US3673465A - Stabilizing magnetic fields - Google Patents

Stabilizing magnetic fields Download PDF

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US3673465A
US3673465A US156241A US3673465DA US3673465A US 3673465 A US3673465 A US 3673465A US 156241 A US156241 A US 156241A US 3673465D A US3673465D A US 3673465DA US 3673465 A US3673465 A US 3673465A
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coil
magnetic field
field
arrangement
current
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US156241A
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Werner Tschopp
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    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F7/00—Regulating magnetic variables

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  • ABSTRACT A method and apparatus for automatically stabilizing the magnetic field produced by a coil, the stabilization being etTected by connecting between the ends of the coil an electronic device which provides a negative resistance which is equal in 10 Claims, 10 Drawing Figures P'A'TENTEDJum I972 3, 673 .465
  • the present invention relates to a method and apparatus for stabilizing a magnetic field produced by coils, for example, in magnetic nuclear resonance spectrometers.
  • the invention particularly relates to systems in which a disturbing voltage produced across the coils by changes in the field or in the flux is used to produce an opposite polarity, equal amplitude voltage in electronic means, acting as a negative resistance, so that the resulting voltage across the coils is zero.
  • This state is maintained by the production, in the coils, of a correction current which creates a correction field of such an amplitude that the field or flux change is fully compensated, i.e., reduced to zero.
  • Another object of the invention is to simplify the structure required for achieving such stabilization.
  • a further object of the invention is to provide a method and apparatus for enabling and causing the excitation coil of such a system to function simultaneously as the deviation detection coil and the correction coil.
  • the method of the present invention for stabilizing a magnetic field produced by one or a plurality of coils utilizes the excitation coil, or coils, simultaneously as the detector coil, or coils, and the correction coil, or coils, by causing deviation voltages produced by undesired changes in the field or flux across a field-producing excitation coil to produce a correction current in electronic'means provided in the short circuit connection of the coil, i.e., the electronic means are connected to form a loop with the coil, the electronic means presenting a negative resistance which is so dimensioned that the resulting voltage across the inductance of the coil is compensated to zero so that the field or flux changes are also compensated.
  • the apparatus for stabilizing a magnetic field by one or a plurality of coils is provided with no additional magnetic field detection elements and/or magnetic field correction coils other than the excitation coil and contains electronic means in the short-circuit connection of the excitation coil which means-constitute a negative resistance whose absolute value is equal to that of the internal resistance of the excitation coil.
  • the excitation coils themselves take over the function of detection as well as of correction of field or flux fluctuations so that further detection and/or correction coils become unnecessary.
  • the described self-contained correction system may be combined with other stabilization methods such as, for example, stabilization by means of nuclear magnetic resonance.
  • FIG. 1 is an equivalent circuit diagram of a coil used in describing the principles of the present invention.
  • FIG. 2 is an equivalent circuit diagram further illustrating the principles of the invention.
  • FIG. 3 is an equivalent circuit diagram illustrating a circuit according to the invention.
  • FIGS. 4-6 are block circuit diagrams of preferred embodiments of the invention.
  • FIG. 7 is a circuit diagram of another preferred embodiment of the invention.
  • FIG. 7a is an equivalent circuit diagram of the coil of FIG. 7.
  • FIGS. 8 and 9 are circuit diagrams of further preferred embodiments of the invention.
  • the present invention is based on the following considerations: If an ideal lossless coil, for example a superconductive coil, is short-circuited, i.e., has its ends effectively connected directly together, in a field to be stabilized, each change in the field will induce a voltage in the coil which itself produces an additional current in the coil. This current flows in such a direction that the field produced thereby opposes the original field change. Since in a lossless coil any arbitrarily small change in the field would produce an arbitrarily large current, the net resulting field change, representing the superposition of the initial field change and opposing field of the coils, approaches zero.
  • FIGS. l-3 illustrate the conditions for such a coil in a simple manner and facilitate an understanding of the basic principles of the present invention.
  • FIG. 1 shows the field flux components and the induced currents and voltages in a short-circuited coil with internal resistance R, w 0.
