US2871326A - Precision potentiometers - Google Patents

Description

Jan. 27, 1959 J. w. WEIDENMAN' ET-AL PRECISION POTENTIOMETERS 2 Sheets-Sheet 1 Filed July 23, 1956 .4'4/1455 14 WE/OE/VMAA/ 04 W0 5. 42A TAJE IN V EN TORS BY 7 gbaa q 922. W

ATTOI/VEY Jan. 27, 1959 J. w. WEIDENMAN ETAI. 2,871,326

PRECISION POTENTIOMETERS Filed July 23, 1956 2 Sheets-Sheet 2 JAMES W. WE/DE/VMA/V OAV/D s. 2471 -05 INVENTORS MW M Arroe/ gy PRECISION PBTENTIOMETERS James W. Weidenman, Westhury, and David S. Rathje, Yonkers, N. Y., assignors to Litton Industries of Caiifornia, Beverly Hills, Calif.

Application July 23, 1956, Serial No. 599,601

20 Claims. (Cl. 201-56) This invention relates to precision otentiometers, and more particularly to precision multiturn potentiometers of exceptional stability wherein the resistance element is wound upon a dimensionally stable and non-hygroscopic coil form and in which the associated slider assembly is guided by a helical element which is Wound over the resistance element.

The term multiturn potentiometer denotes a particular class of potentiometers which are operative to present an output impedance which is variable in accordance with the rotation of an associated input shaft as the shaft is rotated through a plurality of revolutions. As a general rule potentiometers of this type have relatively high resolution as expressed as a percentage of their entire resistance range, and may be designed to provide a variable impedance which varies linearly with rotation of the input shaft, or in accordance with some other predetermined function.

In the prior art, one common structural form which has been used in multiturn potentiometers is disclosed in U. S. Patent 2,495,321, issued January 24, 1950, for a Variable Resistor" to Gibbs et al. in this form of potentiometer a resistance element previously formed in the shape of a compound helix is potted in a suitable plastic, or thermosetting resin, to form an insulative housing cylinder into whose interior the resistance element helix projects. A slider element in sliding engagement with the resistance element is then carried by an input shaft positioned within the housing cylinder, the slider element being moved axially in accordance with shaft rotation by either a traveling not or by a guide slot molded into the housing cylinder between adjacent resistance element turns.

Although this form of prior art has been employed extensively in the prior art, its use is inherently limited by a number of serious disadvantages. Firstly, the insulative outer housing is prone to warping with age. Secondly, even in normal use the housing is very sensitive to moisture and temperature variations and exhibits dimensional instability as a result thereof. Thirdly, the process by which the outer housing is molded does not permit easy reproducibility of the electrical characteristics desired in the completed potentiometer, and in fact high linearity is extremely difficult to achieve regardless of the care exercised in the winding of the resistance element, since the linearity of the completed device is critically dependent upon the potting process. In addition, it is very difiicult to insure that the resistance element helix will be concentric with the input shaft even when those factors afiecting dimensional stability are very accurately controlled. As a consequence of the foregoing factors, this form of prior art potentiometer is completely unsuitable for many applications, and moreover, its life expectancy is limited in even those applications wherein its accuracy and linearity are initially sufliciently precise to merit its utilization.

in still another prior art multitum potentiometer an ates tet effort has been made to eliminate some of the foregoing disadvantages by winding the resistance element in a helical groove formed in the external periphery of an insulative coil form which is produced by casting or molding a thermosetting resin or plastic. In this form of potentiometer the slider assembly is guided axially by either a secondary guide groove formed in the periphery of the coil form or by a special form of slider show which fits around the resistance element and utilizes the resistance element per se as a guide.

Although this latter type of multiturn potentiometer has succeeded to some extent in eliminating those disadvantages found in the first type of multiturn potentiometer, its electrical characteristics are still variable with changing enviromental conditions and aging owing to the fact that the coil form is still dimensionally unstable. In addition, regardless of which form of slider guide arrangement is employed, wear on either the coil form or on the resistance element per se frequently produces potentiometer failure. Still another disadvantage of both of the foregoing prior art potentiometer types is that the end limits of the potentiometers electrical and mechanical rotational ranges are difiicult to adjust with precision, and the stop mechanisms employed to delimit these ranges are frequent causes of failure when the potentiometer input shaft is urged against the associated stops.

