Apparatus For Producing A Region Free From Interfering Magnetic Fields

Abstract

Magnetic field generating coils are arranged at opposite sides of a region to be made free of interference magnetic fields along each of the three coordinate axes. A set of field sensing probes or transducers are also provided along the respective axes with compensation coils to prevent interaction with the field generating coils arranged along the other axes. As a further aspect, undesired interaction of the probes with control circuits of other equipment or devices being used in the region free from interference fields is suppressed by installing at each of the probe locations one or more compensating coils for nullifying the effect of these fields on the probes. Optionally, control amplifiers may be manually adjusted to compensate for fixed fields generated by other equipment being used in the region.

Description

FIELD OF THE INVENTION

The present invention relates generally to apparatus for producing within a given region an environment free from interference magnetic fields originating from external sources, and, more particularly, to such apparatus for producing such a magnetic field free region within which magnetically sensitive devices may be operated.

There are many electrical apparatus having a high sensitivity to ambient magnetic fields and which, if not compensated for in some manner, severely influence the apparatus operation. For example, modern electron microscopes have a very high resolution which under ideal conditions can approach the theoretical limit of 2.3 Angstroms where by ideal conditions is meant that the region within which the electron microscope is operated is substantially free from all interfering externally generated magnetic fields, even the magnetic field of the earth. The importance of this will be appreciated when it is noted that magnetic fields as low as 0.5-1 millioersteds produce detectable deflections in the electron beam of an electron microscope. With such apparatus constant interference fields will only produce a displacement of the image and can be compensated for as long as the relative orientation of the interference field and the microscope are maintained unchanged. However, a more serious problem is created when the interference field is alternating, in that it will affect image definition and in that way the ultimate resolution obtainable by the microscope.

Moreover, it has been found that considerable distortion in color is obtained in color television tubes when the electron beam is shifted by even a very small amount, and, for this reason, those involved in research and development of such picture tubes require areas within which to work with the tubes that are free from external magnetic fields. Similarly, spectroscopes must be free from interference fields for optimal performance.

One method of suppressing interference fields for relatively large enclosures in the past has been by shielding the regions against external fields. This was done by enclosing the area with several layers of a material such as mu-metal or some other highly conductive metal. However, although this approach is satisfactory for many situations, the cost can be objectionably high, particularly where the shielded region is relatively large.

Another known technique for achieving a field-free region is to arrange individual field generating coils, such as Helmholtz coils, in the X, Y and Z coordinates encompassing the region. In addition, separate field sensing probes are arranged along the same X, Y and Z coordinates for detecting the presence of interfering magnetic fields and controlling associated power circuits to the field generating coils for producing a field counter to the interference field. That is, the apparatus in accordance with this technique senses the presence of an interfering magnetic field and an oppositely directed field of the same magnitude is generated thereby bringing the resultant field within the controlled region to zero.

Although the counter field technique just described is satisfactory, it has several serious drawbacks. First of all, the field sensing probes must be located sufficiently far from the monitored region to prevent interaction with the compensating coils on the probes. That is, the probes are not measuring the field within the magnetic free region alone, but a larger space that includes the region. Moreover, although a remote location of the sensors can be tolerated for relatively homogeneous interference fields, such as the magnetic field of the earth, this may not be possible where the fields are generated by such things as motors, generators, or electric current conducting lines, for example. Moreover, to operate satisfactorily, it is necessary that the probe signals and associated circuitry driving the compensating coils be very stable since any change in amplification of any one of the probe channels could result in a severe unbalance in the system. Finally, if the space or region to be maintained interference free is to be monitored continuously, it is necessary in the practice of this technique that special probes be installed within the space or region, which is a disadvantage.

It is, therefore, a primary object and aim of the present invention to provide apparatus for establishing a region or working space that is free from interfering magnetic fields, all of which is obtained inexpensively and reliably.

A further object is the provision of apparatus for producing a magnetic field free region having field counteracting means producing a resultant zero field within the region even where the operational characteristics of the various apparatus component elements vary within broad limits.

Another object is the provision of apparatus for producing a field free region as in the above objects in which monitoring of interference fields within the region can be accomplished relatively easily and inexpensively.

A still further object is the provision of apparatus for creating a region free from interfering magnetic fields in which field sensors include compensation coils to obviate interaction with counter-field generating coils.

In the practice of the present invention, magnetic field generating coils are provided, arranged at opposite sides of the region to be made free of magnetic field and along each of the three coordinate axes. A set of field sensing probes or transducers are provided having compensation coils to prevent interaction with the field generating coils arranged along other axes.

As a further aspect, undesired interaction of the probes with control circuits of other equipment or devices being used in the region free from interference fields is suppressed by installing at each of the probe locations one or more compensating coils which nullifies the effect of these fields on the probes.