  • FIG. 2 shows the provision of a negative resistance (-R2) in a positive feedback path to be effectively in series with the equivalent resistance R, of the coil.
  • FIG. 3 shows the field flux and the induced currents and voltages in a coil provided with the arrangement of the present invention for compensating the coil losses.
  • each block contains a designation of its associated transfer function.
  • the method according to the present invention employs a negative resistance to realize such conditions.
  • This negative resistance -R is to be built into the shortcircuit connection of the coil.
  • the total equivalent resistance of the coil will thus be R, R, and Equation (1 will be modified as follows:
  • Equation [R,l R, I is thus characteristic for a device according to the present invention.
  • a negative resistance can be provided in the short-circuit connection of the coil with the aid for example, of a positive feedback.
  • the diagram of FIG. 2 contains, in addition to the already described parameters R R i and U,, the voltage U R i, which represents the voltage fed back to the field coil.
  • R is a normal resistor which acts as a negative resistance R because of its position in the circuit, which yields the following relation
  • electronic means are required which themselves are disposed in this short-circuit connection and which unavoidably introduce an additional external resistance R into the short-circuit connection.
  • At the very least a feedback amplifier is required, R representing the input and output resistance of this electronic means as well as other parasitic contact resistances.
  • the diagram of FIG. 3 shows the combination of the short-circuited coil of FIG. 1 with the feedback amplifier of FIG. 2. Ignoring for the moment the added value i the following relation results:
  • R is the internal resistance of the coil.
  • the additional negative resistance is no longer R but R R so that the negative resistance is again equal to the internal resistance of the coil.
  • the coil under consideration is also to be the excitation coil, a possibility must be provided which permits the production of a field or intended changes in the field in spite of the compensating effect of the negative resistance. This can be realized by feeding a current i,, to the coil as shown in FIG. 3. The total resulting field flux da then becomes:
  • Equation (5) Equation (5)
  • the total flux 41 can also unequivocally be controlled or, e.g., modulated, even in the stabilized state, the resulting flux being proportional to the integral of the control current i,,,. v
  • Coil 1 produces, for example, a magnetic field within the area which it encloses, which field is to be stabilized.
  • Electronic means 2 are connected in the shortcircuit connection of the coil, i.e., between its terminals, and constitute a negative resistance whose absolute value is equal to that of the internal resistance of the field-producing coil 1.
  • the embodiment is characterized in that no additional magnetic field detection means and/or magnetic field correction coils are provided in addition to coil 1 in the field stabilization system of the apparatus in which the coil is disposed.
  • FIG. 5 A second embodiment is shown in FIG. 5.
  • the field of a pair of Helmholtz coils l, l is to be stabilized.
  • Helmholtz coils An essential characteristic of Helmholtz coils is that the currents through the two coils be identical. This can be achieved by connecting them in series or by connecting them in parallel, as shown, and providing suitable, well-known means for regulating the currents through them.
  • the electronic means 2 of the present invention are disposed between the ends of the parallel-connected coils 1, 1. To feed the coils, a current is produced in a current supply source 3 and applied to the coils. This current can also be used to modulate the intensity of the magnetic field, for example, to give it a sinusoidal or sawtooth time variation, if the current supply source is provided with suitable current modulators.
  • FIG. 6 illustrates a further embodiment in which two or more, for example series-connected, coils 1,1 constitute the excitation coils of an electromagnet having a magnetic core 4, presenting pole pieces separated by an air gap the field in the air gap between the pole pieces of the magnetic core being intended to be stabilized.
  • This is accomplished, as in the previous embodiments, by connecting electronic means 2 between the ends of the series coil arrangement.
  • the coils are fed by the current supply source 3 via the electronic means.
  • This embodiment is characterized by the fact that the above-mentioned magnetic field is produced by coils havingan iron core.
  • FIG. 7 shows a more detailed circuit diagram of a further embodiment in which coil 1 represents the one or more coils of any one of the above-described field-producing coils.
  • Coil 1 may be represented by an inductance L and its internal resistance R as shown in 7a.
  • the electronic means constituted by an operational amplifier 5 and resistors R R and R
  • a positive feedback for the amplifier is provided by the resistor R and the variable resistor R the resistors R and R together forming a voltage divider.
