristic to be improved in this range. By employing a combination of capacitors andseries resistors, it was found that still better control of the frequency characteristic in the mid frequency range could be obtained by properly choosing the values of these two components.

Therefore, the above described new construction has resulted in a light beam modulating galvanometer which, when used in a sound recording system, improves the quality or fidelity of the recorded {sound track.

I claim as my invention:

1. A galvanometer comprising means for forming a magnetic field, a vibratory armature mounted in -said field, said armature terminating in a knife edge, a coil mounted around said armature,

is applied to the input of the associated electrical I network. Curve 13 represents the variation in the amplitude of galvanometer deflection with frequency when constant current is passed through the galvanometer. The-reason that the curve is of this shape is explained in detail in my article entitled "Galvanometers for variablearea recording," published in the Journal of the Society of Motion Picture Engineers, volume XV, No.4, 1930, pages 428 to 438. Fig. 2 of this article shows the response curves for a damped vibrating mechanical system where various degrees of damping are employed. The theory upon which the curves are based is given in the paper.

For instance, if Y represents the ordinates of curve D, then the curve is represented by the relation where A is the amplitude of vibration of the galvanometer and I is the current through the modulation coil. If we multiply these two equations together, we obtain an equation which represents the curve shown in 0, whose amphtude is In other words, curve C represents the amplitude obtained when constant voltage is applied to the input of the network associated with the galvanometer.

Across the winding 26 a shunt circuit is shown, comprising a resistor 40 and a condenser ll having a value of 1200 ohms and .005 mf., respectively, the purpose thereof being to improve the frequency response characteristic by partially neutralizing the inductance of the modulating coil in the mid-frequency range around 3000 cps. Although only a capacitor was shown across the bias winding in my Patent No. 2,189,311, for this purpose I found it desirable to add a resistor in series with the capacitor. The purpose of this combination is to improve the frequency response a vibratory element rotatably mounted on said knife edge, and a line damper mounted on said vibratory element.

2. A galvanometer in accordance with claim 1 in which said armature is formed of two percent vanadium permendur annealed laminations substantially .011 inch thick.

3. A galvanometer in accordance with claim 1 in which said magnetic field includes a pair of air gaps of substantially .050 inch and .010 inch.

4. A galvanometer comprising a pair of pole pieces, means for magnetizing said pole pieces, an armature having one end held between two opposite ends of said pole pieces and spaced between two other'ends of said pole'pieces, an element mounted on the free end of said armature and adapted to be torsionally vibrated by movement of said armature, and a line damper mounted on said element.

5. A galvanometer in accordance with claim 4 in which said damper includes an elongated re- 7 silient member and a fastening member, said resilient member having one end cemented to said fastening member.

6. A galvanometer comprising means for producing a magnetic field, a vibratory armature mounted in said field, a vibratory element adapted to be driven by said armature, a coil surrounding a portion of said armature, said coil being tapped, means for impressing a signal on one portion of said coil, means for impressing all of said coil with another signal, and a reactance circuit in shunt to said last mentioned means.

7. A galvanometer in accordance with claim 6 in which said first mentioned portion of said coil is substantially one half thereof and is impressed with an alternating current signal and all of said coil is impressed with a rectified current signal of said alternating current signal.

8. A galvanometer in accordance with claim 6 in which a resilient line damper is mounted on said vibratory element.

9. A galvanometer comprising means for producing a magnetic field, a vibratory armature mounted in said field. a vibratory element adapted to be driven by said armature, a coil surrounding a portion of said armature, means for impressing one portion of said coil with one signal, means for impressing all of said. coil with another signal, and a resistor and condenser connected in series in said means for impressing one portion of said coil with said first mentioned signal.

10. A galvanometer in accordance with claim 9,

in which said first mentioned signal comprises 7 the instantaneous values bf a signal and said second mentioned signal comprises the average value of said first signal.

GLENN L. DIMMICK.

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

8 unmm sums PATENTS Number Nsmm Date Harrison Oct. 30, 1928 Curtis May 9, 1933 Llndenberg Oct. 2, 1934 Curtis Jan. 12, 1937 Kellog Dec. 6, 1938 Williams May 13,1941

Badmalefl July 1, 1947

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