The present invention, on the other hand, obviates the above and other disadvantages of the prior art by providing a class of multiturn potentiometers which are exceptionally stable and insensitive to aging and which retain their stability under extreme variations in environmental conditions. According to the basic concept of the invention, the multiturn potentiometers herein disclosed utilize a core assembly which include a non-hygroscopic and dimensionally stable coil form, which may be composed of ceramic, for example, the resistance element being wound thereover in a helical pattern. A guide for the associated slider is then provided by an insulating strip which is Wound in the helical groove formed by adjacent turns of the resistance element, the guide helix thereby formed protruding above the resistance element to thus provide a second helical groove bounded at the sides by the guide helix and at its bottom by the resistance element.

In the preferred embodiment of the invention the coil form is concentrically mounted over a dimensionally stable metal hub which in turn is also used for journaling the input shaft to which the potentiometers slider assembly is coupled. Consequently, uniform slider operation with substantially no eccentricity is insured over the entire potentiometer range, and in addition, potentiometer linearity is enhanced.

As will be described in detail hereinbelow, the novel core assembly employed in the multiturn potentiometers of the invention may utilize a conventional compound helix form of resistance element in which a resistance Wire is wound over an associated mandrel, or may simply employ a length of resistance wire wound directly over the coil form to provide an infinite resolution device. in either instance the coil form per se has a relatively smooth outer periphery and is composed of dimensionally stable material which is insensitive to either high humidity, large temperature variations, or aging.

In those potentiometers employing compound helix resistance elements, the resistance element is preferably wound upon the coil form first, the spacing between adjacent turns of the resistance element being of the same order of magnitude as the diameter of the resistance element; the guide helix is then wound thereover in the groove formed by the resistance element, the guide helix 3 preferably being formed of a wire insulated with Teflon (KEL-F), for example, and'having a diameter larger than the spacing between adjacent turns of the resistance element but smaller than the pitch of the resistance element helix. On the other hand, if an infinite resolution potentiometer is desired wherein the resistance element comprises a length of relatively fine resistance wire, the guide helix may first be wound upon the coil form with sufficient spacing between turns to permit the resistance element to be subsequently wound upon the coil form.

Still another feature 'of the invention is the utilization of a novel traveling nut stop mechanism which provides positive metal-to-metal stops at either end of the potentiometers mechanical range, and the inclusion of means for adjusting the mechanical range 'to within one or two degrees or" are.

It is therefore an object of the invention to provide multiturn potentiometerswhose electrical and me chanical characteristics are extremely stable and which are capable of withstanding severe environmental changes.

it is another object of the invention to provide highly stable multiturn potentiometers wherein a non-hygroscopic coil form is employed for mounting an associated resistance element.

Still another object of the invention is to provide multiturn potentiometers wherein the core assembly comprises a ceramic coil form, resistance element helically wound thereover, and a slider guide helically wound over the resistance element in the helical groove defined by ad jacent turns thereof.

A further object of the invention is to provide both high resolution and infinite resolution potentiometers wherein the resistance element is wound over a smooth ceramic coil form and wherein the associated slider is guided by a guide helix which projects radially beyond the resistance element and which is interwound between adjacent turns thereof.

Another object of the invention is to provide multiturn potentio-meters of high linearity wherein the core assembly is concentrically mounted over a metal hub in which the input shaft is concentrically journaled, thereby to eliminate eccentricity between a helical resistance element carried by the coil form and an associ ted slider assembly mounted on the input shaft.

It is also an object of the invention to provide multiturn potentiometers having a metal-tc-rnetal stop mecha nism for positively delimiting the potentiometers mechanical range, and means for adjusting the ends of the range to within several degrees of arc.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understoo from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. it is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not ir tended as a definition of the limits of the invention.

Fig. 1 is an elevation view, partly in section, of a precision multiturn potentiometer constructed in accordance with the invention;

Fig. 2 is an isometric view of a ceramic coil fo m which may be employed in the embodiment of Fig. 1'

Figs. 3, 4 and 5 are views illustrating one manner in which the potentiometer resistance element slider guide may be mounted on the coil form of Fig. 2;

Figs. 6 and 7 are respectively an isometric view and cross-sectional view of one form of slider assembly which may be utilized in the precision multiturn potentiometers or" the invention; and H Fig.8 is a diagrammatic view illustrating the applica- 13 tion of the invention to an infinite resolution multiturn potentiometer.