  • a voltage -R i is created from point P to the point E, where i is the current between the ends of coil 1.
  • the voltage drop across R can be expressed as x (R i,), where x is R /R and has a value which can be set by adjusting the variable resistor, R Since the input voltage between the two terminals and of the operational amplifier approaches zero due to the inherent characteristics of an operational amplifier, the voltage between point E and ground consists essentially only of the voltage across resistor R i.e., only of x(R).
  • the impedance between point E and ground with respect to the operational amplifier is equal, by definition, to the quotient of the input voltage (xR i and the input current (i,), and thus is given by -xR.,.
  • the electronic means constitute a negative resistance which is disposed between the coil terminals and which is produced by means of the positive feedback of an amplifier.
  • the compensation requirement is met when R xR, O, i.e. when the absolute value of the negative resistance [xR is equal to that of the internal resistance [R,].
  • the feeding of the generating or modulation current i,,, into the short-circuit connection is also shown in FIG. 7.
  • the electronic means of this embodiment includes operational amplifiers 6 and 7 having respective feedback resistors R and xR-,, a series resistor R, between the amplifiers, and a resistance R representing the output resistance of amplifier 7.
  • the electronic means are here also disposed in the connection between the ends of coil 1, shown in heavy lines.
  • a negative resistance whose value is equal to the quotient of the voltage across these two points (xR.,,) and the input current (i it thus is xR
  • the value of x can be selected by adjusting resistor xR,.
  • Resistor R represents the output resistance of the operational amplifier (7) and this decreases the effective negative resistance to JtR R,. compensation requirement is thus: R xR R 0.
  • the embodiment is characterized in that a plurality of amplifiers each having one grounded input are contained in the electronic means.
  • the modulation current i introduces a voltage i 'XR into the short circuit connection. This introduces a current im XR4/jwL1 into coil 1, which current produces a flux change i X R,/ jwN This relation is valid provided that the compensation requirement is satisfied.
  • the present method is subject to a time constant which limits the effect of the method to a certain disturbance frequency range. It is thus desirable, as in other stabilizing methods, to combine different methods.
  • FIG. 9 One embodiment for such a combination of stabilization techniques is shown in FIG. 9.
  • the coils 1 feed an electromagnet having a core and pole pieces 4. Feeding by device 3 and stabilization by means 2 occur as described above.
  • a nuclear magnetic resonance measuring head 8 In the field between the pole pieces there is a nuclear magnetic resonance measuring head 8 whose output leads to a measuring device with a transmitter 9.
  • a disturbance signal is produced which originates from a deviation in the transmitting frequency of the nuclear resonance head 8 from the resonant frequency of the employed material in the given magnetic field.
  • This disturbance signal is fed to the coils l in the form of a disturbance current and thus effects a correction of the field.
  • the embodiment is distinguished by the fact that a disturbance current signal which originates from another stabilization device, mainly from a nuclear resonance field stabilization device, is fed into the short-circuit connection of the coil.
  • the great practical advantage of the method and apparatus of the present invention lies in the fact that the elimination of other detection elements and/ or correction coils permits a significant saving in space as well as in manufacturing cost.
  • a method for stabilizing the magnetic field produced by a coil, in which disturbances in the field enclosed by the coil induce a reaction voltage in the coil comprising the steps of connecting to the current flow path of the coil electronic means constituting a negative resistance, and giving the negative resistance a magnitude equal to that of the ohmic resistance of the coil current flow path, whereby the resulting voltage across the coil is compensated to approximately zero and such field disturbances are substantially compensated by the opposing field generated by the correction current resulting from the reaction voltage.
  • Apparatus for stabilizing the magnetic field produced by a coil in which disturbances in the field enclosed by the coil induce, in the coil, a reaction current which produces a magnetic field opposing the disturbance, comprising electronic means connected between the ends of said coil and constituting a negative resistance whose magnitude is substantially equal to that of the internal resistance of said coil, whereby the opposing magnetic field produced by said coil in response to a disturbance substantially completely compensates that disturbance 4.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Automation & Control Theory (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
US156241A 1970-10-13 1971-06-24 Stabilizing magnetic fields Expired - Lifetime US3673465A (en)