In yet another aspect of the invention, means are provided for manually biasing control amplifiers to compensate for fixed fields generated by other equipment being used in the region.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in schematic form the apparatus of the present invention illustrated particularly for the elimination of interfering magnetic fields in a region including an electron microscope.

FIG. 2 discloses a system similar to FIG. 1 including a modified probe.

FIG. 3 depicts a means for producing dynamic control for either of the versions of FIGS. 1 or 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to the drawings and particularly FIG. 1, there is depicted in schematic form the circuit apparatus and coil arrangement of the subject invention for providing a region substantially completely free from interfering, externally generated magnetic fields. More particularly, the region is seen to include, for illustrative purposes only, in its central portion an electron microscope vacuum cylinder 10 and a substantial volume immediately adjacent thereto. The region being monitored and treated by the apparatus to be described is encompassed by a three-dimensional set of Helmholtz coils 11, including pairs of coils 12, 13, 14, 15, 16 and 17, each pair aligned in one of the X, Y and Z coordinate directions. That is, with reference to the coordinate axes diagram, the coils 12 and 13, when energized will provide a field parallel to the X-axis, coils 14 and 15 parallel to the Y-axis, and coils 16 and 17 parallel to the Z-axis.

In addition, a set of X, Y and Z oriented magnetic field sensing probes 18, 19 and 20 are located within the region to be maintained free from interference fields and closely adjacent the cylinder 10. Preferably, each of the probes 18-20 can include a magnetometer, e.g., a flux-gate magnetometer which generates a signal of value related to the strength of the magnetic field existing in the direction of the respective probe axis. Signals from each of the probes 18-20 are connected via leads 21, 22 and 23, respectively, to processing circuits 24, 25 and 26 for producing signals generally proportional to the probe signals The processing circuits are connected to driving amplifiers 27, 28 and 29, the outputs of which are fed over leads 30, 31 and 32, to the pairs of Helmholtz coils for generating fields within the region opposite to that detected by the probes. For simplicity of illustration, the Helmholtz coils 12-17 have been shown as comprising a single turn, however, it is to be understood that in a practical embodiment of the invention, such coils will usually each comprise a considerably larger number of windings.

The term Helmholtz, as applied to the various magnetic field generating coils of this invention, refers to the fact that the coils of each pair, i.e., 12 and 13 for the X-axis, are maintained spaced at a distance substantially equal to the effective radius of the coils. It can be shown that with such an arrangement the magnetic field so generated is highly uniform throughout the space between the coils.

Referring to the output from amplifier 27, for example, the leads 30 are seen to provide energizing power to each of Helmholtz coils 12 and 13 such that the magnetic field generated therein will have a resultant field in a single direction normal to the planes of the two fields and parallel to the X-axis. In a similar manner, closed loop energization of the field generating coils for the Y- and Z- axes is also available. It can be shown that if the probes are maintained relatively close to the cylinder 10, the magnetic field sensed by the probes will be substantially the same as that in the vacuum cylinder 10.

Each of the pairs of the field generating coils, its associated processing circuit and driver amplifier, form a closed loop system. Using the X system as an illustration, if no interfering field is present, and thus none sensed by the X probe 18, no signal is applied to the processing circuit 24 or to the amplifier 27, so that the X-axis Helmholtz coils 12 and 13 will not be energized. On the other hand, when an interference magnetic field is detected by the X probe 18, a signal of polarity corresponding to the direction of the interference field will be applied to the processing circuitry 24, which will produce at the output lines 30 of the amplifier 27 a driving current applied to the Helmholtz coils 12 and 13 of such magnitude and in such direction as to direct a magnetic field in the region of the vacuum cylinder 10 opposite to that of the interference field. Specifically, the resultant of the interference field with that generated by coils 12 and 13 responsive thereto, is ideally zero. Operation for the other axes is the same. In the usual case, an interference field will not be directed exactly along any of the orthogonal axes, but rather at an angle thereto such that more than one probe is affected by the respective interference field component.

It is important to note that changes in sensitivity of the probes, amplification of the driver amplifiers, or field generating coil efficiency have practically no influence on the resultant operation of the described apparatus. There must, however, be sufficient overall sensitivity and amplification in each of the closed loops to provide counteraction for the lowest magnitude of interference field that can adversely affect operation of other equipment in the work region.

Although operation of the FIG. 1 embodiment is generally satisfactory, certain difficulties are encountered in practical operation. First of all, the magnetic field generating coils of one coordinate axis may produce component fields in the probes of the other coordinate axes, mainly because an accurate arrangement of the probes cannot always be practically achieved. Also, when the probes are located outside the region of uniform magnetic field of the Helmholtz coils, the problem is accentuated.