With reference now to the drawings, wherein like or corresponding parts are designated by the same reference characters throughout the several views, there is shown in Fig. 1 a multiturn potentiometer, according to the invention, which is operable in response to rotation of an input shaft it) for conjugally varying the resistance between an output terminal 12 and each of a pair of input terminals 14 and 16, across which a resistance element is connected. According to the fundamental concept of the invention the potentiometer of Fig. 1 includes four basic elements, namely, a central integral hub 18 through which shaft m is journaled, a traveling nut stop mechanism generally designated 2% for delimiting the rotational movement of the input shaft, a core assembly generally designated 22 which is concentrically mounted over hub 18 and which includes the potentiometer resistance element, and a slider assembly generally designated which is rotatable in accordance with rotation of input shaft ill and functions to provide a sliding tap connection to the resistance element.

The central integral hub 18 is preferably constructed of aluminum or some other metallic material to pro vide a dimensionally stable and temperature insensitive reference for concentrically mounting the core assembly, the input shaft and the wiper assembly. Consequently all of the operational elements which affect potentiometer linearity and stability have a single support and accuracy reference, and eccentricity orrun-out between cooperating elements is substantially eliminated.

in the particular embodiment of the invention shown in Fig. 1, input shaft it is journaled within hub 18 by a pair of ball bearings 26 and 23, respectively, the outer races of these bearings engaging a pair of C rings 3t; and 32 which are seated in slots provided therefor in hub 18. The inne races of bearings 26 and 28, in turn, are respectively engaged by a retainer ring 34 seated in a slot in shaft la and by a shouldered mounting ring 36 which is afiixed to shaft 10 by a set screw or the like, not shown. As will be described in more detail hereinbelow, mounting ring 36 also functions as the mounting element for insulatively supporting the associated slider arm assembly 2 so that it may rotate in accordance with the input shaft.

In accordance with the invention, stop mechanism 29 provides a rugged metal-to-metal stop at each end of the potentiomete"s range, and comprises a traveling nut 38 whose periphery has a slot 4t} machined therein to receive a pair of traveling nut guide pins 42 and 4-4 which are threaded into hub 155, the traveling nut being operative to move axially along shaft lit over a threaded portion 46 thereof. This threaded region may be either machined onthe shaft per se or may be a sleeve afiirrcd thereover, and has a pair of tapered dowel pins id and 50' seated therein at opposite ends. The stop mechanism is completed by a stop pin 52 which is mounted in and carried by the traveling nut, the length of the stop pin and the a.- and radial position of the dowel pins 43 and 58 being preselected to provide the desired range of potentiometer adjustment.

More particularly, if it is assumed for purposes of illustration that the potentiometer of Fig. l is to provide an effective electrical angle of 3600 or ten turns, with a mechanical overtravel of 90 at each end, then the dowel pins should be so disposed radially relative to each other that they are engaged by stop pin 52 at opposite ends of a total shaft rotational angle of approximately 3780'. Thereafter the rotational overtravel at opposite ends of the potentiometers range may be accurately controlled to within one or two degrees of are by employing traveling nut guide pins whose tip diameters are only preselected fractionsof the-width of slot 40 in the traveling nut. Thus when-the -in'put shaft isturned to its limit in the clockwise direction 2,8?1, see

only pin 42 rides in the traveling nut and stop pin 52 engages dowel 48, whereas at the end of the counterclockwise range pin 44 rides in the traveling nut slot and the stop pin engages dowel 50, the reduced tip diameter of the guide pins functioning to provide a predetermined play in traveling nut which provides the desired rotational overtravel at each end of the potentiometers range.

It is clear, therefore, that the stop mechanism employed in the potentiometers of the invention permits excep tionally precise control of the rotational range cone-omitant with positive metal-to-metal stops which are independent of the potentiometers resistance element and core assembly. Consequently no extra forces are eX- erted on slider assembly 24 and core assembly 22 at the ends of the potentiometers range, as is the case in many prior art multiturn potentiometers.

Consider now the novel core assembly employed in the precision potentiometers herein disclosed and the manner in which the resistance element and the guide for the wiper arm assembly are afiixed thereon. In accordance with the invention the core assembly includes a nonhygroscopic coil form 54 which is preferably made of ceramic, and an associated resistance element 56 which is wound thereover, the ceramic coil form providing essentially complete mechanical stability or in other words, freedom from warping or other dimensional changes which might otherwise occur from moisture, aging and operation at elevated temperatures. More specifically, the coil forms which the invention contemplates have relatively small coefficients of thermal expansion, are relatively insensitive to high humidity. are chemically inert, and are capable of operating over a temperature range approaching 200 centigrade. As a result of the foregoing factors, the potentiometers of the invention maintain their high stability of electrical characteristics under the most extreme environmental conditions and over long periods of time.

