US3518595A - Variable inductor - Google Patents

Description

June 1970 s. L. DAWSON ETAL 3,518,595

VARIABLE INDUGTOR Filed Oct. 21, 1968 SAM/1054. LEE D4 4306 NoQ/vm/v B02254 RELKMEE INVENTORS p mdazll, L ,Zuz

71 oQ Q United States Patent 3,518,595 VARIABLE INDUCTOR Samuel Lee Dawson and Norman Darrel Felkner, Los Angeles, Calif., assignors to Wyle Laboratories, El Segundo, Calif., a corporation of California Filed Oct. 21, 1968, Ser. No. 768,985 Int. Cl. H01f 21/06 US. Cl. 336-434 6 Claims ABSTRACT OF THE DISCLOSURE Variable inductors which have high stability and resist influence by external magnetic fields, comprising a toroidal core with an air gap, and an armature of ferromagnetic material which can be moved into the gap to bridge it, or out of the gap. A coil wound about the toroidal core displays an inductance which depends upon the position of the armature.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to variable inductors.

Description of the prior art A typical variable inductor employs a tubular shell with multiple windings of a conductor thereon. A cylindrical armature of ferromagnetic material is mounted to slide in and out along the axis of the tube, to thereby change the inductance displayed by the windings. Such types of variable inductors are readily influenced by changing external magnetic fields, and changes in the position of iron bodies around them. This is due to the fact that the lines of magnetic flux lie partially outside of the windings and ferrite armature. Changes in the immediate environment which alter the reluctance of the path taken by these lines of magnetic flux, or which add or subtract magnetic flux at these outside areas can change the effective inductance.

In order to reduce uncontrolled variations in inductance, prior art variable inductors have generally required shielding. The shielding added weight and cost and made heat dissipation more difficult. Although shielding increases stability and freedom from external fields and materials, the cylindrically shaped variable inductors were still substantially sensitive to changes in ambient temperature, due to expansion and contraction of the core in relation to the winding.

OBJECTS AND SUMMARY OF THE INVENTION One object of the present invention is to provide a variable inductor of maximum stability.

Another object is to provide a variable inductor whose inductance can be varied over a wide range.

In accordance with the present invention, a variable inductor is provided which includes a toroidal core of high permeability material, which has an air gap. The core has windings thereabout which display an inductance dependent upon the reluctance of the air gap. An armature positioned at the location of the air gap includes a portion of high permeability such as a ferrite, and a portion of low permeability such as air or certain non-ferrous metals such as copper. The armature is mounted for movement toward and away from a position where it bridges the air gap in the toroid, to thereby change the inductance displayed by the windings.

In one embodiment of the invention, the ends of the toroid on either side of the air gap are flat. The armature comprises a fiat disc which is almost as thick as the air gap. The disc has 180 of ferrite material and 180 of 3,518,595 Patented June 30, 1970 a non-ferromagnetic material such as copper or aluminum which is electrically conductive. The disc is rotatably mounted, so that it can be turned from a position wherein the gap is bridged entirely by ferrite material, entirely by copper, or partially by each. The non-ferromagnetic but electrically conductive material enables large changes in inductance, such as 15%, as compared with the change between the inductance level when there is a ferrite in the gap and when the gap is empty.

The use of a toroidal core with a small gap concentrates the magnetic flux to positions near the gap, even when a ferromagnetic armature is not present in the gap. This limits the effect of external magnetic fields or changes in the position of ferromagnetic materials in the environment, as compared with previous cylindrical variable inductors. The variable inductors are useful in a wide range of applications, such as in tuned circuits to vary the tuned frequency.

The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a variable inductor constructed in accordance with the invention;

FIG. 2 is an exploded, partially sectional perspective view of a variable inductor constructed in accordance with a second embodiment of the invention;

FIG. 3 is a partially sectional perspective View of a variable inductor constructed in accordance with a third embodiment of the invention;

FIG. 4 is a partially sectional perspective view of a variable indutcor constructed in accordance with a fourth embodiment of the invention; and

FIG. 5 is a perspective view of a variable inductor constructed in accordance with a fifth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a variable inductor comprising a core 10 of generally toroidal shape. An air gap 12 is formed in the core, and an armature 18 is disposed partially within the gap. Multiple windings of an insulated conductor 14 are disposed about the core, and the inductance displayed by the windings varies in accordance with the position of the armature 18.

The ends 15 and 16 of the core on either side of the gap are flat. The armature 18 is in the form of a short cylinder or flat disc with a thickness approximately equal to that of the air gap 12 of the core. A portion of the armature is in the gap, but its cylindrical axis is outside of, or displaced from, the gap. The armature comprises a portion 20 of a ferromagnetic material (i.e. a material with a permeability at least several times that of free space) such as a ferrite, and another 180 portion 22 of a non-ferromagnetic material (Le. a material with a permeability approximately equal to that of free space) which is also a good electrical conductor, such as copper. The two portions 20 and 22 are sectors of the cylinder, so they extend throughout the cylinder length.

