US3339166A - Rotary-contact potentiometers - Google Patents

Description

Aug. 29, 1967 J. MAURICE ROTARY-CONTACT POTENTIOMETERS Filed April 16, 1965 5 Sheets-Sheet 1 Maw/m? Aw/v MAMe/cA- g- 29, 1967 J. MAURICE ROTARY-CONTACT POTENTIOMETERS 3 Sheets-Sheet 2 Filed April 16, 1965 M ViA IM 5, JW J0 0- Aug. 29, 1967 J. MAURICE ROTARY-CONTACT POTENTIOMETERS 3 Sheets-Sheet 5 Filed April 16, 1965 Awnvraf c/f/IN M4 Me/ae- United States Patent 3,339,166 ROTARY-CONTACT POTENTIOMETERS Jean Maurice, 7 Rue Defly, Nice, France Filed Apr. 16, 1965, Ser. No. 448,705 Claims priority, application France, Apr. 29, 1964, 972,760, Patent 1,402,082 8 Claims. (Cl. 338-155) ABSTRACT OF THE DISCLOSURE A rotary contact potentiometer has a base plate with a ring resistor on it and a pair of electrically conductive discs in tangential contact with the ring resistor. A rotary member having a resilient arm bears against the discs and causes them to wobble about rolling contact with the ring resistor; but a pin-in-slot connection prevents axial rotation of the discs. The use of plural discs reduces the contact pressure of the discs on the resistor.

The present invention relates to a rotary-contact potentiometer of the type comprising a resistor having the shape of an open ring which is fixed to an insulating base-plate and on which a moving contact is designed to travel.

The improvements in accordance with this invention relate more especially to the mode of construction of the moving contact-or sliderof the potentiometer, and its control.

In the potentiometers of the type at present used, the moving contact usually consists of a flexible metallic blade, the extremity of which may be fitted with a contactpoint which is either of graphite or precious metal and which is applied in rubbing action on the surface of the resistor. The wear caused by repetition of said rubbing action produces a variation in the electrical characteristics of the instrument which may be important.

Such a variation, which is acceptable in potentiometers of commercial types, is highly undesirable when it is desired to obtain good stability of control operations. This is true especially in the case of miniature potentiometers, in Which the resistor is a very delicate component and is therefore liable to become rapidly damaged as a result of wear. Consequently, the service life of such potentiometers can be of particularly brief duration.

Attempts have been made to limit the wear of the resistor by interposing a flexible metallic strip between the moving contact and the resistor. Since the strip is subjected to elastic deformation in order to localize the contact at the point at which the spring of the moving contact exerts its pressure, the service life of the mechanism is shortened by the excessive mechanical fatigue to which the blade is subjected. In addition to this fact, premature wear of the control members results from the strain induced by the high pressure exerted by the spring on the said blade.

It is therefore an object of the instant invention to obviate the above drawbacks.

Another object of this invention is to provide a potentiometer in which the rotary contact is capable of moving over the resistor practically without friction, thereby making the presence of an intermediate flexible contact member merely optional.

These and further objects of the invention will become apparent as the specification proceeds.

In accordance with the present invention, the rotarycontact potentiometer, which comprises a ring-resistor mounted on an insulating base-plate and a control shaft fitted with a flexible blade for the purpose of actuating the moving contact is mainly characterized in that the said contact comprises an electrically conductive disc which is arranged at an oblique angle relative to the resistor base-plate and which is held applied against and tangentially to the said resistor by the blade spring of the said control shaft, a coupling system being additionally provided between the resistor base-plate and the contact disc in such a manner that the said disc is capable of rolling without sliding over the said resistor upon angular displacement of the control shaft.

In accordance with a preferred embodiment of the invention, the disc which serves as a moving contact is applied through the intermediary of a rounded member against a cupped socket formed at the center of the insulating base-plate of the resistor. The rounded member and the socket may also be used for the electric connection of the moving contact.