The utilization of coil forms of ceramic or similar stable materials in the potentiometers constructed in accordance with the invention is permitted primarily b cause the surface of the coil form may be relatively smooth and does not require grooves for seating the resistance element or for guiding the associated wiper element, as in the potentiometer of the prior art. The elimi nation of the foregoing requirement is in turn permitted by another novel feature of the core assembly, namely, the use of an insulating guide helix, designated 58 in Fig. l, which is wound over resistance element 56 and which seats in the grooves formed by adjacent turns of the resistance element. As shown in Fig. 1, the guide helix is supported by adjacent turns of the resistance element, and protrudes radially above the outermost surface of the turns thereof. Consequently the guide helix performs two different functions, namely, it maintains the spacing between adjacent turns of the resistance element, thereby eliminating the need for a helical seating groove in the coil form, and serves as guide for the associated potentiometer slider, thereby eliminating the need for either a guide groove in the coil form or for a follower element which employs the resistance element per se as a guide.

With reference now to Figs. 2 through 5, the various steps which are employed in the formation of the coil assembly, including the mounting of the resistance element on the coil form and the winding thereover of the guide helix, will be described. Referring now with particularity to Fig. 2 there is shown an isometric view of the coil form employed in the particular embodiment of the invention shown in Fig. 1, the coil form being preferably cylindrical in shape although it will be appreciated that other configurations, such as a conical frustum, could be employed if a non-linear function were to be simulated.

Although the coil form may be composed of any suit- Lil able material which has the characteristics previously enumerated, the fabrication of the coil form by ceramic casting has been found especially satisfactory, one suitable ceramic being steatile offered for sale by American Lava. The coil form may then be ground if necessary to provide a smooth periphery for subsequently mounting the resistance element and to assure minimum eccentricity when the completed coil assembly is later mounted on the potentiometers central hub. For the particular embodiment of the invention herein shown and described the coil form may also be ground on one end at two points spaced approximately 180 apart to provide a pair of notches 60 and 62 which are employed for later clamping the core assembly in its operational position.

In addition to the foregoing operations, a plurality of apertures are also ground or drilled through the coil form at predetermined points, as shown in Fig. 2, after which a pair of resistance element clamping lugs 64 and 66 are swaged in two of the apertures and a pair of identical clamping lugs 68 and 70 are swaged in two adjacent apertures for subsequently mounting the guide helix. One of the remaining apertures, designated 72 in Fig. 2, is left open to facilitate the distribution of the potentiometers electrical input conductors, while a pair of low impedance plugs 74 and 76 which may be composed of beryllium copper, for example, are swaged into the remaining apertures in the coil form.

The relative positions of the apertures in the coil form are not particularly critical, and may be readily determined by looking to the electrical and mechanical specifications which the potentiometer is designed to meet, as for example, the total mechanical angle of the input shaft and the actual effective electrical angle of the potentiometer. More specifically, clamping lugs 64 through 70 should be positioned a sufficient distance apart, in view of the number of resistance turns required and the pitch of the resistance element, to permit the potentiometer input shaft to traverse its entire mechanical range without driving the arm to the end of the resistance element. The low impedance plugs 74 and 76, on the other hand, are provided for tapping the resistance element at opposite ends of the potentiometers electrical range, and must therefore be properly located both axially and radially with respect to each other in view of the potentiometers electrical range and the pitch of the resistance element.

After the coil form has been fabricated, resistance element 56 is wound thereon, as shown in Fig. 3, the end of the resistance element being held by a clamp 78 which in turn is held by the lug designated 64 in Fig. 2. As shown in Fig. 3 by way of illustration, the resistance element here comprises a compound helix wherein a resistance wire is first wound upon a mandrel wire after which the combination is wound on the coil form, care being taken to maintain constant the spacing between adjacent turns thereof. At the conclusion of this operation the other end of the resistance element is then afiixed to the coil form by a clamp 80, as seen in Fig. 5.

After the resistance element is wound upon the coil form electrical connections are made between the plugs 74 and 76 and the resistance elements at opposite ends of its electrical range. As shown in Fig. 4, which is a magnified View of a portion of the coil form 54 and resistance element 56, the electrical connections to the resistance element are preferably made by tap welding a relatively fine jumper wire 82 first to plug 74 and then to a turn of the resistance wire which, together with a mandrel wire 84, forms the resistance element. The same process is then repeated at the opposite end of the potentiometers electrical range, the welding process preferably being carried out with the aid of a suitable optical magnifier to assure that the electrical connections are properly made and to provide the precise electrical range desired.

