US2507301A

US2507301A - Apparatus for controlling magnetic fields

Info

Publication number: US2507301A
Application number: US722585A
Authority: US
Inventors: Harry W Fulbright
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1947-01-17
Filing date: 1947-01-17
Publication date: 1950-05-09
Anticipated expiration: 1967-05-09

Description

y 1950 H. w. FULBRIGHT ,301

APPARATUS FOR CONTROLLING MAGNETIC FIELDS 2 Sheets-Sheet 1 Filed Jan. 17, 1947 Patented May 9, 1950 2,507,301 APPARATUS FOR CONTROLLING MAGNETIC FIELDS Harry W. Fulbright, Princeton, N. J., assignorto the United States of America as represented by the United States Atomic Energy Commission Application January 17, 1947, Serial N0. 722,585

7 Claims.

This invention relates to a magnetic field controller, and is concerned with the provision of a novel improved apparatus whereby the controller automatically controls a magnetic field by varying the current through the windings of an associated electromagnet.

The invention is of particular utility in the maintenance of a constant magnetic field when operating a magnetic momentum selector. In the utilization of the controller in conjunction with a magnetic momentum selector, such as a mass spectrograph, it is desirable to maintain a constant magnetic field over a long period of exposure time; and when operating the selector as a spectrometer, it is desirable that a reproducible magnetic field be maintained when making successive exposures. Inherent difficulties which arise due to the hysteretic losses produced in the iron core of the electromagnet will make the conventional field current measurements of the electromagnet substantially inaccurate. Therefore, it is of importance that the measurement of the magnetic field be made directly and not rely on secondary current measurements.

It is an object of the present invention to provide a magnetic field controller which will automatically compensate for a variation in a mag netic field.

It is a further object to provide a device of the above nature having a compensating optical-electrical network which will automatically vary the current through the windings of an electromagnet corresponding to changes in a varying magnetic field.

Other features of the invention will be hereinafter described and claimed.

In the drawing Figure 1 is a vertical crosssectional view, partially in elevation, of the galvanometer unit embodying the principles of the invention;

Figure 2 is a diagrammatic view of the controller showing the inter-relationship between the associated optical and electrical system and the galvanometer unit.

Referring now to the drawings, the numeral l generally designates an electromagnet consisting of pole pieces l2 and I4 of ferro-magnetic material and an electrical winding II which is wound around the periphery of said pole pieces 12 and I4. A portion 16 of a magnetic momentum selector is disposed in the air gap between the associated pole pieces I2 and I4 and said portion l6 of the selector being defined by the side walls l8 constructed of non-magnetic material.

2 A re-entrant chamber 20 is disposed in the side walls l8 of the magnetic momentum selector.

The galvanometer unit generally described by numeral 30 is encased in a unit housing 32 having an upper portion inserted in the re-entrant chamber 20 of the magnetic momentum selector. The galvanometer unit 30 consists of two galvanometer elements generally indicated by

numerals

29 and 39 wherein the first galvanometer element 29 is defined by a coil winding 34 and the magnetizable structure of the electromagnet ID. This combination including the coil winding 34 and the electromagnet Ill shall be designated, for purposes of illustration, as the "first galvanometer element. A second galvanometer element 39 is defined by a coil winding 40 and a permanent magnet 42 consisting of a U-shaped yoke 4| of ferro-magnetic material defining two pole pieces 44 and 4B. The U-shaped yoke is attached to the lower portion of the housing 32 by means of

stud bolts

41 and 48 mounted to a base plate 49.

The galvanometer unit 30 comprises an upper member 3| consisting of two similar upright members 33 which are constructed of light tubular material, such as aluminum. Attached to the upper portion of said members 33 is the first coil winding 34 which is disposed in the magnetic field produced by the electromagnet, said field being the magnetic field which is to be controlled. The upright members 33 are inter-connected by a plurality of cross-members 35 and attached to a central member 36 0f the galvanometer unit 30. The central member 36 is attached to a suspension fiber 31 made of a non-magnetic material such as phosphor copper. The upper extremity of said fiber 31 is attached to a crosspiece 38 which is rigidly mounted to the galvanometer housing 32. A cross-member 50 is attached at the lower portion of the center member 36 and attached thereto is the second coil winding 40. Said cross member 50 is adapted to perform as a stopping bar so as to prevent large angular rotation of the galvanometer unit 30; and the angular rotation of the galvanometer unit 30 may be limited by adjusting the stopping screws 5| mounted in the associated

pole pieces

44 and 46 respectively. At the lower portion of the central member 36 a lower fiber suspension 52 is attached thereto; and the lower extremity of said fiber 52 is attached to a mounting 54 which is fixed to the inner poi-tion of the yoke 4|. The fiber 52 affords a lower mounting means so as to prevent translational motion of thegalvanometer unit 30.