In accordance with another preferred embodiment of the invention, the angle which is made with the plane of the contact resistor by the moving disc is smaller than 45, the diameter of the disc being close to the diameter of the ring formed by the resistor. The elastic blade which is made fast for rotation with the control shaft is located on the side opposite to the resistor and is inclined to the disc at a relatively small angle, the contact between the blade and the disc being intended to take place at a point located approximately mid-way between the center and the periphery of the said disc.

Provision is made for a number of different forms of the coupling system between the disc and the resistor base-plate in order to ensure the rolling motion of the moving contact without sliding. In accordance with one of the forms referred-to, the said system comprises a substantially radial partition wall mounted on the baseplate and engaged with play within a recess provided for this purpose in the disc.

This arrangement can be replaced by a stud which is carried by the disc and engages Within a notch formed in a substantially radial wall which is arranged on the base-plate.

In accordance with a further alternative embodiment of the invention, the coupling system comprises a toothed annulus which is fixed to the rotary disc and which is adapted to engage with a second toothed annulus mounted on the base-plate of the resistor. This arrangement can in turn be provided in a number of different practical forms of execution.

Further characteristic features of the invention will become apparent from the following description.

In the accompanying drawings which are given solely by way of example without implied limitation, there have been shown four particular forms of the invention.

FIG. 1 is a view in elevation of the first embodiment and in cross-section through the casing and associated components.

FIG. 2 is a plan view in cross-section taken along the line 11-11 of FIG. 1.

FIG. 3 is a view in diametral cross-section of the potentiometer base-plate and of the moving contact, this cross-section being taken along the line IIIIII of FIG. 2.

FIG. 4 is a view which is similar to FIG. 1, showing the second embodiment.

FIG. 5 is a similar view of the third embodiment, the casing being assumed to have been removed.

FIG. 6 is a corresponding simplified plan view of FIG. 5.

FIG. 7 shows in elevation and in axial half-section another embodiment of the invention, the control device being assumed to have been removed.

FIG. 8 is a plan view of the base-plate of FIG. 7 after the rotary disc has been removed.

Referring now to FIGS. 1 to 3, there can be seen at 1 the insulating base-plate of circular shape of the potentiometer on which is disposed a layer 2 of resistance material in the form of a split ring, the ends of which are shown at 3 (FIG. 2). At this point, the base-plate 1 is provided with contact-studs 4 which are connected to the ends of the resistor 2. The contact-studs have extensions in the form of connector-pins 5.

The base-plate 1 is normally covered with a casing 6 which is fixed by any suitable means, especially by crimping, pasting or soldering. The casing 6 is fitted with a bearing 7 within which is rotatably fitted a control shaft 8, the axis of which corresponds to the axis of the resistor track 2. The shaft 8 terminates in a projecting block 9 housed within the casing 6, there being mounted on said block an elastic blade 11 which is intended to apply the moving contact 10 against the resistor.

In accordance with the present invention, said moving contact 10 consists of a disc 13 provided on that face thereof which is directed towards the resistor 2 with a frusto-conical rolling edge 14 obtained, for example, by means of an annular chamfer of the metal which forms the disc. On the opposite side of the rolling edge 14, the disc 13 is covered by a thin insulating disc 15 against which the blade spring 11 is applied.

The moving contact as thus designed is rotatably mounted at an oblique angle relative to the axis of the potentiometer through the intermediary of a joint assembly consisting of a conducting ball 16 supported within a cupped socket 17 formed in a raised portion 18 which is provided at the center of the resistor 2.

In addition, the conducting disc 13 is pierced by a spherical central opening 19 having the same radius as the ball 16, thereby permitting the accurate fitting together of the two components, as shown in FIG. 3.

The raised portion 18 is preferably metallic and forms one piece with a connector-pin 21 disposed along the center line of the potentiometer. The pin 21 constitutes together with the two pins the three terminal connectors of the potentiometer.