Following the tap welding operation the guide helix ecrees 58, which is employed for maintaining the turn spacing of the resistance element, .is woundthereover and is staked at opposite ends, as shown in Fig. 5, we pair of clamps 86 and 38. In accordance with the invention, as pointed out hereinbefore, the guide helix is preferably an insulator; one form of guide helix which has been found to function satisfactorily comprises Teflon insulated wire, the Teflon functioning to provide a lubricating action with respect to the slider element when the potentiometers input shaft is rotated. The final operation in the fabrication of the core assembly is to tap weld a pair of electrical conductors 9t and 92 to the internal ends of low impedance plugs 74 and 76, the conducto-rs'being brought out through aperture 72 in the coil form, substantially as shown in Fig. 5. The core assembly is now completed and may be mounted over. the potentiometefs central hub.

With reference once more to the elevational view of Fig. 1, coil form 54 seats in an annular groove in hub 18, and is clamped to the hub at its opposite end by a pair of clamps 94 and 96 which engage in notches 6d and 62 in the coil form. it will be noted that the coil form is spaced from the central portion of hub 18 to provide access for conductors 90 and 92 within the coil form, the external ends of the conductors being in turn connected to input terminals 14 and after passing through an aperture 98 in an outer housing member 1%. As viewed in Fig. l the housing member is affixed to a shouldered rim of hub 18 by suitable fasteners, not shown, and in turn supports an insulating terminal box 162 in which the input terminals are mounted, as by swaging.

Consider now the structure and operation of slide assembly 24. It will be recalled that mounting ring 36 which is affixed to the right hand end of shaft it}, as viewed in Fig. 1, functions simultaneously as a stop for the inner race of bearing 28 and as the mounting element of the slider assembly. The slider assembly per se comprises a number of elements including a slider arm 1% which is insulatively mounted concentric with shaft 1% by a pair of insulating washers 166 and 10S and a pair of screws, designated 110, which seat in mounting 36, the slider arm having at least one longitudinally extending arm which is disposed parallel to the outer periphery of the coil form and adjacent its resistance element. In sliding engagement with this arm is a slider block 112 to which is affixed a slider or wiper 114 one end of which slideably engages the resistance element between adjacent turns of guide helix 58, the other end of slider 114 eing held in sliding engagement with slider arm 1%. The slider assembly is completed by a slip ring 116 to which output terminal 12 is appended and with which a pair of brush members 113, electrically and mechanically connected to slider arm 1%, are in sliding engagement. As shown in Fig. -l, slip ring 115 is seated in and is affixed to a rear cover plate 129 which in turn is aihxed to outer housing member 1% with suitable fasteners not shown.

With reference now to Figs. 6 and 7 there is shown in more detail the structure of slider arm 10%, slider block 112, and brush members 118. As shown in Fig. 6 with particularity, slider block 112 is formed from sheet material, such as beryllium copper, and is cut away on one side to provide a pair of guides 12% for the free end of slider 114. The slider in turn is affixed to the slider lock at one or more points, as by soldering as indicated at point 122, and is so shaped to provide a sliding low impedance electrical connection to wiper arm 104- at point 124.

As shown in Fig. 7, the lower portion of wiper 114 is cantilevered from the wiper block and is shaped to ride in the groove formed between adjacent turns of guide helix 53 and to engage element 56 at substantially a point contact. It is clear, of course, that slider 114 should be constructed of a low'iinpedance, non-oxidizing and'mechanically resilient material, one such material being sold under the trade name Paliney by the I. M. Ney Co. of Hartford, Connecticut.

Returning now toPig; 1, it may be seen that as input shaft lld is rotated over its range, slider 114 continuously engages resistance element 56, and in addition, bears against the side of guide helix 5S and is thereby forced axially with respect to input shaft it to move slider block 112 along slider arm 164. It is also apparent that at the ends of the input shaft range stop mechanism 243 functions to resist further rotational movement without imparting any additional forces whatsoever to the slider assembly. Consequently the stability and accuracy of potentiometers constructed in accordance with the invention are even further enhanced.

