US2357623A - Feedback method and system for electromechanical devices - Google Patents

Description

Sept. 5, 1944. F. G. ALBIN FEEDBACK METHOD AND SYSTEM FOR ELECTRO-MECHANICAL DEVICES Filed May 26, 1943 MQW ATTo-RNEY.

Patented Sept. 5, 1944 FEEDBACK DIETHOD AND SYSTEM FOR ELECTROMECHANICAL DEVICES Frederick George Albin, Los Angeles, Calif., assignor to Radio Corporation of America, a corporation of Delaware Application May 26, 1943, Serial No. 488,606

15 Claims.

This invention relates to electrical control circuits and particularly to a feedback circuit arrangement for a device for translating electrical energy into mechanical motion.

In the art of sound recording, a Well-known method of and system for modulating a light beam being impressed upon the sound track area of a lm is by the use of a galvanometer having a moving armature which tilts a mirror for deflecting the beam with respect to the film. The armature of the galvanometer is actuated by a magnetic field produced by a coil connected to the output of the speech or audio amplifying equipment which in turn may be connected to a microphone or other translating device such as a disc or film phonograph pickup. The armature of the galvanometer is usually tensioned, damped, and otherwise influenced to provide a displacement proportional to the amplitude of the currents at all frequencies, but such influencing introduces a mechanical resonance which abnormally increases or decreases the movement in one or more frequency bands in the audio spectrum. Also, there may frequently be one or more electrical resonant points somewhere in the system which alters the optimum proportionality.

A typical galvanometer frequency response curve is illustrated in Wolfe Patent No. 2,270,367, of January 20, 1942, wherein one form of correction system is disclosed and claimed. In view of these conditions, distortion of the sound being impressed upon the nlm is obtained, this type of distortion being eliminated only when the overall frequency-response characteristic curve of the fgalvanometer is flat over the entire audio frefquency range. The present invention reduces this distortion of the galvanometer to a minimum and thereby permits of greater tolerances in the .manufacture of the galvanometer and the electrical circuit used therewith. The operation of the invention is such that only those voltages which are directly and solely proportional to the armature movements are detected and fed back into the speech input amplifying system.

In addition to the above-mentioned Wolfe patent, other feedback systems for eliminating distortion in electro-mechanical devices are known, such as disclosed in Wilhelm Patent 2,194,175, of March 19, 1940. In order to obtain a voltage truly representative of the motion of the movable member, Wolfe suggests no inductive coupling between the actuating coil and the pickup coil, while Wilhelm neutralizes the mutual inductance by a transformer or he employs a separate generator for the feedback voltages. It has been (Cl. 17g-100.3)

found. however, that because of the peculiar saturation condition of iron core material encountered in galvanometers and certain other electromechanical devices at high current levels. that transformer neutralization or separate generation as shown by Wilhelm is not wholly sufficient to produce the desired feedback voltages. Also, that it is diilicult to practically prevent all mutual inductance between coils on the same core as shown by Wolfe.

The present invention overcomes these difliculties by the generation of the feedback voltages with a second galvanometer identical with the modulating galvanometer but Whose armature is held stationary. In this manner, the voltage introduced by the mutual coupling between the two coils on the galvanometer winding may be exactly balanced out by combining these two voltages in amplitude and phase opposition before introduction into the speech input equipment. The modulating coils of the two galvanometers are connected in series aiding while the pickup coils are connected in series opposing. Potentiometers are provided for each pickup coil to balance amplitudes While variable resistances are provided for balancing the phase. By the use of two identical galvanometers the exact voltages will be generated in each pickup` coil due to the mutual inductance between the coils on the same core.

The principal object of the present intention, therefore, is to improve the operation of an electro-mechanical device.

Another object of the invention is to maintain a constant linear relationship between the amplitude of the input voltage and the mechanical displacement of an element actuated thereby at all input frequencies.

A further object of the invention is to provide a feedback system for a sound recording galvanometer wherein the feedback voltages are generated in two identical galvanometers.

-A further object of the invention is to re-introduce into an amplifying system, voltages generated by armature movements which are not directly proportional to the electrical currents originating the movements.

Although the novel features which are believed to be 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 descriptionread in conjunction with the accompanying drawing forming a part hereof, in which:

Fig. 1 is a diagrammatic view of a sound record ing system embodying the invention, and,

Fig. 2 is a graph showing the characteristics o one element of said system.