The armature 18 is fixed to a shaft 24 which is rotatably mounted on bearings 26 and 28. The end 25 of the shaft is enlarged to hold it in place, and a domed or menisius-shaped spring washer 29 is provided to press against the shaft end 30. The washer 29 provides an appreciable but limited resistance to turning of the shaft 24, to maintain the armature in any position to which it is turned. The end 30 of the shaft has a slot 31 for receiving a screwdriver to facilitate turning of the shaft.

The entire apparatus may be potted with resin or the like to provide a complete package. In the position shown in FIG. 1, the entire gap is bridged by the ferromagnetic portion 20, thereby providing a minimum reluctance across the gap and a maximum inductance for the device. A half rotation of the armature places the non-ferromagnetic material entirely within the gap 12. This increases the reluctance of the gap, thereby decreasing the inductance to a lower level.

The non-ferromagnetic material of portion 22 has a permeability almost equal to that of free space. Thus, the static reluctance of the path across the gap in the core is reduced to that existing when nothing is in the gap. However, the electrical conductance of the material of portion 22 results in the induction of currents therein when the magnetic flux in the core is changing. These currents induced in the material at 22 oppose the change in flux, and result in an effective decrease of the reluctance of the air gap to an even smaller level than exists for free space. This effect is very noticeable when the windings 14 carry high frequency currents, and enables a decrease of inductance to a low level. For example, at a frequency on the order of 1 mHz. the change in inductance achieved by turning the armature 180 can be on the order of :15 when the portion 22 is of a good conductor such as copper. If free space is substituted for copper, the change is only about '-10% The toroidal variable inductors are relatively insensitive to external magnetic fields or bodies, and have been found to display good stability under changes of temperature. The path of flux is of low reluctance, so that the device is efiicient and a large inductance is provided in a small volume. In addition, a high Q is realizedso that a large inductance is provided with a low resistance of the windings.

Toroidal cores generally must be wound in special machines which repeatedly thread a conductor through the hole in the core. However, in many cases the toroidal cores of the invention can be wound by allowing the wire to pass through the air gap during winding. This can reduce the cost of constructing the variable inductors and enable more turns to be received.

FIG. 2 illustrates a second embodiment of the invention wherein the core 40 has concavely rounded depressions 42 and 44 of part-cylindrical shape on either side of the gap 46 in the core. The depressions face in the same direction and are aligned with each other, and each depression faces substantially perpendicular to the other end of the core. An armature 48 of elongated cylindrical shape is disposed in the depressions to bridge the gap. The armature has a portion 50 of ferromagnetic material defining a half cylindrical or 180 portion, and a portion 51 of non-ferromagnetic material comprising the other 180 of the cylinder. Each portion 50 and 51 extends throughout the cylindrical length of the armature, although each comprises only a portion of the armature cross-section.

The armature 48 is mounted in a pair of bearings 52 and 54 at either end, and a slotted boss 56 is fixed to one end of the armature to facilitate turning. Turning of the armature brings all or part of the ferromagnetic portion 50 closely against the surface of the core at the depression, to cause variations in the inductance of the device. The embodiment of FIG. 2 is especially resistant to external fields or bodies because the entire ferromagnetic portion 50 is always a relatively small distance from the depressions 42 and 44 in the core. The limited distance assures that nearly all flux lines not passing through the non-ferromagnetic portion 51, will pass through the ferromagnetic portion 50, rather than through any iron or other bodies in the environment.

FIG. 3 illustrates a third embodiment of the invention which comprises a ferromagnetic core 60 with a gap 62, and annular or rounded depressions 64 and 66 on either side of the gap. The depressions 64 and 66 face each other to closely surround a cylindrical armature 68 on opposite annular sides thereof. The armature has a cylindrically shaped portion 70 of ferromagnetic material and another cylindrically shaped portion 72 of the same size of non-ferromagnetic material, the two portions disposed end-to-end and in alignment.

The armature 68 is constrained to movement in and out of the gap 62 in the core by a plastic tube 74 surrounding the armature. A rod 76 joined to the portion 70 can be pushed or pulled to move the armature parallel to its cylindrical axis. Of course, the inductance is greatest when the ferromagnetic portion 70 is fully within the gap, and is least when it is fully withdrawn and the nonferromagnetic portion 72 is therein. This embodiment enables the direct conversion of a linear motion to a change in inductance, as is the case in the variable inductors known heretofore which comprised a cylindrical winding. However, the embodiment of FIG. 3 is more stable, more insensitive to external fields and bodies, and more compact than the prior art types.

FIG. 4 illustrates a fourth embodiment of the invention which utilizes a threadably mounted armature 80' to vary the gap 82 in a toroidal core 84. One end 86 of the core has a rounded depression partially encircling the armature, and the other end 88 is fiat. The armature, which is constructed entirely of ferromagnetic material, is of cylindrical shape with a flat end 89, to closely engage the ends of the core. The armature is fastened to a threaded member 90 with a slotted end 92, which is threadably engaged with a nut-like member 94. Rotation of the threaded member 90 moves the armature end 89 toward and away from the flat end 88 of the core, to-

increase and decrease, respectively, the inductance displayed by the core winding 96.