In the foregoing description of the potentiometers em bodiment shown in Figs. 1 through 7 there is disclosed the adaptation of the novel teachings of the invention to multiturn potentiometers wherein the resistance element constitutes a compound helix, and wherein, therefore, resolution is limited. It is to be expressly understood, however, that the teachings of the invention are also applicable to other forms of multiturn potentiometers, such as infinite resolution otentiometers and potentiometers capable of dissipating relatively large amounts of power.

Referring now to Fig. 8, there is shown a modified type of core assembly which may be employed in the potentiometer of Fig. l to provide infinite resolution, the core assembly again including a coil form 54, a resistance element 56 which here comprises a length of resistance wire, and a guide helix 5% which is again employed for guiding the associated potentiometer slide assembly. In the construction of this unit a very light film of an epoxy resin is first brushed on the coil form, after which guide helix 53 is wound thereover and clamped. A single strand of resistance wire of the desired resistance per unit length is then wound into the groove formed by adjacent turns of the guide helix, substantially as shown, after which the entire assembly is baked in an oven to solidify the resin. The completed core assembly may then be used directly in the potentiometer structure of Fig. l, the only modification required being the addition of a small contact shoe to the slider wire 114 so that the contact shoe performs the actual contacting function. addisupport for the resistance wire, and allows tap connection to be made almost anywhere in the windi Still another modification of the invention, may also be described with reference to Fig. 8, is the utilization of a heavily anodized aluminum cylinder as the coil form, together with a metallic guide helix, to provide a slide-wire potentiometer capable of operating-at extremely high temperatures and/or dissipating a relatively large amount of power. The principal advantage of this construction, in addition to those advantages previously set forth, is that it eliminates substantially all-organic materials in the potentiometers cooperating elemen a while still providing a dimensionally stable and accurate reference base which new functions as an excellent heat sink as well.

In view of the foregoing description it will he recognized that numerous other modifications and alterations may be made in the potentiometers herein disclosed with out departing from the spirit and scope of the invention. For example, it is clear that the slider assembly could employ a pair of sliders and a pair of slip rings for providing output signals at two output terminals simultaneously. Again, it will be recognized that the coil form could carry dual resistance elements each with its own slider and helical guide therefor, in which instance a pair of slip rings would again be provided, one for each of the resistance elements. Accordingly, it is to be expressly understoodthatthe scope of the invention is to 9 be limited only by the spirit and scope of the appended claims.

What is claimed as new is:

1. In a multiturn potentiometer wherein a slider ele ment is rotatable in accordance with the rotation of an input shaft to provide a variable resistance as the shaft is rotated through a plurality of revolutions, a di1nensionally stable core assembly comprising: a coil form of substantially cylindrical configuration and formed from a non-hygroscopic material; a resistance element wound over said coil form in a helical pattern; and an insulating guide helix wound between adjacent tu ns of said resistance element, the cylinder defined by the outermost periphery of said guide helix having a diameter larger than that of the cylinder defined by the outermost periphery of said resistance element whereby the slider element is slidably engageable with the outer periphery of said resistance element and is guidable axially with respect to said coil form by adjacent turns of said guide helix.

2. The combination defined in claim 1 wherein said coil form is composed of ceramic.

3. The combination defined in claim 2 wherein said resistance element com-prises a mandrel wire and a resistance wire helically wound over said mandrel wire, said mandrel wire being wound over said coil form in said helical pattern.

4. The combination defined in claim 3 wherein said insulating guide helix comprises an insulated wire having a diameter larger than the spacing between adjacent turns of said resistance element but smaller than the pitch of said helical pattern whereby adjacent turns of said guide helix are spaced from each other to permit the slider eleme t to siidably engage said outer periphery of said resistance element.

5. The combination defined in claim 4 wherein said insulated wire of said guide helix is insulated with Teflon.

6. The combination defined in claim 1 wherein said coil form is composed of aluminum and wherein at least the outer surface or" said coil form is heavily anodized to provide insulation with respect to said resistance element.

7. The combination defined in claim 1 wherein said resistance element comprises a resistance wire wound in said helical pattern.

8. In a multiturn potentiometer, a dimensionally stable core assembly substantially impervious to moisture and capable of operation at relatively high temperatures, said core assembly comprising: a coil form composed of non-hygroscopic material and having an axis, the periphery of said coil form being relatively smooth and defining a surface of revolution with respect to said axis; a metallic resistance element helically wound over said coil form in the configuration of a helix; and an insulating member wound over said coil form and said resistance element, said insulating member being Wound in the helical groove defined by adjacent turns of said resistance element and extending radially from said axis beyond the surface of the adjacent turns of said resistance element thereby to provide a helical groove whose sides are bounded by said insulating member and whose bottom is bounded by said resistance element.