Referring now to Fig. l, a microphone 5 is connected to an amplier 6 which, in turn, is connected to an amplifier 3 feeding the modulating coils of two identical galvanometers A and B over conductors I2, i3, and i4. The structures of these galvanometers may be that of any standard two coil type such as disclosed and claimed in Dimmick U. S. Patent No. 1,936,833, of November 28, 1933. The galvanometer A is the galvanometer for modulating the light beam in a sound recording system shown diagrammatically by a light source il, a lens and mask unit I6, an optical slit assembly 20 and a iilm 2l, the light being deected by the mirror i9 of the galvanometer A. The modulating coils connected to the output of amplifier 8 are connected in series/aiding relationship, these windings being mounted on respective armatures or cores l5 and i6, core I5 being movable for actuating the mirror I9 While core I6 is locked in a stationary position.

A second detecting or pickup winding 22 is shown on core l5 and a second pickup winding 23 is shown on core I6, these windings being connected in a series opposing relationship. Across the winding 22 is connected a potentiometer 25 and a variable resistor 26, while across winding 23 is connected a potentiometer 29 and a variable resistor 30, one of the conductors 24 and one of the conductors 28 being interconnected by conductor 21. The potentiometers 25 and 29 are for the purpose of balancing the amplitude of the voltages generated in the windings 22 and 23, respectively, while the variable resistors 26 and 30 are provided to balance the phase of the voltages generated in the respective windings.

The output of potentiometers 25 and 29 is fed over conductors 32 and 33 to an equalizer 35 over a variable resistor 34 for controlling the magnitude of the feedback amplitude. The output of the equalizer 35 is connected over conductors 31 to amplifier 8.

In the above arrangement, the voltage introduced in the winding 22 of the modulating galgalvanometer is a vector sum of the voltage induced by the mutual coupling with modulation coil I of galvanometer A plus the voltage induced by the motion of the armature modulating the Ilux through the winding 22. It is only the voltage due to the motion of the armature or core I which should be fed back out of phase with the original voltages to minimize distortion. rIhis voltage is generally diilicult to obtain because the peculiar variation in the saturation of the core makes the mutual coupling voltage difficult to eliminate. This latter voltage can be accurately balanced or neutralized, however, by generating another mutual coupling voltage with a galvanometer of the same construction, but one in which no motion voltage is generated. This is accomplished by locking the armature or core I6 of galvanometer B stationary as shown, the constuction of galvanometer B being identical to that of galvanometer A. By adjustment of potentiometers 25 and 29 to balance the amplitudes of the mutual induced voltages, and by adjustment of resistors 26 and 30 to balance the phase of these voltages, a voltage due solely to the exact motion of armature l5 is impressed on the equalizer 35, the amplitude of which 'is controllable by variable resistor 34.

The equalizer 35 is for the purpose of restoring the original frequency-response charactertic to the galvanometer which is destroyed by the feedback action. It is well-known that in any electromagnetic device for translating electrical currents into mechanical motion, the velocity of the motion is constant and independent of frequency for a given amplitude. That is, the product of frequency and amplitude is constant and when the frequency of a constant amplitude current goes up the amplitude of motion or displacement goes down. In constructing a galvanometenthis displacement is made proportional to amplitude at all frequencies by reducing the efficiency of the device in the lower frequency range of the audio spectrum.

When-feedback is introduced into such a system, however, the device again reverts to a constant velocity action and in order to overcome or correct for this reversion, an equalizer is provided having a characteristic of the reverse nature to to that of the characteristic of the galvanometer. Such an equalizer is provided at 35, but may also be provided between the amplifiers 6 and 8 as well.

To illustrate this feature, reference is made to Fig. 2 wherein the horizontal dot and dash line shows the desired characteristic of the equalizer. When feedback is introduced, the galvanometer functions as a constant velocity system and the drooping characteristic shown by the solid line is the result, the rate of amplitude decrease with frequency being 6 db. per octave. To restore the fiat frequency-response characteristic, the equalizer 35 is provided with the reverse characteristic shown by the dotted curve whose variation is also at the rate of 6 db. per octave. The above system, therefore, produces feedback voltages directly proportional to the amplitude of displacement of the modulating galvanometer armature and an over-al1 nat frequency-response characteristic. Although the invention is illustrated in connection with a sound recording galvanometer, it is to be understood that it is applicable to any electro-mechanical device such as a loud speaker or other similar device.

