US3721984A

US3721984A - Magnetometer employing magnetically suspended body

Info

Publication number: US3721984A
Application number: US00161894A
Authority: US
Inventors: J Codina
Original assignee: GRAVIMETRICS Inc
Current assignee: GRAVIMETRICS Inc
Priority date: 1971-07-12
Filing date: 1971-07-12
Publication date: 1973-03-20
Anticipated expiration: 1990-03-20

Abstract

A levitational magnetic field suspends a magnetic body in space and when shielded from the earth''s magnetic field, its upward pull and the pull of gravity are equal and opposite and the body is suspended. When unshielded, the electromagnet is additionally exposed to the earth''s magnetic field as well as to the levitational field and the body remains suspended. In both cases, an electrical signal dependent upon the intensity of the levitational field is produced. The difference between the value of this signal when the electromagnet is shielded and when the electromagnet is not shielded provides a measure of the intensity of the earth''s magnetic field.

Description

United States Patent [1 1 Codina 1March 20, 1973 [54] MAGNETOMETER EMPLOYING MAGNETICALLY SUSPENDED BODY [21] Appl.No.: 161,894

Related US. Application Data [63] Continuation of Ser. No. 831,280, June 5, 1969,

abandoned.

[52] U.S. Cl ..324/43 R, 73/382, 308/10 [51 Int. Cl. ..G01r 33/02 [58] Field of Search ..324/43 R; 73/382,505, 517; 308/10 [56] References Cited UNITED STATES PATENTS 1,746,627 2/1930 Babbitt ..73/382 2,691,306 10/1954 Beams et al. 2,856,240 10/1958 Braezeale et al. 2,942,479 6/1960 Hollmann 3,090,239 5/1963 Dacus ..308/1O 4MP! F/ER 3,112,962 12/1963 Lautzenhiser ..3l7/l23X OTHER PUBLICATIONS Mcllwraith et al., Improved Magnetic Suspension System, Rev. of Scien. lnst., V01. 29, No. 11, November 1958, pp. 1029-1033.

Primary Examiner-Robert J. Corcoran Attorney-Bacon & Thomas [5 7] ABSTRACT A levitational magnetic field suspends a magnetic body in space and when shielded from the earth's magnetic field, its upward pull and the pull of gravity 10 Claims, 3 Drawing Figures S PPL Y PATENTEDHARZOIHYS SHEET 10F 2 NJW Q ma Nomi: 0

Quibt w s 5 mm m Mm v6 m Wm 2 MAGNETOMETER EMPLOYING MAGNETICALLY SUSPENDED BODY This is a continuation of application Ser. No. 831,280, filed June 5, 1969, now abandoned.

SUMMARY OF THE INVENTION In my invention a toroidally shaped magnet core is disposed in a vertical plane and possesses a small short gap at its lowest vertical point. An excitation winding is disposed around the core, thus forming a ring-shaped electromagnet. When the winding is energized, a levitational magnetic field is produced which is concentrated within and around the gap. An output signal is supplied to the winding to establish the levitational field.

A magnetic body is disposed adjacent but below the gap. This body is subject both to the upward lift of the levitational magnetic field and the downward pull of the oppositely directed gravitational force. Unless shielding is employed, the electromagnet is subjected to the influence of the earths magnetic field. The body is positioned within a predetermined vertical zone spaced apart from and below the gap. Body position indication means responsive to momentary disturbances of the body in the zone, generates an output signal varying with changes in the vertical position of the body in the zone, this signal being supplied to the excitation winding.

Additional means coupled between the indication means and the winding derives a feedback signal from the output signal. This feedback signal is combined with a reference signal by the indication means to produce the output signal. When the downwardly directed forces decrease, the output signal decreases to decrease the intensity of the levitational field.