9. The combination defined in claim 8 which further includes fastening means for staking the ends of said resistance element and of said insulating member to said coil form.

10. The combination defined in claim 9 wherein said coil form is composed of ceramic and wherein said fastening means comprises metallic plugs swaged in apertures in said coil form adjacent to the end of said resistance element and of said insulating member.

11. In a linear multiturn potentiometer, the combination comprising: a ceramic cylinder; a compound helix resistance element positioned over said cylinder,

adjacent turns of said resistance element defining a first helical groove and being spaced apart by a predetermined distance; an insulated wire wound over said ceramic cylinder and sealed in said first groove, the diameter of said insulated wire being larger than said predetermined distance but smaller than the pitch of said resistance element whereby adjacent turns of said insulated wire define a second helical groove bounded at the bottom by said resistance element; and means including a slider element selectively moveable in said sec- 0nd groove and in contact with said resistance element for providing a moveable electrical tap point to said resistance element.

12. A multiturn potentiometer comprising: a hollow metallic hub including a cylindrical portion and an outwardly extending flange portion integral with one end of said cylindrical portion; an input shaft journaled in said hub concentrically therewith; a core assembly including a non-hygroscopic coil form concentrically mounted over said cylindrical portion of said hub, a resistance element helically wound over said coil form, and an insulating guide helix wound over said coil form and seated in a first helical grove defined by adjacent turns of said resistance element, the outer periphery of said guide helix extending radially beyond the outer periphery of said resistance element whereby said guide helix defines a second helical groove bounded at the bottom by said resistance element; means for applying an electrical potential to said resistance element adjacent to the opposite ends thereof; and a slider assembly coupled to said in ut shaft and rotatable therewith, said slider assembly including slider means positioned in said second helical groove in sliding engagement with said resistance element, said guide helix functioning to move said slider means axially with respect to said hub when said slider means is rotated with respect to said hub by rotation of said input shaft whereby the movement of said slider means defines a helical path in said second helical groove.

13. A multiturn potentiometer comprising: a hollow metallic hub including a cylindrical portion and an outwardly extending flange portion integral with one end of said cylindrical portion; an input shaft journaled in said hub concentrically therewith; a core assembly including a non-hygroscopic coil form concentrically mounted over said cylindrical portion of said hub, a resistance element helically wound over said coil form. and an insulating guide helix wound over said coil from and seated in a first helical groove defined by adjacent turns of said resistance element, the outer periphery of said guide helix extending radially beyond the outer periphery of said resistance element whereby said guide helix defines a second helical groove bounded at the bottom by said resistance element; means for applying an electrical potential to said resistance element adjacent to the opposite ends thereof; and a slider assembly coupled to said input shaft and rotatable therewith, said slider assembly including a slider arm insulatively connected to said input shaft and rotatable therewith, said slider arm having a straight end portion extending longitudinally along said core assembly adjacent to the outer periphery thereof and being rotatable about said core assembly upon rotation of said input shaft, a slider block slidably mounted on said straight end portion of said slider arm, and slider means connected to said slider block and positioned in said second helical groove in slide engagement with said resistance element, said guide helix functioning to move said slider means axially with respect to said hub when said slider means is rotated with respect to said hub by rotation of said input shaft whereby the movement of said slider means defines a helical path in said second helical groove.

14. The potentiometer defined in claim 13 which further includes brush means mechanically mounted on said slider arm and electrically connected to said slider means, and a slip ring fixedly and concentrically mounted with respect to said hub and engaged by said brush means thereby to provide a low impedance electrical path between said slip ringand the point on said resistance element engaged by said slider means.

15. The potentiometer defined in claim 12 wherein said coil form comprises a ceramic cylinder one end of which abuts said flange portion of said hub.