The angle a made with the resistor track 2 by the disc 13 as measured by the corresponding angle made by the axis XX of the potentiometer with the axis YY of the moving contact is preferably within the range of a few degrees to 45. This angle is chosen in consideration of constructional possibilities which are in turn determined by the maximum dimensions of the instrument. Similarly, the width of the frusto-conical rolling edge 14 is determined according to the composition and dimensions of the surface of the resistor 2. In the example considered, the width of the resistor 2 is 0.5 mm., the width of the chamfer 14 is 0.2 mm. and the angle a is 30.

The coupling system provided between the moving contact as thus designed and the base-plate 1 comprises in the embodiment contemplated a radial partition wall 22 which is disposed between the ends 3 of the resistor 2 and which extends parallel to the axis XX. Said partition wall penetrates into the interior of a radial recess or notch 23'which is formed in the disc 13. The notch 23 is located in the vincinity of the highest point of the disc 13 when the chamfered edge 14 is in contact with the central portion of the resistor 2.

The respective dimensions of the partition wall 22 and of the recess 23 are such that a slight clearance is formed between these two surfaces for the reason which will become apparent hereunder.

The potentiometer being initially in the position shown in FIGS. 1 to 3, as characterized by the fact that the moving contact is disposed symmetrically with respect to the wall 22 and that the blade 11 is applied against the insulating disc 15 in the vicinity of the zone of tangential contact of the rolling edge 14 with the resistor track 2, it will be understood that, if the shaft 8 is caused to rotate either in one direction or in the other, the blade 11 which exerts pressure on the disc 15 is driven in rotation in asymmetric manner with respect to the partition wall 22, thus imparting a rocking torque to the moving contact 10. Under the action of this torque and by reason of the fact that the partition wall 22 prevents the contact 10' from rotating about its own axis, the rolling edge 14 is caused to roll without sliding over the resistor track 2 in the same direction as the direction of rotation of the flexible blade 11.

The angle of inclination of the contact 10 relative to the plane of symmetry of the partition wall 22 is permitted on account of the clearance which is allowed between said partition wall and the recess 23. In the case of predetermined dimensions as hereinabove specified and when a thrust of the order of 150 grams is exerted by the blade spring 11, satisfactory results are accordingly obtained when a clearance of approximately 0.3 mm. is allowed between the surfaces 22, 23. In the case of a clearance of this order, the generator-line of contact between the resistor track 2 and the rolling edge 14 can be displaced along the resistor track over a range of motion which corresponds in practice to the opening of the resistor track 2 itself.

Even if the clearance provided between the surfaces 10 causes a very slight extraneous sliding motion, such a sliding motion, by reason of its very small amplitude, rather tends to produce a favourable result by ensuring the self-cleaning of the portions which are in contact. Moreover, experience has shown that resistors which have a sintered carbon base or which are fabricated of conductive ceramic materials are capable of withstanding without difiiculty frictional forces having values which are determined by the clearances of the order of magnitude considered.

In the embodiment of FIG. 4, the moving contact 10 consists of two superposed discs 31, 32. The disc 31 is provided with a cone-shaped joint 33 and is surmounted by a washer 34 through the center of which projects a pivot-pin 35.

The disc 32 is provided with an axial bore 36 which is mounted with play about the pivot-pin 35. The chamfered edges which are intended to form the frusto-conical rolling surfaces 37, 38 are formed around the periphery of the discs 31 and 32.

The diameters of the discs 31, 32 are so calculated that, after positioning over the insulating base-plate 1, the angle which is made by the disc 32 with the surface of the resistor track 2 is slightly greater than the angle made by the disc 31.

Under these conditions, and if a suflicient clearance exists between the pivot-pin 35 and the bore 36, the disc 32 is applied on the one hand against the periphery of the washer 34 and on the other hand against the resistor 2 by virtue of the thrust exerted by the elastic blade 11 which is fitted in this example with an insulating shoe 39 for the purpose of maintaining the insulation of the control shaft 8.

The advantage of such a design as described above is that the pressure applied by the moving contact 10 against the resistor 2 is reduced. In fact, if the pressure which is exerted by the blade 11 is 200 grams, calculations show that the contact pressure between the disc 32 and resistor 2 is approximately grams whereas the contact pressure exerted by the disc 31 is only 50 grams.