Another problem arises from the fact that equipment operating within the region being maintained free from interfering fields, such as an electron microscope for example, frequently generates a magnetic field of its own which is sensed by the probes and interpreted thereby as an interference field resulting in a further counteracting field being generated in the manner already described. For example, typically, an electron microscope will produce a magnetic field which at the outer surface of the vacuum cylinder may attain a magnitude as great as 10 oersteds. Accordingly, special measures must be taken when such equipment is operated within the work region to obviate an erroneous counter field being generated resulting from the detection of the equipment generated field. A particularly effective technique for this purpose and the one described herein, is the introduction of compensating windings onto the probes.

Turning now to FIG. 2 of the drawings, probes 33, 34 and 35 for monitoring the field condition in a given region, are assumed oriented as in the FIG. 1 arrangement to detect fields parallel to the X-axis, Y-axis and Z-axis, respectively. As before, conventional electric connections are provided from these probes to processing circuits 36, 37 and 38, the modulation of which are individually controlled in a conventional manner by adjustment of the devices 39, 40 and 41. The output of each of the processing circuits 36-38 is fed via leads 42, 43 and 44 to power or driver amplifiers 45, 46 and 47, which amplifiers, as in the first described embodiment, generate currents functionally related to the magnetic fields interacting with the respective probes 33-35.

Current from the amplifiers 45-47 is directed along leads 48, 49 and 50 to drive the X, Y and Z field generating coils 51, 52 and 53, the latter being shown schematically as a single turn each. One lead of each of paired leads 48-50 includes a serially arranged resistor 54, 55 and 56. The voltage drops produced in the resistors 54-56 by driving currents are applied across compensating windings wound on the probes for each of the other two coordinate axes. That is, the voltage drop across resistor 54 (which is in the X-axis circuit) is applied through a variable resistor 57 to a compensating winding 58 on the X probe, and also via a further variable resistor 59 to a compensating winding 60 on the Z probe 35. Similarly, the voltage across resistor 55 in the circuit to the Y coil is applied through variable resistor 61 to a compensating winding 62 on the Z probe, as well as through a variable resistor 63 to a compensating winding 64 on the X probe. Finally, voltage developed across resistor 56 by current powering the Z coils is applied under control of the variable resistor 65 to a compensating winding 66 on the X probe and via a variable resistor 67 to a compensating winding 68 on the Y probe.

Three other compensation windings for the X, Y and Z probes, respectively, are identified by the numerals 69, 70 and 71, and which are fed by current from the field current of the equipment being used, such as the field current of the magnetic lense system of an electron microscope 72, for example. Threshold adjustment for the windings 69-71 is under the individual control of variable resistors 73, 74 and 75, specific adjustment of which will be described.

Operation of the apparatus depicted in FIG. 2 is generally the same as that in the embodiment of FIG. 1 already described, except that compensation of stray field components detected by the probes from operation of the different coils is achieved. Initially the three separate closed circuits for the X, Y and Z coils are temporarily disconnected at the lines 42, 43 and 44. Also, at this time, the lens systems of the electron microscope 72 is turned off and only one of the sets of Helmholtz coils is maintained in operation, the X coil 51, for example. A prescribed amount of current is caused to flow in the X coil 51 and the devices 40 and 41 are referred to to determine if stray field effect is produced in either the Y or Z probes as a result of the X field generation. Assuming that there is such an interaction, then proper adjustment of variable resistors 57 and 59 as well as insuring correct polarity by the reversing connections to the terminals 76 and 77 if needed, will zero the readings in the devices 40 and 41 whereby all effect of the X coil field on the Y and Z probes is counteracted. This same procedure is followed with respect to the other probes by sequential energization of the Y and Z coils. By this technique which can be referred to as "cross compensation," all effect of components from the X, Y and Z coil magnetic fields on the probes is eliminated.

For compensating or counteracting any fixed field generated by the electron microscope 72, the leads 42-44 are temporarily disconnected as before. When the electron microscope is switched on, the devices 39-41 will experience a deviation from zero as a result of the field produced by the microscope lens system. Zeroing of the devices 39-41 is accomplished by individual adjustment of the variable resistors 73, 74 and 75, and, where needed, reversal of connection at terminals 78-80 to produce the correct polarity for required compensation.

After initial calibration or compensation for stray fields from the Helmholtz coils and from the equipment being used in the field-free region (electron microscope), the leads 42, 43 and 44 are again connected as shown in FIG. 2. The apparatus is now ready for general use to counteract the effect of interference magnetic fields generated by sources located externally of the region encompassed by the Helmholtz coils.

In certain circumstances it may be possible to avoid interaction of the probes with fields generated by the electron microscope by locating the probes where the effect of the electron microscope's field is minimal.