16. A multiturn potentiometer comprising: a hollow metallic hub including a cylindrical portion and an outwardly extending flange portion integral with one-end of said cylindrical portion; an input shaft journaled in said hub concentrally therewith; a core assembly including a nonhydroscopic coil form concentrically mounted over said cylindrical portion of said hub, a resistance element helically wound over said coil form, an insulating guide helix wound over said coil form and seated in a first helical groove defined by adjacent turns of said resistanceelement, the outer periphery of said guide helix extending radially beyond the outer periphery of said resistance element whereby said guide helix defines a second helical grove bounded at the bottom by said resistance element; means for applying an electrical potential to said resistance element adjacent to the opposite ends thereof; a slider assembly coupled to said input shaft and rotatable therewith, said slider assembly including slider means positioned in said second helical groove in sliding engagement with said resistance element, said guide helix functioning to move said slider means axially with respect to said hub when said slider means is rotated with respect to said hub by rotation of said input shaft whereby the movement of said slider means defines a helical path in said second helical groove; and a. stop mechanism for delimiting the range over which said input shaft is rotatable, said stop mechanism comprising a metal thread carried by said input shaft and positioned within the cylindrical portion of said hub, a traveling nut threaded on said thread and having an axially extending slot in its periphery; at least one guide pin seated in said cylindrical portion of said hub and extending inwardly into said slot in said traveling nut, a pair of dowel pins seated in said input shaft adjacent opposite ends of said thread; and a stop pin extending axially through said traveling nut predetermined distances on both sides thereof, said stop pin being engageable by one of said dowel pins to arrest rotation of said shaft when said shaft is rotated continnously in one direction and being engageable by the other of said dowel pins to arrest rotation of said shaft when said shaft is rotated continuously in the opposite direction.

17. In a multiturn potentiometer, a metal-to-metal stop mechanism for delimiting the rotational range of an associated input shaft, said stop mechanism comprising: a metallic hub having a cylindrical region; means for journaling the input shaft within the cylindrical region of said hub and concentric therewith, the portion of the input shaft extending within said cylindrical region being threaded; a traveling nut threaded on said threaded portion of said shaft and having an axially extending slot in its peripher; traveling nut guide means seated in said cylindrical region of said hub and extending inwardly into said slot in said traveling nut; a pair of dowel pins seated in said input shaft adjacent opposite ends of said threaded region and extending radially from said shaft; and a stop pin extending axially through said traveling nut predetermined distances on both sides thereof, said stop pin being engageable by one of said dowel pins to arrest rotation of said shaft when said shaft is rotated continuously and being engageable by the other of said dowel pins to arrest rotation of said shaft when said shaft is rotated continuously in one direction in the opposite direction.

18. The combination defined in claim 11 which further includes; at least two low impedance metallic plugs imbedded in and extending through said cylinder at two spaced points; a pair of welded connections between said metallic plugs and two spaced points on said resistance element; and a pair of electrical conductors respectively connected to said metallic plugs within said ceramic cylinder.

19. in a multiturn potentiometer wherein a slider element is rotatable in accordance with the rotation of an input shaft to provide a variable resistance as the shaft is rotated through a plurality of revolutions, a core assembly comprising: a coil form of substantially cylindrical configuration and composed of an insulating material; a resistance element wound over said coil form in a helical pattern; and an insulating guide helix wound between adjacent turns of said resistance element, the cylinder defined by the outermost periphery of said guide helix having a diameter larger than that of the cylinder defined by the outermost periphery of said resistance element whereby the slider element is slidably engageable with the outer periphery of said resistance element and is guidable axially with respect to said coil form by adjacent turns of said guide helix.

20. A multiturn potentiometer comprising: a hollow metallic hub including a cylindrical portion and an outwardly extending flange portion integral with one end v of said cylindrical portion; an input shaft journaled in said hub concentrically therewith; a core assembly including an insulative coil form concentrically mounted over said cylindrical portion of said hub, a resistance element helically wound over said coil form, and an insulating guide helix wound over said coil form and seated in a first helical groove defined by adjacent turns of said resistance element, the outer periphery of said guide helix extending radially beyond the outer periphery of said resistance element whereby said guide helix defines a second helical groove bounded at the bottom by said resistance element; means for applyingan electrical potential to said resistance element adjacent to the opposite ends thereof; and a slider assembly coupled to said input shaft and rotatable therewith, said slider assembly including slider means positioned in said second helical groove in sliding engagement with said resistance element, said guide helix functioning to move said slider means axially with respect to said hub when said slider means is rotated with respect to said hub by rotation of said input shaft whereby the movement of said slider means defines a helical path in said second helical groove. 9

References tCited in the file of this patent UNITED STATES PATENTS 644,311 Anderson Feb. 27, 1900 644,312 Anderson Feb.'27, 1900 2,495,321 Gibbs et al. Jan. 24, 1950 2,662,955 Erltelens Dec. 15, 1953 2,665,355 Van Alen et al. Ian. 5, 1954 2,696,544 Poch Dec. 7, 1954 2,724,034 Altieri Nov. 15, 1955

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