US 3673465 A
A method and apparatus for automatically stabilizing the magnetic field produced by a coil, the stabilization being effected by connecting between the ends of the coil an electronic device which provides a negative resistance which is equal in magnitude to the resistance of the path over which current flows through the coil, so that the resistance between the coil ends is essentially zero and flux disturbances are automatically compensated by the counterflux induced in the coil by such disturbances.
Claims available in
Description (OCR text may contain errors)
United States Patent 1 June 27, 1972 Tschopp  STABILIZING MAGNETIC FIELDS  Inventor: Werner Tschopp, Eggenbergstrasse ll,
Forch, Switzerland  Filed: June 24, 1971 [211 App]. No.: 156,241
 Foreign Application Priority Data Oct. 13, 1970 Switzerland ..15ll1/70  U.S.Cl ..,3l7/123  Field ofSearch ..317/123  References Cited UNITED STATES PATENTS 3,080,507 3/1963 Wickerham et a] ..317/l23 3,234,435 2/1966 Hempstead et a]. ..317/123 3,389,333 6/1968 Wolff et al ..3l7/l23 3,489,955 1/1970 Freebom ..3 17/123 Primary Examiner-L. T. Hix Attorney-Spencer & Kaye  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
INVENTOR Werner Tschopp ATTORNEYS.
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INVENTOR Werner Tschopp W i /%f.' BY
BACKGROUND OF THE INVENTION 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.
Previously known methods and apparatus for stabilizing magnetic fields produced by coils all employ separate means for detecting field or flux changes, i.e., means independent of the excitation coil such as, for example, separate detection coils, magnetic field dependent resistors or magnetic nuclear resonance measuring heads. The error signal produced by these means itself produces, after appropriate processing, a correction signal. This signal, which is to compensate the measured deviation in the field or flux, is usually applied to separate correction coils. It is known, however, to use the excitation coil itself as the correction coil. Systems having the general form discussed above are disclosed for example, in Swiss Pat. No. 376,291 and US. application Ser. No. 122,960, filed by Toni Keller on Mar. 10, 1971, titled Finely Stabilizing the Magnetic Field of a Magnetic Nuclear Resonance Device."
However, it has not thusfar been suggested to use the excitation coil not only as the correction coil but simultaneously also as the detection coil.
SUMMARY OF THE INVENTION It is a primary object of the present invention to improve the magnetic field stabilization of such devices.
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. l 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.
In the apparatus of the present invention 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.
2 BRIEF DESCRIPTION or THE DRAWINGS 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.
DESCRIPTION OF THE PREFERRED EMBODIMENTS 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.
In the case of real, i.e., non-ideal, short-circuited coils, the internal resistance of the coil limits the magnitude of the induced current. 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.
In FIGS. l-3, each block contains a designation of its associated transfer function.
With a certain disturbance in the total flux 4; enclosed by the short-circuited coil, a change Ada is produced in the total flux d) which, according to FIG. 1, is equal to the superposition of counter-flux dz, and disturbing flux The diagram of FIG. 1 provides the following equation:
m 1 l j 1) 4M where R, is the internal resistance, N, the number of turns and L, the inductance of the coil, 1', the current in the short-circuit connection of the coil and U, the induced voltage across the coil. This equation indicates that the total change in flux Ada, induced by a flux disturbance becomes smaller as the factor R,/(R, jmL,) becomes smaller. If it were possible by some method to bring R, to zero, the resulting field fluctuations would also be compensated to zero and the field would be stabilized.
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:
It is evident at once that the total change in flux Ada then becomes equal to zero when the negative resistance R, becomes equal in magnitude to the internal resistance R,. The
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 In order to be able to build the negative resistance into the short-circuit connection, 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:
It becomes e vh'leiitat once that R R, R must now be provided in order to meet the compensation requirement. As before, R is the internal resistance of the coil. The additional negative resistance, however, is no longer R but R R so that the negative resistance is again equal to the internal resistance of the coil.
Since 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:
With a full compensation brine fi.1ani;.;a.;;;;'i;., for R, R R2; Equation (5) is reduced to:
This means that 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
A first embodiment of the apparatus of the present invention is shown in FIG. 4. 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.
A second embodiment is shown in FIG. 5. Here, for example, the field of a pair of Helmholtz coils l, l is to be stabilized.
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. In the connection between the coil ends, shown in solid lines in FIG. 7, there is interposed 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.
The entire electronic circuit constitutes a negative resistance. 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.,.
Thus it is seen that 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.
Current i has the effect of introducing a voltage i 'R into the short-circuit. This acts to generate a current equal to i R ljw in coil 1, resulting in a flux change =i,,,'R,,/jwN,. This relation is valid provided that the compensation requirement is satisfied.
Ifthere are available only operational amplifiers with inputs that are grounded on one side, at least two of these amplifiers are required in order to realize the required negative resistance. An embodiment of this type is illustrated in FIG. 8. 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. Between points E and A along the path containing the operational amplifiers there is presented 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.
As in every stabilizing method, 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.
This is possible without any difficulties when a signal representing the disturbance resulting from the utilization of another stabilization method is fed in. in the form of a differentiated current signal at the same point where the excitation current i,,, is fed in. Of course the known laws for the superposition of different control circuits must here be observed particularly as regards the frequency response.
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. 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. In this measuring device 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.
It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
1. 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.
2. A method as defined in claim 1 wherein the negative resistance is provided by a positive feedback of an amplifier.
3. 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. An arrangement as defined in claim 3 wherein said electronic means comprise an amplifier across which a positive feedback is connected.
5. An arrangement as defined in claim 3 wherein said coil constitutes the only coil utilized for stabilizing the magnetic field.
6. An arrangement as defined in claim 3, further comprising a current supply source connected to said coil for supplying thereto a current for producing the magnetic field.
7. An arrangement as defined in claim 3, further comprising an iron core surrounded by said coil and cooperating therewith to produce the magnetic field.
8. An arrangement as defined in claim 3, further comprising an additional field stabilizing device producing a correction current signal for acting on the same magnetic field, said additional device being connected to deliver its correction current signal to said coil.
9. An arrangement as defined in claim 8 wherein said additional field stabilizing device operates according to the principle of the nuclear magnetic resonance.
10. An arrangement as defined in claim 3 wherein there are a plurality of coils connected together for producing the magnetic field.