In either situation, the body is maintained in equilibrium, and the output signal is a monotonic function of the levitational field intensity. When the electromagnet is shielded from the earths magnetic field, the levitational field intensity is equal to the downward pull of the gravitational force. When the electromagnet is not so shielded, the levitational field intensity is equal to the downward pull of the gravitational force plus the effect of the earths magnetic field. Thus, the difference between the value of the output signal when shielding is present, and the value of this signal when shielding is absent, is a monotonic function of the intensity of the earth's magnetic field at the point where the magnetometer is located.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of my magnetometer;

FIG. 2 is an enlarged cross sectional side view of one physical embodiment of my magnetometer; and

FIG. 3 is a top view of the magnetometer of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIGS. 1 3, a magnetic toroid or magnetic core is disposed in a vertical plane with a small short non-magnetic gap 12 disposed at the lowest vertical point on the core. Four

different windings

14, 16, 18 and 20 are disposed on the core 10. Winding 14 is the excitation winding and is coupled to the output of amplifier 22. The input of amplifier 22 is connected to the output of differential amplifier 24. Winding 20 is the feedback winding and is coupled to one input of differential amplifier 24. The other input of this amplifier is connected to the output of a reference signal generator 26 which produces a reference signal of constant frequency and amplitude.

Winding 14 is also connected via a meter 28 (current reading) to the power supply 30, preferably a DC. power supply, to provide bias on coil 14. Winding 16 is coupled to terminals 32 which are used for recording rapid changes in position of body 38 as explained below. Winding 18 is connected via a potentiometer 34 and switch 36 to the power supply.

The core with its windings is disposed in a housing 50 rotatable about the vertical axis of the core whereby the core can be rotated about a vertical axis to any desired angular position.

Housing 50 is adapted to be detachably enclosed by a shield 52, formed for example of mu metal, which, when in a position as shown, shields the electromagnet from the earth's magnetic field and, when removed, removes this shielding and whereby the body and electromagnet are both exposed to the earths magnetic A ferromagnetic ball or other body 38 is disposed adjacent but below gap 12.

When the magnetic flux in the core approaches the nonmagnetic gap, it detours therearound through the body which tends to close the magnetic gap. The levitational magnetic field producing the flux is produced by winding 14. Winding 14 is excited by amplifier 22. Amplifier 22 receives as an input the output signal from differential amplifier 24. The output signal from amplifier 24 is proportional to the difference between the feedback signal developed in winding 20 as a result of the flow of flux around the coil and a reference signal of constant amplitude and frequency yielded by generator 26.

When the apparatus is first put in operation the reference signal generator starts and its output is amplified in 24 and 22 and fed to the excitation winding 14 to produce a levitational magnetic field at the gap 12. The body 38, originally resting on any suitable support, is then drawn toward the gap 12 and in so moving it changes the reluctance of the toroid thereby changing the signal induced in sensing coil 20 and thus altering the output of amplifier 24. This results in a decrease in the levitational field so that body 38 achieves a stable position suspended below the magnet.

When the upward lift of the levitational magnetic field counterbalances the downward pull of the oppositely directed gravitational force on the body, the body is held in fixed position. The flux in the core is held constant, the winding excitation is constant and the difference between the feedback and reference signals is constant. Any change in position caused by variation of field or gravitational force will cause the body to move closer to or further from the core. This action will produce changes in the exciting field and in the feedback signal acting in a direction to overcome the effect of the variation and to return the body to its original position. The net result is that the body is always heldin a predetermined vertical zone below the p- The system operates as follows. Assuming switch 36 open an shield 52 removed, the body will be suspended in the zone, with the feedback levitational field plus the earths magnetic field counter-balancing the pull of the force of gravity. Any disturbance tending to move the body up or down produces a variation of the feedback signal in such direction as to restore the body to its original position. The core is rotated in housing 50 until the reading of the meter 28 is a minimum. This action permits the angular position of the earths magnetic field to be determined with respect to a reference position of the core, i.e., the angular position of the core is aligned with the angular position of the earths magnetic field.

The shield 52 is then placed in position. This action changes the meter reading. The difference in the two readings is monotonically related to the intensity of the earths magnetic field, the intensity of which can be computed by subtracting one reading from the other. Alternatively, the switch 36 may be closed after the shield is in position and potentiometer 34 is then adjusted to vary the current in such manner as to return the meter reading to its original value. Since the potentiometer adjustment is'itself a monotonic function of the intensity of the earths magnetic field, a readout device can be coupled to the potentiometer for displaying-the field intensity reading.