The first coil winding 34 is electrically connected to a central electrical connection block 89,

which is firmly mounted to the housing 32. The lead wires 6 la and 62a. of said winding 34 are connected to the binding posts 6! and 62 respectively.

lead wires

63a and 64a, to the binding

posts

63 and 64 respectively.

A third coil winding 59 is attached to the lower extremity of the central member 36 and mounted in the field of the standard permanent magnet 62. For purposes of the illustration in Fig. l the third winding 59 is superimposed upon the second wind- 1 ing and appears to be a composite coil except for the

lead wires

65a and 65!; which extend there= from and are attached to the block 60 by means rect current will be allowed to pass through the second coil 49. Since a small amount of current is allowed to pass through each coil 34 and do in such a manner that the torques exerted on the gaivanometer unit 39 by the magnetic forces act- 'The second winding 49 is connected by means of g lie to the winding ii of the electromagnet it. The current amplifier stage generally indicated by numeral 89, in the particular instance, employs a 2A3 thermionic tube wherein a fraction of theoutputof the amplifier stage 98 is employed as the field current of the electromagnet it]. The balance of the output currentin said amplifier 80 is dropped through a plate load resistor Ht. In actual operation four'2A3s are normally used and these tubes can handle approximately 600 milliamperes without operating beyond the conventional ratings. Normally the plate load resistor Hi l is adjusted so that the cur- V rent amplifier 99 will carry approximately one ing on the first coil 3 and the second coil so are mutually in opposition, a null position of the galvanometer unit 30 may be attained. For any ratio of currents in the two windings 34 and so, there is. one and only one value of field strength of the electromagnet It, for which the suspended galvanometer unit 39 will be in equilibrium at its null position. Thus, it is necessary only to adjust the field current flowing through the winding H of the electromagnet it so as to keep the suspended galvanometer unit 30 in equilibrium at its null position in order to keep the magnetic field strength developed across the air gap of the electromagnet. l0 constant. This manner of control is accomplished by the use of an electronic amplifier generally indicated by numeral 80, controlled by means of a dual photocell circuit generally indicated by numeral 90, said photocell circuit 90 being actuated by an associated optical system 99 wherefrom a beam of light is reflected from a mirror I31 which is attached centrally to the central member 38.

In operation a source'of light from a light bulb I09 is directed through a condensing lens E02 and reflected from the mirror I31 onto a second condensing lens I04 so as to concentrate the light beam at a point intermediate between dual cathodes of the photoelectric cells 9| and 92 of the photoelectric tube 90A. This position intermediate between the cathodes of the photoelectric tube is considered the null position of the galvanometer unit 30. An electrical bridge circuit 93 consisting of

batteries

94 and 95 comprising adjacent balancing arms of fixed voltage ratio, is connected in series with the current paths of the photoelectric cells 9| and 92, the latter comprising adjacent balancing arms of variable voltage ratio. The

conjugate points

96 and 91 of the bridgeare connected by conductors through a conventional direct voltage amplifier I6. The

output of the amplifier I96 is coupled to the grid H2 of the thermionic amplifier tube 0. The current output of the current amplifier tube III is transformer coupled by means of a transformer to two hundred milliamperes, since the field current developed usually increases as the potential developed by the battery H9 in the associated circuit drops. The normal operating voltage of the battery H8 is 112 volts, and if the voltage drops to 90 volts or less the sensitivity of the controller correspondingly decreases.

The galvanometer unit 36 in conjunction with the associated photoelectric circuit 99, the optical system 99 and the current amplifier stage at are generally designated as the magnetic field controller.