As will be understood, the values referred-to are given solely by way of example. Other values can be obtained according to the diameter of the washer 34 and the circumference along which the shoe 39 exerts its pressure on the disc 32.

In the example considered, the system of coupling between the moving contact 10 and the base-plate 1 of the potentiometer consists of a cylindrical stud 41 which is fixed to the periphery of the disc 32. Said stud is engaged within a slot 42 which is parallel to the axis XX and which is formed in the radial partition Wall 43. The height of the members 42, 43 is sufficient to ensure complete guiding to the stud 41 over the entire range of travel of this latter.

Obviously, the number of contact discs is not limitative and provision can be made for moving contacts which comprise three, four or even a greater number of discs. When it is desired to construct potentiometers of very small size, the discs 31, 32 have dimensions such that the thickness thereof permits a suflicient flexibility to dispense with the need for any articulation between discs. It is merely necessary to ensure that the end discs 32 is of suitable thickness to withstand without appreciable deformation the stress which is produced by the blade spring 11.

In the embodiment of FIGS. 5 and 6, the moving contact consists of a disc 51 provided at its periphery 52 with a sharp edge which is in contact with the resistor track 2. The disc 51 is electrically conductive and is provided at its center with a projecting portion 53 terminating in a ball-shaped member 54 which is applied against a socket 17 of similar type to that described above.

The coupling system between the moving contact as thus designed and the base-plate 1 consist in this example, in the case of the moving contact, of a projecting disc 55 which is provided with a peripheral set of teeth 56. This set of teeth is adapted to engage with a toothed annulus 57 which is arranged parallel to the axis X-X at the periphery of the base-plate 1.

As in the previous form of embodiment, an insulating shoe 39 which is fixed to the blade 11 is applied against the disc 55.

In order to provide a strictly reliable connection between the central connector-pin 21 and the contact 10, it is intended, in this example, to place between said connectorpin and the disc 51 a hair-pin type flexible strip 58. The flexibility of said strip is sufiicient to enable it to counteract the twisting eflort which results from the rotation of the moving contact 10. The engagement of the set of teeth 56 with the toothed annulus 57 serves to prevent any sliding of the disc 51 on the resistor track 2. Similarly, the strip 58 alone can prevent any angular displacements of the contact disc relative to the resistor.

In the embodiment of FIGS. 7 and 8, the insulating base-plate 1 of the resistor track 2 is provided at its center with a toothed annulus consisting of a notched cup 61 traversed by an electrically conductive rod 62 which serves as a connection for the moving contact 10. The rod 62 is also provided with an annular shoulder 63 for the purpose of clamping the cup 61 against the base-plate 1 and is extended by a frusto-conical boss 64 which constitutes one of the elements of a knuckle joint system.

In addition, the moving contact 10 is fitted with a rigid insulating disc 65 which carries a flexible metallic disc 66 and a toothed annulus consisting of a second cup 67, the concavity of which is opposite to that of the cup 61. The teeth of the cups 61, 67 are orientated in such manner as to engage with each other. The assembly of components 65, 66, 67 is traversed by a bush 68, the ends of which are upset in such a manner as to constitute a fixing rivet.

Since the slope of the chamfer of the boss 64 corresponds to the angle between the axes XX and YY of the members 1 and 65, the contact between said boss and the bush 68 is of the tangential type, which is conducive to electrical continuity.

As is evident, the potentiometer is again provided with an elastic blade which is integral with the control shaft disposed as in the previous examples along the axis XX. These members, which are similar to those which have been described earlier, have not been shown for the sake of clarity. The operation of the system is the same as before, the toothed cups 61, 67 being designed to prevent any sliding of the disc 66 which is deformed elastically in the vicinity of its point of contact with the resistor track 2. This arrangement, which takes up very little space and is simple to construct, produces highly satisfactory results. The articulated coupling system 64, 68 ensures high reliability of contact and positioning.