Another version of the invention for removing the influence of relatively constant fields produced by equipment such as an electron microscope, is that shown in FIG. 3. As illustrated there, coupling capacitors 81, 82 and 83 are serially arranged, respectively, between the processing circuits 36-38 and their associated driver amplifiers 45-47. In addition, the input of each amplifier 45-47 is connected via a manually adjustable slidewire contact of a resistance potentiometer arranged across a D. C. source: potentiometer 84, amplifier 45; potentiometer 85, amplifier 46; and potentiometer 86, amplifier 47. The remainder of circuit apparatus can be the same as in FIG. 2.

Manual compensation for the field produced by the electron microscope or other such equipment in the FIG. 3 embodiment is effected by manually adjusting each of the slide-wire contacts of the potentiometers 84-86 until zero is indicated on each of the devices 39-41. After zeroing in this manner, the apparatus of the invention is now fully compensated for all constant magnetic fields existing at the location of the probes which, although it has been assumed are generated by the electron microscope or other equipment, can in actuality be any constant field such as, for example, the magnetic field of the earth. The capacitors 81-83 in conjunction with the input resistance of the respective amplifiers 45-47 form a time constant as is well known in the electronic arts. It is advisable that the value of these capacitors be chosen in order that the lower frequency limit be in the range 0.1 Hz, which will exclude slower changes of magnetic field from being balanced out by the system. However, since the measuring time required for most electron microscope operation is usually below 10 seconds, a frequency limit approximating the lower frequency limit specified above is most feasible, while changes of magnetic field at a higher rate which could impair resolution of the electron microscope will, on the other hand, be satisfactorily controlled.

Claims

What is claimed is:

1. Apparatus for producing a space free from magnetic interference fields within which space systems sensitive to interference fields are located, such as electron microscopes, spectroscopes, and the like, comprising:

probe means responsive to magnetic fields being located in the space to be free from interference fields for receiving the sum total of all the magnetic fields existing at the probe means location, said probe means generating electric signals generally proportional to the ambient magnetic field;

means connected with said probe means for generating an electric current as a function of the sum total of all the magnetic fields received by the probe means; and

coil means powered by the electric current for producing further electric fields in directions and of respective magnitudes such that the sum total of all the magnetic fields existing at the probe means location is substantially zero.

2. Apparatus as in claim 1, in which the probe means are located in the immediate vicinity of the systems sensitive to interference fields.

3. Apparatus as in claim 1, in which the probe means are arranged to sense magnetic fields preferentially along the directions of an orthogonal set of coordinates and the coil means are arranged so that magnetic fields generated thereby are directed along the field-sensitive directions of the probe means.

4. Apparatus as in claim 1, in which interaction of the coil means with the probe means is suppressed by the provision of compensating winding means on said probe means, and means providing a current to said compensating winding means of such polarity and magnitude as to cancel out the interaction.

5. Apparatus as in claim 1, in which interactions of fields produced by said systems sensitive to interference fields with the probe means are suppressed by compensating winding means wound on said probe means, and further means are provided for directing an electric current through said compensating winding means of such magnitude and polarity as to counteract the systems generated fields.

6. Apparatus for eliminating interference magnetic fields from a prescribed region in which field generating equipment is operated, comprising:

first, second and third coil means arranged along the respective x, y and z orthogonal axes of said region;

first, second and third magnetic field probe means located within said prescribed region and each respectively arranged to sense magnetic fields along one of the orthogonal axes of said region;

individual processing circuits connected to said probe means for providing electric signals substantially proportional to the magnetic fields sensed by the associated probe means; and

separate amplifying means interconnecting each processing circuit and the associated coil means for powering said coil means to produce a magnetic field in said region cancelling interference fields sensed by the probe means.

7. Apparatus as in claim 6, in which each probe means is provided with compensating winding means, and an individually selectively variable source of electric power is connected to each said compensating winding means for cancelling out in each probe means the effect of fields generated by the coil means located in the other coordinate axes.

8. Apparatus as in claim 6, in which each probe means is provided with compensating winding means, and an individually selectively variable source of electric power is connected to each said compensating winding means for cancelling out in each probe means the effects of fields produced by said field generating equipment.

9. Apparatus as in claim 6, in which there are provided first and second compensating windings on each probe means, and individual selectively adjustable electric power sources connected to each compensating winding the adjustment of which effectively cancels out for each probe means both the field effect of coil means in the other coordinate axes and that of the field generating equipment.

10. Apparatus as in claim 6, in which individual selectively variable electric power source means are connected to each amplifying means, individual adjustment of which compensates for relatively constant interference fields.

11. Apparatus as in claim 6, in which capacitor means interconnect each processing circuit with its associated amplifying means so that current changes in the coil means are only produced on changes in magnetic fields sensed by the probe means.