Winding l6 responds very rapidly to changing fluxand produces an output proportional thereto at terminals 32.

While I have described my invention with particular reference to the drawings, other embodiments of the invention will occur to those skilled in the art.

What I claim is:

1. A magnetometer comprising:

an electromagnet in the form of a toroidal core arranged in a vertical plane and having an excitation winding thereon and a gap in said core at the bottom thereof;

a magnetic body disposed adjacent but below said gap and held in a predetermined vertical zone by a levitational magnetic field produced by said excitation winding; I

body position detecting and control means responsive to changes in reluctance in the magnetic flux path of said core to generate an output signal varying with changes in said reluctance caused by changes of vertical position of said body in said zone said position changes being caused by the external magnetic field to which said body is exposed;

a power supply connected to one terminal of said excitation winding to provide a bias thereon and means for applying said output signal to the other terminal of said excitation winding to produce said levitational magnetic field to maintain said body in said predetermined zone;

said detecting and control means including a second winding on said core responsive to said changes in reluctance in the magnetic flux path for producing a feedback signal, means for generating a reference signal, and combining means for combining said feedback signal with said reference signal to produce said output signal; and

indicator means operatively connected to said excitation winding for indicating the amount of energy being supplied to said excitation windin said indication being a measure of the magnetic f1%ld to which said magnetometer is exposed.

2. A magnetometer as defined in claim 1 wherein said means for generating said reference signal generates a reference signal of constant amplitude and frequency.

3. A magnetometer as defined in claim 1 wherein said indicator means comprises a meter connected in series between said power supply and said excitation winding.

4. A magnetometer as set forth in claim 1 including a further winding on said toroid for responding to rapid changes in position of said body.

5. An apparatus as set forth in claim 1 wherein said toroid is rotatable about a vertical diameter thereof.

6. An apparatus as set forth in claim 1 further including selectively removable shielding for said toroid and body to selectively shield said toroid and body from the earths magnetic field or to expose the same thereto.

7. An apparatus as set forth in claim 6 wherein said shielding is formed of mu metal.

8. An apparatus as set forth in claim 1 wherein said combining means includes a differential amplifier having inputs coupled respectively to said reference signal generator and to said second winding.

9. An apparatus as set forth in claim 8 wherein said means. for applying'said output signal to the other terminal of said excitation winding includes an amplifier coupled at its input to the output of the differential amplifier and at its output to the excitation winding.

10. An apparatus as set forth in claim 1 further including a calibration winding on said toroid, a potentiometer coupled across the power supply, the variable tap of the potentiometer coupled to one end of the calibration winding, with the other end of the winding being connected to one terminal of the power supply.

Claims

1. A magnetometer comprising: an electromagnet in the form of a toroidal core arranged in a vertical plane and having an excitation winding thereon and a gap in said core at the bottom thereof; a magnetic body disposed adjacent but below said gap and held in a predetermined vertical zone by a levitational magnetic field produced by said excitation winding; body position detecting and control means responsive to changes in reluctance in the magnetic flux path of said core to generate an output signal varying with changes in said reluctance caused by changes of vertical position of said body in said zone said position changes being caused by the external magnetic field to which said body is exposed; a power supply connected to one terminal of said excitation winding to provide a bias thereon and means for applying said output signal to the other terminal of said excitation winding to produce said levitational magnetic field to maintain said body in said predetermined zone; said detecting and control means including a second winding on said core responsive to said changes in reluctance in the magnetic flux path for producing a feedback signal, means for generating a reference signal, and combining means for combining said feedback signal with said reference signal to produce said output signal; and indicator means operatively connected to said excitation winding for indicating the amount of energy being supplied to said excitation winding said indication being a measure of the magnetic field to which said magnetometer is exposed.

6. An apparatus as set forth in claim 1 further including selectively removable shielding for said toroid and body to selectively shield said toroid and body from the earth''s magnetic field or to expose the same thereto.

9. An apparatus as set forth in claim 8 wherein said means for applying said output signal to the other terminal of said excitation winding includes an amplifier coupled at its input to the output of the differential amplifier and at its output to the excitation winding.