In the embodiment considered, a radial notch 71 is formed in the base-plate 1 so as to permit the correct positioning of said base-plate within the casing. This notch also serves to limit the ends of the resistor 2 on which are mounted the terminal connectors (not shown). The adjustment of the ohmic value of the resistance can be obtained by machining a peripheral chamfer 69 after the resistor 2 has been placed in position on the baseplate 1.

In all the foregoing embodiments, the edges of the discs which are intended to come into contact with the resistor are necessarily free of rough excrescences of any kind. For this reason, the rolling contact provided in accordance with this invention is preferably made on a chamfered surface. However, the machining operation which this entails is not necessary if a mechanical or electrolytic polishing operation is carried out on the moving contact. Striations can also be formed in the said moving contact or this latter can be sand-blasted in order to increase the contact pressure per unit area which is exerted on the resistor. The moving contact can be formed of metal or of any other electrically conductive material such as graphite. The rolling surfaces can also be covered with metallic deposits of precious metals or with a conducting film which may either be rigid or flexible. It will also be understood that the flexible coupling strip 58 which is fitted between the moving contact and the central connector-pin of the potentiometer can also be provided in the other forms of embodiment which have been previously described.

I claim:

1. A rotary contact potentiometer comprising an assembly consisting of an insulating base plate having a ring resistor mounted thereon, at least a first and a second disc in tangential contact with said assembly with at least said second disc being electrically conductive and in tangential contact with said ring resistor, means fitted substantially at the center of said ring resistor for supporting said first disc in an oblique position with respect to said base plate, said second disc being adjacent to said first disc, in tangential contact with a part thereof and fitted with play with respect thereto, rotating control means for resiliently engaging said second disc on the side thereof opposed to said resistor and adjacent to the region which is in tangential contact with said ring resistor, and coupling means for coupling said base plate and said second disc and for preventing said second disc from sliding when tilted by the rotation of said rotating control means.

2. A rotary contact potentiometer according to claim 1, and supporting means for said second disc comprising a cupped socket provided substantially at the center of said insulating base plate, a rounded member in tangential contact with said socket and in supporting engagement with said second conductive disc, said potentiometer further comprising terminals fitted on said base plate in electrical connection with said resistor and with said cupped socket.

3. A rotary contact potentiometer according to claim 1 wherein said rotating control means comprise a control shaft in axial relation with said ring resistor, a blade spring fitted on said shaft and having at least a part thereof in parallel relation with said second disc, and means on said blade spring to slidably engage said second conductive disc in a region thereof located approximately midway between the center and the periphery of said disc.

4. A rotary contact potentiometer according to claim 1 wherein said rotating control means comprise a control shaft in axial relation with said ring resistor, a flexible blade integral with said control shaft, and an insulating shoe borne by said blade in sliding and resilient contact with the surface of said second conductive disc which is opposite to said ring resistor.

' 5. A rotary contact potentiometer according to claim 1 wherein at least one of said discs comprises a frustoconcial rolling edge in tangential contact with said resistor, said rolling edge consisting of a chamfer provided at the periphery of said disc.

6. A rotary contact potentiometer according to claim 1 wherein said coupling means between said base plate and said second electrically conductive disc comprise a substantially radial partition Wall projecting from said base plate, a recess formed in said wall, and a stud carried by said second disc, projecting therefrom and in engagement with said recess, play being provided between said stud and recess.

7. A rotary contact potentiometer as claimed in claim .1, both of said discs being electrically conductive and in tangential contact with said ring resistor.

8. A rotary contact potentiometer as claimed in claim 1, said tangential contact between said discs taking place substantially closer to the center of the discs than the tangential contact of said discs with said assembly.

References Cited UNITED STATES PATENTS 9/1930 Bruno 338-155 FOREIGN PATENTS 514,931 11/ 1920 France. 386,261 9/1930 Great Britain.

OTHER REFERENCES French Add. 35,216 7-1929.

RICHARD M. WOOD, Primary Examiner.

J. G. SMITH, Assistant Examiner.

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