US2823287A - Potentiometer - Google Patents

Description

Feb. 11, 1958 A. J. KLOSE 2,823,287

POTENTIOMETER Filed Aug. 10, 1953 ALFRED 67f M035 IN V EN TOR.

Unitcd States Patent POTENTIOMETER Alfred .J. Klose, Rolling Hills, Calif, assignor to G. M. Giannini & Co. Inc., Pasadena, Calif., a corporation of New York Application August 10, 1953, Serial No. 373,411

4 Claims. (Cl. 201-59) This invention is concerned generally with electrical potentiometers of the type in which a 'brush is movable continuously along the length of a resistance element that is supported on a convex surface.

In such potentiometers the support surface is ordinarily cylindrical in form, and the resistance element may typically extend only part way around the cylinder, or may comprise a plurality of helically wound turns. Particularly in the latter case, difficulty may be experienced in driving the brush sufficiently accurately along a corresponding helical path to insure that contact is made only with a single turn of the winding. To insure correct following of the resistance element by the brush relatively precise and hence expensive following mechanism is usually required, and even then the adjacent turns of the resistance element must be generously spaced to prevent spurious contact. When the winding comprises resistance wire, such difiiculties become increasingly serious the smaller the diameter of the wire.

Moreover, when the brush is required to follow the winding accurately, as in previous potentiometers of the described type, the winding always engages the same point, or substantially the same point, of the brush. That frequently leads to excessive wear of the brush, with consequent short life or unreliable operation of the instrument. I

In accordance with the present invention, means are provided for shifting a limited portion of the resistance winding out of alignment with the body of the winding, and the brush makes contact with the resistance element only within that shifted portion. Control means are provided, by which the winding shifting means and the brush (which may comprise a unitary structure) are movable longitudinally with respect to the winding, so that the portion of the winding that is shifted and the position of the brush contact may be selectively varied in a continuous fashion.

With that novel type of brush engagement, adjacent turns of the winding, in those instances in which a pinrality of turns is employed, may be wound on the supporting form in closely spaced relation. If the brush is moved helically in correspondence to the Winding, that helical movement need not be particularly accurate, since only a single turn of the winding is shifted into position in which it can engage the brush. Furthermore, no axial movement of the brush need be provided at all, greatly simplifying the mechanical structure of the device and reducing the possibility of imperfect electrical connection between the brush and the external electrical circuit.

Whereas the invention offers particularly important advantages in connection with otentiometers having relatively long helical windings, it is useful also in other types of instrument, including, for example, those in which the winding extends angularly only through a relatively small part of a circle.

A full understanding of the invention and of its further objects and advantages will be had from the following description of an illustrative embodiment, of which deice scription the accompanying drawings form a part. That description is intended only as illustration, and many changes may be made in the particulars of the described embodiment without departing from the scope of the invention; It will be understood, for example, that relative movement of the resistance winding and'the brush structure may be obtained in any convenient manner, movement of one of those elements and holding the other element fixed being merely illustrative. In the present embodiment the brush has been illustratively selected as the fixed element. The term potentiometer is used in the sense of including variable resistances, such as are frequently called rheostats.

In the drawings:

Fig. 1 is an'axial section of an illustrative embodiment of the invention; and

Fig. 2 is a transverse section on line 22 of Fig. 1 at enlarged scale.

In the present illustrative embodiment of the invention, the resistive element 10 comprises a single wire of relatively small diameter, typically of the order of 0.001", which may therefore provide a relatively high electrical resistance per unit length. That wire is mounted in insulated'relation on a support 12, which may be of any type that permits the winding to be locally displaced transversely from its normal position. That support is shown for illustration as a substantially solid cylindrical body of dielectric material having in its convex surface 15 a helical channel 14, which typically may comprise a relatively fine thread. The winding lies freely in channel 14, its ends being connected to electrical terminals in'any suitable manner. The length of. the winding may be only a part of one full turn, or may occupy a relatively large number of consecutive turns of channel 14.

An illustrative contacting structure is indicated generally at 20, and comprises a single piece of resilient metal of suitable type having a mounting portion 22 and an arm portion 24, which forms the brush proper, to be further described. Mounting portion 22 of the brushassembly comprises a resilient strip, one end of which is rigidly mounted in insulated relation, as by the bushing 23, on a support, shown as frame plate 30. The other end of resilient strip 22 carries brush 24, which extends substantially parallel to the axis of winding support 12 and closely spaced radially from its convex surface. A single turn of winding 16 passes over brush 24, so that a portiononly of the winding is supported by the brush in spaced-relation to support 12, as indicated at 25. That portion 25 of the winding is displaced out of alignment with the body of the winding, and sli'dingly engages at its center the radially outer face of the brush at 26,

Contacting structure 20, and particularly its fiat mounting portion 22, is so formed that when under no constraint its brush arm extends obliquelyv outwardly, as indicated schematically in phantom lines at 24a. The turn of the winding that passes over the brush arm deflects the latter inwardly from its natural position 24a to an actual Working position such as that shown in solid lines at24 in the drawings, the resilience of the brush when so deflected being such as to maintain the winding under a light yielding tension.