I claim as my invention:

1. In a system for translating electrical energy into mechanical energy, a source of electrical energy, a rst translator of said electrical energy into mechanical energy, said translator having a movable element, means for impressing said electrical energy on said translator for producing motion of said element, a second translator similar to said first translator, means for simultaneously impressing said electrical energy on said second translator, the normally movable element of said second translator` being locked in a stationary position, means for generating voltages in phase opposition in each of said translators, means for combining said generated voltages, and means for feeding said combined voltages into said impressing means.

2. An electrical system in accordance with claim 1, in which said impressing means Comprises a modulating coil for each of said translators connected in a series aiding relationship and said generating means comprises a second coil for each of said translators connected in a series opposing relationship, means being provided across said generating coils for balancing the amplitude and phase of said generated voltages.

3. An electrical system in accordance with claim 1, in which means are provided in said feeding means for varying said generated volt- 2,857,623 age as applied to said impression means at the rate of 6 db. per octave.

4. An electro-mechanical system comprising a source of electrical energy, a translator having a moving element for translating said electrical energy into motion, the motion of said element being substantially proportional to the amplitude of said electrical energy at all frequencies in the audio spectrum, a second translator of a similar construction to said iirst translator, the normally moving element of said second translator being adapted to be locked in a stationary position, means for impressing said electrical energy simultaneously on both of said translators, a coil in each of said translators for simultaneously conducting said electrical energy thereto, a second coil on each of said translators adapted to have voltages induced therein, the second coil in said iirst translator having voltages induced therein in accordance with the mutual inductance between said coils in said translator and the motion of its moving element and the second coil in said second translator having voltages induced therein in accordance with the mutual inductance between the coils in said second translator, means for combining the voltages induced in said second coils on said translators, and means for feeding said combined voltages to said irst mentioned coils in said translators.

5. An electro-mechanical system in accordance with claim 4 in which said f lrst mentioned coils are connected in a series aiding relationship and said second coils on said translators are connected in a series opposing relationship.

6. An electro-mechanical system in accordance with claim 4 in which means are connected across `said second coils of said translators for balancing the amplitude and phase of the voltages generated therein in accordance with the mutual inductance between the coils in the respective translators.

7. An electro-mechanical system in accordance with claim 4, in which an equalizer is provided in said feeding means for varying the frequency response characteristic of saidvoltages in a ratio of 6 db. per octave.

8. The method of maintaining a fiat frequency response characteristic in an electro-mechanical device comprising impressing electrical energy on an operating electro-mechanical device and simultaneously on a similar electro-mechanical device locked to prevent its operation, generating voltages in each of said electro-mechanical devices, the voltages generated in said operating electro-mechanical device being proportional to the mutual inductance therein and the motion of lits armature, and the voltages generated in said non-operating device being proportional to the mutual inductance therein, balancing the generated voltages due to said mutual inductances and impressing said resultant voltage on said devices. y

9. The method in accordance with claim 8, in which the resultant generated voltages are varied at the rate of 6 db. per octave before impression on said electro-mechanical devices.

l0. The method of obtaining mechanical motion of the moving element of an electro-mechanical device proportional to the amplitude of the voltages impressed thereon at all frequencies within the range oi operation comprising generating electrical voltages, translating said voltages into movements of a mechanical element, generating a voltage corresponding to the said move-- ments and in accordance with the mutual inductance in said device, generating a second voltage corresponding only to the mutual inductance in said device, balancing said voltages correspond ing to said mutual inductance, and combining the voltage resulting from said balancing with said originallygenerated voltages.

11. The method in accordance with claim 10 in which the amplitude of said voltages corresponding to said movements are increased as the frequencyincreases at the rate of substantiallir 6 db. per octave. n

12. A system of translating electrical energy in'to mechanical motion comprising a signal source, a irst translator having an actuating coil, a pickup coil and an armature adapted to move proportionally with the amplitude of said signal,

a second translator having an actuating coil, a pickup coil and an armature locked in a stationary position, means for connecting said signal source with said actuating coils, and means for connecting said pickup coils to said connecting means between said signal source and said actuating coils.

13. A system in accordance with claim 12 in which said actuating coils are connected in a series aiding relationship and said pickup coils are connected in a series opposing relationship.

14. A system in accordance with claim 12 in which said last mentioned connecting means includes a potentiometer connected in shunt across each of said pickup coils for balancing the amplitudes of the voltages generated therein and a variable resistor connected in shunt across each or `said pickup coils for balancing .the4 phase of the voltages generated therein.

15. A system in accordance with claim' 12 in which said last mentioned means includes an equalizer for said generated voltages and means for controlling the amplitude thereof.

FREDERICK G. ALBIN.

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