The embodiment of FIG. 4 enables changes in inductance to continue through several turns of the armature which can facilitate fine adjustments. In addition, the tendency of the core to move the armature to the position of greatest inductance is resisted by the threadable mounting, even for relatively course threads. The ends 88 and 89 of the core and armature, respectively, are preferably mated to enable a large area of contact when moved together, to provide a high maximum inductance. However, instead of flat ends, other shapes such as tapered configurations can be employed to vary the manner of change of inductance.

FIG. 5 illustrates a fifth embodiment of the invention wherein the armature is of short cylindrical shape with a band-like portion 102 of ferromagnetic material and two portions 104 and 106 of non-ferromagnetic material on either side. The armature is rotatably mounted along its cylindrical axis 108, by a rod and bearings (not shown). This embodiment of the invention enables a variation from maximum to minimum inductance with only a quarter-turn of the armature.

While each of the foregoing embodiments of the invention have peculiar advantages, all of them have the advantages resulting from the toroidal shape with an armature which can substantially completely close the limited gap in the toroid. These include the reduced sensitivity to external fields and bodies, and increase stability, compactness, and ease of winding. While toroids of substantially constant rectangular cross-section are showing a variety of toroidal-like shapes can be utilized, so long as the core defines a substantially closed path for the flux except for a gap which can be closed by the movable armature.

Variable inductors have been constructed in accordance with the above by winding a conductor around a core and molding or potting them in an epoxy. Many variable inductors of the types shown in FIGS. 1 and 2 have been constructed utilizing a ferrite for the ferromagnetic portions of the core, and copper or aluminum for the non-ferromagnetic portions. Small variable inductors with an outside diameter of 4 inch, and air gaps approximately inch long were constructed with inductances of microhenries to 1 millihenry. The inductors were utilized at frequencies of 100 kHz. to 25 mHz., and displayed changes in inductance on the order of +-IS%. The inductors operated stably without the necessity for magnetic shielding or close control of the environment.

Although particular embodiments of the invention have been described and illustrated herein, it is recog nized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

What is claimed is:

1. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having ends forming a gap in said core;

a winding disposed about said core to generate magnetic flux therein;

an armature comprising a first portion of ferromagnetic material, and a second portion of nonferromagnetic material which is of high electrical conductivity; and

means for positioning said armature in a plurality of positions, including a first position wherein said ferromagnetic portion substantially completely bridges said gap while substantially none of said material of high electrical conductivity is in said gap, and a second position wherein said material of high electrical conductivity substantially completely bridges said gap while substantially none of said ferromagnetic material is in said gap.

2. The variable inductor described in claim 1 wherein:

said ends of said core at said gap are substantially flat and parallel to each other;

said ferromagnetic portion of said armature comprises a portion of a disc which has a thickness approximately equal to the separation of said ends of said core and an axis displaced from said gap; and

said means for positioning said armature comprises means for rotating said ferromagnetic portion about said disc axis and maintaining it in any of a plurality of rotational positions thereabout.

3. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having ends forming a gap in said core, each of said ends having trough-like concave depressions which face in the same direction;

a winding disposed about said core to generate magnetic flux therein;

an elongated cylindrical armature with its rounded perimeter mated to and disposed within said concave depressions, said armature including a ferromagnetic portion thereof extending throughout the length of said cylinder along only a portion of the cross-section thereof; and I means for rotating said armature about its cylindrical axis.

4. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having ends forming a gap in said core, said ends including a first end having a trough-like depression and a second end in line with an imaginary extension of said trough-like depression;

a winding disposed about said core to generate magnetic flux therein;

an armature comprising a substantially cylindrically shaped portion of ferromagnetic material for sub stantially bridging said gap, to provide a substantially closed magnetic path through said core; and

means for threadably mounting said armature for rotation about the axis of its cylindrically shaped portion to advance it toward and away from said second of said end of said core while maintaining it in constant engagement with said first end of said core.

5. A variable inductor comprising:

a toroidal-like core of ferromagnetic material, said core having ends forming a gap in said core, each of said ends having a trough-like depression;

a winding disposed about said core to generate magnetic flux therein;

a substantially cylindrical armature having a substantially strip-like portion of ferromagnetic material extending substantially diametrically through said armature and along its length to substantially bridge said gap in said core, said armature also having portions on diametrically opposite sides of said cylinder which are free of ferromagnetic material; and

means for rotating said armature about the axis of said cylinder, said strip-like portion being thin enough so that none of it is directly opposite either of said core ends of at least one position of said armature, whereby the inductance changes between a maximum and a minimum with a substantially quarter-rotation of said armature.

6. The variable inductor described in claim 5 wherein:

said portions of said armature which are free of ferromagnetic material are constructed of nonferromagnetic material of high conductivity.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 3 1939' Austria. 11/ 1956 Germany.

1948 Great Britain. 875,468 8/ 1961 Great Britain. 239,091 12/ 1945 Switzerland.

THOMAS J. KOZMA, Primary Examiner U.S. Cl. X.R.

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