When the current flowing through coils 3d and d6 which is supplied by the 3. /2 volt battery it, exceeds 1 milliampere the operation of the con troller is substantially unafiected by slight current variations since it is the ratio of the currents flowing through the coils 3t and id respectively which controls the field strength. This is important, since it permits a simple unregulated source of. current to be used in the coils 34 and it. When the current flowing through the coils 3t and 40, respectively, is increased approximately 50 times, the sensitivity of the controller will increase greatly. However, it is not possible to raise the current flowing through the coil 34 and All beyond a few miliamperes because of periodic oscillations which will develop. These oscillations are due to the mechanical angular vibrations set up in the galvanometer unit 30. This difiiculty is somewhat. eliminated by using a large,

developed across the electromagnet ill, the unit 30 will swing the light source image, thereby unbalancing the current flowing through the two cells 9i and 92 of the phototube 99a. stops 5! are provided so that the unit 30 cannot rotate beyond the range of the photocells 9! or 92. When the galvanometer unit 30 is deflected because of a change in the direct current potential of the magnet power supply, which may be due to an unbalancing current from the phototube 90a amplified many times, current developed by the amplifier is impressed upon the winding II of the electromagnet ill in the proper sense so as to restore the galvanometer unit an to a null position. If one of the field current circuit conditions is changing constantly, such as a fluctuating voltage in the battery 8, the galvanometer unit 30 will hunt back and forth continuously around the null position. In actual operations changes in the circuit conditions are relatively slow and generally monotonic functions of time, so that the controller hunts only over an extremely small hysteresis loop because of statistlcal variations in conditions. Because of the possibility that hunting of the galvanometer unit 30 might break into violent oscillations, a certain amount of degenerative coupling between the magnet field current circuit and the galvanometer unit 30 is provided. A third coil 59 is wound along in a composite manner with the second coil 40 at the lower end of the galvanometer unit 30 so that damping currents may be supplied to the third coil winding 59 so as to suppress oscillations. These currents are obtained from inductive coupling of the winding 59 with the field current circuit through a transformer H6. A potentiometer II! is shunted across the secondary winding of the transformer H6 so as to provide a varying current in the feed-back circuit of the third coil 59. It has been found that the controller will not operate satisfactorily without the use of the third damping winding 59. By disconnecting the third coil winding 59 when the controller is in operation, violent oscillations occur, which can be immediately suppressed by re-connecting the third coil winding 59. The third coil winding 59, is transformer coupled to the electrical winding ll of the electromagnet Ill, so that for any given change in the amount of current flowing through the winding H of the electromagnet l there will be a transient current transmitted through the third coil winding 59 which will exert a transient deflecting force suificient to suppress the oscillatory motion of the galvanometer unit 30.

The response of the controller to changes in the magnetic field developed across the electro magnet in is fairly rapid; approximately one second being required for a complete readjustment of the field current transmitted through the electromagnet Ill in order to offset the effect of shorting out 10% of the field winding II. This rapid response is due to the fact that the magnetic torques operating on the galvanometer unit are large compared with the restoring torque of the fiber suspension 31; and the fact that the movement has a relatively small moment of rotation inertia. The inductance of the magnet field winding II also lengthens the response time somewhat. Shorting out about 25% to of the field winding ll leads to a change of less than 0.2% in the field strength measured with a conventional flip coil.

For spectrographic operation a controller can be set to maintain field strength, over a large energy range from about 20 KEV to 10 MEV. In order to change the field strength, it is only necessary to change the shunting resistance supplied by the potentiometer l2 and resistor 14, thereby altering the ratio of the current in the

coil windings

34 and 40, respectively. These are the only changes required over this large energy range. For spectroscope use the magnetic field can be adjusted by changing resistance value of the rheostat I4, which will vary the ratio of the currents flowing through coils 34 and 40: and this change can be calibrated in terms of field strength. This manner of calibration will do away with the necessity of checking individual field strengths.

In general, it may be said that the device disclosed in the present application is illustrative, rather than limitin in scope, and that all of the numerous modifications which would naturally occur to those skilled in the art are included in the scope of the present invention. Only such limitations as are indicated in the appended claims should be imposed on the scope of this invention.

What is claimed is:

1. Apparatus of the class described comprising a plurality of similar galvanometer elements each having a magnetizable structure and an electrical winding movable in the field thereof and each having a constant current in the winding thereof, said windings being mechanically interconnected so as to transmit a. positive torque therebetween, means adapted to produce in the field of one of the magnetizable structures magnetic fluxes having a unidirectional component substantially opposite to the magnetic fluxes developed in the field of the other magnetizable structure, and means responsive to the angular motion of said galvanometer elements to vary the current through said electromagnet.