The brush is mounted for controlled movement longitudinally with respect to the winding, it being in large part immaterial whether the brush or the winding is actually movable in space, or whether both may be movable relative to the frame of the instrument. As illustratively shown, the brush is mounted rigidly, save for its inherent resilience, on the frame plate 30, and the winding is movable longitudinally past the brush. Winding support 12 is fixedly mounted coaxially on the control shaft 34, as by the metal hub 32 and the setscrew 33. Shaft 34 is journaled, as by the ball bearings indicated at 36 and 37, in a through bore 38 normal to the plane of frame plate 30, the forwardly projecting end of the shaft preferably carrying a control knob 39 by which the winding and its support can be rotated bodily about the shaft axis. Such rotation causes relative movement of brush 24 longitudinally of the helical winding.

Electrical connections between the respective ends of the wmding and terminals fixed on the frame may be made many suitable manner, highly flexible wires being sufficient in many instances. Particularly when the winding includes a large number of turns, it is preferred to provide sliding connections, for example by means of slip rings, as schematically indicated at 40 and 41, embedded in an axial face of support 12 and connected by wires 42 and 43 through suitably placed bores 44 and 45 in the support to the winding ends. Brushes 46 and 47 are mounted in insulated relation on frame plate 30, as by the insulating bushings 48 and 49, and resiliently engage the respective slip rings 40 and 41 in all rotational positions of shaft 34. Connection terminals 50 and 51 are shown in position to be accessible from the front of frame plate 30, and a similar connection terminal for the brush is indicated at 53.

In operation of the device, rotation of control shaft 34 causes the winding to rotate with its support 12, and to move longitudinally with respect to the brush. From another viewpoint, the brush moves along the winding, shdingly engaging the radially inner face of the wire, and thereby establishing a movable electrical contact. action causes the wire to be lifted progressively free of the drum surface ahead of the brush, and simultaneously permits the wire to return into engagement with the drum surface behind the brush. When, as in the present embodiment, a helical channel 14 is provided in the support surface, the wire is lifted out of the channel in which it is lying ahead of the brush, and is returned to that channel behind the brush. Therefore relative movement of the brush past any point of the winding produces no permanent change in the condition of the winding at that point However, as the brush passes any point of the wlndmg, the wire is locally deflected into spaced relatron with the support and out of alignment with the body of the winding, permitting the brush to pass between it and its normally supporting surface. The point 26 of 'br -ush contact moves continuously along the length of the w nding circumferentially of the support, dividing the winding electrically into two portions of continuously variable ratio. Since the wire, at the point of brush engagement, has been deflected out of alignment with the body of the winding, no difliculty is encountered in limitmg brush contact to a single one of the turns of the w nding. There is no possibility of accidental contact with any non-deflected turn of the winding.

The axial position of contact 26 between the lifted turn of the winding and the brush is not defined by the structure of the brush itself, which in the present embodiment presents a smoothly polished surface to the wire, but is determined rather by the action of channel 14 of the support in guiding the wire at each side of the brush. Thus, in the present embodiment, in which the relative movement of brush and winding is circular while the winding itself is helical, longitudinal movement of the winding across the brush also involves movement of the contact point 26 along the brush arm in an axial direction. On the other hand, the relative brush movement may be made helical, for example by replacing bearings 36 and 37 by threads between shaft 34 .and frame plate 30, the pitch of those threads being the same as the pitch of winding channel 14. With such mounting of the resistance support, the resistance element moves only longitudinally with respect to the brush, with no axial movement, and the position of contact does not move along the brush.

Whereas the brush structure in the present embodiment is basically very simple, the detailed form of the brush That arm is .an important aspect of the invention. Thatbrush arm is shown in transverse section in Fig. 2. As may be seen from that figure, the working surface 64 of the brush is symmetrical about an axial plane of the winding support, indicated by the line 60, and is curved cylindrically and substantially circularly about an axis of curvature, indicated at 62, which lies in that plane of symmetry. The radius of curvature of the brush, indicated at r, is taken in the present illustrative embodiment to be approximately one third of the radius of the helical winding, indicated at R. The width of the brush, indicated as d in Fig. 2, is substantially one half of r. And the spacing between thte brush working surface and channel 14, indicated at h, is preferably made as small as is conveniently possible. That spacing is shown illustratively as approximately one fifth of the brush radius of curvature r, but with careful manufacture may be made considerably less than that. The extreme edges of the brush, at which the resistance wire enters and leaves contacting relation, are slightly broken, as indicated at 63. But that departure from the circular form of the brush, as shown, is limited to the extreme edge portion, the curvature being relatively constant over the greater part of the Working surface of the brush. That entire working surface 64 is smoothly polished to reduce friction and wear of the winding, and to provide smooth and uniform electrical contact.