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Application Number Priority Date Filing Date Title
CH1511170A CH540528A (de) 1970-10-13 1970-10-13 Vorrichtung zur Erzeugung eines Magnetfeldes mittels einer oder mehreren Spulen und zur Konstanthaltung des erzeugten Magnetfeldes

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FR (1) FR2112288B1 (OSRAM)
GB (1) GB1315777A (OSRAM)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3769554A (en) * 1971-09-15 1973-10-30 Spectrospin Ag Apparatus for varying the strength of a stabilized magnetic field
US3925711A (en) * 1974-08-29 1975-12-09 Us Air Force Series tuned spin coil supply
US4607225A (en) * 1983-07-19 1986-08-19 Regents Of The University Of California Apparatus and method for reducing spurious currents in NMR imaging apparatus induced by pulsed gradient fields
US5225999A (en) * 1990-07-06 1993-07-06 The Trustees Of The University Of Pennsylvania Magnetic environment stabilization for effective operation of magnetically sensitive instruments
US5586064A (en) * 1994-11-03 1996-12-17 The Trustees Of The University Of Pennsylvania Active magnetic field compensation system using a single filter
RU2182721C2 (ru) * 1995-04-07 2002-05-20 Дисковижн Ассошиейтс Устройство для регулирования величины магнитного поля

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3080507A (en) * 1961-06-08 1963-03-05 Gulf Research Development Co Apparatus for stabilizing magnetic fields
US3234435A (en) * 1963-07-09 1966-02-08 Bell Telephone Labor Inc Magnetic field stabilizer for a superconductive device
US3389333A (en) * 1964-02-10 1968-06-18 Sperry Rand Corp Control system for maintaining a desired magnetic field in a given space
US3489955A (en) * 1967-09-13 1970-01-13 Honeywell Inc Amplifier apparatus

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3080507A (en) * 1961-06-08 1963-03-05 Gulf Research Development Co Apparatus for stabilizing magnetic fields
US3234435A (en) * 1963-07-09 1966-02-08 Bell Telephone Labor Inc Magnetic field stabilizer for a superconductive device
US3389333A (en) * 1964-02-10 1968-06-18 Sperry Rand Corp Control system for maintaining a desired magnetic field in a given space
US3489955A (en) * 1967-09-13 1970-01-13 Honeywell Inc Amplifier apparatus

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3769554A (en) * 1971-09-15 1973-10-30 Spectrospin Ag Apparatus for varying the strength of a stabilized magnetic field
US3925711A (en) * 1974-08-29 1975-12-09 Us Air Force Series tuned spin coil supply
US4607225A (en) * 1983-07-19 1986-08-19 Regents Of The University Of California Apparatus and method for reducing spurious currents in NMR imaging apparatus induced by pulsed gradient fields
US5225999A (en) * 1990-07-06 1993-07-06 The Trustees Of The University Of Pennsylvania Magnetic environment stabilization for effective operation of magnetically sensitive instruments
US5586064A (en) * 1994-11-03 1996-12-17 The Trustees Of The University Of Pennsylvania Active magnetic field compensation system using a single filter
RU2182721C2 (ru) * 1995-04-07 2002-05-20 Дисковижн Ассошиейтс Устройство для регулирования величины магнитного поля

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DE2106051B2 (de) 1973-02-01
FR2112288A1 (OSRAM) 1972-06-16
GB1315777A (en) 1973-05-02
FR2112288B1 (OSRAM) 1976-09-03
DE2106051A1 (de) 1972-04-20
DE2106051C3 (de) 1975-08-28
CH540528A (de) 1973-08-15

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