2. In a device of the class described, in combination, two galvanometer elements each having a stationary magnetizable structure and an electrical winding defiectable in the field thereof and each having a constant current in the winding thereof, said windings being mechanically interconnected so as to form an integral defiectable unit, means for producing in the field of one magnetizable structure a constant magnetic flux having a unidirectional component substantially opposite to the magnetic flux produced in the field of the other magnetizable structure, said other magnetizable structure being an electromagnet, and means responsive to the deflection of said unit to compensate for variation in the magnetic field of the electromagnet by varying the electrical current flowing therethrough.

3. Apparatus for controlling the magnetic field of an electromagnet, said apparatus comprising, in combination, a first winding disposed in said magnetic field, a standard magnetic field, a second winding disposed in said standard magnetic field, means for mechanically coupling said windings so that a rotative force exerted upon one winding constitutes a rotative force upon the other winding, means for flowing a constant current through both of said windings so that the rotative forces exerted upon said windings by a given magnetic field of the electromagnet and the standard magnetic field, respectively, are equal and opposite, and means responsive to an in equality of said forces to change the current through said electromagnet.

4. In an apparatus of the class described, in combination, two galvometer elements, one having a stationary electromagnet and a coil winding defiectable in the field thereof and the other having a stationary permanent magnet and a coil winding defiectable in the constant magnetic field thereof, means for mechanically coupling said coils to a rigid member so as to form an integrally defiectable unit, means for flowing a constant current through both of said coils so that the rotative forces exerted upon said coils by a given magnetic field of the electromagnet and the constant magnetic field, respectively, are equal and opposite, and means responsive to an inequality of said forces to vary the current through said electromagnet.

5. Apparatus for controllin the magnetic field of an electromagnet, said apparatus comprising, in combination, a first winding disposed in said magnetic field, a permanent magnet producing a standard magnetic field, a second winding disposed in said standard magnetic field, means for mechanically coupling said windings so that a rotative force exerted upon one winding constitutes a rotative force upon the other winding, said mechanical means comprising a rigid member interconnecting said windings thereby defining a unit, means for flowing a constant current through both of said windings so that the rotative forces exerted upon said windings by the magnetic field of the electromagnet and the standard magnetic field, respectively, are equal and opposite, electrical damping means comprising a third winding adapted to prevent periodic oscillatory motion of the galvanometer unit, said third winding being firmly attached to said unit and mounted in the field of the standard magnet, and means responsive to an inequality of said forces to change the current through said electromagnet.

8. Apparatus as recited in claim 5 wherein the last-mentioned means comprises a reflecting surface rigidly attached to said unit, a photoelectric circuit adapted to produce a. current in response to the angular deflection of said reflecting surface from an original null position, an electrical direct voltage amplifier means responsive to the current output of the photoelectric circuit. and a current amplifier means, said direct voltage amplifier being coupled to said current amplifier, and said current amplifier being connected to the windings of the associated electromagnet and adapted to vary the current therethrough.

7. In an apparatus of the class described, in combination, two galvanometer elements, the first element having a stationary electromagnet and a coil winding deflectable in the field thereof, and the second element having a stationary permanent magnet and a coil winding deflectable in the constant magnetic field thereof, means for mechanically coupling said coils to a rigid member so as to form an integrally deflectable unit, means for flowing a constant current through both of said coils so that the rotative forces exerted upon said coils by a given magnetic field of the electromagnet and the constant magnetic fleld, respectively, are equal and opposite, means responsive to the inequality of said forces to change the current through the electrical winding of the electromagnet, and a third coil winding being rigidly attached to said deflectable unit and mounted in the field of the standard magnet, said third coil winding being transformer coupled to the electrical winding of the electromagnet so that for any given change in the amount of current flowing through the winding of the electromagnet there will be a transient current transmitted through the third coil winding adapted to exert a transient deflecting force suflicient to suppress oscillatory motion of said deflectable unit.

HARRY W. FULBRIGHT.

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

UNITED STATES PATENTS Number Name Date 1,677,378 Albrecht July 17, 1928 1,857,452 Hausman May 10, 1932 2,113,436 Williams Apr. 5, 1938 2,216,301 Sparrow Oct. 1, 1940 2,220,788 Lehman Nov. 5, 1940 2,264,256 Fairchild Nov. 25, 1941 2,422,240 J ackes June 17, 1947 2,436,039 Fay Feb. 17, 1948 2,440,503 Fay Apr. 27, 1948