An important feature of the described structure is the relatively large radius of curvature r of the working surface of the brush. That relatively large radius of curvature prevents the resistance wire from bending over the brush so sharply as to produce a permanent set or to cause fatigue effects that might unduly shorten the life of the instrument. The required radius must be larger the greater the diameter of the wire, the lower the yield stress and the higher the modulus of elasticity, each of those relations being approximately linear. It is preferred that the spacing h between the brush and support be as small as is practicable. The brush radius may effectively be from about one fifth to about one half of the radius R of the winding support, and is illustratively shown with the preferred value of about one third of R.

The portion of the winding that is lifted clear of support 12 by the brush, indicated at 25, forms on each side of the brush a free bight, which is held substantially straight by the tension of the winding. One end of that bight 25 is at 65 where it meets the edge of brush working surface 64; and the other end is at 67 where it meets the surface of support 12. The radii r and R have been drawn to those respective points, and form angles with the plane of symmetry which are indicated in Fig. 2 at 70 and 72, respectively. Due to the limited width 0? of the brush with respect to its radius of curvature'r, the angle 70 is less than the angle 72. It will be seen that angle 72 represents the total change of direction of the winding between point 67 and the plane of symmetry 60. And angle 70 represents the part of that change of direction that corresponds to the uniform curvature of the brush surface between point and plane 60. The fact that angle is less than angle 72 therefore means that, whereas winding bight 25 contacts support 12 at 67 strictly tangentially, its contact at 65 brush surface 64 is not tangential with respect to the primary curvature of the brush (represented by radius r). Instead, the winding is caused to bend more sharply immediately adjacent its contact with each brush edge than over the intermediate portion of working surface 64 of the brush. That sharper curvature near the points 65 results directly from the described proportions of the parts, particularly the dimensions denoted in the figure as r, d, h and R. It has the advantage of providing increased contact pressure between the brush and the winding immediately adjacent the brush edges, insuring positive and uniform electrical contact at those points, and preventing any uncertainty, and hence variation, in the point of effective e ectrical contact of the brush with the winding. Due to that positive contact at the two points 65, the brush effectively shorts out the portion of winding between those points. With respect to its electrical function the brush may therefore be considered to engage substantially a single point of the winding.

I claim:

1. A potentiometer comprising support means defining a generally helical channel, an electrically conductive brush near the channel and longitudinally movable with respect to the channel, and a resistance element normally lying freely in the channel, a limited portion of the resistance element adjacent the brush passing over the outer face of the brush in spaced relation to the channel, said outer face being curved with an effective radius of curvature between about one-fifth and about one-half the radius of the helical channel, both edges of the brush in contact with said element being more sharply curved and providing increased contact pressure between the brush and the element.

2. In a variable resistance device of the type including support means defining a generally helical channel and a resistance element normally lying freely in the channel and wherein a limited portion of the resistance element is lifted from the channel in spaced relation to the channel to engage electrical conductive brush means, that improvement comprising: electrically conductive brush means defining an outer contact face in engagement with said resistance element, said contact face being smoothly curved and having a large effective radius of curvature r relative to the radius R of said helical channel and whose center is positioned within said support means, the extent of said outer face d engaged by said resistance element being substantially one-half of r and the edges of said outer face curving more sharply and engaging the resistance element with increased contact pressure.

3. In a variable resistance device of the type including support means defining a generally helical channel and a resistance element normally lying freely in the channel and wherein a limited portion of the resistance element is lifted in spaced relation to the channel for purposes of making longitudinally movable electrical contact therewith, that improvement prolonging the life of the resistance element by protecting it from unduly sharp bending comprising electrically conductive brush means having an outer smoothly curved face said outer face electrically engaging said resistance element and lifting said limited portion thereof, said outer face having an effective radius of curvature 1' that is large relative to the radius of curvature R of said helical channel, the center of curvature of the radius r lying between the axis of the helical channel and the portion of the helical channel adjacent to said brush means, said outer face being spaced above said helical channel by a distance 11 no more than onefifth of the effective radius of curvature r.

4. In a variable resistance device of the type including support means defining a generally helical channel having a radius R, an electrically conductive brush member near the channel and longitudinally movable with respect to the channel, and a resistance element normally lying freely in the channel and wherein a limited portion of the resistance element adjacent the brush is lifted in spaced relation to the channel and passes over the brush member, that improvement protecting the resistance element from sharply bending in passing over the brush member which includes a radially outer smoothly curved face on said brush member lifting and electrically con tacting the resistance element and having a large effective radius of curvature r relative to the radius R of said helical channel and wherein r is less than R with the center of curvature for the radius r being positioned between the axis of the helical channel and the portion of the helical channel adjacent to said brush member.

References Cited in the file of this patent UNITED STATES PATENTS 1,918,013 Bermann July 11, 1933 2,442,469 Palya June 1, 1948 2,662,150 Mairs Dec. 8, 1953

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